4.heat load calculations211207

7/23/2019 4.Heat Load Calculations211207

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Heat Load Calculations


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Building Survey

The orientation of the building is thefirst parameter that a hvac engineer

should know before designing the

system required. The south westcorner of the building will susceptible

to the most intense heat rays from the

sun if it is e!posed" compared to allother corners. So the heat transfer

through that part will be higher.


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#ield sketches complete

architectural drawings civilplans are the basic aspects of

the good survey. $very H%&C

engineer should be aware

about the importance of

building survey.


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'se of Space

&pplication of the conditioned

space is another parameter.

(ffice Hospitals restaurantsfactories machine shops etc.

&ccording to the application

the air conditioning system will

differ.


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)hysical *imensions of &ir

conditioning Space Length breadth height of the

space+ distance between floor

and ceiling+ distance betweenfloor and false ceiling is also

an important factor. The area of

the space to be air conditioned

is a very important data.


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Columns and Beams

Si,e depth location of

beams and columns


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Construction material

Type of material used forconstruction is another data

with which the heat transfercan be calculated. *ifferent

type of materials will have

different type of heattransmission coefficient.


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Heat transmission

coefficient is defined as therate of heat transferredthrough the building barrier.

-t is determined by ' /012/3 24 3 25 3 66667 where

2 is the resistance offeredby different type ofmaterials.


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Surrounding conditions

-t should be checkedwhether the e!terior of walls

windows and roofs areshaded by ad8acentbuildings or e!posed tosunlight whether thead8acent space is

conditioned or not.


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Location of *oors and

9indows

Type si,e and #requency of

use of the doors and

windows will affect theamount of heat transfer.


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Stairways Lift corridors

$scalators and open to non

air conditioned areas

-t is not easy to air

condition open spaces as

the level of infiltration will be

very high. This will result inwastage of air conditioning.


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(ccupancy

Total number of people in thespace to be air conditioned. $ach

and every human being will emit

a lot of heat energy due to themetabolic activity of human body.

*uration of the occupancy and

nature of activity should also be

known.


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Lighting

Type of lighting usedinside the space will be afactor as the lights usedwill emit lot of heatenergy. There are

different types of lightsaccording to which theheat emitted differs.


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The types of lightscommonly used areincandescent fluorescentrecessed and e!posed.

The amount of air flowshould be known or elsethe estimate should be

done on th basis ofwatts 0sq.ft.


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&ppliances

There will be differenttypes of appliances and

equipments used in the space

to be air conditioned Theequipment will deliver some

heat inside the space to beconditioned.


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Business machines

electronic equipment etc canbe present in the area to be

conditioned. The locationwattage rating and whether it is

hooded or not are the different

types of parameters to bechecked.


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%entilation required There are different standards set for

the different type of requirements. C#:0sq. ft

C#:0person

C#:0room

These parameters should also be

taken in to account. %entilationrequirement will in turn play aimportant role in maintaining good

indoor air quality.


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-ndoor unit and outdoor unit

location )arameters for indoor unit location

are There should be no obstruction to

air flow and distribution ofconditioned air. There should enough space for

servicing. )iping and wiring work can be easily

connected. There should be a solid wall or

ceiling to mount the unit.


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)arameters for outdoor unitlocation are

There should be good aircirculation

There should be no other ma8orheat interference.

The e!haust air should not besucked in once again for

recirculation. The unit should not be kept

very close to corrosive

materials or environment.


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$stimation of heat load

#or summer air conditioning the airconditioned space has to be maintained

at a lower temperature than the

surrounding temperature. The moisturelevel also should be maintained at a lowe

level compared to the atmospheric level.

So there has to be heat transfer betweenthe conditioned space and the

surroundings.


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&fter building survey a H%&C

engineer should know the amount

of heat transfer that should take

place between the space to be

conditioned and the surroundings.#or this heat load calculation and

estimation is required. Hence it is

required for the design andselection of air conditioning

equipment.


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;eneral )rocedure

To calculate a design

cooling or heating load

information about buildingdesign and weather data at

design conditions should beprepared.


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(utside design conditions

The first important step is to

fi! the peak outside

condition. ie+ the date andtime of year when the peak

conditions can prevail.


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-nside design conditions

-ndoor air temperature to be

maintained in each space

should be dictated by theapplication.

S l t i i i th h


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Solar transmission gain through

walls roofs glass and partitions

&s a result of absorption of directradiation of sun temperature of wall

roof glass and partitions rises

above the ambient temperature.This results in large temperature

difference with respect to the airconditioned space. The heat

transfer coefficient or '


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Then compute the heat

transmission loss for each kind of

wall glass floor ceiling and roof in

each building by multiplying the

heat transfer coefficient in eachcase by area of the surface and

temperature difference between

indoor air and outdoor air orad8acent lower temperature spaces.

-nfiltration


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-nfiltrationCompute the heat loss associated with

infiltration through cracks and openingsof doors and windows. These valuesdepend on the kind of crack and temp.difference of indoor and outdoor air. Theair blowing in the opposite direction ofthe building which is higher in moisturecontent and temperature to infiltrate

through the cracks and openings. Thisresults in the sensible and latent heattransfer to the conditioned space and

locali,ed heat gains.


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% til ti


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%entilation

9hen positive ventilation

using outside air is providedto the conditioned space

energy required to conditionthis air to the temperaturerequired must be provided by

the unit. So associated loaddue to fresh air should also becalculated. This is consider as

a forced infiltration.


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-nternal Heat ;ain

-nternal heat gain throughoccupancy lights and otherappliances should also be

calculated. Human body throughmetabolism generates heat withinitself and releases it by radiation

convection and evaporation fromthe surface. -lluminants will convertelectrical power to light and

sensible heat.


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9hy we require fresh airopening in the air

conditioning system=

- d &i > lit


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-ndoor &ir >uality

%entilation or outside fresh air in to

the air conditioned space is veryimportant. -t is to replace the o!ygenconsumed and to dilute the carbon

dio!ide and pollutants in the air.)ollutants are like bacteria fungusdust materials chemicals etc.Therefore it is important to removethe contaminants so that we canimprove the indoor air quality.

#resh air intake will never remove


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#resh air intake will never removethe contaminants but they willdilute them so that thecontaminants can be kept in thepermissible limits according to thestandard levels. (dour removal is

handled by introducing freshoutdoor air in to the air handlingunit. This air is mi!ed with the

return air to improve the air quality.This should be be done beforegetting the air treated at the

cooling coil.


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&pplication C#:1%ol.of#resh air7

(ffice Space 4?

2estaurants 4?

Bars 5?

Conference room 4?(perating rooms 5?

Supermarkets /@

&uditoriums /@

Laboratries 4?

Hotel rooms 5?


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The following steps can improve

indoor air quality

:aintain a slight positive pressure

inside the conditioned space so

that infiltration can be prevented. &ir filtration is very important so

that the particulates can be

prevented from passing on to theconditioned space.


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Cleaning of supply return and fresh airducts at the time of commissioning and a

regular intervals of operation. The condensate should be prevented

from getting stagnant in the drain pan.

The condensate will become a breedingpoint of bacteria and fungi.

&void fibre glass lined &H'As+ since it isinert by itself once it becomes wet thenit will become a collection area ofparticulate material and microbial growth.


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)2($CTS

4.heat load calculations211207

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