12 cooling load calculations
TRANSCRIPT
Contents
• Principle of cooling load
• Why cooling load & heat gains are different
• Design conditions
• Understand CLTD/CLF method
• An example
Cooling Load • It is the thermal energy that must be removed
from the space in order to maintain the desired comfort conditions
• HVAC systems are used to maintain thermal conditions in comfort range
Purpose of Load Estimate
• Load profile over a day
• Peak load (basis for equipment sizing)
• Operation Energy analysis
• HVAC Construction cost
• Enclosure heat transfer characteristics– Conduction
– Convection
– radiation
• Design conditions– Outdoor & indoor
• Heat Gains– Internal
– External or Solar
• Thermal capacity
Principles of cooling Load Estimate
Space Characteristics
• orientation
• Size and shape
• Construction material
• Windows, doors, openings
• Surrounding conditions
• Ceiling
Space Characteristics
• Occupants (activity, number, duration)
• Appliances (power, usage)
• Air leakage (infiltration or exfiltration)
• Lighting (W/m2)
Indoor Design ConditionsBasic design parameters
• Air temperature
– Typically 22-26 C
• Air velocity
– 0.25 m/s
• Relative humidity
– 30-70 %
• See ASHRAE 55 – 2004 Comfort Zone
Indoor Design Conditions• Indoor air quality
– Air contaminants
– Air cleaning
• Acoustic requirements
• Pressurization requirements
Outdoor Design Conditions
• Weather data required for load calculation
– Temperature & humidity
– Wind speed, sky clearness , ground reflectance etc
• Design outdoor conditions data can be found in ASHRAE Fundamentals Handbook
Outdoor Design Conditions
• ASHRAE Fundamentals 2001
– Design severity based on 0.4%, 1%, & 2% level annually (8760h)
– For example at 1% level, the value is exceeded in 0.01x8760h = 87.6 h in a year
Outdoor Design For Cooling
Criteria: 0.4% DB and MWB
Station Cooling DB/MWB
MiriMalaysia
0.4% 1% 2%
DB (˚C ) MWB ( ˚C )
DB MWB DB MWB
32.2 26.3 31.8 26.3 31.4 26.2
Source: ASHRAE Fundamentals 2001
Terminology• Space- a volume without partition or a group
of rooms
• Room- an enclosed space
• Zone- a space having similar operating
characteristics
Heat Gain • Space Heat gain
– The instantaneous rate at which heat enters into , out of, or generated within a space. The components are:
• Sensible gain
• Latent gain
Heat gains Convective(%)
Radiant (%)
Solar radiation with internal shading
42 58
Fluorescent lights
50 50
People 67 33
External wall 40 60
Cooling Load
• Space Cooling load
– The rate at which heat must be removed from a space to maintain air temperature and humidity at the design values
• Cooling load differs from the heat gain due to
– delay effect of conversion of radiation energy to heat
– Thermal storage lag
Extraction Rate
• Space Heat extraction rate
– The actual heat removal rate by the cooling equipment from the space
– The heat extraction rate is equal to cooling load when the space conditions are constant which is rarely true.
The principal terms of heat Gains/Losses are indicated below.
(Source: ASHRAE Handbook Fundamentals 2005)
Heat Balance
Coil Load
• Cooling coil load
– The rate at which energy is removed at the cooling coil
– Sum of:
• Space cooling load (sensible + latent)
• Supply system heat gain (fan + supply air duct)
• Return system heat gain (return air duct)
• Load due to outdoor ventilation rates (or ventilation load)
External Loads
1. Heat gains from Walls and roofs
– sensible
2. Solar gains through fenestrations
– Sensible
3. Outdoor air
– Sensible & latent
Cooling Load Components
• Space cooling load
– Sizing of supply air flow rate, ducts, terminals and diffusers
– It is a component of coil load
– Bypassed infiltration is a space cooling load
• Cooling coil load
– Sizing of cooling coil and refrigeration system
– Ventilation load is a coil load
Profiles of Offshore Systems Cooling Loads
Components % Load LQ (L)
%LoadLQ (U)
%Load CCR
%LoadSG/MCC
Solar Transmission 3 4 7 4
Occupants 3 3 3 0
Lights 5 5 8 4
Equipment 10 1 29 21
Outdoor air bypassed 7 8 5 6
Outdoor air notbypassed
72 79 48 64
Total 100 100 100 100
Heat Load Components
Outdoor air & Electrical Equipment loads (77-85% )
People: 3%
Lighting: 4-8%
Solar Transmission: 3-7%
Infiltration : 5-8%
Calculation Methods
1. Rule of thumb method
– Least accurate
– eg 100 btu/ft2 for a space
2. Static analysis (Room temperature is constant)
– CLTD/CLF method
3. Dynamic analysis
– Computer modeling
CLTD/CLF Method
• Cooling load is made up of – Radiation and conduction heat gain– Convection heat gain
• Convective gain is instantaneous– No delay– Heat gain equals cooling load
• Conductive and radiation heat gains are not instantaneous– Thermal delay – Heat gain is not equal to cooling load– Use CLTD & CLF factors
CLTD/CLF Method (ASHRAE 1989)
Cooling load due to solar & internal heat gains
• Glazing (sensible only)– Radiation & conduction
– Convection (instantaneous)
• Opaque surface ( wall, floor, roof) load (sensible only)– Conduction
– Convection (instantaneous)
• Internal loads (sensible & latent)– Radiation & conduction
– Convection (instantaneous)
Cooling Load Temperature Difference CLTD
CompareQ transmission = UA (T o – T i )Q transmission = UA (CLTD)
• CLTD is theoretical temperature difference defined for each wall/roof to give the same heat load for exposed surfaces to account for the combined effects of radiation, conductive storage, etc – It is affected by orientation, time , latitude, etc– Data published by ASHRAE
Cooling Load Factor (CLF)
• This factor applies to radiation heat gain
• If radiation is constant, cooling load = radiativegain
• If radiation heat is periodical, than
Q t = Q daily max (CLF)
CLF accounts for the delay before radiative gains becomes a cooling load
Glazing
• Q = A (SC) (SHGF) (CLF)A= glass areaSC= shading coefficientSHGF= solar heat gain factor,
tabulated by ASHRAECLF= cooling load factor,
tabulated by ASHRAE
• Q = U x A x CLTDU= surface U-factorA= surface areaCLTD= cooling load temperature
difference
transmitted
absorbed
reflected
Solar ray
glass
Opaque Surfaces
• Q 2 = UA (CLTD)U= surface U-factor
A= surface area
CLTD= cooling load temperature difference
• Tabulated or chart values for CLTD can be referred
• Offshore enclosure– Light weight
– Metal frame with insulation
– Group G wall with U-value about 0.5-1.0 W/m2 K
Opaque Surface Calculations
• Use Table for wall CLTD• Use Table for roof CLTD
– Select wall/roof type– Look up uncorrected CLTD– Correct CLTDCLTD c=(CLTD+LM)+ (25.5-t r) + (t m-29.4)
• LM= latitude /month correction (Table )• T r = indoor temperature (22C)• T m= average temperature on the design day = (35+22)/2 =
28.5 C Eg. If CLTD=40 C, LM=-1.7 (west face)CLTD c= (40-1.7) + (25.5-22)+ (28.5-29.4) = 40.9 C
Types of Internal Load
• Internal loads are– People
– Lights
– Equipment or appliances
• Consist of convective and radiant components– Light (mostly radiant)
– Electrical heat (radiant and convective)
– People (most convective)
• Time-delay effect due to thermal storage
Internal Load- Lighting
Area Light Power
Density W/m2
Office 25
Corridor 10
Sleeping 10
CCR 25
MCC/SG 25
Kitchen 25
Recreation 20
•Heat gain (lighting)= 1.2 x total wattage x CLFOr based on light power density ranging from 10-25 W/m2(average density, say=20 W/m2)•Where light is continuously on, CLF=1
Internal Loads- People
• Q people-s = No x sensible heat gain/p x CLF
• Q people-L = No x latent heat gain/p
Internal Load – Equipment Heat
• Cooling of electrical equipment in MCC/SG is an important function of HVAC system offshore. The components include:
• Transformers• Motors• Medium/high voltage switchgears• Cables & trays• Motor starters• Inverters• Battery chargers• Circuit breakers• Unit panel board etc
• Heat dissipation from these equipments are mainly based data published by the manufacturers
Typical Outdoor & Indoor Design Conditions Used Here
Conditions Dry-bulb temperature (C)
% RH Moisture content, kg/kg
Outdoor air 35 70 0.025
Indoor air 22 55 0.009
Difference 13 0.016
ASHRAE fundamental Handbook published data, at 0.4%, 1% and 2% design level. At 0.4% design level, Miri has only 35h (out of 8760 h a year) at 32.2 DB & 26.3 WB or higher
Infiltration Air is Cooling Load
• Load due to Ventilation air into the space
Sensible load, (W)
= mass flow rate x specific heat x (∆T)
= 1.23 x l/s x (To – T i) or (1.08 x cfm x ∆T)
Where To = Outside temperature, C
Ti = indoor air temperature, C
Ventilation Cooling Load
Ventilation latent load, (W)
= mass flow rate x latent heat of vaporization x (humidity difference)
= 3010 x l/s x (∆ẁ) or (4840 x cfm x ∆ẁ)
Where
∆ẁ = Inside-outside humidity ratio difference of air ( kg/kg)
Total Cooling Load
• This is also call the Grand total load
• Sum of
– Space heat gain
– System heat gain
– load due to outdoor air supplied through the air handling unit
• Air bypassed the coil
• Air not bypassed the coil
Room Total Load
System Heat Gain
• These are sometimes external to the air conditioned space
• HVAC equipment also contributes to heat gain
– Fan heat gain
– Duct heat gain
Bypass Factor
Bypass factor is an important coil characteristic on moisture removal performance .
It’s value depends on:
• Number of rows/fins per inch
• Velocity of air
Bypass Factor of the coil
• When air streams across the cooling, portion of air may not come into contact with the coil surface
• BPF = un-contacted air flow
total flow
BPF is normally selected at 0.1 for offshore cooling and dehumidification.
Typical Coil Bypass Factor
Row Deep 14 fins/inch
Face velocity=
2 m/s
2.5 m/s 3 m/s
1 0.52 0.56 0.59
2 0.274 0.31 0.35
4 0.076 0.10 0.12
6 0.022 0.03 0.04
Source: Refrigeration and Air Conditioning by CP Arora
Effect of Bypass Factor on Ventilation Load
• Coil load due to outdoor air
SH= (OASH)(1-BPF)
LH= (OALH)(1-BPF)
• Effective room load
ERSH=RSH+(OASH)(BPF)
ERLH=RLH + (OALH)(BPF)
Cooling Load Classroom Exercise
• Estimate the cooling load of a portal cabin shown here:
• Assuming that– Outdoor condition is 35C,
70% RH– Indoor condition is 22C ,
55 % RH– U-factor=0.5 W/m2 K– Occupied by 2 persons– Electrical equipment heat
is 3 kW– 100l/s leakage due to
pressurization
PlatformLower Deck
4 x 4 x 3 h
N
Cooling Load CalculationsItems Procedures
Transmission- sensibleWall- West sideWall- East sideWall – North Wall- SouthRoofFloorTotal (T1)
Q = UA (CLTD)
Internal load- sensiblePeopleEquipmentLightTotal (T2)
Safety Factor (5% of T1+ T2)Fan heat & supply Duct Gain (7 % of T1+T2)
RSH (Total of the above)
Cooling Load CalculationsItems Procedures
Design conditions Outdoor 35C, 70% RHIndoor 22C, 55 RH
Ventilation- sensibleBypass air (0.1 bypass factor)Sensible heat of bypass air
10% x outdoor air
Ventilation - LatentLatent heat of bypass air
Cooling Load CalculationsItems Procedures
Design conditions Outdoor 35C, 70% RHIndoor 22C, 55 RH
ERSHRSHSensible heat of air bypassEffective Room Sensible Heat
ERLHPeopleLatent heat of air bypassEffective Room Latent Heat
Effective Room Total Heat (ERTH)ERSH+ESLH
Coil Load CalculationItems Procedures
Design conditions Outdoor 35C, 70% RHIndoor 22C, 55 RH
Coil Load – SensibleEffective Room Sensible HeatSH of Outdoor air not bypassedTotal (Coil Sensible heat)
Coil Load – LatentEffective Room Latent HeatLH of Outdoor air not bypassedTotal (Coil latent heat)
Total coil load (GTH)
Sensible Heat Factor (SHF)
• Ratio of sensible to total heat – SHF = Sensible heat/ total heat
= SH/ (SH + LH)
A low value of SHF indicates a high latent heat load, which is common in humid climate.
• In the above example, – Calculate the SHF of the room (RSHF)
– Calculate the effective room sensible heat factor (ESHF)
– Calculate the SHF of the coil (GSHF)