Cooling Load calculations

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

• Enclosure heat transfer characteristics

– Conduction – Convection – radiation

• Design conditions

– Outdoor & indoor

• Heat Gains

– Internal

– External or Solar

• Thermal capacity

Space Characteristics

• orientation

• Size and shape

• Construction material

• Windows, doors, openings • Surrounding conditions

Space Characteristics

• Occupants (activity, number, duration) • Appliances (power, usage)

• Air leakage (infiltration or exfiltration)

Indoor Design Conditions

Basic design parameters • Air temperature – Typically 22-26 C • Air velocity – 0.25 m/s • Relative humidity – 30-70 %

Indoor Design Conditions

• Indoor air quality

– Air contaminants – Air cleaning

• Acoustic 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

Terminology

• Space

of rooms

• Room

• Zone

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

Heat Gain = Cooling Load

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)

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

Internal Loads

1. People

– Sensible & latent

2. Lights

– sensible

3. Appliances

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

Outdoor air bypassed 7 8 5 6

Outdoor air not bypassed

72 79 48 64

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

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

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 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 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

CLTD for Sunlit Wall Group G

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)

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)

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 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

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

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

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

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 Platform Lower Deck 4 x 4 x 3 h

N

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 Procedures

Room Latent Heat (RLH) People

Room Total Heat RSH + RLH

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

10% x outdoor air

Ventilation - Latent

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)

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)

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.