Joint Comprehensive Certificate Course on HVAC&R System, 2012 

2012年度暖通空調及製冷系統綜合証書課程

Fundamentals of HVAC&R Part 1

Presented by:Ir Dr. Sam C. M. Hui

February 28, 2012

About the Lecturer

• Dr. Sam C. M. Hui• PhD, BEng(Hons), CEng, CEM, MASHRAE, MCIBSE,

MHKIE, MIESNA, LifeMAEE, AssocAIA• ASHRAE Distinguished Lecturer (2009-2011)• CEng = Chartered Engineer• CEM = Certified Energy Manager• LifeMAEE = Life Member, Associatn of Energy Engineers• Worked in 1998 as a visiting researcher in the Asia Pacific

Energy Research Centre, Japan• Research interests: energy efficiency in buildings and

sustainable building technologies

Joint Comprehensive Certificate Course on HVAC&R System, 2012Feb-Apr 2012

Dr. Sam C. M. HuiDepartment of Mechanical Engineering

The University of Hong KongE-mail: cmhui@hku.hk

Fundamentals of HVAC&R Part 1

Contents

• Introduction

• Psychrometry

• Thermal comfort

• Load and energy calculations

Introduction

• Terminology• Heating, ventilating, air-conditioning and refrigerating

(HVAC&R) 暖通空調及製冷• Heating, ventilating and air-conditioning (HVAC) 暖通空調

• Mechanical ventilating and air-conditioning (MVAC or ACMV) 機械通風及空調

• Air conditioning and refrigeration (AC&R) 空調及製冷• Environmental control systems (ECS) 環境控制系統

• Misused word in HK:• Air cond. “冷氣” (= cold air)

Introduction

• Definition (from ASHRAE*)• Air conditioning is the process of treating

air so as to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the requirements of the conditioned space.• Basic processes: Cooling and Heating

• Comfort air conditioning• To meet comfort requirements of occupants

(*ASHRAE = American Society of Heating, Refrigerating & Air-conditioning Engineers, Inc.)

(Source: www.howstuffworks.com/ac.htm)

See also: “How Air Conditioners Work” (1:07)http://youtu.be/nKZ2DPvvua8 (Source: www.howstuffworks.com/ac.htm)

(Source: www.howstuffworks.com/ac.htm)

A typical air conditioner

Air conditioning with a chilled water system

Chilledwater

system

Refrigerantcycle

What arethe major

components?

Multiple chiller variable flow chilled water system(Source: ASHRAE HVAC Systems and Equipment Handbook 2004)

Introduction

• Air Conditioning and Refrigeration• No. 10 on the list of the [Greatest Engineering

Achievements of the 20th Century]• http://www.greatachievements.org

• These cooling technologies have altered some of our most fundamental patterns of living

• Buildings are climate-controlled & comfortable• Fresh foods & milk are kept in refrigerators/freezers• Building designs are changed completely• Environment for industrial processes are controlled

Introduction

• The History of Air Conditioning• www.air-conditioners-and-

heaters.com/air_conditioning_history.htm• 1830: Dr. John Gorrie (ice for cooling hospital rooms)• 1881: James Garfield (device w/ melted ice water)• Late 19th century: “manufactured air” (controlling

humidity in textile mills)• Early 1900s’: Willis Carrier (designed modern A/C

systems for offices, apartments, hotels, hospitals)• 1917-1930: movie theatres were kept cool by A/C

Introduction

• Common types of air conditioning systems• Centralised air systems

• Constant volume (CV), variable air volume (VAV), Displacement ventilation

• Partially centralised air/water systems• Fan coils, chilled beams, chilled ceilings, room based

heat pumps• Local systems

• Split units, variable refrigerant flow (VRF) or variable refrigerant volume (VRV) [??]

Individual room air-conditioning system

(Source: EnergyWitts newsletter, EMSD)

Primary air fan coil unit (PA-FCU) system

Variable-air volume (VAV) package system

Psychrometry

• Psychrometry• The study of atmospheric air and its associated

water vapour• Dry air and moist air

• Dalton’s law of partial pressures• Standard atmospheric pressure = 101.325 kPa• Saturated vapour pressure

• Max. pressure of water vapour that can occur at any given temperature

Psychrometry

• Psychrometric Chart: parameters• Moisture content (g), or absolute humidity (w)• Relative humidity (rh or RH)• Percentage saturation (μ)• Wet-bulb temperature (twb)• Specific volume (v)

• (See the illustration on psychrometric chart)

Psychrometric ChartCan you read them

from the chart? Wet-bulb temperature

Enthalpy

Dew-point temperature

Relative humidity

Humidity ratio

Specific volume

Dry-bulb temperature

Psychrometry

• Common processes:• Sensible cooling / sensible heating• Cooling and dehumidification / heating and

humidification• Humidification / dehumidification• Evaporative cooling / chemical dehydration

• Typical devices:• Cooling/heating coils• Humidifiers / dehumifiers

Basic psychrometric processes

35

1

84

6

2

7

Process 0-1: Sensible heatingProcess 0-2: Sensible coolingProcess 0-3: HumidifyingProcess 0-4: DehumidifyingProcess 0-5: Heating and humidifyingProcess 0-6: Cooling and dehumidifyingProcess 0-7: Cooling and humidifyingProcess 0-8: Heating and dehumidifying

Psychrometric processes

Sensible cooling/heating Cooling and dehumidification

Evaporative coolingAdiabatic dehumidification

Cooling coil

Entering air

Leaving air

Cooling and dehumidificationSimple air conditioning cycle

Can you draw such a cycle for Hong Kong summer conditions?- Outdoor: DBT = 33 ºC; WBT = 28 ºC; flow = 20% of supply air- Indoor: DBT = 25 ºC; %RH = 50%- Air leaving cooling coil: DBT = 13 ºC; %RH = 95%

Psychrometry

• Further reading & learning:• Air Conditioning: Psychrometrics

[www.bsenotes.com]• www.arca53.dsl.pipex.com/index_files/psy1.htm

• CIBSE Journal CPD Programme: The psychrometrics of air conditioning systems (Mar 2010), www.cibsejournal.com/cpd/2010-03/

• Daikin's Free Psychrometrics tool• www.daikin.eu/binaries/Psychrometric%20diagram%2

0viewer%20V210_tcm24-133157.zip

What is Thermal Comfort?

- That condition of mindwhich expresses satisfactionwith the thermal environment.

ISO 7730

Body Temperature• Normal body core temperature: 37 oC.• We have separate Heat- and Cold-

sensors.• Heat sensor is located in hypothalamus. Signals when temperature is higher than 37 oC.

• Cold sensors are located in the skin. Send signals when skin temperature is below 34 oC.

• Heating mechanism:• Reduced blood flow.• Shivering.

• Cooling mechanism:• Increased blood flow.• Sweating (Evaporation).

Hot Cold

37 oC 34 oC

Perception of Thermal Environment• Heat sensor in

Hypothalamus send impulses when temperature exceeds 37 oC.

• Cold sensors sends impulses when skin temperature below 34 oC.

• The bigger temperature difference, the more impulses.

• If impulses are of same magnitude, you feel thermally neutral.

• If not, you feel cold or warm. Warmimpulses

Coldimpulses Activity

The Energy Balance

• Thermal Comfort can only be maintained when heat produced by metabolism equals the heat lost from body.

HeatLost

HeatProdu-ced

Thermal comfort

• General heat balanceS = M - W - E - (R + C)

whereS = rate of heat storage of human bodyM = metabolic rateW = mechanical work done by human bodyE = rate of total evaporation lossR + C = dry heat exchange through radiation &

convection

Conditions for Thermal Comfort• Two conditions must be fulfilled

to maintain Thermal Comfort:• Heat produced must equal heat lost• Signals from Heat- and Cold-sensors must neutralise each other

• The sweat production is used instead of body core temperature, as measure of the amount of warm impulses.

• Relation between the parameters found empirically in experiments.

• No difference between sex, age, race or geographic origin.

Metabolic Rate

Metabolic Rate

0 1 2 3 4

0 1 2 3 4

2040

60

W/m2

Sw

eat p

rod.

2930

323334

oC.

10080

31

The Comfort Equation

The Comfort Equation (cont’d)Thermal comfort

• Environmental factors:• Dry-bulb temperature (also related to humidity)• Relative humidity (or water vapour pressure)

• Influences evap heat loss and skin wettedness• Usually RH between 30% and 70% is comfortable

• Air velocity (increase convective heat loss)• Preferable air velocity

• Mean radiation temperature• Radiation has great effect on thermal sensation

Mean Radiant Temperature

• The Mean Radiant Temperature (MRT) is that uniform temperature of an imaginary black enclosure resulting in same heat loss by radiation from the person, as the actual enclosure.

• Measuring all surface temperatures and calculation of angle factors is time consuming. Therefore use of Mean Radiant Temperature is avoided when possible.

Actual room Imaginary room

RR’

t1

t2

tr

t3

t4

Heat exchange by radiation:R=R’

• Energy released by metabolism depends on muscular activity.

• Metabolism is measured in Met (1 Met=58.15 W/m2 body surface).

• Body surface for normal adult is 1.7 m2.

• A sitting person in thermal comfort will have a heat loss of 100 W.

• Average activity level for the last hour should be used when evaluating metabolic rate, due to body’s heat capacity.

Metabolic Rate0.8 Met

1 Met

8 Met

4 Met

Calculation of Insulation in Clothing

• 1 Clo = Insulation value of 0,155 m2 oC/W

0,15 Clo0.5 Clo

1.0 Clo

1.2 Clo

Comfort Temperature, tco (typical)

1.7 clo2.5 Met

RH=50%tco=6oC

0.8 clo2.2 Met

RH=50%tco=18oC

0.5 clo1.2 Met

RH=50%tco=24,5oC

Thermal comfort

• Predicted mean vote (PMV)• A complex function of six major comfort parameters• Predict mean value of the subjective ratings of a group

of people in a given environment• Predicted percentage of dissatisfied (PPD)

• Determined from PMV as a quantitative measure of thermal comfort

• ‘Dissatisfied’ means not voting -1, +1 or 0 in PMV• Normally, PPD < 7.5% at any location and LPPD < 6%

Predicted Mean Vote scale

- +3 Hot

- +2 Warm

- +1 Slightly warm

- +0 Neutral

- - 1 Slightly cool

- -2 Cool

- -3 Cold

The PMV index is used to quantify the degree of discomfort

PMV and PPD

• PMV-index (Predicted Mean Vote) predicts the subjective ratings of the environment in a group of people.• 0 = neutral (still 5% people are dissatisfied)

• PPD-index predicts the number of dissatisfied people.

Thermal comfort

• Comfort zones• Defined using isotherms parallel to effective

temperature (ET) or standard ET (SET)• ASHRAE comfort zones for summer and winter

(for typical indoor and seated person)• Proposed comfort zones

• Within 5 to 16 mm Hg water vapour pressure• For summer, 22.8 oC SET 26.1 oC• For winter, 20.0 oC SET 23.9 oC

ASHRAE Comfort Zones(based on 2004 version of ASHRAE Standard 55)

Local Thermal Discomfort

• Draught

• Radiation Asymmetry

• Vertical Air Temperature Differences.

• Floor temperature

Acclimatisation/Adaptation!

When the air conditionsystem fails you canadapt by adjusting yourCLO value

Load & energy calculations

• Thermal 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 outside of the comfort range, HVAC systems are used to bring the thermal conditions back to comfort conditions

Load & energy calculations

• Purpose of HVAC load estimation• Calculate peak design loads (cooling/heating)• Estimate likely plant/equipment capacity or size• Specify the required airflow to individual spaces• Provide info for HVAC design e.g. load profiles• Form the basis for building energy analysis

• Cooling load is our main target• Important for warm climates & summer design• Affect building performance & its first cost Cooling load profiles

Load & energy calculations

• Typical HVAC load design process• 1. Rough estimates of design loads & energy use

• Such as by rules of thumb & floor areas• See “Cooling Load Check Figures”• See references for some examples of databooks

• 2. Develop & assess more info (design criteria, building info, system info)

• Building layouts & plans are developed• 3. Perform detailed load & energy calculations

Cooling Load Components

• External• 1. Heat gain through exterior walls and roofs• 2. Solar heat gain through fenestrations (windows)• 3. Conductive heat gain through fenestrations• 4. Heat gain through partitions & interior doors

• Internal• 1. People• 2. Electric lights• 3. Equipment and appliances

Cooling Load Components

• Infiltration• Air leakage and moisture migration, e.g. flow of

outdoor air into a building through cracks, unintentional openings, normal use of exterior doors for entrance

• System (HVAC)• Outdoor ventilation air• System heat gain: duct leakage & heat gain, reheat,

fan & pump energy, energy recovery

Components of building cooling load

External loads

Internal loads

+ Ventilation load & system heat gains

Load & energy calculations

• Cooling load calculation method• Example: CLTD/SCL/CLF method

• It is a one-step, simple calculation procedure developed by ASHRAE

• CLTD = cooling load temperature difference• SCL = solar cooling load• CLF = cooling load factor

• See ASHRAE Handbook Fundamentals for details• Tables for CLTD, SCL and CLF

Load & energy calculations

• External• Roofs, walls, and glass conduction

• q = U A (CLTD) U = U-value; A = area• Solar load through glass

• q = A (SC) (SCL) SC = shading coefficient• For unshaded area and shaded area

• Partitions, ceilings, floors• q = U A (tadjacent - tinside)

Load & energy calculations

• Internal• People

• qsensible = N (Sensible heat gain) (CLF)• qlatent = N (Latent heat gain)

• Lights• q = Watt x Ful x Fsa (CLF)

• Ful = lighting use factor; Fsa = special allowance factor

• Appliances• qsensible = qinput x usage factors (CLF)• qlatent = qinput x load factor (CLF)

Load & energy calculations

• Ventilation and infiltration air• qsensible = 1.23 Q (toutside - tinside)• qlatent = 3010 Q (woutside - winside)• qtotal = 1.2 Q (houtside - hinside)

• System heat gain• Fan heat gain• Duct heat gain and leakage• Ceiling return air plenum

Load & energy calculations

• Definitions• Space heat gain: instantaneous rate of heat gain

that enters into or is generated within a space• Space cooling load: the rate at which heat must be

removed from the space to maintain a constant space air temperature

• Space heat extraction rate: the actual rate of heat removal when the space air temp. may swing

• Cooling coil load: the rate at which energy is removed at a cooling coil serving the space Conversion of heat gain into cooling load

(Source: ASHRAE Handbook Fundamentals 2005)

Block load and thermal zoning

North

South

West East

Cooling loads due to windows at different orientations

(Source: D.G. Stephenson, 1968)

Load & energy calculations

• From load estimation to energy calculations• Only determine peak design loads is not enough• Need to evaluate HVAC and building energy consumption

• To support design decisions (e.g. evaluate design options)• To enhance system design and operation• To compile with building energy code

• Energy calculations• More complicated than design load estimation• Form the basis of building energy and economic analysis

Load & energy calculations

• Two categories• Steady-state methods

• Degree-day method• Variable base degree-day method• Bin and modified bin methods

• Dynamic methods• Using computer-based building energy simulation• Try to capture dynamic response of the building• Can be developed based on transfer function, heat

balance or other methods

Heating degree-day:

Cooling degree-day:

tbal = base temperature (or balance point temperature)(e.g. 18.3 oC or 65 oF); Qload = Qgain + Qloss = 0

to = outdoor temperature (e.g. average daily max./min.)

* Degree-hours if summing over 24-hourly intervalsDegree-day = Σ(degree-hours)+ / 24

+ Only take the positive values

Buildingdescription

Simulationoutputs

Simulation tool(computer program)

Weatherdata

- physical data- design parameters

- energy consumption (MWh)- energy demands (kW)- environmental conditions

Systems (air-side)

Plant (water-side & refrig.)

HVAC air systems HVAC water systems

Energy input by HVAC plant

Energy input by HVACair/water systems

Energy storage

Energy inputby appliance

Thermal Zone

Building energy simulation process

Load & energy calculations

• Further reading & learning:• Comfort [www.bsenotes.com]

• www.arca53.dsl.pipex.com/index_files/science1.htm• Thermal comfort – Wikipedia

• http://en.wikipedia.org/wiki/Thermal_comfort

• ASHRAE Handbook Fundamentals 2009, Chps. 14-19 (on load and energy calculations)