refr003 - heat load

7/27/2019 REFR003 - Heat Load

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ISHRAE

INSTITUTE OF

EXCELLENCE

ISHRAEINDIAN SOCIETY OF HEATING

REFRIGERATING AND AIR-

CONDITIONING ENGINEERS

ISHRAE INSTITUTE OF EXCELLENCE

# 76, I FLOOR, KASTURI COMPLEX, MISSION ROAD, BANGALORE 560 027, PHONE: 080-22245523, 41495045 WEB SITE: www.iiebangalore.org

HEAT LOAD

ESTIMATIONS


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MESSAGE FROM THE CHAIRMAN

ISHRAE INSTITUTE OF EXCELLENCE (IIE) was conceived after an intense deliberation andpondering over the pros and cons of different seminars and workshops conducted by ISHRAE andASHRAE for the HVAC&R and allied subjects in order to provide a beneficial learning Institute ofExcellence. The aspirants are those who are eager to enhance their professional competency in pace

with & up to date with worldwide technological advancement.

The HVAC & R industry is facing acute shortage of Skilled Manpower at all levels, Further there hasbeen no adequate technical Training and Refresher courses for such Team of Engineers. Keeping thisin mind, IIE, Bangalore has been instrumental in organising Refresher Courses for the WorkingEngineers. The course has been designed in such a way that the programs are conducted in theevenings and during week ends. IIE Bangalore could refresh more than 500 Engineers so far.

It is the wish of IIE Bangalore that such dissemination of Knowledge should not stop at Bangalore andshould spread to all places. As such IIE has consolidated the lecture notes and has prepared a Powerpoint presentation of such lectures so that all IIE Centers in the country can take the benefit. Thenotes and the power point presentation will come in handy for the IIE Centres and the Faculties so

that the courses can be conducted with ease.The Refresher course notes by and large are compiled from the Seminars and Workshops conducted byISHRAE Bangalore Chapter over the years.

Further IIE Bangalore has taken a positive step to work with the Industry and Institutions. IIE inassociation with ISHRAE Bangalore Chapter and ASHRAE South India Chapter is planning to facilitatethe industry to draw Manpower from Engineering Colleges, Polytechnics, ITIs and Cream of ScienceGraduate and train them in such a way that they can be used directly by the industry. This is at atime when the industry is facing shortage of manpower as well as shortage of time in training suchmanpower.

I take this opportunity to thank the Trustees of ISHRAE Foundation Trust, Core Management Committee

members, Faculties and the ISHRAE Head Quarters for their support in the great work of Disseminationof Knowledge.

As Knowledge is Power, please make use of these Refresher course Notes and reap the best of thebenefits.

Wish you all the Best!

D. NIRMAL RAM.CHAIRMAN,IIE, IFT Bangalore

ACKNOWLEDGEMENT

IIE acknowledges with thanks the following eminent personalities whose lectures are used to compilethis refresher course materials.

D. NIRMAL RAM, G.V. RAO, LESLIE DSOUZA, MAHESH KUMAR,

U. V. ACHAR, K. V. PRADEEP, RAKESH SAHAY AND MANY OTHERS

Bibilography :

ISHRAE Hand BookASHRAE Hand BooksCarrier System Design Manual


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Heat Load EstimationsIshrae Institute of Excellence, Chennai

U ndoubtedly one of the primary reasons for failures in

air-conditioning plants is due to improper estimation

of the heat load and failure to take into account various

factors which affect it. T he load estimation is based on

the actual instantaneous peak load. It is not possible tomeasure this actual instantaneous peak load but only

can be estimated. B efore estimati ng this load a

complete survey of the building, if the building exists,

or the plans, incase of a new building, has to be done.

A n accurate survey of the various parameters will result

in a realistic load estimation.

T he following data need to be collected:

1. O rientation of the building and latitude.

2. Application.

3. D imensions of the building.

4. H eight up to ceiling.

5. H eight up to false ceiling

6. Is the roof exposed?

7. D epth of the beam and projections of the

column

8. Size and number of windows.

9. Whether windows are shaded?

10. M aterial of construction of walls, ceiling/ roof.

11. O utside dry and wet bulb temperatures (all

seasons)

12. Inside design dry bulb temperature and relative

humidity.

13. N o. of persons.

14. A re they smoking? T ype of activity

15. L ighting load and type of lights.

16. M achinery loads with diversity

17. O ther additional loads.

18. D uration of operation

19. Space to locate various equipments

20. Ventilation required

21. D etails of exhaust, if any.

22. Level of cleanliness to be maintained

23. A vailability of soft water and electricity

24. O ther relevant information

LOAD COMPONENTS:

1. SO LA R G AIN

a. T hrough Wall

b. T hrough Roof

c. T hrough G lass

2. T RA N SM ISSIO N G A IN

a. T hrough Wall

b. T hrough C eiling

c. T hrough Floor

d. T hrough G lass

3. RO O M IN T ER N A L LO A D

a. People

b. Lighting

c. Equipment

d. Infiltration

e. System gain

f. M iscellaneous Sources

4. O UT D O O R LO AD

a. Fresh A ir System G ain

HEAT LOAD CONCEPTS

A good designer has to calculate the cooling load at

optimum design conditions. T he load so calculated

should not be too high or too less. T he space heat gain

is a resultant effect of sensible and latent heat.T he

sensible heat is the phenomenon of temperature,

whereas the latent heat is the stored heat in the form

of moisture or metabolism rate.

T he other heat load components can be classified into:-

a) Loads originated from heat sources outside or

external to the conditioned space.

b) Loads within the conditioned space.

c) Load occurring from heat gains or losses with

moving cool fluids to and from the conditioned

space.

HEAT LOAD ESTIMATION

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NOTE:

A ir-conditioning load estimations are based on quantity

of ai r required to produce the design conditions. A s

such in high altitudes where air conditioning is required,

when the density decreases, more quantity of ai r is

required to satisfy the given sensible load. T he weight

of the air to meet the latent load decreases owing tothe higher wet bulb temperature and relative humidity,

the wet bulb temperature decreases as the altitude

increases corresponding to the sea level.

Load estimations are based on either normal design

conditions or maximum design conditions. In normal

design condi tions, the outdoor design condi tions

are the simultaneously occurring dry bulb and wet

bulb temperature and humidity which are permitted

to exceed a few times a year for shorter periods.

T his is generally recommended for comfort and

normal industrial applications and it is occasionallypermissible to exceed the inside design conditions.

In cases where inside temperature swings on the

higher side is not tolerable then the design should

be based on the maximum outside design conditions.

T he maximum design dry and wet bulb temperatures

are simultaneous peaks and not individual peaks

that are considered for the load estimation. A constant

temperature is required for many industrial applications

instead of a temperature level.

T he actual cooling load will generally be below the

peak total instantaneous heat gain, thus requiring

a smaller equipment to perform a specific job. If

the equipment is allowed to run at a few degrees higher

than design requirement during peak periods, a smaller

capacity plant will meet the requirement. A smaller

system running for longer duration at full load will result

in saving in power and is more efficient than a bigger

system running at part load conditions for a shorter

duration.

R easons for the difference in the actual heat gain and

the total instantaneous peak heat gain is due to storage

effect, diversity and stratification. If the cooling capacity

supplied to the space matches with the cooling load,

the temperature in the space remains constant. O n

the contrary, i f the cooling capacity supplied to the

space is more than the cooling load then lower

temperatures are maintained. P recooling a space

below the design conditions increases the storage of

heat at the time of peak load. P recooling is useful in

reducing the cooling load in applications such as

churches, theaters and auditoriums.

D iversity of cooling load results from the probable non

occurrence of part of the cooling load such as lighting,

people and equipment load. T he size of the diversity

factor has to be based on the accurate judgment of

the user or his engineer.

H eat may be stratified in rooms with high ceiling and

where the air is exhausted through the ceiling or the

return air is taken above the false ceiling.

OUTDOOR DESIGN CONDITIONS

While calculating the heat load the outside conditions

play a vital role in estimating the heat load.

In A merica ASH R A E data are regarded as the industry

standard. In India ISH R A E has started working on the

project on establishing and compiling authentic

weather data for various places in India.

T he ambient air properties and solar intensities changes

with different elevation, latitude and longitude. While

selecting the refrigeration capacity of the plant for yearround air conditioning the cooling load for summer and

monsoon weather whichever is higher is selected.

In general for Indian climatic conditions 4P M is the

average time for solar heat gain and average daily

range of temperature (M aximum D B M inimum D B

in a day) vary from 15 to 20 degree F (L ocal conditions

are to be referred).

INSIDE DESIGN CONDITIONS:

T he human body considers itself comfortable when it

can maintain an average body temperature between

97 degree F and 100 degree F. It becomes the task of

air-conditioning to maintain the environment around

the body within this comfort zone of conditions.

In general 75 degree F DB and 50% R H is considered

the design conditions for human comfort. H owever,

these conditions may vary depending upon the

environmental requirement and applications.

SOLAR HEAT GAIN

T he primary weather related variable influencing thesensible cooling load for a building is solar radiation.

T he effect of solar radiation is more pronounced on

exposed surfaces.

R oom sensible heat is calculated as under.

T he heat transfer rate q is given by equation.

q= U A (T 1-T 2)

Where q= H eat transfer rate in B tu per hour.

U = C oefficient of overall heat transfer between the

adjacent and the conditioned space in B tu/ h sqft-

deg.F.

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A = A rea of the separating section in sqft.

T 1= A verage air temperature in adjacent space deg. F

T 2= A ir temperature in conditioned space deg. F

U = 1/ R where R= A ddition of thermal resistance of all

the surfaces coming in between the conditioned space

and adjacent space. (R efer tables for T hermalR esistance R of various building and insulating

materials).

SOLAR HEAT GAIN THROUGH GLASS

T he heat from the sun is partly scattered, partly

reflected and partly absorbed by the atmosphere.

T he scattered radia ti on i s called as di ffused

radiation. T he solar heat which directly comes through

the atmosphere is termed as direct radiation. It enters

the air-conditioned space through glass windows and

is absorbed by the objects and air in the conditionedarea. O rdinary glass absorbs a smaller percentage of

the solar heat say round 6% and reflects or transmits

the remaining. T he amount of reflection is dependent

on the angle of incidence which is the angle between

the perpendicular to the glass surface and the sun rays.

M ore heat is reflected and less heat is transmi tted inside

the conditioned area if the angle of incidence is more.

T he total solar heat gain in the conditioned area is the

heat transmitted together with around 40% of the heat

absorbed by the glass windows.

D epending on the latitudes, for each month in a year

and for different exposures and on different timingsthere are tables for the solar heat gain. T his solar heat

gain in B tu / hr/ sqft. area is multiplied with the area of

the glass and the factor depending on the shade. For

ordinary glass the factor is 1. 0 whereas for inside

Venetian blinds of light color the factor is 0.56.

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SOLAR AND TRANSMISSION HEAT GAIN

THROUGH EXPOSED WALLS:

H eat flows from higher level to the lower whenever

there exists a temperature difference. T he rate at

which the heat flows inside varies with the resistance

im posed by that material. T he solar heat gain on

the exposed wall does not become an instantaneous

room load. T he heat is absorbed by the external

wall and is conducted slowly into the inner layers

of the wall and only the convected and radiated

heat from the inner surface of the wall is the room

load. D ue to this unsteady state of heat flow it is a

general practice to consider an equivalent

temp erature dif ference. T he equivalent

temperature difference is the temperature

difference that results in total heat flow through

the structure as caused by the variable solar radiation

and outdoor temperature.

T he reciprocal of the total resistance offered by the

wall is called the transmi ssion coefficient U . It is

the rate at which the heat is transferred through

the wall and is expressed in B T U / hr/ Sq.ft/ deg.F

temp. diff. T he equivalent temperature difference

for different thickness of walls with different

exposures and timings are available in the tables

enclosed. T hese equivalent temperature differences

are worked out with an outside temperature of 95

deg. F and an inside temperature of 80 deg.F. A s

such corrections to equivalent temperatures are to

be made for different conditi ons. U nlik e the heat

gain tables for glass which constitutes only the solar

gain and not the transmission gain, this equivalent

temperature considers the solar heat as well as the

transmission heat gain due to the difference in

temperature between outside and inside conditions.

In addition to the resistance offered by the various

components in the wall, we have to take into account

the film coefficient, when working out the transmission

co-efficient. It is the resistance offered by the film of

air which clings to the surface of the wall. T he

resistance is more when the air is still and is less when

there is wind velocity.

Whenever a false ceiling is provided in a room having

an exposed roof, the space enclosed between thefalse ceiling and the roof is called as attic space. If

this attic space is not properly ventilated the space

temperature may exceed the outside temperature.

T he space temp erature can be work ed out

considering that the rate of heat flow from outside

into the attic space is equal to the rate of flow of

heat from the attic space into the room.

TRANSMISSION GAIN THROUGH

GLASS & PARTITIONS

T here will be heat transmission through the glass apart

from the solar gain due to the difference in

temperature between the conditioned and non-

conditioned space. Simi larly partitions/ ceiling/ floor

will also have heat transmission. T hey are worked

out by considering the area, temperature difference

and the factor.

INTERNAL LOADS

PEOPLEH eat is generated withi n a human body by

metabolism. T he metabolic rate depends on the

nature of activity. T he enclosed table will give the

sensible and latent load due to personnel depending

on the type of activity and the inside temperature.

Before the heat load estimation, the exact number

of persons inside the conditioned area has to be

ascertained properly for an accurate estimation.

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LIGHTS

L ights produce sensible heat and are dissipated by

radiation and convection. A bout 80% of the input

is radiated and around 10% is convected for an

incandescent lamp. For a fluorescent lamp 25% of

the input is radiated 50% is convected. For a

fluorescent lamp, approximately 25% more heat isgenerated than the input and this is due to the

ballast. It is preferred to get the exact number of

lights and its wattage and type. It is also a common

practice to give this load in watts/ sq.ft depending

on the application. T he wattage is multiplied by

3.413 to arrive at the heat dissipated in B T U / hr.

ELECTRIC MOTORS

Electric motors generate sensible heat which is

dissipated inside the conditioned area depending

on the location of the prime mover and the driven

equipment. T he heat dissipated by the motor is

input multiplied by the motor i nefficiency. T he rest

of the heat is dissipated by the driven machinery.

When a motor is overloaded or partially loaded the

heat generated will not obey the above law. A s such

in case of heavy machinery load it is advisable to

measure the input and not to depend on the rated

horse power of the motors. When the motor rating

is in K W i t is multiplied by 3413 and when the

rating is in H P it is multiplied by 2545 to obtain

the heat dissipation in B T U / hr. Suitable diversity

has to be applied to the connected electrical load

depending on the actual running of the motor at a

particular period of time.

O ther internal loads that may constitute the room

load may be gas burners, electric/ steam heaters and

water fountains, hot water/ steam pipes and tanks.

SYSTEM HEAT GAIN

System heat gain constitutes heat added or lost by

the system components such as ducting, pi pi ng,

water pumps and the blower. O ver and above some

safety factor is considered to account for the errors

in the survey or in the estimate. L eakage in the

supply duct will add to the room sensible and latent

heat. Supply ducts running in non conditioned area

will gain heat and as such becomes the room sensible

load. R eturn ducts for the above reasons will add to

the outdoor load.

INFILTRATION AND VENTILATION

Infiltration is not a feature for air-conditioning jobs

which is so for refrigeration. T his is for the simple

reason that for air-conditioning, outdoor air is introduced

which develops a positive pressure inside the

conditioned area and only exfiltration does occur.

H owever infiltration may occur if wind velocity outside

is higher. Infiltration is also a predominant feature for

high rise buildings due to stack effect. Infiltration of air

and by pass of air through the cooling coil becomes aroom load.

O utdoor ai r is introduced into the conditioned area so

as to dilute the odours given off by the people, smoking

and other fumes and contaminations generated inside

the room. T he quantity of fresh air depends upon the

volume of the room or the number of people and the

activity. Ventilation standards for different applications

are shown in the enclosed tabulations. For comfort

applications during the peak load when it is permitted

the outdoor air quantity may be reduced resulting in

smaller equipment. H owever during periods other thanthe peak load the required maximum fresh air has to

be introduced into the room which will do the flushing.

H owever in any case the air quantity during peak load

should not be lesser than 50% of the required air

quantity. Indoor air quality (IAQ) is now talked loudly

by all. M ini mum requirem ent of fresh air for

applications having lesser occupancy is one air change

per hour.

Solar gain through walls, glass, roof and transmission

gain through partition walls, ceiling, floors, internal

loads such as people, light, equi pm ent and

infiltration of fresh air(due to by pass in the cooling

coil) constitute R oom Sensible H eat (RSH ). When the

system gain is added to this, this becomes Effective

R oom Sensible H eat (ERSH ).

H eat gain through infiltration, people and other sources

which adds moisture in the room constitute Room

Latent H eat (RLH ). When system gain is added it

becomes Effective R oom L atent H eat (ERLH ). T he

summation of room sensible / effective room sensible

and room latent / effective room latent heat is calledas R oom Total H eat (RT H )/ Effective R oom Total H eat

(ER T H ). When outdoor sensible and latent heat is added

it becomes G rand Total H eat (G T H ) based on which

the air-conditioning system is designed.

T he effective room sensible heat over the effective

room total heat is called as effective room sensible

heat factor. With this factor and the inside design

conditions, A pparatus D ew Point (A D P ) is calculated.

D ew point is the temperature at which condensation

occurs when the air is cooled and the effective

surface temperature of the coil should match with the

dew point to meet the design parameters. Temperature

rise is the difference in temperature between the room

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and the apparatus dew point multiplied by the factor

(1-bypass factor). Effective room sensible heat over

1.08 and the temperature rise gives the dehumidified

air quantity which has to be pumped into the room to

offset the room load and to meet the design conditions.

In high latent load applications the dehumidified air

quantity will work out to be low. In such cases some airhas to be bypassed across the cooling coil to reduce

the temperature of air entering the room which

otherwise will cause a cold blast on the occupants. T he

dehumidified air quantity and the bypassed air is the

total air quantity on which the equipment is selected.

Similarly for applications such as clean rooms minimum

required air changes are required to be met. D uring

such occasions also more air will be bypassed across

the cooling coil.

A I R Q U A N T IT Y E Q U A T I O N S

cf mda= (1 )1.08 x (1-BF)(t

rm- t

ad p)

ERSH

cf mda= (2 ). 68 x (1-BF)(Wrm- W

adp)

ERLH

cf mda= (3 )4.45 x (1-BF)(h

rm-

hadp)

ERTH

cf mda = (4 )1.08 (t

edb-tl db)

TSH

cf mda = (5 ).68 x (W

ea- W

l a)

TLH

cf mda = (6 )4.45 (h

ea- h

la)

GTH

cf msa= (7 )1.08 x (t

rm-tsa)

RSH

cf msa= (8 ).68 x (W

rm- W

sa)

RL H

cf msa= (9 )4.45 x (h

rm- h

sa)

RT H

cf mba= cf m sa- cfmda (10)

Not e: cfmdawil l be less th an cfmsaonly when ai r is

physical ly bypassed around t he condit io nin g apparatus.

cf msa= cfm oa+ c fmr a (11)

1.08 = 0.244 x60

13.5

where . 24 4 = specific heat of moist air at 70 F db

and 50% rh, B tu/ (deg F) (lb dry air)

6 0 = min/ hr

13 . 5 = specific volume of moist air at

70 F db and 50% rh

0.68 = x

where 60 = min/ hr

13. 5 = specific volume of moist air at

70 F db and 50% rh

10 76 = average heat removal required to

condense one pound of water vapor

from the room air70 00 = grains per pound

4.45 =

where 6 0 = min/ hr

13 . 5 = specific volume of moist air at

70 F db and 50% rh

60 1076

13.5 7000

* RSH S, R LH S and G T H S are supplementary loadsdue to duct heat gain, duct leakage loss, fan and pump

horsepower gains, etc. T o sim pli fy the various

examples, these supplementary loads have not been

used in the calculations. H owever, in actual practice,

these supplementary loads should be used where

appropriate.

When no air is to be physically bypassed around the

conditioning apparatus, cfmda= cfmsa.

** When tm, Wm and hm are equal to the entering

conditions at the cooli ng apparatus, they may be

substituted for tedb, Wea and hea respectively.

D E R IV AT I O N O F A I R C O N S TA N T S

60

13.5

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Cooling & Dehumidifying Heat Load Estimate

Job N ame

A ddress

Space U sed for

Size X = Sq. Ft. X = C u. Ft.

I tem A rea or Sun G ain or Factor Btu/ H our

Q uanti ty T emp. D iff.

E sti mated for L ocal T i me P eak L oad L O C A L T I M E

SU N T I M E

C O N D I T I O N S D B WB % R H D P G r/ Lb

O utside

R o o m

D i fference X X X X X X X X

S OLAR GAIN GLASSG lass Sq Ft X X

G lass Sq Ft X X

G lass Sq Ft X X

G lass Sq Ft X X

Sky light Sq Ft X X

S OLAR & TRANS . GAIN - WALLS & ROOFWall Sq Ft X X

Wall Sq Ft X X

Wall Sq Ft X X

Wall Sq Ft X X

Wall Sq Ft X X

R oof Sun Sq Ft X X

R oof Shaded S q Ft X X

TRANS GAIN EXCEPT WALLS & ROO FA ll G lass Sq Ft X X

P arti ti on Sq Ft X X

C ei lli ng Sq Ft X X

Floor Sq Ft X X

INFILTRATION AND OUTSIDE AIRI nf i ltrati on C fm X X 1.08

O utside A ir C fm X F X BF X 1.08

INTERNAL HEATP eople P eople X

P ower H .P./ K W X

L I ghts W atts X 3.4

A ppliances, Etc X 3.4

X

EFFECTIVE ROOM SENSIBLE HEAT (A)ROO M LATENT HEAT

Infi ltration C fm X gr/ lb X 0 . 68

O utside-A i r C fm X gr/ lb X B F X 0.68P eople P eople X

Steam lb/ hr X 1080

ROOM SENSIBLE HEAT

H eat G ai n% L eak L oss%

D uct D uct H .P.% Factor

Supply Supply F an S afety

Vapor Tran.

A ppliances, Etc

R oom L atent H eat SubT otal

SU P P LY D U C T % + SA FET Y FA C T O R %

L EA K A G E L O SS

EFFECTIVE ROOM LATENT HEAT (B)EFFECTIVE ROOM TOTAL HEAT (C)=(A+B)

O U T S ID E A I R H E A TSensible: C fm X F X (1-BF) X 1.08

L atent: C fm X gr/ lb X (1-B F) X 0.68

Grand Total Heat Sub-Total (D) = (C+Outside Air Heat)

GRAND TO TAL HEAT (GTH) (E) = (D +Lo ss es )

To n s = E / 1 2 , 0 0 0

Selected R oom C ondi tions D B WB % R H

VENTILATIONP eople X C fm/ P erson=

INFILTRATION

Sq. Ft.X C fm/ sq. ft=

C fm Ventilat ion *

S WINGING/

R EV O LV IN G D O O R S - P EO P L E X C FM / P ER SO N =O pen doors X C FM / D O O R =

Exhaust Fan

C rack Feet X C fm/ Ft =

C F M O U T S ID E A I R T H R U A P P A R A T U S *

SENSIBLE HEAT FACTOR & APPARATUS DEWPOINT

(A ) Eff. room Sens. H eat

(C ) Eff. room total Heat

Indicated A D P F Selected A D P F

(1-B F ) X (R oo m Tem p-A D P ) = D ehum idi fi ed ri seF

D ehumidified C FMR oom Sensible heat =

1.08 X D ehumidified rise

R eturn R eturn P ump

D uct D uct H .P.% %H eat G ai n% L eak L oss

NOTES

(ESH F)

Sens H eat Factor

Estimated by : D ate :

C hecked by : P age N o. : ______ of ______

=

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DISCLAIMER

Ishare Foundation Trust, Bangalore and IIE Bangalore confirm that the materials are compiled fromvarious lectures, seminars, workshops conducted by various ISHRAE members, faculties of reputefrom ISRHAE Bangalore Chapter. This is not a book but a collection of course materials to refresh and

train the freshers and others belonging to the HVAC & R and allied fraternity.

refr003 - heat load

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