joint comprehensive certificate course on hvac&r system...
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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: [email protected]
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)