Final Load Calculations

8/3/2019 Final Load Calculations

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Chapter 02

Load Estimation Procedure

2.1General Procedure for Load Estimation

Following procedure is adapted to calculate Cooling load of the building.

1. Solar Gain through Glass:Glass, being the major cladding material of most building, provides the most direct route for

entry of solar radiation. For this reason, the proper estimation of heat gain through glass is

imperative. Glass may be considered opaque to radiation for sources of surface temperature less

than 2000C.Of the energy that is incident upon the glass, some is reflected and lost, some is transmitted

through the glass, and some is absorbed by the glass as the energy passes through it. This small

amount of energy raises the temperature of the glass, and the glass eventually transmits this heat

by convection partly to the room and partly to the exterior.

External Shading

This is the best way of excluding the entry of direct solar radiation into a building. If the shade is

completely opaque, all the direct radiation is prevented from entering through the window,

although scattered radiation from the ground and the sky can still enter and must be taken into

account.

Internal Shading and Double-Glazing

In tropical and subtropical climates, fixed, horizontal and external shades are effective because

the sun is high in the sky for most of the day but in the temperate zone the lower solar altitudes

that prevail for the majority of the time make them as much less used. For this reason, internal

shading is much adopted and is essential for sunlit windows if air conditioning is to be

satisfactory.

Internal shades are often of the Venetian blind type and for these to give most benefit; they

should be of a white or aluminum color and should have polished surfaces. They should be

adjusted so that they reflect the rays of the sun back to the outside and so that no direct rays pass


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between the slots. The blinds absorb some radiation, warm up and convex and radiate heat into

the room. Blinds are most effective when fitted between sheets of double-glazing.

The effectiveness of various combinations of shades and glazing is expressed in terms of a solar

heat gain coefficient

Where,

SHGF is solar heat gain factor (BTU/hr)

A is corresponding area ()F is overall factor for solar heat gain through glass [1]

Correction for SHGF [1]:

Steel sash or not

Multiply with 1.17

Haze conditions

For 10% hazy conditions

Altitude

+0.7% per 1000Ft

For 1914 ft

So correction factor will be

Dew point increases from 66.8F

Dew point difference


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So,( )

Thus correction factor for dew point

Corrected solar heat gain factor:

Overall Factor for solar heat gain through glass:

Using table 16 of Carrier hand book

Or

Using ASHRAE table 3.22

2. Conduction through Wall, Roof and Floor:Basically any building is composed of structures like walls, roofs, and floor. The heat transfer

through wall and roof is similar as both structures are of the same nature and exposed to the same

type of boundary conditions. For floors directly in contact with ground or over an underground

basement that is neither ventilated nor conditioned, heat transfer may be neglected for cooling

load estimates.

Where

Q is conduction gain (Btu/hr)

U is overall heat transfer coefficient (table33)

A is area of glass TD is temperature difference)

3. Infiltration and Ventilation:Outdoor air that flows through a building is often used to dilute and remove indoor air

contaminants. However, the energy required to condition this outdoor air could be a significant

portion of the total space-conditioning load. The magnitude of the outdoor airflow into the

building must be known for proper sizing of the HVAC equipment and evaluation of energy


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consumption. For buildings without mechanical cooling and dehumidification, proper ventilation

and infiltration airflows are important for providing comfort for occupants.

Air exchange of outdoor air with the air already in a building can be divided into two broad

classifications: infiltration and ventilation

Infiltration is uncontrolled flow of air through unintentional openings such as cracks in the walls

and ceilings and through perimeter gaps of windows and doors driven by wind, temperature

difference and internally induced pressures. It is caused by a greater air pressure on the outside

of the building than the inside. The amount of in filtered air depends on the pressure difference,

the number, size and the shape of cracks involved; the number, the length and width of the

perimeter gaps of the windows and doors; and the nature of the flow in the crack or gap (laminar

or turbulent).

Ventilation

Ventilation is the intentional introduction of air from the outside into a building; it is further

subdivided into natural ventilation and forced ventilation.

Natural ventilation is the intentional flow of air through open windows, doors, grilles, and other

planned building envelope penetrations, and it is driven by natural and/or artificially produced

pressure differentials.

Forced ventilation is the intentional movement of air into and out of a building using fans andintake and exhaust vents; it is also called mechanical ventilation.

Most of the time the air introduced in to the building by ventilation is expressed in terms of air

exchange per hour (ACH) [2].

Sensible

Where

Latent

Where

Is grains intensity difference (Grains/lb)


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4. People (occupants):Human beings give off heat at a metabolic rate, which depends on their rate of working. The

sensible and latent heat proportion of the heat librated for any given activity depends on the

value of the ambient dry bulb temperature; the lower the dry bulb temperature the larger theproportion of sensible heat dissipated. Typical values of sensible and latent heat librations of heat

are given in Table 3.4(ASHARE)

Sensible

Where

Is human sensible heat gain Is number of people Is cooling load factor

Latent

Where

Is latent human heat gain

5. LIGHTS:As lighting is often the major space load component, a precise account of this heat gain is

required. The computation of this heat load is often very difficult due to the rate of heat gain at

any given time being quite different from the heat equivalent of the power supplied

instantaneously to these lights.

Only part of the energy from light is in the form of convective heat that is picked up

instantaneously by the air conditioning apparatus. The remaining portion is in the form of

radiation which affects the conditioned space only after having been absorbed and rereleased by

walls, floors, furniture, etc. This absorbed energy contributes to the space cooling load only after

a time lag, with some part of energy still present and reradiating after the lights are switched off.


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Where

Is light intensity

6. Ventilation bypass:Nearly 10% air that passes from the cooling coil by passes without conditioned. This by-pass air

adds load in the room for the cooling purposes.

Sensible

Where

Is bypass factor usually taken as 0.1 (10%)Latent

7. Outdoor air heat gain:This includes the amount of energy required to condition the outdoor air to the required values of

temperature and humidity.

8. Total sensible heat:


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9. Total latent heat:

10.Total Load

2.2 Load estimation for 2nd floor NANDO:

Length Width Height

ft. ft. ft.

32.33 39.12664 7

Area 1264.964397Volume 8854.750781no. of people 40

1. Solar Gain through Glass:

Table2.1 SHGF [1]

Direction SHGF(Btu/hr)

Latitude30 Latitude40 Latitude34(interpolation)

NE 14 14 14

SE 22 42 35.33

Correction for SHGF:

Steel sash or not

Multiply with 1.17

Haze conditions

For 10% hazy conditions


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Altitude

+0.7% per 1000Ft

For 1914 ft

So correction factor will be

Dew point increases from 66.8F

Dew point difference

So,( )Thus correction factor for dew point

Corrected solar heat gain factor:

NE SE

Overall Factor for solar heat gain through glass:

Glass specifications:

Size Type single glass(single glaze)

Light color

Inside blend

Thickness 10mm

Using Table 16[1]

Factor (F) =0.56


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Or

Using ASHRAE Table 3.22[2]

Factor =0.55

Table2.2 Overall factor (F) for solar heat gain through Glass (3.22) [2]

Now, using

SOLAR GAIN GLASS(radiation) in Btu/hr

NE 224 14.11 0.55 1738.35

SE 212.31 35.61 0.55 4158.20

2. Conduction Through Glass(wall)


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U is overall heat transfer coefficient =0.83

Table2.3 Over all heat transfer coefficient (U) for glass in

A is the area of all glass

Conduction through Roof:

Overall heat Transfer coefficient for roof is calculated as [3]


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U-Factor calculations for Roof

Sr.BUILDING

COMPONENTS

THICKNESS

THERMAL

RESISTANCE/INCH

THERMAL

RESISTANCE

-in (INCHES) summer winter summer winter

1

outside

air(moving) 0.25 0.172 top side plaster 3 0.2 0.1 0.6 0.3

3 RCC 6 0.08 0.08 0.48 0.48

4

bottom side

plaster 0.75 0.2 0.1 0.15 0.075

5 inside air (still) 0.92 0.61

6Total Resistance (Rt) 2.4 1.635

7Overall U- factor (U) =1/Rt 0.42 0.61

Higher value of U =0.61

Now,

Total conduction,


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3. Infiltration: Sensible

Where

This gives

Latent

4. People:


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Table 2.4 Heat gain by occupants w.r.t activity [2]

Sensible

Latent

5. LIGHTS:

6. Ventilation bypass: Sensible


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Where

And

Is bypass factor usually taken as 0.1 (10%)

Latent

7. Outdoor air heat gain:

8. TOTAL ROOM SENSIBLE HEAT:


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Leakage losses =1%

Now, Effective Room Sensible heat:

Total sensible heat:

9. ROOM LATENT HEAT

Leak losses = 1%

Effective room latent heat:

Total latent heat:

10.Total Load

Safety factor= 10%

Hence Total Load:


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2.3 Load Calculation for Basement Floor

Length 20.6 Width 18.83 Height 8.25 Area 387.89 Volume 3200.16 No of people 22

sensible latent

BTU/hr

Infiltration950.45 1347.37

people6050 2310

lights3306.84

ventilation bypass1176.12 1667.292

outdoor air heat gain 10585.8 15005.63

Total room sensible23212.59

Total room latent23040.408

Total room cooling

load 46252.998

Tons of Refrigeration 3.8544165


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2.4 Load Calculation for Ground Floor

Length 21.4 ft

Width 33.97 ft

Height 8.25 ft

Area 726.96 ft2

Volume 5997.40 ft3

No of people 22

sensible latent

BTU/hr

solar radiation(glass) 6389.302

conduction (glass) 13867.83


people6050 2310

lights14933.65

appliances 12000

ventilation bypass 1176.12 1667.292

outdoor air heat gain10585.8 15005.63



Total room cooling load101587.51



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2.5 Load Calculation for 1st. Floor

Length 31.66 ft

Width 55.33 ft

Height 8.25 ft

Area 1751.75 ft2

Volume 14451.92 ft3

No of people 22

sensible latent

BTU/hr

solar

radiation(glass)2739.328

conduction (glass) 6826.5


people6050 2310

lights6197.317

ventilation bypass1176.12 1667.292

outdoor air heat gain 10585.8 15005.63



Total room cooling

load 59965.59



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2.6 Heating Load calculations:

1. Solar Gain through Glass:2. Conduction through Wall, Roof and Floor:3. Infiltration and Ventilation:

4. People (occupants):5. LIGHTS:6. Ventilation bypass:7. Outdoor air heat gain:8. Total Load

Heat gain:

Solar Gain through Glass:

Conduction through Wall, Roof and Floor:

People (occupants):

LIGHTS:

Appliances

Heat loss:

Infiltration and Ventilation:

Ventilation bypass:

Outdoor air heat gain:

Total Heating Load:

Design conditions for heating load calculations:

Month DecemberDate 15th

Winter design DB 35.6 F

Winter design WB 32.18

Relative humidity 67.40%

humidity ration 20.6grains/lb

Dew point 26.6 F


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References:

[1]-Handbook of Air Conditioning System Design by Carrier Air Conditioning Company

[2]-ASHRAE Fundamentals 1985

[3]-Building Energy Code of Pakistan (ENERCON February 2008)

[4]-Transient Analysis of Cooling Load of A Single Zone Building By Korbaga Fantu

(Addis Ababa University School of Graduate Studies)

Final Load Calculations

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