cooling load calcualtions - ashrae bangalore...ashrae cooling load temperature deference method...

Copyright 2020 © Eng. Chandana Dalugoda.

Cooling Load Calculations ASHRAE CLTD/SCL/CLF Method

1

By

Eng. Chandana Dalugoda, CEng MIE(SL), FASHRAE, MCIBSE, GCGI (UK), MConsE (SL)

Managing Partner, Chandana Dalugoda ConsultantsColombo, Sri Lanka

COOLING LOAD CALCUALTIONS - April 25, 2020

Copyright 2020 © Eng. Chandana Dalugoda. 2

Purpose of Cooling Load CalculationsPrimary objective of the cooling load calculations is to provide data required to design a HVAC system and to select HVAC equipment.

Rules of Thumb or check figures should not be used to determine the cooling load, since cooling load poses many variables, that varies building to building. Check figures are used to verify the accuracy of final cooling load.

Calculations are usually carried out using computer based software programs; ASHRAE Radiant Time Series (RTS) method, Transfer Function Method. Manual method is ASHRAE Cooling Load Temperature Deference method (CLTD/SCL/CLF). Advantage is all calculations are apparent unlike computer programs.

This lecture will be based on ASHRAE CLTD/SCL/CLF method. CLTD is cooling load temperature difference, SCL is solar cooling load & CLF is cooling load factor



External & Internal Loads

• Cooling load is the rate of energy removal required to maintain indoor environment at a desired temperature and humidity condition.

• The amount of cooling at any particular time varies widely, depending on external and internal factors.



Moisture Removal & the Latent Load

• Three primary sources of humidity within buildings include People, appliances and outside air from ventilation & infiltration.

• To maintain constant level of humidity within the space, the cooling system must remove moisture at a rate equal to the moisture addition.



LOADS IN A TYPICAL AIR CONDITIONING SYSTEM



SENSIBLE COOLING LOAD & LATENT COOLING LOADS

Cooling load comprises of both Sensible & Latent loads. Actually, only sensible heat or the sensible load occurs in the room and Latent Load is not part of the room. Latent heat is part of change of state, so inside the room it appears only as moisture but no latent heat exist or transfer. When moisture from the return air is condensed back to water on the cooling coil, latent heat of condensation is absorbed & is called the latent heat load.

Return air

Supply air

Sensible load in the room

Latent load in the coil



Time Delay EffectEnergy absorbed by walls, floor, furniture, etc., contributes to space cooling load only after a time lag. Some of this energy is still present and re-radiating even after the heat sources have been switched off or removed, as shown in the figure below



Heat Transfer Calculations



Most significant heat gain for the majority of modern buildings is from the sun, SOLAR heat gain. To understand the effect of sun has an influence on the building, we need to appreciate the principles of solar geometry.

Heat from where?



Sun will be directly overhead Tropic of Cancer on June 21st and Tropic of Capricorn on December 21st. So countries situated between Tropic of Cancer & Capricorn are known as Tropical climates or Tropics. During December South Pole has 24 hr daylight and during June North Pole has 24 hr daylight.

Seasonal Changes In Solar Declination



Temperature&RH%

» Comparisonbetweentropicsandtemperateclimate˃ FigurebelowshowsLondonoutdoorconditionsareclosertostandardindoorconditionsinsummercooling.

Unit Colombo Chennai Delhi London

drybulb °C 31.3 38.5 43.8 28.3

wetbulb °C 26 28.3 29.6 19.8

RH % 66 46 36 5203/03/2012



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03/03/2012

Tropicalweather

Tropicofcancer23.5º

India & Sri Lanka



03/03/2012

Sun Path Diagram

Sun path diagram of Jayawardanepura, Sri Lanka, 7ºN

Sun path diagram of London, UK, 52ºN

Actual Sun Path Model



Solar Angles For Vertical & Horizontal Surfaces



When it is colder on one side of an envelope element, such as a wall, roof, floor or window, heat will conduct from warmer side to colder side. Heat conduction is driven by temperature difference and represents a major component of the heating & cooling load calculations.

U-Factor The U factor is the rate of steady-state heat flow. In SI units, it is the amount of heat in Watts that flows through a one square meter area with one degree Celsius temperature difference.

R-Value R-value is the thermal resistance to hate flow.

U = 1/R (w/m2K) R = R1 + R2 + R3+….Rn

Understanding U-Factor & R-Value



Typical Wall U-Factor Calculation

Masonry wall section. This is an insulated masonry cavity wall, popular in other countries.



Typical Roof U-Factor Calculation



FLAT MASONRY ROOF SECTION

Commercial flat masonry roof section is shown. 2- corrugated metal deck 3- concrete slab light weight aggregate 50mm 4- Rigid roof deck insulation 5- Built-up roofing (metal blast)



Thermal Prophets of Material

Thermal properties of common building materials are given in Table-1, Building and Insulating Materials: Design Values, 2013 ASHRAE Handbook-Fundamentals (SI). This table is arranged into 8 basic families of materials, starting with insulating materials.

When Resistance is not available :

R = x / k x = thickness (m) k = conductivity W/mK



Thermal Capacitance & Sensible Heat Transfer

Three terms that will be used throughout this course are the thermal capacitance, sensible energy transfer and latent energy transfer. Basic equation used to calculate thermal capacitance is;

Q = m Cp ΔT Where; Q = Heat transfer, W m = mass of the material kg Cp = specific heat, KJ/Kg.ºK ΔT = temperature change in the material as a result of the heat transfer, ºK



Outdoor Design ConditionsWhen calculating the thermal loads for building, it is very important that the heating & cooling equipment system be capable of maintaining adequate comfort under all reasonable conditions.

But sizing cooling system for hottest temperature ever recorded will result in an oversized air conditioner that will more expensive than necessary to buy and often less efficient to operate.

Extensive record of climatic condones for various countries can be found in 1997 ASHAR Handbook-Fundamentals. 2009 & 2013 ASHRAE Handbook Fundamentals CD Rom has additional details for most of the cities which has WMO weather station number.

Design Conditions & Water Data



ISHRAE published Sri Lanka Weather Data 2019 book, which provides weather data for 20 cities in Sri Lanka. Our sincere gratitude goes to Vishal Garg, Jyotirmay Mathur & ISHRAE President Vikram Murthy for this grate work.



Indoor Design Conditions 2017 ASHRAE Handbook



LOAD ESTIMATING SPREAD SHEET FOR CLTD/SCL/CLF METHOD

Load calculation spread sheet, use (area x U x CLTD) to determine the load in Watts. U-factor for each composite structure needs to calculated and all physical dimensions of the building fabric should be obtain from drawings or actual measurements. Factors such as SHGF, CLTD, SC & properties of material could be found in ASHRAE Handbooks. Load calculations precedes with building survey with clearly labelled sketches. All the construction details, power, water availability, water quality reports & air quality should be obtain.



Roof Load

q = U A (CLTD)

A = Area of the roof (m2) U = U- factor roof (W/m2K) CLTD = cooling load temperature difference

Roof load can be calculated inclusive of the ceiling. If so, separate ceiling load need not be calculated. CLTD is not general temperature difference, it consider roof construction, & solar time.



Wall Load

q = U A (CLTD)

A = Area of wall (m2) U = U- factor wall (W/m2K) CLTD = cooling load temperature difference

Wall area should be net area for a particular orientation, excluding all windows and doors. Each orientation load is calculated separately. CLTD is not general temperature difference, it consider wall construction, & solar time.



Window Load

q = U A (CLTD)

A = Net Glass Area (m2) U = U- factor Glass (W/m2K) CLTD = cooling load temperature difference

Glass area is including frame & take all glasses without considering orientation. CLTD is not general temperature difference, it is for heat conduction through glass & consider only solar time.

Glass- Conduction



ASHRAE U-FACTORS FOR GLASSES



q = A . SC . SHGF . CLF

A = Area calculated from building plans, m2 SC = shading coefficient, dimensionless SHGF= solar cooling load factor, W/ m2 CLF = Cooling Load Factor

Window LoadGlass- Solar

Glass area is including frame for a particular orientation. Each orientation load is calculated separately. SC is for single glazing, double glazing and insulated glasses. SHGF is for particular latitude & orientation. CLF is based on orientation, room construction and orientation.



Solar Heat Gain Through Glasses



Cooling Load Against Type Of Construction

Actual Cooling Load, Solar Heat Gain, Light, Medium and Heavy Construction



Heat Storage



Cooling Load From Partitions, Ceiling & Floor

Partitions : a partition is a wall adjacent to unconditioned area Ceiling : could be part of the roof-U calculations. N/A Floors : Floor on grade does not count for heat transfer.

q = U . A . TD

U = design heat transfer coefficient (U-factor) W/(m².K) A = Area calculated from building plans, m2 TD = ta - tr

ta = temp. of adjacent space, °C

tr = room design temp., °C



Occupancy Load

People Sensible q sensible = N F d q shg (CLF)

N = number of people in space F d = diversity factor

q shg = sensible heat gain per person, W/person

CLF = cooling load factor by hour of occupancy



People Latent q latent = N F d q lhg N = number of people in space F d = diversity factor

q lhg = latent heat gain per person, W/person


Occupancy Load



Heat Gain From People



Load from Lighting Lights q el= INPUT . Ful Fsa (CLF) INPUT = watts input from electrical plans, W Ful = lighting use factor

Fsa = lighting special allowance factor

CLF = cooling load factor based on total hours of operation & time

Lighting Load



Appliances Sensible Load

Appliances Sensible q sensible = HEAT GAIN . (CLF) heat gain= sensible heat gain per appliance, W q lhg = latent heat gain per appliance, W

CLF = cooling load factor



Appliances Latent q latent = HEAT GAIN

heat gain= latent heat gain per appliance, W

Appliances Latent Load



Load From Motors

Power q p= 2545 P EF (CLF) P = horse power ratings from electrical plans EF = efficiency factor




InfiltrationInfiltration contribute sensible & latent heat to the thermal load of commercial buildings. It is practically impossible to accurately predict infiltration on theoretical grounds. There are two main forces driving infiltration;

1. Prevailing wind 2. Natural draft

Prevailing wind causes high pressure on one side of the structure and a slight negative pressure on the opposite side. Providing air lock or air curtains at entrances reduces the infiltration. Natural draft is due to stack effect, hot air rises through the building and escapes through cracks in the top of the building.



Two methods to estimate infiltration rates have been developed; 1. Air Change method (ACH) 2. Effective leakage area method

ACH method For residential & small commercial buildings ACH method is used. Usually 0.5 ACH is used, assuming good construction practices are used. Area x hight x ACH gives infiltration rate in m3/h.

Infiltration Air Flow

Construction ACHTight 0.3Medium 0.6Loose 0.9



Infiltration Load

Infiltration Load Infiltration of air Sensible q s= 1.23 (ύ) (to-tr) ύ = volume flow rate of infiltrating air, L/s to = outdoor temp. °C

tr = room design temp. °C



Infiltration Load Latent q l= 3.0 (ύ) (wo-wr) ύ = volume flow rate of infiltrating air, m3/s wo = moisture content for outdoor air, kg/kg

wr = moisture content for room air, kg/kg

Infiltration Load

DB °C WB °C w Kg/Kg

OUTDOOR 32 27 0.0206

INDOOR 24 17 0.0093

DIFFE 8 10 0.0113



Ventilation Load Ventilation air Sensible q s= 1.23 (ύ) (to-tr) ύ = volume flow rate of ventilation air, L/s to = outdoor temp. °C

tr = room design temp. °C

Ventialtion Load



Ventialtion Load

Ventilation Load Latent q l= 3.0 (ύ) (wo-wr) ύ = volume flow rate of ventilation air, L/s wo = moisture content for outdoor air, kg/kg

wr = moisture content for room air, kg/kg

DB °C WB °C w Kg/Kg

OUTDOOR 32 27 0.0206

INDOOR 24 17 0.0093DIFFE 8 10 0.0113



Breathing Zone Outdoor Airflow.

The design outdoor airflow required in the breathing zone of the occupiable space or spaces in a zone, i.e., the breathing zone outdoor airflow (Vbz), shall be determined in accordance with following Equation

Vbz = (Rp x Pz) + (Ra x Az)

Where: Az = zone floor area: the net occupiable floor area of the zone m2, (ft2).

Pz = zone population: the largest number of people expected to occupy the zone during typical usage. default occupant density listed in Table 6-1.

Rp = outdoor airflow rate required per person as determined from Table 6-1. Note: These values are based on adapted occupants.

Ra = outdoor airflow rate required per unit area as determined from Table 6-1.

Ventialtion



Load From HVAC System

Heat gain in air distribution system q = U As (ta-td) U = design heat transfer coefficient (U-factor) W/(m².K) A = outside surface area of duct, m2 ta = temp. of duct surface, °C

tr = room design temp., °C

NOTE: The heat gains of duct system must be considered when the ducts are not in the conditioned space. Total heat gain from ducts would be 1 to 2 % of the room sensible heat gain, when the ducts are not in the conditioned space and insulate.



Summary of Cooling Load1. Roof Load 9563 2. Wall Load

1. NE 1433 2. SE 520 3. NW 278

3. Glass Conduction 1. Windows 1515 2. Doors 107

4. Glass Solar 1. SE 2137 2. NW 2667

5. Partition- Board 699 6. Internal Heat

1. People (sensible) 8960 2. Lights 946 3. Appliances 820

7. Room Sensible Heat 32,527 8. Latent Load

1. People 7200 9. Room Latent Heat 7,200 10. Ventilation

1. Sensible 6452 2. Latent 22303

11. GRAND TOTAL HEAT 68,483



To find the supply air volume in we can use the following formula

q= m Cp ΔT qs= v/vs Cp ΔT (mass flow rate

is replaced with density & volume flow rate) Then; v= qs vs/ Cp ΔT

ΔT= t2 - t5

Volume flow rate v = 32.5 x 0.854 / [1.02 x (24-12)] = 2.26 m3 /s

Supply air flow 2.26 m3 /s (2,267 L/s, 4,800 CFM)

Supply Air Volume Flow Rate Calculations

Psychrometric analysis of HVAC cycle neglecting all heat losses & gains



Single-duct single zone HVAC system, plot on the psychrometric chart.

Coil process, fan heat, duct heat is also shown above.

Air Side System Calculations With Psychrometric Chart



1. Cooling Coil Capacity (Total)

2. Cooling Coil Capacity (Sensible)

3. Coil Conditions (on coil/off coil temperatures DB & WB)

4. Supply Air Volume

5. Outdoor air volume for IAQ

Air Conditioner Specifications



Zoning

ZONES Separating a building into zones enables the temperatures in each zone to be controlled more accurately, particularly when there are varying heat gains between zones.

Each zone will have its own system or part of the system that cools the area. As an example, AHU’s serving each zone, or VAV system with VAV terminals for every room or a group of fan coil for the purpose of cooling only that zone.

ZONE HEAT GAINS The heat gains to each area needs to be calculated; this will include heat gains through internal walls if there are temperature within a single building.



LOAD CALCULATIONS USING COMPUTER ALGORITHMS

HOURLY COOLING LOADS

The zone loads can be calculated hourly basis. Plotting the values on a graph enable the peak simultaneous load to be determined.

It is not essential to calculate and plot all the zone cooling loads for each and every hour of the day if the approximate peak time of loads in the building is known.

For example if the peak time is likely to be 14.00 hr, then checking 13.00 hr and 15.00 hr would be useful.



Sample hourly loads for each zones Calculations

By adding the loads for the east & west zone at each hour, the building load at each hour can be identified. From these totals, the peak simultaneous load can be found and as above it is 145 kW



Cooling Load Check FiguresOCCUPANCY COOLING LOAD W/m²

(AIR-CONDITIONED AREA)

Apartments, residence Auditorium Banks Hairdresser Beauty Shop

120 280 175 215 260

Cafeteria Classroom Clinic Clothing store Computer room

350 95

190 165 480

Conference room Department Store: Basement Main floor Upper floors

275 125 150 125

Factory- Light manufacture Factory- Heavy manufacture Food stores Hotel & Motel rooms Laboratory

275 490 160 120 130

Library Mall Medical Offices Milk Bars, Fast foods Office- General (Perimeter)

150 135 185 270 170

Office- General (Interior) Office- Private Post office Restaurant Shoe store

100 180 180 330 185

Super Market Theatre

160 260

Cooling load check figures are used for verification after proper calculations are made.

Also they use in preliminary estimates during schematic design to size the builders shafts, plant rooms etc..

A detailed load estimate based on an accurate survey of the space to be carried out to size the HVAC equipment


cooling load calcualtions - ashrae bangalore...ashrae cooling load temperature deference method...

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