AIR –CONDITIONING

Principles and Concepts

Air conditioning is the process whereby the condition of Air, as defined by its

temperature and moisture content, is changed. In practice other factors must also

be taken into account especially cleanliness; odor; velocity & distribution pattern.

Principles of Air- Conditioning:

Human comfort

Inevitably 'comfort' is a very subjective matter. The Engineer aims to ensure

'comfort' for most people found from statistical surveys .Most people (90%) are

comfortable when the air temperature is between 18-22°C and the %sat is

between 40-65%. This zone can be shown on the psychometric chart. And is

known as the comfort zone.

Outside air is quite likely to be at a different condition from the required comfort

zone condition. In order to bring its condition to within the comfort zone we may

need to do one or more of the following:-heat it; cool it; dehumidify it; humidify

it; or mix it.

Dry air mass flow

In order to use the psychometric chart for air-conditioning work we need to find

& use dry air mass flows. However, in practice air-flows are frequently measured

in terms of volume flow. In order to find dry air mass flow we need to use the

specific volume of the air.

Specific volume = volume/mass

The specific volume of the air is given from the

Psychometric chart in m³/kg of dry air, therefore the Mass flow will be in terms of

dry air mass flow.

Obviously the condition of the air must be known (Typically d.b. temp. & %sat) in

order to find the specific Volume.

Air heating

The heating process can be illustrated on the psychometric chart thus:

Cooling/Dehumidification

In the case of cooling, the mixture will firstly be sensibly cooled to the point of

saturation (called the dew point) then liquid water will precipitate if we cool

further. Because moisture is removed dehumidification is achieved.

The cooling/dehumidification process can be illustrated on the psychometric

chart thus:

Humidification

The process of humidification allows the air to mix with extra water. A sufficient

contact time between the air and water will normally result in the air reaching

100%Saturation. The process is very close to the evaporation from a wet bulb. It

therefore follows a line of constant wet bulb Temperature.

Mixing

Often, instead of exhausting 'stale' air completely some of it is filtered, de-

odorized and mixed with fresh incoming air. This conserves energy and narrows

the operating conditions for the air-conditioning system.

Heat Transfer

Heat is a form of energy. Every object on earth has some heat energy. The less

heat an object has, the colder we say it is. Cooling is the process of transferring

heat from one object to another. When an air-conditioning system cools, it is

actually removing heat and transferring it somewhere else. This can be

demonstrated by turning on a Spot Cooler and placing one hand in front of the

cold air nozzle and the other over the warm air exhaust. You will feel the action of

the transfer of heat.

Sensible and Latent Heat

There are two forms of heat energy: sensible heat and latent heat.

Sensible heat is the form of heat energy which is most commonly understood

because it is sensed by touch or measured directly with a thermometer. When

weather reporters say it will be 90 degrees, they are referring to sensible heat.

Latent heat cannot be sensed by touch or measured with a thermometer. Latent

heat causes an object to change its properties. For example, when enough latent

heat is removed from water vapor (steam or humidity), it condenses into water

(liquid).

If enough latent heat is removed from water (liquid), it will eventually freeze. This

process is reversed when latent heat is added.

Change of State

An object that changes from a solid to a liquid or liquid to vapor is referred to as a

change of state. When an object changes state, it transfers heat rapidly.

Humidity

Moisture in the air is called humidity. The ability of air to hold moisture directly

relates to its temperature.

The warmer air is, the more moisture it is capable of holding. Relative humidity is

the percentage of moisture in the air compared to the amount of moisture it can

hold. A moisture content of 70°F air with 50% relative humidity is lower than 80°F

air with 50% relative humidity.

When the humidity is low, sweat evaporates from your body more quickly. This

allows you to cool off faster. High humidity conditions do not allow sweat to

evaporate as well because the air is at its maximum capacity.

Humidity is also a form of latent heat. When air contains more humidity, it has

more latent heat.

REFRIGERANT

Refrigerants are substances used by air conditioners to transfer heat and create a

cooling effect. Air-conditioning systems use specially formulated refrigerants

designed to change state at specific temperatures providing optimum cooling.

Portables use a refrigerant called R-22 or HCFC-22. HCFC stands for

hydrochlorofluorocarbon.This is currently the most common refrigerant used by

air-conditioning systems.

REFRIGERANT PHASE-OUT

Many of the current forms of refrigerants used today are being phased out based

on concern for depletion of the ozone layer. Portables use R-22, which has been

deemed acceptable for use by the EPA until the year 2010. By that time, an

ozone-friendly refrigerant that can be easily substituted for R-22 will be readily

available.

PSYCHOMETRIC CHART

Psychometric Chart

The principles of psychometric chart apply to any physical system consisting of

gas-vapor mixtures. The most common system of interest, however, are mixtures

of water vapor and air because of its application in heating, ventilating, and air-

conditioning and meteorology.

Psychometric ratio

The psychometric ratio is an important property in the area of psychometrics as it

relates the absolute humidity and saturation humidity to the difference between

the dry bulb temperature and the adiabatic saturation temperature.

Mixtures of air and water vapor are the most common systems encountered in

psychometric. The psychometric ratio of air-water vapor mixtures is

approximately unity which implies that the difference between the adiabatic

saturation temperature and wet bulb temperature of air-water vapor mixtures is

small. This property of air-water vapor systems simplifies drying and cooling

calculations often performed using psychometric relationships.

A psychometric chart is a graph of the physical properties of moist air at a

constant pressure (often equated to an elevation relative to sea level). The chart

graphically expresses how various properties relate to each other, and is thus a

graphical equation of state. The thermo physical properties found on most

psychometric charts are:

Dry-bulb temperature (DBT) is that of an air sample, as determined by an

ordinary thermometer, the thermometer's bulb being dry. It is typically the

abscissa, or horizontal axis of the graph. The SI units for temperature are Celsius;

other units are Fahrenheit.

Wet-bulb temperature (WBT) is that of an air sample after it has passed through

a constant-pressure, ideal, adiabatic saturation process, that is, after the air has

passed over a large surface of liquid water in an insulated channel. In practice,

this is the reading of a thermometer whose sensing bulb is covered with a wet

sock evaporating into a rapid stream of the sample air. The WBT is the same as

the DBT when the air sample is saturated with water. The slope of the line of

constant WBT reflects the heat of vaporization of the water required to saturate

the air of a given relative humidity.

Dew point temperature (DPT) is that temperature at which a moist air sample at

the same pressure would reach water vapor saturation. At this saturation point,

water vapor would begin to condense into liquid water fog or (if below freezing)

solid hoarfrost, as heat is removed. The dew point temperature is measured easily

and provides useful information, but is normally not considered an independent

property. It duplicates information available via other humidity properties and the

saturation curve.

Relative Humidity (RH) is the ratio of the mole fraction of water vapor to the

mole fraction of saturated moist air at the same temperature and pressure. RH is

dimensionless, and is usually expressed as a percentage. Lines of constant RH

reflect the physics of air and water: they are determined via experimental

measurement. Note: the notion that air "holds" moisture, or that moisture

dissolves in dry air and saturates the solution at some proportion, is an erroneous

(albeit widespread) concept

Humidity Ratio (also known as Moisture Content, Mixing Ratio, or Specific

Humidity) is the proportion of mass of water vapor per unit mass of dry air at the

given conditions (DBT, WBT, DPT, RH, etc.). It is typically the ordinate or vertical

axis of the graph. For a given DBT there will be a particular humidity ratio for

which the air sample is at 100% relative humidity: the relationship reflects the

physics of water and air and must be measured. Humidity Ratio is dimensionless,

but is sometimes expressed as grams of water per kilogram of dry air or grains of

water per pound of air.

Specific Enthalpy symbolized by h, also called heat content per unit mass, is the

sum of the internal (heat) energy of the moist air in question, including the heat

of the air and water vapor within. In the approximation of ideal gasses, lines of

constant enthalpy are parallel to lines of constant WBT. Enthalpy is given in (SI)

Joules per kilogram of air or BTU per pound of air.

Specific Volume, also called Inverse Density, is the volume per unit mass of the air

sample. The SI units are cubic meters per kilogram of air; other units are cubic

feet per pound of dry air.

The versatility of the psychometric chart lies in the fact that by knowing three

independent properties of some moist air (one of which is the pressure), the

other properties can be determined. Changes in state, such as when two air

streams mix, can be modeled easily and somewhat graphically using the correct

psychometric chart for the location's air pressure or elevation relative to sea level.

For locations at or below 2000 ft (600 m), a common assumption is to use the sea

level psychometric chart.

How to read the chart

The most common chart used by practitioners and students alike is the "ω-t"

(omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as

the abscissa and the humidity ratios (ω) appear as the ordinates.

In order to use a particular chart, for a given air pressure or elevation, at least two

of the six independent properties must be known (DBT, WBT, RH, Humidity Ratio,

Specific Enthalpy, and Specific Volume). This gives rise to 15 possible

combinations.

DBT : This can be determined from the abscissa

DPT : Follow the horizontal line from the point where the line from the horizontal

axis arrives at 100% RH, also known as the saturation curve.

WBT : Line inclined to the horizontal and intersects saturation curve at DBT point.

RH : Hyperbolic lines drawn asymptotically with respect to the saturation curve

which corresponds to 100% RH.

Humidity Ratio : Marked on Ordinate axis.

Specific Enthalpy : lines of equal values, or hash marks for, slope from the upper

left to the lower right.

Specific Volume : Equally spaced parallel family of lines.

REFRIGERATION CYCLE

Refrigerant

Refrigerants are substances used by air conditioners to transfer heat and create a

cooling effect. Air-conditioning systems use specially formulated refrigerants

designed to change state at specific temperatures providing optimum cooling.

Portables use a refrigerant called R-22 or HCFC-22. HCFC stands for

hydrochlorofluorocarbon.This is currently the most common refrigerant used by

air-conditioning systems.

Refrigerant Phase-Out

Many of the current forms of refrigerants used today are being phased out based

on concern for depletion of the ozone layer. Portables use R-22, which has been

deemed acceptable for use by the EPA until the year 2010. By that time, an

ozone-friendly refrigerant that can be easily substituted for R-22 will be readily

available.

In the refrigeration cycle, a heat pump transfers heat from a lower temperature

heat source into a higher temperature heat sink. Heat would naturally flow in the

opposite direction. This is the most common type of air conditioning. A

refrigerator works in much the same way, as it pumps the heat out of the interior

into the room in which it stands.This cycle takes advantage of the universal gas

law PV = nRT, where P is pressure, V is volume, R is the universal gas constant, T is

temperature, and n is the number of moles of gas (1 mole = 6.022×1023

molecules).

In the refrigerator, the cycle is continuous. In the following example, provided

that the refrigerant being used is pure ammonia, which boils at -27 degrees F. This

is what happens to keep the refrigerator cool:

• The compressor compresses the ammonia gas. The compressed gas heats

up as it is pressurized (orange).

• The coils on the back of the refrigerator let the hot ammonia gas dissipate

its heat. The ammonia gas condenses into ammonia liquid (dark blue) at

high pressure.

• The high-pressure ammonia liquid flows through the expansion valve.

Expansion valve can be considered as a small hole. On one side of the hole

is high-pressure ammonia liquid. On the other side of the hole is a low-

pressure area (because the compressor is sucking gas out of that side).

• The liquid ammonia immediately boils and vaporizes (light blue), its

temperature dropping to -27 F. This makes the inside of the refrigerator

cold.

• The cold ammonia gas is sucked up by the compressor, and the cycle

repeats.

The Refrigeration Cycle

A=Inside the refrigerator

B=Compressor

C=Expansion Valve

Heat pump and refrigeration cycle

Thermodynamic heat pump and refrigeration cycles are the models for heat

pumps and refrigerators. The difference between the two is that heat pumps are

intended to keep a place warm and refrigerators designed to cool it. Technically a

refrigerator cycle is also a heat pump cycle.

A heat pump is when heat is removed from a low-temperature space or source

and rejected to a high-temperature sink with the help of external mechanical

work.

The inverse of the heat pump cycle is the thermodynamic power cycle. In the

power cycle, heat is supplied from a high-temperature source to the heat engine,

part of the heat being used to produce mechanical work and the rest being

rejected to a low-temperature sink. This satisfies the second law of

thermodynamics. A heat pump describes the changes that take place in the

refrigerant as it alternately absorbs and rejects heat as it circulates through a

refrigerator. It is also applied to HVACR work, when describing the "process" of

refrigerant flow through an HVACR unit, whether it is a packaged or split system.

Heat naturally flows from hot to cold. Work is applied to cool a living space or

storage volume by pumping heat from a lower temperature heat source into a

higher temperature heat sink. Insulation is used to reduce the work and energy

required to achieve and maintain a lower temperature in the cooled space. The

operating principle of the refrigeration cycle was described mathematically by

Sadi Carnot in 1824 as a heat engine.

The most common types of heat pump systems use the reverse-Rankine vapor-

compression refrigeration cycle although absorption heat pumps are used in a

minority of applications.

Heat pump can be classified as:

• Vapor cycle,

• Gas cycle, and

• Stirling cycle

Vapor cycle refrigeration can be classified as:

• Vapor compression refrigeration

• Gas absorption refrigeration

Vapor-compression cycle

The vapor-compression cycle is used in most household refrigerators as well as in

many large commercial and industrial refrigeration systems. The following Figure

provides a schematic diagram of the components of a typical vapor-compression

refrigeration system.

Thermodynamics of the cycle can be analyzed on a diagram .In this cycle, a

circulating refrigerant such as Freon enters the compressor as a vapor. From point

1 to point 2, the vapor is compressed at constant entropy and exits the

compressor superheated. From point 2 to point 3 and on to point 4, the

superheated vapor travels through the condenser which first cools and removes

the superheat and then condenses the vapor into a liquid by removing additional

heat at constant pressure and temperature. Between points 4 and 5, the liquid

refrigerant goes through the expansion valve (also called a throttle valve) where

its pressure abruptly decreases, causing flash evaporation and auto-refrigeration

of, typically, less than half of the liquid.

That results in a mixture of liquid and vapor at a lower temperature and pressure

as shown at point 5. The cold liquid-vapor mixture then travels through the

evaporator coil or tubes and is completely vaporized by cooling the warm air

(from the space being refrigerated) being blown by a fan across the evaporator

coil or tubes. The resulting refrigerant vapor returns to the compressor inlet at

point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapor-compression refrigeration cycle,

and does not take into account real-world effects like frictional pressure drop in

the system, slight thermodynamic irreversibility during the compression of the

refrigerant vapor, or non-ideal gas behavior (if any).

Vapor absorption cycle

In the early years of the twentieth century, the vapor absorption cycle using

water-ammonia systems was popular and widely used but, after the development

of the vapor compression cycle, it lost much of its importance because of its low

coefficient of performance (about one fifth of that of the vapor compression

cycle). Nowadays, the vapor absorption cycle is used only where waste heat is

available or where heat is derived from solar collectors.

The absorption cycle is similar to the compression cycle, except for the method of

raising the pressure of the refrigerant vapor. In the absorption system, the

compressor is replaced by an absorber which dissolves the refrigerant in a

suitable liquid, a liquid pump which raises the pressure and a generator which, on

heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some

work is required by the liquid pump but, for a given quantity of refrigerant, it is

much smaller than needed by the compressor in the vapor compression cycle. In

an absorption refrigerator, a suitable combination of refrigerant and absorbent is

used. The most common combinations are ammonia (refrigerant) and water

(absorbent), and water (refrigerant) and lithium bromide (absorbent).

Gas cycle

When the working fluid is a gas that is compressed and expanded but doesn't

change phase, the refrigeration cycle is called a gas cycle. Air is most often this

working fluid. As there is no condensation and evaporation intended in a gas

cycle, components corresponding to the condenser and evaporator in a vapor

compression cycle are the hot and cold gas-to-gas heat exchangers in gas cycles.

The gas cycle is less efficient than the vapor compression cycle because the gas

cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As

such the working fluid does not receive and reject heat at constant temperature.

In the gas cycle, the refrigeration effect is equal to the product of the specific heat

of the gas and the rise in temperature of the gas in the low temperature side.

AIR CYCLE

Air is by nature the safest and cheapest refrigerant. Environmental concerns

about ozone depletion, global warming and increasingly stringent legislation have

renewed interest in alternative refrigeration technologies.

Air cycle systems have specific advantages that apply to all potential applications:

• The working fluid (air) is free, environmentally benign, totally safe and non-

toxic.

• Air cycle equipment is extremely reliable, thereby reducing maintenance

costs and system down-time.

• The performance of an air cycle unit does not deteriorate as much as that

of a vapor-compression unit when operating away from its design point.

• When operating in a refrigeration cycle, an air cycle unit can also produce

heat at a useful temperature.

The use of air as a refrigerant is based on the principle that when a gas expands

isentropically from a given temperature, its final temperature at the new pressure

is much lower. The resulting cold gas, in this case air, can then be used as a

refrigerant, either directly in an open system, or indirectly by means of a heat

exchanger in a closed system. The efficiency of such systems is limited to a great

extent by the efficiencies of compression and expansion, as well as those of the

heat exchangers employed. Originally, slow speed reciprocating compressors and

expanders were used. The poor efficiency and reliability of such machinery were

major factors in the replacement of such systems with vapor compression

equipment. However, the development of rotary compressors and expanders

greatly improved the isentropic efficiency and reliability of the air cycle. Advances

in turbine technology, together with the development of air bearings and ceramic

component offer further efficiency. Combining this with newly available compact

heat exchangers with greatly improved heat transfer characteristics makes

competition with many existing vapor compression, and certainly liquid nitrogen

systems, quite feasible.

Environmental control in buildings Until recently the use of air cycle has been largely restricted to aircraft cabin air

conditioning systems. A recent trial has demonstrated the advantages that air

cycle technologies can offer to passenger train air conditioning systems. An

important conclusion of this trial was that air cycle train air conditioning systems

will have lower overall life cycle ownership costs than comparable vapour

compression systems. The successful demonstration of these units in Germany’s

ICE2.2 high speed trains by Normalair-Garrett Ltd. led to the company receiving

the Engineering Council’s Environmental Award for Engineers in 1996.

Studies carried out by the Buildings Research Establishment (BRE) and frperc have

demonstrated that air cycle systems in buildings would have a number of

advantages. These include -

• Lamination of the need to use environmentally damaging CFC, HCFC or

other alternative refrigerants in building air conditioning systems

• Use of high grade heat recovery from air cycle cooling systems resulting in

lower energy consumption

• Improved reliability and reduced maintenance compared with conventional

systems

• Maintenance of near full load efficiency at part load conditions

• No susceptibility to refrigerant leakage

Food freezing system

Currently frperc are working on the design, construction and installation of an air

cycle fluidized bed freezer for food freezing. The air cycle plant will operate with

air as the refrigerant delivering it to the freezer bed at -75°C.

Fluidized beds have a number of useful characteristics. Heat and mass transfer

rates to and within the bed are high and there is a good uniformity of treatment

of the particles to yield high quality individually quick frozen products. Freezing

food faster can increase turnover on an existing footprint, reduce the freezing

cost and produce a higher quality of frozen food. Freezing food with an air cycle

refrigeration plant has two advantages;

• The air can replace toxic, inflammable or environmentally unfriendly

refrigerants and replace it with a safe and replaceable refrigerant

• It is capable of producing freezing temperatures far colder than vapor

compression plant for less energy consumption, size and cost. Freezing

temperatures as low as those produced by cryogenic refrigeration are

possible but without the high running costs and energy consumption

inherent in such systems.

CFC free heat pumps

The objective of the project is to develop heat pump systems, to be used in

existing as well as new buildings, using air as the environmentally benign working

fluid to improve the primary energy ratio of heating and cooling systems. To

improve the efficiency of air cycle systems the (isentropic) efficiency of the

rotating equipment (expanders and compressors) is crucial. High efficiency

equipment is available in other application fields such as pressurized air systems

and energy recovery systems .

COMFORT COOLING SYSTEMS

The need for heating and cooling in buildings:

The prime requirement in respect of the indoor climate in a building is that room

temperature should be at a comfortable level, regardless of the weather

conditions outside. In addition, the indoor air must be acceptably clean, lighting

and acoustic conditions must be good etc.Nevertheless, the first and foremost

condition for a building to be usable at allies that the indoor temperature

inacceptable. As soon as the ambient temperature is lower than the

Indoor temperature, heat flows out from the building through its boundary

surfaces (the building envelope). At the same time, the building also loses heat

through air infiltration, i.e. the inward leakage of outdoor air into the building

through gaps and cavities in walls, roofs, doors and windows. Bearing in mind the

fact that the indoor temperature in most buildings is maintained at a little over20

°C, this means, throughout most of the year, the building is losing heat to its

surroundings.

The internal heat generation in commercial premises and some industrial

buildings, on the other hand, is often relatively great. In combination with the fact

that construction standards have been developed and improved, so that buildings

are nowadays well insulated and airtight, this means that the heat losses through

the building envelope are small. If we consider new office buildings, department

stores, hospitals and similar buildings within the commercial premises and

industrial sector, we find that heat deficits usually occur only during the night and

at weekends, while there is nearly always ahead surplus during working hours.

Such buildings require only simple heating systems to meet the modest heat

deficits, as opposed to the considerably more extensive systems needed in order

to deal with the substantial heat surpluses, and to prevent the indoor

temperature becoming unacceptably high during working hours.

In general terms, the greater the heat surplus, and therefore the greater the

capacity of the cooling system, the more difficult it is to produce an indoor

climate that is good in all respects. It is therefore always important to attempt to

design the building in general so that there will be only a low heat surplus.

Comfort cooling

The surplus heat that has to be removed from buildings in order to maintain the

indoor temperature below some previously determined maximum permissible

temperature is referred to as the cooling requirement. In other words, the cooling

requirement of the building is exactly the same as its heat.

The climate control system in building has to maintain both the thermal climate

and the air quality. Maintaining the thermal climate consists primarily of keeping

the temperature of the indoor air within given limits. Maintaining the air quality

consists of controlling the ‘cleanliness’ of the indoor air by supplying a sufficient

quantity of outdoor air to ventilate the interior of the building. Maintenance of air

quality sometimes also includes ensuring that given

Concentrations of particles and/or gases are not exceeded.

The cooling system must be able to deal with variations in the cooling

requirement, whether over the day or over the year. The two basic types of all-air

cooling systems are the constant air flow system and the variable air flow system,

Although there are also combinations of the two methods. The need for comfort

cooling arises, therefore, when requirements in respect of the thermal climate

also include requirements in respect of maximum permissible indoor

temperatures. In general, HVAC (Heating, Ventilation and Air-Conditioning)

systems used in order actively to cool buildings can be divided up into three main

types:

_ all-air cooling systems

_ all-water cooling systems

_ combined systems

(With cooling supplied both by air and by water)

All-air cooling systems

The design air flow rate in these systems, and thus the necessary sizes of

ventilation ducts, is determined by the design cooling requirement. In other

words, it is the thermal requirements, and not the air quality requirements, that

determine the necessary air flow rate. In existing buildings, it is normally both

difficult and expensive to replace the ventilation duct system. If the existing ducts

cannot transport sufficiently large air quantities to meet the cooling

requirements, all-water-cooling systems will usually be installed in connection

with Conversion or modernization.

Constant air volume systems (CAV systems)

In such systems, the temperature of the air supplied to the building can vary, but

the air flow rate is kept constant. Such systems are referred to as Constant Air

Volume (CAV) systems. It is the rooms having the greatest cooling requirement

that normally determine the supply air temperature delivered by the central air

conditioning unit: the air may, if necessary, be heated before supplied to other

rooms. Although a CAV system supplies air at a constant flow rate, the fans are

sometimes powered by two-speed motors, running at the lower speed when the

building cooling requirement falls. The air flow rate is then reduced in proportion

to the fan speed.

Variable air volume systems (VAV systems)

The air flow rate to each room is varied as necessary, but with maintenance of a

constant supply temperature, i.e. the supply temperature does not change even if

the load changes. However, the supply air temperature is normally changed in

step with the time of year, as a function of the ambient temperature.

Systems of this type are referred to as Variable Air Volume (VAV) systems.

The air flow to each individual room is controlled by dampers in some form of box

(VAV-box) in the immediate vicinity of the supply point to the room, while the

central supply and exhaust air fans are controlled by variable inlet vanes or by

adjustable speed drive controlled motors, usually of the frequency-inverter type.

The control system normally maintains a constant static pressure in the supply air

duct. The flow rate varies from a maximum, during the hottest days, down to

perhaps 20 % of maximum flow rate during the coldest days of the year, when the

purpose of the air is only to maintain the air quality.

All-water cooling systems

Systems of this type supply all-water cooling to the individual rooms, with the

ventilation system designed purely to maintain the air quality. Systems of this

type are often chosen in connection with conversion or renovation projects.

There is usually space above the false ceilings to install the water pipes needed

for distribution of cold water throughout the building.

Combined systems

All-air and all-water cooling systems can be combined in many ways. One such

need for a combined system is if all-air cooling is used, but the cooling

requirement is so great that an all-air cooling system alone is not capable of

dealing with it satisfactorily, as such high air flow rates would be required that

draughts would be unavoidable. It is also possible to combine all-air cooling

systems so that certain parts of the building, or certain rooms, are cooled by a

VAV system, while other parts of the building are cooled by a CAV system.

Cooling supply devices

Cooling can be supplied to a room in a number of different ways. The following

are brief descriptions of how chilled beams, cooling panels, fan coil units and

induction units operate. Fan coil units and induction units are normally positioned

below windows in the outside walls.

Chilled beams

These are units which, by natural convection from a finned heat exchanger, cool

the air in the room. They may also be combined with the supply air terminal

device in order to provide both functions and, in many cases, to increase the

cooling capacity of the baffle. Some chilled beams can also incorporate a heating

function.

Cooling panels

Cooling panels can be hung from the ceiling. Cold water flows through an

aluminium plate, which transfers heat from the air to the cold water. The panel

cools the warm room air and also cools the room surfaces by low-temperature

radiation. These panels are produced in a number of versions, e.g. for mounting

flat against the ceiling, hanging, or for integration in a false ceiling. Most of their

cooling capacity is provided by radiation.

Fan coil units

These are units by which both heating and cooling can be supplied to a room

(although not at the same time).

A fan coil unit incorporates a fan which circulates the room air through the unit, in

which the air is either heated or cooled as required. The two heat exchangers

(heating and cooling) are supplied with hot or cold water from a central unit in

the building. This type of room cooler unit can meet the highest cooling

requirements, but it also has the highest noise level.

Induction units

These are units by which both heating and cooling can be supplied to a room

.When in use, the ventilation air for the room is supplied through the induction

unit. It flows through a nozzle with high velocity, which therefore has the effect of

inducing air from the room through the heating or cooling heat exchangers.

AIR CONDITIONING

The term air conditioning most commonly refers to the cooling and

dehumidification of indoor air for thermal comfort. In a broader sense, the term

can refer to any form of cooling, heating, ventilation or disinfection that modifies

the condition of air.[1] An air conditioner (AC or A/C in North American English,

aircon in British and Australian English) is an appliance, system, or mechanism

designed to stabilize the air temperature and humidity within an area (used for

cooling as well as heating depending on the air properties at a given time) ,

typically using a refrigeration cycle but sometimes using evaporation, most

commonly for comfort cooling in buildings and transportation vehicles.

The concept of air conditioning is known to have been applied in Ancient Rome,

where aqueduct water was circulated through the walls of certain houses to cool

them. Similar techniques in medieval Persia involved the use of cisterns and wind

towers to cool buildings during the hot season. Modern air conditioning emerged

from advances in chemistry during the 19th century, and the first large-scale

electrical air conditioning was invented and used in 1902 by Willis Haviland

Carrier.

Air conditioning applications:

Air conditioning engineers broadly divide air conditioning applications into

comfort and process.

Comfort applications aim to provide a building indoor environment that remains

relatively constant in a range preferred by humans despite changes in external

weather conditions or in internal heat loads.

The highest performance for tasks performed by people seated in an office is

expected to occur at 72 °F (22 °C) Performance is expected to degrade about 1%

for every 2 °F change in room temperature.[6] The highest performance for tasks

performed while standing is expected to occur at slightly lower temperatures. The

highest performance for tasks performed by larger people is expected to occur at

slightly lower temperatures. The highest performance for tasks performed by

smaller people is expected to occur at slightly higher temperatures. Although

generally accepted, some dispute that thermal comfort enhances worker

productivity, as is described in the Hawthorne effect.

Comfort air conditioning makes deep plan buildings feasible. Without air

conditioning, buildings must be built narrower or with light wells so that inner

spaces receive sufficient outdoor air via natural ventilation. Air conditioning also

allows buildings to be taller since wind speed increases significantly with altitude

making natural ventilation impractical for very tall buildings. Comfort applications

for various building types are quite different and may be categorized as:

• Low-Rise Residential buildings, including single family houses, duplexes, and

small apartment buildings

• High-Rise Residential buildings, such as tall dormitories and apartment

blocks

• Commercial buildings, which are built for commerce, including offices,

malls, shopping centers, restaurants, etc.

• Institutional buildings, which includes hospitals, governmental, academic,

and so on.

• Industrial spaces where thermal comfort of workers is desired.

In addition to buildings, air conditioning can be used for comfort in a wide variety

of transportation including land vehicles, trains, ships, aircraft, and spacecraft.

Process applications aim to provide a suitable environment for a process being

carried out, regardless of internal heat and humidity loads and external weather

conditions. Although often in the comfort range, it is the needs of the process

that determine conditions, not human preference. Process applications include

these:

• Hospital operating theatres, in which air is filtered to high levels to reduce

infection risk and the humidity controlled to limit patient dehydration.

Although temperatures are often in the comfort range, some specialist

procedures such as open heart surgery require low temperatures (about 18

°C, 64 °F) and others such as neonatal relatively high temperatures (about

28 °C, 82 °F).

• Clean rooms for the production of integrated circuits, pharmaceuticals, and

the like, in which very high levels of air cleanliness and control of

temperature and humidity are required for the success of the process.

• Facilities for breeding laboratory animals. Since many animals normally only

reproduce in spring, holding them in rooms at which conditions mirror

spring all year can cause them to reproduce year round.

• Aircraft air conditioning. Although nominally aimed at providing comfort for

passengers and cooling of equipment, aircraft air conditioning presents a

special process because of the low air pressure outside the aircraft.

• Data processing centers

• Textile factories

• Physical testing facilities

• Plants and farm growing areas

• Nuclear facilities

• Chemical and biological laboratories

• Mines

• Industrial environments

• Food cooking and processing areas

In both comfort and process applications the objective may be to not only control

temperature, but also humidity, air quality, air motion, and air movement from

space to space.

Humidity control

Refrigeration air conditioning equipment usually reduces the humidity of the air

processed by the system. The relatively cold (below the dew point) evaporator

coil condenses water vapor from the processed air, (much like an ice cold drink

will condense water on the outside of a glass), sending the water to a drain and

removing water vapor from the cooled space and lowering the relative humidity.

Since humans perspire to provide natural cooling by the evaporation of

perspiration from the skin, drier air (up to a point) improves the comfort

provided. The comfort air conditioner is designed to create a 40% to 60% relative

humidity in the occupied space. In food retailing establishment’s large open

chiller cabinets act as highly effective air dehumidifying units.

Some air conditioning units dry the air without cooling it, and are better classified

as dehumidifiers. They work like a normal air conditioner, except that a heat

exchanger is placed between the intake and exhaust. In combination with

convection fans they achieve a similar level of comfort as an air cooler in humid

tropical climates, but only consume about a third of the electricity. They are also

preferred by those who find the draft created by air coolers discomforting.

Energy use

It should be noted that in a thermodynamically closed system, any energy input

into the system that is being maintained at a set temperature (which is a standard

mode of operation for modern air conditioners) requires that the energy removal

rate from the air conditioner increase. This increase has the effect that for each

unit of energy input into the system (say to power a light bulb in the closed

system) requires the air conditioner to remove that energy. In order to do that

the air conditioner must increase its consumption by the inverse of its efficiency

times the input unit of energy. As an example presume that inside the closed

system a 100 watt light bulb is activated, and the air conditioner has an efficiency

of 200%. The air conditioners energy consumption will increase by 50 watts to

compensate for this, thus making the 100 W light bulbs utilize a total of 150 W of

energy.

Portable air conditioners

A portable air conditioner or portable A/C is an air conditioner on wheels that can

be easily transported inside a home or office. They are currently available with

capacities of about 6,000 to 60,000 BTU/h (1800 to 18 000 watts output) and

with and without electric resistance heaters. Portable air conditioners come in

three forms, split, and hose and evaporative:

A split system has an indoor unit on wheels connected to an outdoor unit via

flexible pipes, similar to a permanently fixed installed unit.

Hose systems Air-to-Air and Monoblock are vented to the outside via air ducts. A

function of all cooling that use a compressor, is to create water as it cools the air.

The "monoblock" version collects the water in a bucket or tray and stops when

full. The Air-to-Air version re-evaporates the water and discharges it through the

ducted hose and can hence run continuously.

A single duct unit draws air out of the room to cool its condenser. This air is then

replaced by hot air from outside or other rooms, thus reducing efficiency.

However, modern units run on approximately 1 to 3 ratio i.e., to produce 3 kW of

cooling this will use 1 kW of electricity.

Air cooled portable air conditioners are compressor-based refrigerant system that

uses air to exchange heat, similar to a car or typical household air conditioner.

With this type of system the air is dehumidified as it is cooled.

Evaporative air conditioners do not have a compressor or condenser. Instead,

liquid water is poured in and released as vapor. Because they do not have a

condenser which needs cooling, they do not need hoses or pipes, allowing them

to be truly portable.

As a rule of thumb, 400 square feet (37 m²) can be cooled per 12,000 BTU/h (3.5

kW or one ton of air conditioning) by a refrigerative air conditioner. However,

other factors will affect the total heat load. Evaporative air conditioners use much

less energy.

Types of air conditioner equipment

• Window and through-wall units

Many traditional air conditioners in homes or other buildings are single

rectangular units used to cool an apartment, a house or part of it, or part of a

building. For an example, see the photos to the right. Hotels frequently use PTAC

systems, which combine heating into the same unit. Air conditioner units need to

have access to the space they are cooling (the inside) and a heat sink; normally

outside air is used to cool the condenser section. For this reason, single unit air

conditioners are placed in windows or through openings in a wall made for the air

conditioner; the latter type includes portable air conditioners.

Window and through-wall units have vents on both the inside and outside, so

inside air to be cooled can be blown in and out by a fan in the unit, and outside air

can also be blown in and out by another fan to act as the heat sink. The controls

are on the inside.

Evaporation coolers

In very dry climates, so-called "swamp coolers" are popular for improving comfort

during hot weather. This type of cooler is the dominant cooler used in Iran which

has the largest number of units than anywhere else in the world, hence some

referring to "swamp coolers" as Persian coolers. An evaporative cooler is a device

that draws outside air through a wet pad, such as a large sponge soaked with

water. The sensible heat of the incoming air, as measured by a dry bulb

thermometer, is reduced. The total heat (sensible heat plus latent heat) of the

entering air is unchanged. Some of the sensible heat of the entering air is

converted to latent heat by the evaporation of water in the wet cooler pads. If the

entering air is dry enough, the results can be quite comfortable. These coolers

cost less and are mechanically simple to understand and maintain.

There is a related, more complex process called absorptive refrigeration which

uses heat to produce cooling. In one instance, a three-stage absorptive cooler first

dehumidifies the air with a spray of salt-water or brine. The brine osmotically

absorbs water vapor from the air. The second stage sprays water in the air,

cooling the air by evaporation. Finally, to control the humidity, the air passes

through another brine spray. The brine is reconcentrated by distillation. The

system is used in some hospitals because, with filtering, a sufficiently hot

regenerative distillation removes airborne organisms.

Absorptive chillers

Some buildings use gas turbines to generate electricity. The exhausts of these are

hot enough to drive an absorptive chiller that produces cold water. The cold

water is then run through radiators in air ducts for hydronic cooling. The dual use

of the energy, both to generate electricity and cooling, makes this technology

attractive when regional utility and fuel prices are right. Producing heat, power,

and cooling in one system is known as trigeneration.

Central air conditioning

Central air conditioning, commonly referred to as central air (US) or air-con (UK),

is an air conditioning system which uses ducts to distribute cooled and/or

dehumidified air to more than one room, or uses pipes to distribute chilled water

to heat exchangers in more than one room, and which is not plugged into a

standard electrical outlet.

With a typical split system, the condenser and compressor are located in an

outdoor unit; the evaporator is mounted in the air handling unit (which is often a

forced air furnace). With a package system, all components are located in a single

outdoor unit that may be located on the ground or roof.

Central air conditioning performs like a regular air conditioner but has several

added benefits:

• When the air handling unit turns on, room air is drawn in from various parts

of the house through return-air ducts. This air is pulled through a filter

where airborne particles such as dust and lint are removed. Sophisticated

filters may remove microscopic pollutants as well. The filtered air is routed

to air supply ductwork that carries it back to rooms. Whenever the air

conditioner is running, this cycle repeats continually.

• Because the central air conditioning unit is located outside the home, it

offers a lower level of noise indoors than a free-standing air conditioning

unit.

Thermostats

Thermostats control the operation of HVAC systems, turning on the heating or

cooling systems to bring the building to the set temperature. Typically the heating

and cooling systems have separate control systems (even though they may share

a thermostat) so that the temperature is only controlled "one-way". That is, in

winter, a building that is too hot will not be cooled by the thermostat.

Thermostats may also be incorporated into facility energy management systems

in which the power utility customer may control the overall energy expenditure.

In addition, a growing number of power utilities have made available a device

which, when professionally installed, will control or limit the power to an HVAC

system during peak use times in order to avoid necessitating the use of rolling

blackouts.

Equipment capacity

Air conditioner equipment power in the U.S. is often described in terms of "tons

of refrigeration". A "ton of refrigeration" is defined as the cooling power of one

short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This

is equal to 12,000 BTU per hour, or 3517 watts

(http://physics.nist.gov/Pubs/SP811/appenB9.html). Residential "central air"

systems are usually from 1 to 5 tons (3 to 20 kW) in capacity.

The use of electric/compressive air conditioning puts a major demand on the

nation's electrical power grid in warm weather, when most units are operating

under heavy load.

Health implications

Air conditioning has no greater influence on health than heating—that is to say,

very little—although poorly maintained air-conditioning systems (especially large,

centralized systems) can occasionally promote the growth and spread of

microorganisms, such as Legionella pneumophila, the infectious agent

responsible for Legionnaire's disease, or thermophilic actinomycetes.Conversely,

air conditioning (including filtration, humidification, cooling, disinfection, etc.) can

be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating

rooms and other environments where an appropriate atmosphere is critical to

patient safety and well-being. Air conditioning can have a positive effect on

sufferers of allergies and asthma.

In serious heat waves, air conditioning can save the lives of the elderly. Some local

authorities even set up public cooling centers for the benefit of those without air

conditioning at home.

Properly maintained air-conditioning systems do not cause or promote illness,

despite superstitions that air-conditioning is unconditionally dangerous to one's

health. As with heating systems, the advantages of air conditioning generally far

outweigh the disadvantages.

The internal section of the same unit. A modern Americool window air-

conditioner internal section

External section of a typical AC

Air Conditioning Units

How Air –Conditioners work

Air conditioners and refrigerators work the same way. Instead of cooling just the

small, insulated space inside of a refrigerator, an air conditioner cools a room, a

whole house, or an entire business. Air conditioners use chemicals that easily

convert from a gas to a liquid and back again. This chemical is used to transfer

heat from the air inside of a home to the outside air.

The machine has three main parts. They are a compressor, a condenser and an

evaporator. The compressor and condenser are usually located on the outside air

portion of the air conditioner. The evaporator is located on the inside the house,

sometimes as part of a furnace.

• The working fluid arrives at the compressor as a cool, low-pressure gas

called Freon. The compressor squeezes the fluid. This packs the molecule of

the fluid closer together. The closer the molecules are together, the higher

its energy and its temperature.

• The working fluid leaves the compressor as a hot, high pressure gas and

flows into the condenser. If you looked at the air conditioner part outside a

house, look for the part that has metal fins all around. The fins act just like

a radiator in a car and help the heat go away, or dissipate, more quickly.

• When the working fluid leaves the condenser, its temperature is much

cooler and it has changed from a gas to a liquid under high pressure. The

liquid goes into the evaporator through a very tiny, narrow hole. On the

other side, the liquid's pressure drops. When it does it begins to evaporate

into a gas.

• As the liquid changes to gas and evaporates, it extracts heat from the air

around it. The heat in the air is needed to separate the molecules of the

fluid from a liquid to a gas.

• The evaporator also has metal fins to help in exchange the thermal energy

with the surrounding air.

• By the time the working fluid leaves the evaporator, it is a cool, low

pressure gas. It then returns to the compressor to begin its trip all over

again.

• Connected to the evaporator is a fan that circulates the air inside the house

to blow across the evaporator fins. Hot air is lighter than cold air, so the hot

air in the room rises to the top of a room.

• There is a vent there where air is sucked into the air conditioner and goes

down ducts. The hot air is used to cool the gas in the evaporator. As the

heat is removed from the air, the air is cooled. It is then blown into the

house through other ducts usually at the floor level.

• This continues over and over and over until the room reaches the

temperature you want the room cooled to. The thermostat senses that the

temperature has reached the right setting and turns off the air conditioner.

As the room warms up, the thermostat turns the air conditioner back on

until the room reaches the temperature.

A-Expansion Valve

B-Compressor

Schematic diagram of an air-conditioner

Window AC Units

A window air conditioner unit implements a complete air conditioner in a small

space. The units are made small enough to fit into a standard window frame. It

contains:

• A compressor

• An expansion valve

• A hot coil (on the outside)

• A chilled coil (on the inside)

• Two fans

• A control unit.

The fans blow air over the coils to improve their ability to dissipate heat (to the

outside air) and cold (to the room being cooled).

BTU and EER

Most air conditioners have their capacity rated in British thermal units (BTU).

Generally speaking, a BTU is the amount of heat required to raise the

temperature of one pound (0.45 kg) of water 1 degree Fahrenheit (0.56 degrees

Celsius). Specifically, 1 BTU equals 1,055 joules. In heating and cooling terms, 1

"ton" equals 12,000 BTU.

A typical window air conditioner might be rated at 10,000 BTU. For comparison, a

typical 2,000-square-foot (185.8 m2) house might have a 5-ton (60,000-BTU) air

conditioning system, implying that you might need perhaps 30 BTU per square

foot. (Keep in mind that these are rough estimates. To size an air conditioner for

your specific needs, contact an HVAC contractor.)

The energy efficiency rating (EER) of an air conditioner is its BTU rating over its

wattage. For example, if a 10,000-BTU air conditioner consumes 1,200 watts, its

EER is 8.3 (10,000 BTU/1,200 watts). Obviously, you would like the EER to be as

high as possible, but normally a higher EER is accompanied by a higher price.

Let's say that you have a choice between two 10,000-BTU units. One has an EER

of 8.3 and consumes 1,200 watts, and the other has an EER of 10 and consumes

1,000 watts. Let's also say that the price difference is $100. To understand what

the payback period is on the more expensive unit, you need to know:

• Approximately how many hours per year you will be operating the unit

• How much a kilowatt-hour (kWh) costs in your area

Let's say that you plan to use the air conditioner in the summer (four months a

year) and it will be operating about six hours a day. Let's also imagine that the

cost in your area is $0.10/kWh. The difference in energy consumption between

the two units is 200 watts, which means that every five hours the less expensive

unit will consume 1 additional kWh (and therefore $0.10 more) than the more

expensive unit.

Assuming that there are 30 days in a month, you find that during the summer you

are operating the air conditioner:

4 mo. x 30 days/mo. x 6 hr/day = 720 hours

[(720 hrs x 200 watts) / (1000 watts/kW)] x $0.10/kWh = $14.40

Split-system AC Units

A split-system air conditioner splits the hot side from the cold side of the system,

like this:

The cold side, consisting of the expansion valve and the cold coil, is generally

placed into a furnace or some other air handler. The air handler blows air through

the coil and routes the air throughout the building using a series of ducts. The hot

side, known as the condensing unit, lives outside the building.

The unit consists of a long, spiral coil shaped like a cylinder. Inside the coil is a fan,

to blow air through the coil, along with a weather-resistant compressor and some

control logic. This approach has evolved over the years because it is low-cost, and

also because it normally results in reduced noise inside the house (at the expense

of increased noise outside the house). Besides the fact that the hot and cold sides

are split apart and the capacity is higher (making the coils and compressor larger),

there is no difference between a split-system and a window air conditioner.

• In warehouses, businesses, malls, large department stores and the like, the

condensing unit normally lives on the roof and can be quite massive.

Alternatively, there may be many smaller units on the roof, each attached

inside to a small air handler that cools a specific zone in the building.

• In larger buildings and particularly in multi-story buildings, the split-system

approach begins to run into problems. Either running the pipe between the

condenser and the air handler exceeds distance limitations (runs that are

too long start to cause lubrication difficulties in the compressor), or the

amount of duct work and the length of ducts becomes unmanageable.

Chilled-water and Cooling-tower AC Units

In a chilled-water system, the entire air conditioner lives on the roof or behind the building. It cools water to between 40 and 45 F (4.4 and 7.2 C). This chilled water is then piped throughout the building and connected to air handlers as needed. There is no practical limit to the length of a chilled-water pipe if it is well-insulated.

A-Expansion valve

B-Compressor

C-Heat Exchanger

D-Chilled water to the building

Cooling Towers

In all of the systems described earlier, air is used to dissipate the heat from the

outside coil. In large systems, the efficiency can be improved significantly by

using a cooling tower. The cooling tower creates a stream of lower-

temperature water. This water runs through a heat exchanger and cools the

hot coils of the air conditioner unit. It costs more to buy the system initially,

but the energy savings can be significant over time (especially in areas with

low humidity), so the system pays for itself fairly quickly.

Cooling towers come in all shapes and sizes. They all work on the same

principle:

• Generally, the water trickles through a thick sheet of open plastic mesh.

• Air blows through the mesh at right angles to the water flow.

• The evaporation cools the stream of water.

• Because some of the water is lost to evaporation, the cooling tower

constantly adds water to the system to make up the difference.

Air-Distribution Systems

There are various types of air-distribution systems, like fans, filters, ductwork,

outlets, dampers etc.

HVAC

HVAC is an initialism/acronym that stands for "heating, ventilating, and air

conditioning". HVAC is sometimes referred to as "climate control" and is

particularly important in the design of medium to large industrial and office

buildings such as sky scrapers and in marine environments such as aquariums,

where humidity and temperature must all be closely regulated whilst maintaining

safe and healthy conditions within.

Heating, ventilating, and air conditioning is based on the basic principles of

thermodynamics, fluid mechanics, and heat transfer The invention of the

components of HVAC systems goes hand-in-hand with the industrial revolution,

and new methods of modernization, higher efficiency, and system control are

constantly introduced by companies and inventors all over the world.

The three functions of heating, ventilating, and air-conditioning are closely

interrelated. All seek to provide thermal comfort, acceptable indoor air quality,

and reasonable installation, operation, and maintenance costs. HVAC systems can

provide ventilation, reduce air infiltration, and maintain pressure relationships

between spaces. How air is delivered to, and removed from spaces is known as

room air distribution.

In modern buildings the design, installation, and control systems of these

functions are integrated into one or more HVAC systems. For very small buildings,

contractors normally "size" and select HVAC systems and equipment. For larger

buildings where required by law, "building services" designers and engineers,

such as mechanical, architectural, or building services engineers analyze, design,

and specify the HVAC systems, and specialty mechanical contractors build and

commission them. In all buildings, building permits for, and code-compliance

inspections of the installations are the norm.

HVAC systems use ventilation air ducts installed throughout a building that supply

conditioned air to a room through rectangular or round outlet vents, called

"diffusers"; and ducts that remove air through return-air "grilles.

Heating

Heating systems may be classified as central or local. Central heating is often used

in cold climates to heat private houses and public buildings. Such a system

contains a boiler, furnace, or heat pump to heat water, steam, or air, all in a

central location such as a furnace room in a home or a mechanical room in a large

building. The system also contains piping or ductwork to distribute the heated

fluid, and radiators to transfer this heat to the air. The term radiator in this

context is misleading since most heat transfer from the heat exchanger is by

convection, not radiation. The radiators may be mounted on walls or buried in the

floor to give under-floor heat.

In boiler fed or radiant heating systems, all but the simplest systems have a pump

to circulate the water and ensure an equal supply of heat to all the radiators. The

heated water can also be fed through another heat exchanger inside a storage

cylinder to provide hot running water.

Forced air systems send heated air through ductwork. During warm weather the

same ductwork can be reused for air conditioning. The forced air can also be

filtered or put through air cleaners. Most ducts cannot fit a human being (as they

do in many films) since this would require a greater duct-structural integrity and

create a potential security liability.

Heating can also be provided from electric, or resistance heating using a filament

that glows hot when you cause electricity to pass through it. This type of heat can

be found in electric baseboard heaters, portable electric heaters, and as backup

or supplemental heating for heat pump (or reverse heating) system.

The heating elements (radiators or vents) should be located in the coldest part of

the room and typically next to the windows to minimize condensation. Popular

retail devices that direct vents away from windows to prevent "wasted" heat

defeat this design parameter. Drafts contribute more to the subjective feeling of

coldness than actual room temperature. Therefore, rather than improving the

heating of a room/building, it is often more important to control the air leaks.

The invention of central heating is often credited to the ancient Romans, who

installed a system of air ducts called "hypocaust" in the walls and floors of public

baths and private villas. The ducts were fed with hot air from a central fire.

Generally, these heated by radiation; a better physiologic approach to heating

than conventional forced air convective heating.

Ventilating

Ventilating is the process of "changing" or replacing of air in any space to remove

moisture, odors, smoke, heat, dust and airborne bacteria. Ventilation includes

both the exchange of air to the outside as well as circulation of air within the

building. It is one of the most important factors for maintaining acceptable indoor

air quality in buildings. Methods for ventilating a building may be divided into

mechanical/forced and natural types. Ventilation is used to remove unpleasant

smells and excessive moisture, introduce outside air, and to keep interior building

air circulating, to prevent stagnation of the interior air.

Air-Handling unit

HVAC Systems Design and Safety

Heating, ventilating and air-conditioning (HVAC) systems can play several roles to

reduce the environmental impact of buildings. The primary function of HVAC

systems is to provide healthy and comfortable interior conditions for occupants.

Well-designed, efficient systems do this with minimal non-renewable energy and

air and water pollutant emissions. Cooling equipment that avoids

chlorofluorocarbons and hydro chlorofluorocarbons (CFCs and HCFCs) may

eliminate a major cause of damage to the ozone layer.

However, even the best HVAC equipment and systems cannot compensate for a

building design with inherently high cooling and heating needs. The greatest

opportunities to conserve non-renewable energy are through architectural design

that controls solar gain, while taking advantage of passive heating, day lighting,

natural ventilation and cooling opportunities. The critical factors in mechanical

systems' energy consumption - and capital cost - are reducing the cooling and

heating loads they must handle.

Air Change per Hour (ACH)

The number of times per hour that the volume of a specific room or building is

supplied or removed from that space by mechanical and natural ventilation.

Air handler, or air handling unit (AHU)

Central unit consisting of a blower, heating and cooling elements, filter racks or

chamber, dampers, humidifier, and other central equipment in direct contact with

the airflow. This does not include the ductwork through the building.

British thermal unit (BTU)

Any of several units of energy (heat) in the HVAC industry, each slightly more than

1 kJ. One BTU is the energy required to raise one pound of water one degree

Fahrenheit, but the many different types of BTU are based on different

interpretations of this “definition”. In the United States the power of HVAC

systems (the rate of cooling and dehumidifying or heating) is sometimes

expressed in BTU/hour instead of watts.

Chiller

A device that removes heat from a liquid via a vapor-compression or absorption

refrigeration cycle. This cooled liquid flows through pipes in a building and passes

through coils in air handlers, fan-coil units, or other systems, cooling and usually

dehumidifying the air in the building. Chillers are of two types; air-cooled or

water-cooled. Air-cooled chillers are usually outside and consist of condenser coils

cooled by fan-driven air. Water-cooled chillers are usually inside a building, and

heat from these chillers is carried by recirculating water to outdoor cooling

towers.

Controller

A device that controls the operation of part or all of a system. It may simply turn a

device on and off, or it may more subtly modulate burners, compressors, pumps,

valves, fans, dampers, and the like. Most controllers are automatic but have user

input such as temperature set points, e.g. a thermostat. Controls may be analog,

or digital, or pneumatic, or a combination of these.

Fan-coil unit (FCU)

A small terminal unit that is often composed of only a blower and a heating

and/or cooling coil (heat exchanger), as is often used in hotels, condominiums, or

apartments.

Condenser

A component in the basic refrigeration cycle that ejects or removes heat from the

system. The condenser is the hot side of an air conditioner or heat pump.

Condensers are heat exchangers, and can transfer heat to air or to an

intermediate fluid (such as water or an aqueous solution of ethylene glycol) to

carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as

with cooling towers).

Constant air volume (CAV)

A system designed to provide a constant air volume per unit time. This term is

applied to HVAC systems that have variable supply-air temperature but constant

air flow rates. Most residential forced-air systems are small CAV systems with

on/off control.

Damper

A plate or gate placed in a duct to control air flow by introducing a constriction in

the duct.

Evaporator

A component in the basic refrigeration cycle that absorbs or adds heat to the

system. Evaporators can be used to absorb heat from air (by reducing

temperature and by removing water) or from a liquid. The evaporator is the cold

side of an air conditioner or heat pump.

Furnace

A component of an HVAC system that adds heat to air or an intermediate fluid by

burning fuel (natural gas, oil, propane, butane, or other flammable substances) in

a heat exchanger.

Fresh air intake (FAI)

An opening through which outside air is drawn into the building. This may be to

replace air in the building that has been exhausted by the ventilation system, or

to provide fresh air for combustion of fuel.

Grille

A facing across a duct opening, usually rectangular is shape, containing multiple

parallel slots through which air may be delivered or withdrawn from a ventilated

space.

Heat load, heat loss, or heat gain

Terms for the amount of heating (heat loss) or cooling (heat gain) needed to

maintain desired temperatures and humidities in controlled air. Regardless of

how well-insulated and sealed a building is, buildings gain heat from warm air or

sunlight or lose heat to cold air and by radiation. Engineers use a heat load

calculation to determine the HVAC needs of the space being cooled or heated.

Louvers

Blades, sometimes adjustable, placed in ducts or duct entries to control the

volume of air flow. The term may also refer to blades in a rectangular frame

placed in doors or walls to permit the movement of air.

Makeup air unit (MAU)

An air handler that conditions 100% outside air. MAUs are typically used in

industrial or commercial settings, or in once- through (blower sections that only

blow air one-way into the building), low flow (air handling systems that blow air

at a low flow rate), or primary-secondary (air handling systems that have an air

handler or rooftop unit connected to an add-on makeup unit or hood) commercial

HVAC systems.

Packaged terminal air conditioner (PTAC)

An air conditioner and heater combined into a single, electrically-powered unit,

typically installed through a wall and often found in hotels.

Roof-top unit (RTU)

An air-handling unit, defined as either "recirculating" or "once-through" design,

made specifically for outdoor installation. They most often include, internally,

their own heating and cooling devices. RTUs are very common in some regions,

particularly in single-story commercial buildings.

Variable air volume (VAV) system

An HVAC system that has a stable supply-air temperature, and varies the air flow

rate to meet the temperature requirements. Compared to CAV systems, these

systems waste less energy through unnecessarily-high fan speeds. Most new

commercial buildings have VAV systems.

Thermal zone

A single or group of neighboring indoor spaces that the HVAC designer expects

will have similar thermal loads. Building codes may require zoning to save energy

in commercial buildings. Zones are defined in the building to reduce the number

of HVAC subsystems, and thus initial cost. For example, for perimeter offices,

rather than one zone for each office, all offices facing west can be combined into

one zone. Small residences typically have only one conditioned thermal zone, plus

unconditioned spaces such as unconditioned garages, attics, and crawlspaces, and

unconditioned basements.

Coils

The selection of hot and chilled water coils will have a substantial impact on the

fan energy use.

Thin coil design

Traditional AHU design specifies coil sizes assuming a face velocity of between

400 and 500 feet per minute. A new design technique called low face velocity,

high coolant velocity or LFV/HCV has been researched at the University of

Adelaide, Australia. This technique uses a "thin" coil design that is roughly half the

number of tubes in depth as in conventional designs but double the coil face area.

The net result is a face velocity in the range of 150 to 200 feet per minute (FPM)

with much higher heat transfer efficiency and lower pressure drop than in

conventional designs. Because the coil's pressure loss is proportional to the

velocity at a square rate, face velocity reduction can result in pressure drops of

one-fourth or less compared to the equivalent, traditionally designed coil.

Preheat coils

A preheat coil is commonly used to control condensation inside the HVAC system

for laboratories that use 100 percent outside air or when the outside air

temperature falls below freezing. If a heating coil is used downstream, the

preheat coil should become inactive to save energy when outdoor temperatures

reach 45 degrees F. Preheat coils are also used to warm the outside air stream,

assuring better air stream mixing and providing free humidification.

Damper

A damper is a valve or plate that stops or regulates the flow of air inside a duct,

chimney, VAV box, air handler, or other air handling equipment. A damper may be

used to cut off central air conditioning (heating or cooling) to an unused room, or

to regulate it for room-by-room temperature and climate control. Its operation

can be manual or automatic. Manual dampers are turned by a handle on the

outside of a duct. Automatic dampers are used to regulate airflow constantly and

are operated by electric or pneumatic motors, in turn controlled by a thermostat

or building automation system.

In a chimney flue, a damper closes off the flue to keep the weather (and birds and

other animals) out and warm or cool air in. This is usually done in the summer,

but also sometimes in the winter between uses. In some cases, the damper may

also be partly closed to help control the rate of combustion. The damper may be

accessible only by reaching up into the fireplace by hand or with a wood poker, or

sometimes by a lever or knob that sticks down or out. On a wood burning stove or

similar device, it is usually a handle on the vent duct as in an air conditioning

system. Forgetting to open a damper before beginning a fire can cause serious

smoke damage to the interior of a home, if not a house fire.

Opposed blade dampers in a mixing duct

Dampers must be installed in places where airflow needs to be controlled and/or

blocked. Dampers located directly behind an outlet tend to be noisy. A better

location is in the final branch near the connection to the trunk duct. Wherever a

balancing or volume damper is located, it should be accessible. Lay-in ceiling tiles

provide good access; in a fixed ceiling, an access door is needed. Dampers should

not be installed in hood exhaust systems even if the exhaust duct passes through

a firewall. Use the UL approved alternative -- a properly supported, heavy-gauge

steel, unobstructed duct.

Dampers have to withstand the maximum static pressure in a system. The

maximum static pressure is the maximum that can be experienced in a system,

not simply the pressure introduced by the fan during normal operation. Maximum

static pressure usually occurs when all dampers in a system are closed except

those on one flow path.

Automated zone dampers

A zone damper (also known as a Volume Control Damper or VCD) is a specific type

of damper used to control the flow of air in an HVAC heating or cooling system. In

order to improve efficiency and occupant comfort, HVAC systems are commonly

divided up into multiple zones. For example, in a house, the main floor may be

served by one heating zone while the upstairs bedrooms are served by another. In

this way, the heat can be directed principally to the main floor during the day and

principally to the bedrooms at night, allowing the unoccupied areas to cool down.

Zone dampers as used in home HVAC systems are usually electrically powered. In

large commercial installations, vacuum or compressed air may be used instead. In

either case, the motor is usually connected to the damper via a mechanical

coupling.

Advantages:

• Cost.

• Power consumption.

Disadvantages:

• Zone dampers are not 100% reliable. The motor-to-open/motor-to-closed

style of electrically operated zone dampers aren't "fail safe" (that is, they

do not fail to the open condition). However, zone dampers that are of the

"Normally Open" type are fail-safe, in that they will fail to the open

condition.

• No inherent redundancy for the furnace. A system with zone dampers is

dependent upon a single furnace. If it fails, the system becomes completely

inoperable.

• The system can be harder to design, requiring both “SPDT” thermostats

(and relays) and the ability of the system to withstand the fault condition

whereby all zone dampers are closed simultaneously.

Fire dampers

Fire dampers are fitted where ductwork passes through fire compartment walls /

fire curtains as part of a fire control strategy. In normal circumstances, these

dampers are held open by means of fusible links. When subjected to heat, these

links fracture and allow the damper to close under the influence of the integral

closing spring. The links are attached to the damper such that the dampers can be

released manually for testing purposes. The damper is provided with an access

door in the adjacent ductwork for the purpose of inspection and resetting in the

event of closure.

Ducts

Ducts are used in heating, ventilation, and air conditioning (HVAC) to deliver and

remove air. These needed airflows include, for example, supply air, return air, and

exhaust air. Ducts also deliver, most commonly as part of the supply air,

ventilation air. As such, air ducts are one method of ensuring acceptable indoor

air quality as well as thermal comfort.

A duct system is often called ductwork. Planning ('laying out'), sizing, optimizing,

detailing, and finding the pressure losses through a duct system is called duct

design.

Duct materials

Like modern steel food cans, at one time air ducts were often made of tin, like 'tin

cans' were made for food. Tin is more corrosion resistant than plain steel, but is

also more expensive. With improvements in mild steel production, and its

galvanization to resist rust, steel 'sheet metal' has replaced tin in ducts as well as

food cans..

Galvanized steel

Ducts are still most often made of galvanized steel. Various fittings allow

transitioning between the various shapes and sizes. A "tee" connection, for

example, is where the air flow can be divided into two or more downstream

branches. Many factory-made shapes and sizes are available but galvanized steel

can easily be cut and bent to form additional shapes when required. Steel ducts

are commonly wrapped or lined with fiberglass thermal insulation, both to reduce

heat loss or gain through the duct walls and water vapor from condensing on the

exterior of the duct when the duct is carrying cooled air. Insulation, particularly

duct liner, also reduces duct-borne noise. Both types of insulation reduce

'breakout' noise through the ducts' sidewalls.

Polyurethane duct board (Preinsulated aluminum ducts)

While as mentioned above, galvanized steel is still very common, always more

rectangular ducts are being manufactured from “duct board”, thanks to the fact

that custom or special shapes and sizes of ducts can easily be shop or field

fabricated. In addition to the fact that ducts made with “duct board” do not need

any further insulation. Among the various types of rigid polyurethane foam panels

available, a new water formulated panel stands out. In this particular panel, the

foaming process is obtained through the use of water instead of the CFC, HCFC,

HFC and HC gasses. The foam panels are then coated with aluminum sheets on

either side, with thicknesses that can vary from 50 micrometres for indoor use to

200 micrometres for external use in order to guarantee the high mechanical

characteristics of the duct. The ducts construction starts with the plotting of the

single pieces on the panel. The pieces are then cut from the panel (with a 45° cut

as explained below), bent if necessary in order to obtain the different fittings, and

finally closed through an operation of gluing, pressing and taping. Having

obtained the various duct sections, they can easily be installed by using an

invisible aluminum flange system.

Fiberglass duct board (Preinsulated non metallic ductwork)

Also the fiberglass panels provide built-in thermal insulation and the interior

surface absorbs sound, helping to provide quiet operation of the HVAC system.

The duct board is formed by sliding a specially-designed knife along the board

using a straightedge as a guide; the knife automatically trims out a "valley" with

45° sides; the valley does not quite penetrate the entire depth of the duct board,

providing a thin section that acts as a hinge. The duct board can then be folded

along the valleys to produce 90° folds, making the rectangular duct shape in the

fabricator's desired size. The duct is then closed with staples and special

aluminum or similar 'metal-backed' tape. Commonly available duct tape should

not be used on air ducts, metal, fiberglass, or otherwise, that are intended for

long-term use; the adhesive on so called 'duct tape' dries and releases with time.

Flexible tubing

Flexible ducts, known as flex, have a variety of configurations, but for HVAC

applications, they are typically flexible plastic over a metal wire coil to make

round, flexible duct. Most often a layer of fiberglass insulation covers the duct,

and then a thin plastic layer protects the insulation. Flexible duct is very

convenient for attaching supply air outlets to the rigid ductwork. However, the

pressure loss through flex is higher than for most other types of ducts. As such,

designers and installers attempt to keep their installed lengths (runs) short, e.g.,

less than 15 feet or so, and to minimize turns. Kinks in flex must be avoided.

Flexible duct is normally not used on the negative pressure portions of HVAC duct

systems.

D U C T D E S I G N O B J E C T I V E S

The objectives of good duct design are occupant comfort, proper air distribution,

economical heating and cooling system operation, and economical duct

installation. The outcome of the duct design process will be a duct system (supply

and return plenums, ducts, fittings, boots, grilles, and registers) that

• Provides conditioned air to meet all room heating and cooling loads.

• Is properly sized so that the pressure drop across the air handler is within

manufacturer and design specifications.

• Is sealed to provide proper air flow and to prevent air from entering the house

or duct system from polluted zones.

• Has balanced supply and return air flows to maintain a neutral pressure in the

house.

• Minimizes duct air temperature gains or losses between the air handler and

supply outlets, and between the return register and air handler.

SUPPLY DUCT SYSTEMS

Supply ducts deliver air to the spaces that are to be conditioned. The two most

common supply duct systems for residences are the trunk and branch system and

the radial system because of their versatility, performance, and economy.

The spider and perimeter loop systems are other options.

TRUNK AND BRANCH SYSTEM

In the trunk and branch system, a large main supply trunk is connected directly to

the air handler or its supply plenum and serves as a supply plenum or an

extension to the supply plenum. Smaller branch ducts and run outs are connected

to the trunk. The trunk and branch system is adaptable to most houses, but it has

more places where leaks can occur. It provides air flows that are easily balanced

and can be easily designed to be located inside the conditioned space of the

house. There are several variations of the trunk and branch system. An extended

plenum system uses a main supply trunk that is one size and is the simplest and

most popular design. The length of the trunk is usually limited to about 24 feet

because otherwise the velocity of the air in the trunk gets too low and air flow

into branches and run outs close to the air handler becomes poor. Therefore, with

a centrally located air handler, this duct system can be installed in homes up to

approximately 50 feet long. A reducing plenum system uses a trunk reduction

periodically to maintain a more uniform pressure and air velocity in the trunk,

which improves air flow in branches and run outs closer to the air handler.

Similarly, a reducing trunk system reduces the cross-sectional area of the trunk

after every branch duct or run out, but it is the most complex system to design.

SPIDER SYSTEM

A spider system is a more distinct variation of the trunk and branch system. Large

supply trunks (usually large-diameter flexible ducts) connect remote mixing boxes

to a small, central supply plenum. Smaller branch ducts or run outs take air from

the remote mixing boxes to the individual supply outlets. This system is difficult to

locate within the conditioned space of the house.

RADIAL SYSTEM

In a radial system, there is no main supply trunk; branch ducts or run outs that

deliver conditioned air to individual supply outlets are essentially connected

directly to the air handler, usually using a small supply plenum. The short, direct

duct runs maximize air flow. The radial system is most adaptable to single-story

homes. Traditionally, this system is associated with an air handler that is centrally

located so that ducts are arranged in a radial pattern. However, symmetry is not

mandatory, and designs using parallel runouts can be designed so that duct runs

remain in the conditioned space (e.g., installed above a dropped ceiling).

PERIMETER LOOP SYSTEM

A perimeter loop system uses a perimeter duct fed from a central supply plenum

using several feeder ducts. This system is typically limited to houses built on slab

in cold climates and is more difficult to design and install.

RETURN DUCT SYSTEMS

Return ducts remove room air and deliver it back to the heating and cooling

equipment for filtering and reconditioning. Return duct systems are generally

classified as either central or multiple-room return.

MULTIPLE-ROOM RETURN SYSTEM

A multiple-room return system is designed to return air from each room supplied

with conditioned air, especially those that can be isolated from the rest of the

house (except bathrooms and perhaps kitchens and mechanical rooms). When

properly designed and installed, this is the ultimate return duct system because it

ensures that air flow is returned from all rooms (even with doors closed),

minimizes pressure imbalances, improves privacy, and is quiet. However, design

and installation costs of a multi-room return system are generally higher than

costs for a central return system, and higher friction losses can increase blower

requirements.

CENTRAL RETURN SYSTEM

A central return system consists of one or more large grilles located in central

areas of the house (e.g., hallway, under stairway) and often close to the air

handler. In multi-story houses, a central return is often located on each floor. To

ensure proper air flow from all rooms, especially when doors are closed, transfer

grilles or jumper ducts must be installed in each room (undercutting interior

doors to provide 1 inch of clearance to the floor is usually not sufficient by itself).

Transfer grilles are through-the-wall vents that are often located above the

interior door frames, although they can be installed in a full wall cavity to reduce

noise transmission. The wall cavity must be well sealed to prevent air leakage.

Jumper ducts are short ducts routed through the ceiling to minimize noise

transfer.

D U C T A N D R E G I S T E R L O C A T I O N S

Locating the air handler unit and air distribution system inside the conditioned

space of the house is the best way to improve duct system efficiency and is highly

recommended. With this design, any duct leakage will be to the inside of the

house. It will not significantly affect the energy efficiency of the heating and

cooling system because the conditioned air remains inside the house, although air

distribution may suffer. Also, ducts located inside the conditioned space need

minimal insulation (in hot and humid climates), if any at all. The cost of moving

ducts into the conditioned space can be offset by smaller heating and cooling

equipment, smaller and less duct work, reduced duct insulation, and lower

operating costs.

There are several methods for locating ducts inside the

conditioned space.

• Place the ducts in a furred-down chase below the ceiling (e.g., dropped ceiling in

a hallway), a chase furred-up in the attic, or other such chases. These chases must

be specially constructed, air-sealed, and insulated to ensure they are not

connected to unconditioned spaces.

• Locate ducts between the floors of a multi-story home (run through the floor

trusses or joists). The exterior walls of these floor cavities must be insulated and

sealed to ensure they are within the conditioned space. Holes in the cavity for

wiring, plumbing, etc., must be sealed to prevent air exchange with

unconditioned spaces.

• Locate ducts in a specially-constructed sealed and insulated crawlspace (where

the walls of the crawlspace are insulated rather than the ceiling). Ducts should not

be run in exterior walls as a means of moving them into the conditioned space

because this reduces the amount of insulation that can be applied to the duct and

the wall itself. A supply outlet is positioned to mix conditioned air with room air

and is responsible for most of the air movement within a room. Occupant comfort

requires that supply register locations be carefully selected for each room. In cold

climates, perimeter floor outlets that blanket portions of the exterior wall (usually

windows) with supply air are generally preferred. However, in today’s better

insulated homes, the need to locate outlets near the perimeter where heat loss

occurs is becoming less important. In hot climates, ceiling diffusers or high wall

outlets that discharge air parallel to the ceiling are typically installed. In moderate

climates, outlet location is less critical. Outlet locations near interior walls can

significantly reduce duct lengths (decreasing costs), thermal losses (if ducts are

located outside the conditioned space), and blower requirements. To prevent

supply air from being swept directly up by kitchen, bathroom, or other exhaust

fans, the distance between supply registers and exhaust vents should be kept as

large as possible. The location of the return register has only a secondary effect

on room air motion. However, returns can help defeat stratification and improve

mixing of room air if they are placed high when cooling is the dominant space-

conditioning need and low when heating is dominant. In multi-story homes with

both heating and cooling, upper-level returns should be placed high and lower-

level returns should be placed low. Otherwise, the location of the return register

can be determined by what will minimize duct runs, improve air circulation and

mixing of supply air, and impact other considerations such as aesthetics.

DESIGN RECOMMENDATIONS AND K E Y D E S I G N E L E M E N T S

In designing the air distribution the following recommendations before finalizing

the design should be considered:

• Design the air distribution system to be located inside the conditioned space of

the house to the greatest extent possible. Do not locate ducts in exterior walls.

• The entire air distribution system should be “hard” ducted, including returns

(i.e., building cavities, closets, raised-floor air handler plenums, platform returns,

wall stud spaces, panned floor joists, etc., should not be used).

• In two-story and very large houses, consider using two or more separate heating

and cooling systems, each with its own duct system. In two-story homes, for

example, upper stories tend to gain more heat in summer and lose more heat in

winter, so the best comfort and performance is often achieved by using separate

systems for the upper and lower stories.

• Consider supply outlet locations near interior walls to reduce duct lengths.

• Locate supply outlets as far away from exhaust vents as possible in bathrooms

and kitchens to prevent supply air from being swept directly up by the exhaust

fans.

• Consider installing volume dampers located at the takeoff end of the duct

rather than at the supply register to facilitate manual balancing of the system

after installation. Volume dampers should have a means of fixing the position of

the damper after the air distribution system is balanced.

• When using a central return system, include (a) a return on each level of a multi-

story house, (b) a specification to install transfer grilles or jumper ducts in each

room with a door (undercutting interior doors to allow 1 inch of clearance to the

floor is usually not sufficient), and (c) if at all possible, a return in all rooms with

doors that require two or more supply ducts.

• Specify higher duct insulation levels in ducts located outside the conditioned

space than those specified by the 2000 International Energy Conservation Code,

especially when variable-speed air handling equipment is being used. Lower air

flows provided by variable-speed heating and cooling systems to improve

operating efficiency increase the resident time of air within the air distribution

system, which in turn increases thermal losses in the winter and thermal gains in

the summer. Attic insulation placed over ducts helps where it is possible.

• Specify that all duct joints must be mechanically fastened and sealed prior to

insulation to prevent air leakage, preferably with mastic and fiberglass mesh.

Consider testing of ducts using a duct blower to ensure that the air distribution

system is tight, especially if ducts are unavoidably located in an unconditioned

space. A typical requirement is that duct leakage (measured using a duct blower

in units of cubic feet per minute when the ducts are pressurized to 25 Pascals)

should not exceed 5% of the system air flow rate.

CONTENTS

� Principles of Air-Conditioning.

� Psychometric Chart

� Refrigeration Cycle

� Vapor Compression cycle

� Vapor absorption cycle

� Air cycle

� Comfort cooling

� Cooling supply devices

� Air conditioning

• Application

• Types of AC units

• Central air-conditioning

• Window AC units

• HVAC

• Air distribution systems