2. Contents
• Principle of cooling load
• Why cooling load & heat gains are different
• Design conditions
• Understand CLTD/CLF method
• An example
3. 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
4. Purpose of Load Estimate
• Load profile over a day
• Peak load (basis for equipment sizing)
• Operation Energy analysis
• HVAC Construction cost
5. Principles of cooling Load Estimate
• Enclosure heat transfer characteristics
– Conduction
– Convection
– radiation
• Design conditions
– Outdoor & indoor
• Heat Gains
– Internal
– External or Solar
• Thermal capacity
6. Space Characteristics
• orientation
• Size and shape
• Construction material
• Windows, doors, openings
• Surrounding conditions
• Ceiling
7. Space Characteristics
• Occupants (activity, number, duration)
• Appliances (power, usage)
• Air leakage (infiltration or exfiltration)
• Lighting (W/m2)
8. 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
9. Indoor Design Conditions
• Indoor air quality
– Air contaminants
– Air cleaning
• Acoustic requirements
• Pressurization requirements
10. 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
11. 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
12. Outdoor Design For Cooling
Criteria: 0.4% DB and MWB
Station Cooling DB/MWB
Miri 0.4% 1% 2%
Malaysia
DB (˚C ) MWB ( DB MWB DB MWB
˚C )
32.2 26.3 31.8 26.3 31.4 26.2
Source: ASHRAE Fundamentals 2001
13. Terminology
• Space- a volume without partition or a group
of rooms
• Room- an enclosed space
• Zone- a space having similar operating
characteristics
14. 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 (%)
Solar 42 58
• Latent gain
radiation
with internal
shading
Fluorescent 50 50
lights
People 67 33
External wall 40 60
16. 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
21. 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.
22. Heat Balance
The principal terms of heat Gains/Losses are indicated below .
(Source: ASHRAE Handbook Fundamentals 2005)
23. 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)
24. External Loads
1. Heat gains from Walls and roofs
– sensible
2. Solar gains through fenestrations
– Sensible
3. Outdoor air
– Sensible & latent
27. 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
28. Refrigeration Load
• The capacity of the refrigeration system to
produce the required coil load.
29. Profiles of Offshore Systems Cooling
Loads
Components % Load %Load %Load %Load
LQ (L) LQ (U) CCR 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 72 79 48 64
bypassed
Total 100 100 100 100
31. 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
32. 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
34. 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
35. 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
36. Glazing
glass
• Q = A (SC) (SHGF) (CLF)
A= glass area
SC= shading coefficient Solar ray
SHGF= solar heat gain factor,
tabulated by ASHRAE
CLF= cooling load factor,
tabulated by ASHRAE
transmitted
• Q = U x A x CLTD reflected
U= surface U-factor absorbed
A= surface area
CLTD= cooling load temperature
difference
37. 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
39. 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
40. 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
41. Internal Load- Lighting
Area Light Power
•Heat gain (lighting) Density W/m2
= 1.2 x total wattage x CLF Office 25
Corridor 10
Or based on light power Sleeping 10
density ranging from 10-25 CCR
MCC/SG
25
25
W/m2 Kitchen 25
(average density, say=20 Recreation 20
W/m2)
•Where light is continuously
on, CLF=1
42. Internal Loads- People
• Q people-s = No x sensible heat gain/p x CLF
• Q people-L = No x latent heat gain/p
43. 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
44. Typical Outdoor & Indoor Design
Conditions Used Here
Conditions Dry-bulb % RH Moisture content,
temperature (C) 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
45. 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
46. 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)
47. Total Cooling Load
• This is also call the Grand total load
• Sum of
– Space heat gain Room Total Load
– System heat gain
– load due to outdoor air supplied through the air
handling unit
• Air bypassed the coil
• Air not bypassed the coil
48. 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
49. 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
50. 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.
51. Typical Coil Bypass Factor
Row Deep 14 fins/inch
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
52. 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)
53. Cooling Load Classroom Exercise
• Estimate the cooling load N
of a portal cabin shown
here:
• Assuming that
– Outdoor condition is 35C,
70% RH
4x4 Platform
– Indoor condition is 22C , x 3 h Lower Deck
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
54. Cooling Load Calculations
Items Procedures
Transmission- sensible Q = UA (CLTD)
Wall- West side
Wall- East side
Wall – North
Wall- South
Roof
Floor
Total (T1)
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)
60. 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)
61. 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.