cooling load calculations

Cooling Load
Contents
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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
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Load profile over a day
Peak load (basis for equipment sizing)
Operation Energy analysis
HVAC Construction cost
Principles of cooling Load Estimate
• Enclosure heat transfer characteristics
– Conduction
– Convection
– radiation
• Design conditions
– Outdoor & indoor
• Heat Gains
– Internal
– External or Solar
• Thermal capacity
Space Characteristics
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orientation
Size and shape
Construction material
Windows, doors, openings
Surrounding conditions
Ceiling
Space Characteristics
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Occupants (activity, number, duration)
Appliances (power, usage)
Air leakage (infiltration or exfiltration)
Lighting (W/m2)
Indoor Design Conditions
Basic 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
Miri
Malaysia
0.4%
1%
2%
DB (˚C ) MWB (
˚C )
DB
MWB
DB
MWB
32.2
31.8
26.3
31.4
26.2
26.3
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:
Heat gains
Convective
Radiant (%)
• Sensible gain
• Latent gain
(%)
Solar
radiation
with internal
shading
42
58
Fluorescent
lights
50
50
People
67
33
External wall
40
60
Heat Gain
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
Heat Gain = Cooling Load
Heat Gain = Cooling Load
Thermal storage and Construction Type
Time of the Day: Solar Radiation
Time-delay Effect: Lighting
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.
Heat Balance
The principal terms of heat Gains/Losses are indicated below .
(Source: ASHRAE Handbook Fundamentals 2005)
Coil Load
• Cooling coil load
– The rate at which energy is removed at the cooling
coil
– Sum of:
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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
Internal Loads
1. People
– Sensible & latent
2. Lights
– sensible
3. Appliances
– Sensible & latent
Total Cooling Load
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
Refrigeration Load
• The capacity of the refrigeration system to
produce the required coil load.
Profiles of Offshore Systems Cooling
Loads
Components
% Load
LQ (L)
%Load
LQ (U)
%Load
CCR
%Load
SG/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 not
bypassed
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
Compare
Q 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 = radiative
gain
• 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
glass
• Q = A (SC) (SHGF) (CLF)
A= glass area
SC= shading coefficient
SHGF= solar heat gain factor,
tabulated by ASHRAE
CLF= cooling load factor,
tabulated by ASHRAE
• Q = U x A x CLTD
U= surface U-factor
A= surface area
CLTD= cooling load temperature
difference
Solar ray
transmitted
reflected
absorbed
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
CLTD for Sunlit Wall Group G
Source: ASHRAE Fundamental
Opaque Surface Calculations
• Use Table for wall CLTD
• Use Table for roof CLTD
– Select wall/roof type
– Look up uncorrected CLTD
– Correct CLTD
CLTD 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
•Heat gain (lighting)
= 1.2 x total wattage x CLF
Or based on light power
density ranging from 10-25
W/m2
(average density, say=20
W/m2)
•Where light is continuously
on, CLF=1
Area
Office
Corridor
Sleeping
CCR
MCC/SG
Kitchen
Recreation
Light Power
Density W/m2
25
10
10
25
25
25
20
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:
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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
Room Total Load
– load due to outdoor air supplied through the air
handling unit
• Air bypassed the coil
• Air not bypassed the coil
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
14 fins/inch
Row Deep
Face velocity=
2.5 m/s
3 m/s
2 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
N
4x4
Platform
x 3 h Lower Deck
Cooling Load Calculations
Items
Procedures
Transmission- sensible
Wall- West side
Wall- East side
Wall – North
Wall- South
Roof
Floor
Total (T1)
Q = UA (CLTD)
Internal load- sensible
People
Equipment
Light
Total (T2)
Safety Factor (5% of T1+ T2)
Fan heat & supply Duct Gain (7 % of T1+T2)
RSH (Total of the above)
Coil Load Calculations
Items
Room Latent Heat (RLH)
People
Room Total Heat
RSH + RLH
Procedures
Cooling Load Calculations
Items
Procedures
Design conditions
Outdoor 35C, 70% RH
Indoor 22C, 55 RH
Ventilation- sensible
Bypass air (0.1 bypass factor)
Sensible heat of bypass air
Ventilation - Latent
Latent heat of bypass air
10% x outdoor air
Cooling Load Calculations
Items
Procedures
Design conditions
Outdoor 35C, 70% RH
Indoor 22C, 55 RH
ERSH
RSH
Sensible heat of air bypass
Effective Room Sensible Heat
ERLH
People
Latent heat of air bypass
Effective Room Latent Heat
Effective Room Total Heat (ERTH)
ERSH+ESLH
Coil Load Calculation
Items
Procedures
Design conditions
Outdoor 35C, 70% RH
Indoor 22C, 55 RH
Coil Load – Sensible
Effective Room Sensible Heat
SH of Outdoor air not bypassed
Total (Coil Sensible heat)
Coil Load – Latent
Effective Room Latent Heat
LH of Outdoor air not bypassed
Total (Coil latent heat)
Total coil load (GTH)
Sensible Heat Factor (SHF)
SHF
RSHF
ESHF
GSHF
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)
Selection of Air Conditioning
Apparatus
• The necessary data required are:
– GTH ( Grand total heat load)
– Dehumidified air quantity
– Apparatus dew point
These determine the size of the apparatus and
refrigerant temperature.