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MEBS6006 Environmental Services Ihttp://www.hku.hk/bse/MEBS6006/
Load Estimation
Dr. Sam C M Hui
The University of Hong Kong
- .
Sep 2009
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Contents
Basic Conce ts
Outdoor Design Conditions
Indoor Design Conditions
Coolin Load Com onents
Cooling Load Principles
Cooling Coil Load
Software Applications
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Basic Concepts
Conduction
Convection
Thermal properties of building materials
Overall thermal transmittance (U-value)
Thermal capacity (specific heat)Q = U A (t)
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Four forms of heat transfer
CONVECTION
(Source: Food and Agriculture Organization of the United Nations, www.fao.org)
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Basic Concepts
level)
Sensible heat transfer rate:
= 1.23 Flow rate L/s t
Latent heat transfer rate:
qlatent
= ow ra e, s w
Total heat transfer rate:
qtotal = 1.2 (Flow rate, L/s) (h)
total sensible latent
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Basic Concepts
The amount of heat that must be added or removed
rom t e space to ma nta n t e proper temperature
in the space When thermal loads push conditions outside
o e com or range, sys ems are useto bring the thermal conditions back to
comfort conditions
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Basic Concepts
Calculate peak design loads (cooling/heating)
Estimate likely plant/equipment capacity or size
. .
Form the basis for building energy analysis
Cooling load is our main target
Affect building performance & its first cost
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Basic Concepts
General rocedure for coolin load calculations
Obtain the characteristics of the building, building
, , .
specifications
, ,
shading (like adjacent buildings)
a n appropr a e wea er a a an se ec ou oor es gn
conditions
e ec n oor es gn con ons nc u e perm ss e
variations and control limits)
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Basic Concepts
General rocedure for coolin load calculations
(contd)
, ,
equipment appliances and processes that would contribute
Select the time of day and month for the cooling load
Calculate the space cooling load at design conditions
Assess the cooling loads at several different time or a
design day to find out the peak design load
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Cooling load profiles
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Basic Concepts
A buildin surve will hel us achieve arealistic estimate of thermal loads
Use of spaces Physical dimensions of spaces
Columns and beams
onstruct on mater a s
Surrounding conditions Windows, doors, stairways
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(Source: ASHRAE Handbook Fundamentals 2005)
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Basic Concepts
People (number or density, duration of occupancy,
nature o act v ty
Li htin W/m2 t e Appliances (wattage, location, usage)
Vent at on cr ter a, requ rements Thermal stora e if an
Continuous or intermittent operation
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Basic Concepts
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
. ,
building info, system info)
Building layouts & plans are developed
3. Perform detailed load & ener calculations
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OutdoorDesign ConditionsOutdoorDesign Conditions
Climatic design information
General info: e.g. latitude, longitude, altitude,
Outdoor design conditions include
Derived from statistical analysis of weather data Typical data can be found in handbooks/databooks,
such as ASHRAE Fundamentals Handbook
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OutdoorDesign ConditionsOutdoorDesign Conditions
Climatic desi n info from ASHRAE
Previous data & method (before 1997)
, ,
Based on 1%, 2.5% & 5% nos. hours of occurrence
ew met o un amenta s + :
Based on annual percentiles and cumulative frequencyo occurrence, e.g. 0.4%, 1%, 2% (o w o e year)
More info on coincident conditions
Findings obtained from ASHRAE research projects Data can be found on a relevant CD-ROM
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OutdoorDesign ConditionsOutdoorDesign Conditions
,
Annual heating & humidif. design conditions
Coldest month
Heating dry-bulb (DB) temp. Humidification dew point (DP)/ mean coincident dry-
bulb tem . MCDB and humidit ratio HR
Coldest month wind speed (WS)/mean coincident dry-
Mean coincident wind speed (MCWS) & prevailing
.
(Latest information from ASHRAE Handbook Fundamentals 2009)
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OutdoorDesign ConditionsOutdoorDesign Conditions
,
Cooling and dehumidification design conditions
Hottest month and DB range
Cooling DB/MCWB: Dry-bulb temp. (DB) + Meancoincident wet-bulb temp. (MCWB)
Eva oration WB/MCDB: Web-bulb tem . WB +
Mean coincident dry-bulb temp. (MCDB)
Dehumidification DP/MCDB and HR: Dew-point temp.
Enthalpy/MCDB
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OutdoorDesign ConditionsOutdoorDesign Conditions
,
Extreme annual design conditions
Monthly climatic design conditions
Tem erature de ree-da s and de ree-hours
Monthly design DB and mean coincident WB
Mean daily temperature range
Clear sky solar irradiance
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OutdoorDesign ConditionsOutdoorDesign Conditions
Joint frequency tables of psychrometric conditions
Annual, monthly and hourly data
De ree-da s coolin /heatin & climatic normals To classify climate characteristics
yp ca year a a se s year: , ours
For energy calculations & analysis
Recommended Outdoor Design Conditions for Hong Kong
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Recommended Outdoor Design Conditions for Hong Kong
ocation ong ong atitu e , ongitu e , e evation m
Weather station Royal Observatory Hong Kong
Summer months June to September (four hottest months), total 2928 hours
Winter months December, January & February (three coldest months), total 2160 hours
Desi n For comfort HVAC based on For critical rocesses based ontemperatures: summer 2.5% or annualised 1% and
winter 97.5% or annualised 99.3%)summer 1% or annualised 0.4% and
winter 99% or annualised 99.6%)
Summer Winter Summer Winter
DDB / CWB 32.0 oC / 26.9 oC 9.5 oC / 6.7 oC 32.6 oC / 27.0 oC 8.2 oC / 6.0 oC
CDB / DWB 31.0 oC / 27.5 oC 10.4 oC / 6.2 oC 31.3 oC / 27.8 oC 9.1 oC / 5.0 oC
Note: 1. DDB is the design dry-bulb and CWB is the coincident wet-bulb temperature withit; DWB is the design wet-bulb and CDB is the coincident dry-bulb with it.
2. The design temperatures and daily ranges were determined based on hourly data
(Source: Research findings from Dr. Sam C M Hui)
-based on extreme values between 1884-1939 and 1947-1994.
Recommended Outdoor Design Conditions for Hong Kong (contd)
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Recommended Outdoor Design Conditions for Hong Kong (cont d)
xtremetemperatures:
ottest mont : u y
mean DBT = 28.6o
C
o est mont : anuary
mean DBT = 15.7o
Ca so ute max. = . o
mean daily max. DBT = 25.7 oC
a so ute min. = . o
mean daily min. DBT = 20.9 oC
Diurnal range: Summer Winter Whole year- Mean DBT 28.2 16.4 22.8
- Daily range 4.95 5.01 5.0
Wind data: Summer Winter Whole year- Wind direction 090 (East) 070 (N 70 E) 080 (N 80 E)
- Wind speed 5.7 m/s 6.8 m/s 6.3 m/s
Note: 3. Wind data are the prevailing wind data based on the weather summary for the 30-year period 1960-1990. Wind direction is the prevailing wind direction in degreesclockwise from north and the wind speed is the mean prevailing wind speed.
(Source: Research findings from Dr. Sam C M Hui)
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IndoorDesign ConditionsIndoorDesign Conditions
Basic desi n arameters: for thermal comfort
Air temp. & air movement
- -
Air velocity: summer < 0.25 m/s; winter < 0.15 m/s
e at ve um ty
Summer: 40-50% (preferred), 30-65 (tolerable)
Winter: 25-30% (with humidifier); not specified (w/ohumidifier)
See also ASHRAE Standard 55-2004
ASHRAE comfort zone
AS A C f
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ASHRAE Comfort Zones
ase on vers on o tan ar
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IndoorDesign ConditionsIndoorDesign Conditions
Indoor air ualit : for health & well-bein
Air contaminants
. . , , ,
Outdoor ventilation rate provided ASHRAE Standard 62-2007
Air cleanliness (e.g. for processing), air movement
Other design parameters: oun eve no se cr er a
Pressure differential between the space &surroundings (e.g. +ve to prevent infiltration)
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(NC = noise critera; RC = room criteria)
(Source: ASHRAE Handbook Fundamentals 2005)
* Remark: buildings in HK often have higher NC, say add 5-10 dB (more noisy).
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Cooling Load Components
1. Heat gain through exterior walls and roofs
2. Solar heat gain through fenestrations (windows)
.
4. Heat gain through partitions & interior doors
Internal.
2. Electric lights
3. Equipment and appliances
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Cooling Load Components
Air leakage and moisture migration, e.g.
flow of outdoor air into a building through
, ,of exterior doors for entrance
System (HVAC) Outdoor ventilation air
,reheat, fan & pump energy, energy recovery
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Cooling Load Components
Sensible cooling load + Latent cooling load
= (sensible items) + (latent items)
c components ave atent oa s c
onl have sensible load? Wh ?
Three major parts for load calculation
External cooling load
Ventilation and infiltration air
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(Source: ASHRAE Handbook Fundamentals 2005)
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(Source: ASHRAE Handbook Fundamentals 2005)
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(Source: ASHRAE Handbook Fundamentals 2005)
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Cooling Load Components
Example: CLTD/SCL/CLF method
It is a one-step, simple calculation procedure developed
by ASHRAE CLTD = cooling load temperature difference
SCL = solar coolin load
CLF = cooling load factor
Tables for CLTD, SCL and CLF
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Cooling Load Components
Roofs, walls, and glass conduction
q = U (CLTD) U= U-value;A = area
Solar load throu h lass q =A (SC) (SCL) SC = shading coefficient
Partitions, ceilings, floors q = U (tadjacent - tinside)
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Cooling Load Components
People
qsensible =N(Sensible heat gain) (CLF)
qlatent =N(Latent heat gain) Lights
ul sa Ful = lighting use factor; Fsa = special allowance factor
pp ances
qsensible = qin ut x usage factors (CLF)
qlatent = qinput x load factor (CLF)
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Cooling Load Components
qsensible = 1.23 Q (toutside - tinside)
qlatent = 3010 Q (woutside - winside)
= -tota . outs e ns e
System heat gain
Fan heat gain
Ceiling return air plenum
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(Source: ASHRAE Handbook Fundamentals 2005)
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Cooling Load Principles
Space a volume w/o a partition, or a partitioned
room, or group o rooms
Room an enclosed s ace a sin le load Zone a space, or several rooms, or units of space
v
operating characteristics
Thermal zoning
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Cooling Load Principles
Definitions
Space heat gain: instantaneous rate of heat gain
Space cooling load: the rate at which heat must beremove rom e space o ma n a n a cons anspace air temperature
Space heat extraction rate: the actual rate of heatremoval when the space air temp. may swing
Cooling coil load: the rate at which energy is
removed at a coolin coil servin the s ace
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Conversion of heat gain into cooling load
(Source: ASHRAE Handbook Fundamentals 2005)
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Cooling Load Principles
Instantaneous heat ain vs s ace coolin loads
They are NOT the same
ect o eat storage
Ni ht shutdown eriod HVAC is switched off. What happens to the space?
- -
When HVAC system begins to operate
Conditioning period
Space air temperature within the limits
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(Source: ASHRAE Handbook Fundamentals 2005)
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Cooling Load Principles
S ace load and e ui ment load
Space heat gain (sensible, latent, total)
pace coo ng ea ng oa a u ng
Space heat extraction rate Cooling / heating coil load [at air-side system]
Instantaneous heat gain Convective heat
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Convective and radiative heat in a conditioned space
(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)
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(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)
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(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)
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(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)
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(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)
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Cooling Load Principles
Shows the variation of space cooling load
Such as 24-hr cycle
What factors will affect load profiles?
Peak load and block load.
Block load = sum of zone loads at a specific time
Cooling load profiles
ota coo ng oa
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ota coo ng oa
(Source: D.G. Stephenson, 1968)
North
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South
Block load and thermal zoning
Cooling loads due to windows at different orientations
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(Source: D.G. Stephenson, 1968)
Profiles of solar heat gain (July) (for latitude 48 deg N)
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(Source: Keith E. Elder)
Solar cooling load vs. heat gain (July, west) (latitude 48 deg N)
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(Source: Keith E. Elder)
C li L d P i i l
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Cooling Load Principles
Two paths:
Moisture migrates in building envelope
Air leakage (infiltration or exfiltration) If slight RH variation is acceptable, then storage
Latent heat gain = latent cooling load (instantaneously)
What happens if both temp. & RH need to be
Cooling Coil Load
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Cooling Coil Load
Space cooling load (sensible & latent)
Supply system heat gain (fan + air duct)
Load due to outdoor ventilation rates (or
vent at on oa
conditioning cycle on a psychrometric chart?
See also notes in Psychrometrics
Typical summer air conditioning cycle
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Cooling coil load ent at on oa
Return system heat gain
Space cooling load
(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)
Cooling Coil Load
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Cooling Coil Load
(kW)loadSensible
(L/s)airflowSupply
To determine supply air flow rate & size of air
system, ucts, term na s, users
It is a com onent of coolin coil load Infiltration heat gain is an instant. cooling load
Cooling coil load
refrigeration system
Remem er, vent at on oa s a co oa
Heating Load
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Heating Load
Max. heat energy required to maintain winter
n oor es gn temp.
Usually occurs before sunrise on the coldest days
Include transmission losses & infiltration/ventilation
All heating losses are instantaneous heating loads Credit for solar & internal heat gains is not included
Latent heat often not considered (unless w/ humidifier)
Thermal storage effect of building structure is ignored
Heating Load
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Heating Load
-
conditions based on Design interior and exterior conditions
No solar effect (at night or on cloudy winter days)
Before the presence of people, light, and
a liances has an offsettin effect
Also, a warm-up/safety allowance of 20-25%
s a r y common
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(Source: ASHRAE Handbook Fundamentals 2005)
Software Applications
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Software Applications
TRACE 600/700 and Carrier E20-II
Commercial programs from Trane and Carrier
Most widely used by engineers DOE-2 (used more for research)
Software Applications
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Software Applications
TRACE 700
TRACE = Trane Air Conditioning Economics
Demon version can be downloaded
p: www. rane.com commerc a
References
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References
Air Conditionin and Refri eration En ineerin
(Wang and Norton, 2000)
ASHRAE Handbook Fundamentals (2009 edition)
Chapter 14 Climatic Design Information
Cha ter 15 Fenestration
Chapter 17 Residential Cooling and Heating Load
Chapter 18 Nonresidential Cooling and Heating Load
References
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References
Load & Energy Calculations inASHRAE
an oo un amenta s
The followin revious coolin load calculationsare described in earlier editions of the ASHRAE
CLTD/SCL/CLF method met o
TFM method