Hvac psychrometry and concepts

PRESENTED BY:AISHWARYA DEOPUJARI

PRERANA DASNISHTHA DUGGAL

VASUNDHRA SINGHSRIDEVI

SECTION-B6TH SEMESTER

ContentsBasic ConceptsPsychrometryOutdoor Design ConditionsIndoor Design CriteriaCooling Load PrinciplesCooling Load ComponentsHeating Load

Basic ConceptsThermal load

The amount of heat that must be added or removed from the space to maintain the proper temperature in the space

When thermal loads push conditions outsider of the comfort range, HVAC systems are used to bring the thermal conditions back to comfort conditions

PSYCHROMETRY

What is PSYCHROMETRY

The field of engineering concerned with the determination of physical and

thermodynamic properties of gas-vapor mixtures.

Study of various properties of air, method of controlling its temperature and

moisture content or humidity and its effect on various materials and human

beings.

Helps in understanding different constituents of air and how they affect each

other.

Air (ordinary) = mixture of various gases + water vapor or moisture.

Air without any water vapor - dry air (ideal condition, not possible)

Composition of air:Nitrogen (78%), Oxygen (21%)Others (1%) – like carbon dioxide, hydrogen,

helium, neon, and argon along with water vapor.

State Point

Air Properties Dry-bulb temperature, which is usually referred to as simply air temperature, is the air property that is most familiar. Dry-bulb temperature, Tdb, can be measured using a standard thermometer or more sophisticated sensors. This temperature is an indicator of heat content and is shown along the bottom axis of the psychrometric chart. The vertical lines extending upward from this axis are constant-temperature lines.

Wet-bulb temperature, Twb, represents how much moisture the air can evaporate. This temperature is often measured with a common mercury thermometer that has the bulb covered with a water-moistened wick and with a known air velocity passing over the wick. On the chart, the wet-bulb lines slope a little upward to the left, and this temperature is read at the saturation line.

Relative humidity, RH, is the ratio of the actual water vapor pressure, Pv, to the vapor pressure of saturated air at the same temperature, Pvs, expressed as a percentage.

Relative humidity is a relative measure, because the moisture-holding capacity of air increases as air is warmed. In practice, relative humidity indicates the moisture level of the air compared to the airs moisture-holding capacity.

Relative humidity lines are shown on the chart as curved lines that move upward to the left in 10% increments. The line representing saturated air (RH = 100%) is the uppermost curved line on the chart.

Dewpoint, Tdp, is the temperature at which water vapor starts to condense out of air that is cooling. Above this temperature, the moisture stays in the air.

This temperature is read by following a horizontal line from the state-point (found earlier) to the saturation line.

Humidity ratio, w, is the dry-basis moisture content of air expressed as the weight of water vapor per unit weight of dry air.

Humidity ratio is indicated along the right-hand axis of a psychrometric chart.

Specific volume represents the space occupied by a unit weight of dry air, in ft3/lb, and is equal to 1/air density. Specific volume is shown along the bottom axis of a psychrometric chart, with constant-volume lines slanting upward to the left.

Enthalpy, h, is the measure of airs energy content per unit weight (Btu/lbda). Wet-bulb temperature and enthalpy are related intuitively. So, enthalpy is read from where the appropriate wet-bulb line crosses the diagonal scale above the saturation curve.

EGEE 102 - Pisupati 16

Humidity in airRelative

HumidityA measure of of

much water is in the air relative to the maximum amount air can hol at that tmperature

EGEE 102 - Pisupati 17http://www.ae.iastate.edu/Ast473/Lectures/%285%29Psychrometric_Chart/sld024.htm

http://www.ae.iastate.edu/Ast473/Lectures/(5)Psychrometric_Chart/sld024.htm

BASIC FACTORS THAT AFFECT HUMAN COMFORT IN THE INTERNAL ENVIRONMENT-

THERMAL COMFORT

Thermal and air qualityWhat affects the surroundings you live in?Air quality is affected by how hot it is outside or

inside your environmentWhat is humidity and what affects humidity?The amount of moisture that is present within

the air will have an effect on humidity, which is linked to the amount of ventilation entering

What is the normal temperature of a human being?

Human temperature maintain an average core temperature of 37º depending on the metabolic rate

Nature of heat• What is the measure of temperature• Temperature is measured in degrees celsius • The lower is 0 fixed at a melting point of ice at a

stand at atmospheric pressure of 101.32kN/m2• The upper point is 100 degrees – temperature of

steam above the boiling point• What is the acceptable value of temperature

taken at normal design?• Normal design temperature are taken at 21

degrees inside and -1 degrees outside on average

Thermodynamic temperature scale• This is another measure of temperature in

degrees Kelvin• 0 degree celsius= 273.16 Kelvin (K)• 100 degree celsius = 317.16 Kelvin• The unit of thermodynamic temperature is the

fraction of the thermodynamic temperature at the triple point water

• (equilibrium point of the temperature and pressure at which three known phases of substance can exist i.e. liquid, water vapour and pure ice)

Quantity of heat

How do we measure the quantity of heat?Heat is measured in joules (J) which is a

measure of work doneThe rate of expenditure of energy or

doing work or of heat loss is measured in watts (W)

1 watt is = 1 Joule per second1 W =1 J/s

Heat transferName three ways heat is transferred from

one mass to another, for instance a person sitting next to a radiator.

ConductionConvectionRadiation

Thermal comfortIn high activity the temperature rises and the

more heat you will give off. Several factors influences the level heat is generated (metabolic rate) including:

Your surface areaAgeGenderLevel of activitye.g. Sleeping heat output 70W. Lifting 440W.

Typical heat output of an adult male

Activity Example Heat output

Immobile Sleeping 70W

Seated Watching TV 115W

Light work Office 140W

Medium work Factory Work 265W

Heavy work Lifting 440W

ClothingThe amount of clothing that we wear

generally depends on the season and affects our thermal comfort

Clothing is measured in a scale called clo value

1 clo= 0.155m2 K/W of insulation to the bodyTypical values vary from 1-4 clo

Typical clothing values

Clo value Clothing Typical comfort temperature when sitting

0 clo Swimwear 29ºC

0.5 clo Light clothing 25ºC

1 clo Suit , jumper 22ºC

2 clo Coat, gloves, hat 14ºC

Heat losses from buildingsComfortable temperature for humans is

provided by balancing the heat lost through conduction and ventilation through the fabric with similar heat

Optimum temperature will depend on material used , type of construction, orientation of the building and degree of exposure to the rain and wind

Room temperaturesWhat would you consider in design to maintain

temperature in buildings?The resistance of a material to the passage of

heat and the thermal conductivity of the material in passing the heat along are the basics of understanding of maintaining a steady temperature and a comfortable thermal indoor environment

In order to maintain a comfortable room temperature the building must be provided with as much heat as is lost through ventilation

What will the loss of heat in buildings depend on?Materials usedType of constructionOrientation of the building in relation to the

sunDegree of exposure to rain and wind

Thermal conductivity (k)The amount of heat loss in one second

through 1m2 of material, whose thickness is 1 metre

The units are W/mK (watts per metre Kelvin)

K-Values

Material K Value (W/mK)

Brickwork (internal/exposed) (1700kg/m3) 0.84

Concrete, dense (2100kg/m3) 1.40

Concrete, lightweight (1200kg/m3) 0.38

Plaster, dense 0.50

Rendering 0.50

Concrete block, medium, weight (1400kg/m3)

0.51

Concrete block, lightweight (600kg/m3) 0.19

Thermal resistivity (r)Thermal resistivity is the reciprocal of

thermal conductivity:R=1/K

Air movementProperties are tested for airtightnessDraught seals are fitted to all openings

to restrict thermal lossesIf warmer air enter a room is not mixed

with cooler air the room becomes hotter near the ceiling and colder at floor level

Humidity & VentilationHumidity- the amount of water or moisture in

the air measured in grams per cubic metre(g/m3)

Relative Humidity or percentage saturation

This the percentage saturation Actual amount of water vapour/maximum

amount of water vapour that can be held X 100% of the temperature

RELATIVE HUMIDITYHumans are used to a relative humidity of

between 40 and 60%. Greater than this we start to describe air as being ‘Humid’.

HEAT LOSS DUE TO VENTILATIONNatural ventilation leads to the complete

volume of air in a room changing a certain number of times in one hour

Type of room Air changes in hrHalls 1.0Bedrooms /lounges 1.5WCs and bathrooms 2.0

HEAT LOSS DUE TO VENTILATIONThe fresh air entering the room will need to be

heated to the internal temperature of the room. This is calculated with the formula:

Volume of room x air change rate x volumetric specific heat for air x temperature difference

The volumetric specific heat for air is approximately 1300j/m3K and is considered a constant in this formula which will give an answer in joules per hour.

This then has to be converted into watts in order to find the rate of heat loss which is achieved by dividing the number of joules by the number of seconds in one hour

Heat loss to ventilationThis then has to be converted into watts in order to

find the rate of heat loss which is achieved by dividing the number of joules by the number of seconds in one hour

Volume of room/building x air changes hr x 1300J x Temperature difference / 3600s = Watts

It is convenient when carrying out heat loss calculations to assume an average internal temperature of 19°C minus average of -1°C in winter which gives 20°C difference between inside and outside temperatures

Theory into practiceCalculate the rate of heat loss due to

ventilation for the building measuring 4.5m x 3.25 in plan and has a ceiling height of 2.6m. The number of air changes in one hour is 1.35. The outside temperature is 6°C and the inside temperature is 19°C.

Calculation{(4.5x3.25x2.6)m3 x 1.35 x 1300J x (19-6)°}/

3600s

240.983 Watts

Theory into practiceA domestic semi-detached dwelling is subject

to 1.5 changes per hour. Calculate the total heat loss due to ventilation. In this example we have removed the circulation space which is uninhabited.

Room DimensionsLounge is 3.5m x 3.5mKitchen/diner is 4.0m x 2.5mBedroom 1 is 3.0m x 3.0mBedroom 2 is 2.75m x 2.75mBathroom 3 is 2.5m x 2mStorey height is 2.4mAir changes for all rooms 1.5 per hourTemperature difference -1°C outside, 19°C

inside.

CalculationLounge 3.5 x 3.5 x 2.4 =29.4Kitchen 4.0 x 2.5 x 2.4 =24.0Bedroom One 3.0 x 3.0 x 2.4 =21.6Bedroom Two 2.75 x 2.75 x 2.4 =18.15Bathroom Three 2.5 x 2.0 x 2.4 =12.0

Total volume = 105.91m3

Calculation

condensationThis is formed when hot , humid air meets a cold

surface, it condenses onto this surface forming droplets of water vapour.

What are the effects of condensation in the internal environment?

Cause timber rotEncourage mould growthProduce cold spotsProduce high humidityCause corrosion to steelworkDampen insulation, reducung its effectiveness

Heat flow through a structure

Acceptable values The acceptable values of heat loss or U-values

is a complicated topic and you will need to refer to the Building regulations Part L Conservation of fuel and power for guidance on the acceptable U- values.

Ventilation is linked to the Building Regulation Part L that it restricts air tightness of modern structure. Forced ventilation has to be provided in form of fans in bathrooms and cooking areas

Thermal conductivity (k)The amount of heat loss in one second through

1m2 of material, whose thickness is 1 metreThe units are W/mK (watts per metre Kelvin)

P= kA (T1-T2)/ x

A= AreaX= thickness in m² and m respectivelyT1-T2= temperature difference in °C or K Which can be written as follows

W=k x m² x °C/m ; k = W x m/(m² x °C) = W/m°C or W/mK

U-ValuesA measurement of the rate of heat loss

through a structureThermal resistivity is the reciprocal of

thermal conductivity:R=1/K

PRINCIPLES OF AIR COOLING

Principle


A. Expansion ValveB. Compressor

Arrangement


TYPES OF AIR CONDITIONERS Room air conditioners Central air conditioning systems Heat pumps Evaporative coolers


Air Conditioning


Room air conditionerRoom air conditioners cool rooms rather

than the entire home. Less expensive to operate than central

unitsTheir efficiency is generally lower than

that of central air conditioners.Can be plugged into any 15- or 20-amp,

115-volt household circuit that is not shared with any other major appliances



Central Air conditioningCirculate cool air through a system of

supply and return ducts. Supply ducts and registers (i.e., openings in the walls, floors, or ceilings covered by grills) carry cooled air from the air conditioner to the home.

This cooled air becomes warmer as it circulates through the home; then it flows back to the central air conditioner through return ducts and registers


Types of Central ACsplit-system

an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator

Packagedthe evaporator, condenser, and

compressor are all located in one cabinet


Large air conditioning systems Outside air is drawn in,

filtered and heated before it passes through the main air conditioning devices. The colored lines in the lower part of the diagram show the changes of temperature and of water vapor concentration (not RH) as the air flows through the system.


Total Air Conditioning


Variable fresh air mixer and dust and pollutant filtration.

Supplementary heating with radiators in the outer rooms and individual mini heater and

Humidifier in the air stream to each room.


Sizing Air Conditioners

how large your home is and how many windows it has;

how much shade is on your home's windows, walls, and roof;

how much insulation is in your home's ceiling and walls;

how much air leaks into your home from the outside; and

how much heat the occupants and appliances in your home generate


Energy ConsumptionAir conditioners are rated by the number

of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the "ton," which is 12,000 Btu per hour.

Room air conditioners range from 5,500 Btu per hour to 14,000 Btu per hour.


Energy EfficiencyToday's best air conditioners use 30% to 50%

less energy than 1970sEven if your air conditioner is only 10 years

old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model


Energy EfficiencyRating is based on how many Btu per hour

are removed for each watt of power it draws

For room air conditioners, this efficiency rating is the Energy Efficiency Ratio, or EER

For central air conditioners, it is the Seasonal Energy Efficiency Ratio, or SEER


Room Air ConditionersBuilt after January 1, 1990, need have an

EER of 8.0 or greater EER of at least 9.0 if you live in a mild climate EER over 10 for warmer climates


Central ACNational minimum standards for central air

conditioners require a SEER of 9.7 for single-package and 10.0 for split-systemsUnits are available with SEERs reaching nearly

17


Energy Saving MethodsLocate the air conditioner in a window or

wall area near the center of the room and on the shadiest side of the house.

Minimize air leakage by fitting the room air conditioner snugly into its opening and sealing gaps with a foam weather stripping material.


Basic ConceptsPurpose of HVAC load estimation

Calculate peak design loads (cooling/heating)Estimate likely plant/equipment capacity or

sizeProvide info for HVAC design e.g. load profilesForm the basis for building energy analysis

Cooling load is our main targetImportant for warm climates & summer designAffect building performance & its first cost

Basic ConceptsHeat transfer mechanism

ConductionConvectionRadiation

Thermal properties of building materialsOverall thermal transmittance (U-value)Thermal conductivityThermal capacity (specific heat)

Basic ConceptsA building survey will help us achieve a

realistic estimate of thermal loadsOrientation of the buildingUse of spacesPhysical dimensions of spacesCeiling heightColumns and beamsConstruction materialsSurrounding conditionsWindows, doors, stairways

Basic ConceptsBuilding survey (cont’d)

People (number or density, duration of occupancy, nature of activity)

Lighting (W/m2, type)Appliances (wattage, location, usage)Ventilation (criteria, requirements)Thermal storage (if any)Continuous or intermittent operation

Outdoor Design Conditions

They are used to calculate design space loads

Climatic design informationGeneral info: e.g. latitude, longitude, altitude,

atm. pressureOutdoor design conditions

Derived from statistical analysis of weather data Typical data can be found in handbooks/databooks,

such as ASHRAE Fundamentals Handbooks


Climatic design conditions from ASHRAEPrevious data & method (before 1997)

For Summer (Jun. to Sep.) & Winter (Dec, Jan, Feb) Based on 1%, 2.5% & 5% nos. hours of occurrence

New method (ASHRAE Fundamentals 2001): Based on annual percentiles and cumulative

frequency of occurrence, e.g. 0.4%, 1%, 2% More info on coincident conditions Findings obtained from ASHRAE research projects

Data can be found on a relevant CD-ROM


Climatic design conditions (ASHRAE 2001):Heating and wind design conditions

Heating dry-bulb (DB) temp. Extreme wind speed Coldest month wind speed (WS) & mean coincident

dry-bulb temp. (MDB) Mean wind speed (MWS) & prevailing wind

direction (PWD) to DB Average of annual extreme max. & min. DB temp. &

standard deviations


Climatic design conditions (ASHRAE):Cooling and dehumidification design conditions

Cooling DB/MWB: Dry-bulb temp. (DB) + Mean coincident wet-bulb temp. (MWB)

Evaporation WB/MDB: Web-bulb temp. (WB) + Mean coincident dry-bulb temp. (MDB)

Dehumidification DP/MDB and HR: Dew-point temp. (DP) + MDB + Humidity ratio (HR)

Mean daily (diurnal) range of dry-bulb temp.


Other climatic info:Joint frequency of temp. and humidity

Annual, monthly and hourly dataDegree-days (cooling/heating) & climatic

normals To classify climate characteristics

Typical year data sets (1 year: 8,760 hours) For energy calculations & analysis

Indoor Design CriteriaIndoor Design Criteria

Basic design parameters: (for thermal comfort)Air temp. & air movement

Typical: summer 24-26 oC; winter 21-23 oC Air velocity: summer < 0.25 m/s; winter < 0.15 m/s

Relative humidity Summer: 40-50% (preferred), 30-65 (tolerable) Winter: 25-30% (with humidifier); not specified (w/o

humidifier)See also ASHRAE Standard 55-2004

ASHRAE comfort zone

(*Source: ASHRAE Standard 55-2004)

Indoor Design CriteriaIndoor Design Criteria

Indoor air quality:Air contaminants

e.g. particulates, VOC, radon, bioeffluentsOutdoor ventilation rate provided

ASHRAE Standard 62-2001Air cleanliness (e.g. for processing)

Other design parameters:Sound levelPressure differential between the space &

surroundings (e.g. +ve to prevent infiltration)

COOLING LOAD PRINCIPLES

Cooling Load Principles

Terminology:Space – a volume w/o a partition, or a

partitioned room, or group of roomsRoom – an enclosed space (a single load)Zone – a space, or several rooms, or units of

space having some sort of coincident loads or similar operating characteristics Thermal zoning


Space and equipment loadsSpace heat gain (sensible, latent, total)Space cooling load / space heating loadSpace heat extraction rateCooling coil load / heating coil loadRefrigeration load

Instantaneous heat gainConvective heatRadiative heat (heat absorption)

Convective and radiative heat in a conditioned space

Conversion of heat gain into cooling load


Instantaneous heat gain vs space cooling loadsThey are NOT the same

Effect of heat storageNight shutdown period

HVAC is switched off. What happens to the space?Cool-down or warm-up period

When HVAC system begins to operateConditioning period

Space air temperature within the limits

Thermal Storage Effect in Cooling Load from Lights


Load profileShows the variation of space loadSuch as 24-hr cycleWhat factors will affect load profile?Useful for operation & energy analysis

Peak load and block loadPeak load = max. cooling loadBlock load = sum of zone loads at a specific

time

Block load and thermal zoning

Cooling Load Components

• Cooling load calculations• To determine volume flow rate of air system• To size the coil and HVAC&R equipment• To provide info for energy calculations/analysis

• Two categories:• External loads• Internal loads


• External loads• Heat gain through exterior walls and roofs• Solar heat gain through fenestrations (windows)• Conductive heat gain through fenestrations• Heat gain through partitions & interior doors• Infiltration of outdoor air


• Internal loads• People• Electric lights• Equipment and appliances

• Sensible & latent cooling loads• Convert instantaneous heat gain into cooling load• Which components have only sensible loads?

[Source: ASHRAE Fundamentals Handbook 2001]


• Cooling coil load consists of:• Space cooling load (sensible & latent)• Supply system heat gain (fan + air duct)• Return system heat gain (plenum + fan + air duct)• Load due to outdoor ventilation rates (or

ventilation load)

• How to construct a summer air conditioning cycle on a psychrometric chart?

Cooling load

Cooling coil load

Schematic diagram of typical return air plenum


• Space cooling load• To determine supply air flow rate & size of air

system, ducts, terminals, diffusers• It is a component of cooling coil load• Infiltration heat gain is an instant. cooling load

• Cooling coil load• To determine the size of cooling coil &

refrigeration system• Ventilation load is a coil load

Heating Load

• Design heating load• Max. heat energy required to maintain winter

indoor design temp.• Usually occurs before sunrise on the coldest days

• Include transmission losses & infiltration/ventilation

• Assumptions:• All heating losses are instantaneous heating loads

• Solar heat gains & internal loads usually not considered

• Latent heat often not considered (unless w/ humidifier)

ReferencesASHRAE Handbook Fundamentals 2001

Chapter 26 – Ventilation and InfiltrationChapter 27 – Climatic Design InformationChapter 28 – Residential Cooling and Heating

Load CalculationsChapter 29 – Nonresidential Cooling and

Heating Load CalculationsChapter 30 – FenestrationChapter 31 – Energy Estimation and Modeling

Methods

ReferencesAir Conditioning and Refrigeration

Engineering (Wang and Norton, 2000)Chapter 6 – Load Calculations

Handbook of Air Conditioning and Refrigeration, 2nd ed. (Wang, 2001)Chapter 6 – Load Calculations

Documents

Hvac psychrometry and concepts