12 Cooling Load Calculations

Cooling Load

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

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 LoadThermal storage and Construction Type

Time of the Day: Solar Radiation

Time-delay Effect: Example 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.

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

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)

%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 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

• 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 CLTD

U= surface U-factorA= surface areaCLTD= cooling load temperature

difference

transmitted

absorbed

reflected

Solar ray

glass

Opaque Surfaces

• Q 2 = UA (CLTD)U= surface U-factorA= surface areaCLTD= 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 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/m2Office 25Corridor 10Sleeping 10CCR 25MCC/SG 25Kitchen 25Recreation 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 airSH= (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- sensible PeopleEquipmentLightTotal (T2)Safety Factor (5% of T1+ T2)Fan heat & supply Duct Gain (7 % of T1+T2)RSH (Total of the above)

Coil Load CalculationsItems Procedures

Room Latent Heat (RLH)People

Room Total HeatRSH + RLH

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 ProceduresDesign conditions Outdoor 35C, 70% RH

Indoor 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 ProceduresDesign conditions Outdoor 35C, 70% RH

Indoor 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)

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.

Sensible Heat Factor (SHF)SHF

RSHF

ESHF

GSHF