COOLING LOAD ESTIMATIONCOOLING LOAD ESTIMATION

004-03-08

004-03-09

BUILDING SURVEY AND LOAD ESTIMATE

The primary function of air conditioning is to maintain conditions that are (1) conducive to human Comfort and for efficiency, or (2) required by a product, or process within a space. To perform this function, equipment of the proper capacity must be installed and controlled throughout the year. The equipment capacity is determined by the actual instantaneous peak load requirements: type of control is determined by the condition$ to be maintained during peak and partial load. Generally, it is impossible to measure either the actual peak or the partial load in any given space; these loads must be estimated. Before the load can be estimated, it it important that a comprehensive survey be made to assure accurate calculation of the load components. If the building facilities and the actual instantaneous load within a given mass of the building arc carefully studied, an economical equipment selection and system design can result, and smooth, trouble free performance is then possible.

The heat gain is the amount or heat instantaneously coming into or going out of the space. The actual load is defined as that amount of heat which is instantaneously removed by the equipment. The instantaneous heat gain and the actual load on the equipment will rarely be equal, because of the thermal inertia or storage effect of the building structures surrounding a conditioned space. BULDING SURVEY

SPACE CHARACTERISTICS AND HEAT LOAD SOURCES

An accurate survey of the load components of the space to be air conditioned is a basic requirement for realistic estimate of cooling and heating loads. The completnuss and accuracy of J.his suroey is the ocry foundation of tlu: estimate, and its importance can not be o.,crcmphasi. ud. Mechanical and architectural drawings, complete field sketches and, in some cases, photographs of important aspects are part o f a good survey. The following physical as peelS must l>e considered:

1. Orientation of building-Location of the space to be air conditioned with respect to:

(a) Compass poinlS--sun and wind effects.

(b) Nearby permanent structures-shading effects.

(c) Reflective surfaces-water, sand, parking areas, etc.

2. Use of space(s)-Office, hospital, department store, laboratory, machine shop, factory, assembly plant, etc.

3. Physical dimensions of spau(s)-Length, width, and height4. Ceiling height-Floor to floor height, floor to ceiling, clearance between suspended ceiling and beams.

5. ColuTTIJ'I.S and beams-Size, depth, also knee braces.

6. Construction maria!s-Materials and thickness of walls,. roof, ceiling, floors and partitions, and their relative position in the structure.

7. Surrounding conditions-Exterior colour of walls and roof, shaded by adjacent buildings or sunlft. Surrounding spa("(s conditioned or unconditioned - temperature of non-conditioned adjacent spaces, such a.S furnace or boiler room, and

kitchens. Floor on ground, crawl space, basement.

8. IVindows...:Size and location, wood or metal sa.sh, single or double glazing, gasketc:d or not. Type of glass and type of shading device. Dimensions of reveals and overhan.

9. Doors-Loc.ation, type, size, and frequency of use.

10. Stairways, Lifts and escalators-Location, tt"mpcrature of space if' open to unconditioned area. Power of machinery, ventilated or uot.

11. People-Number , dw-ationofoccupancy, nature: of aclivity, any special couccntratiun. /\t tim, it is required t estimate the nunthc:r of people on the basis of square metre per person, or on average traffic.12. Lighung-Watr.age at peak. Type-incandcs . cent, ftuoreseent, rcccs.scd, exposed. If the lights arc recased, the type of air fiow over the lights, exhaust, return or supply, should be' anticipated. At times, it is required to estimate the wattage on a basis of watu per square metre due to lack of exact inform:.tion.

13. }l.folors-Location, rated output, and. usage. The latter is of great significance and should be carefully evaluated.

The power input to electric motors is not necessarily equal to the rated output divided by the motor efficiency. Thl=Se motors may be operating under a continuous overload, or may be operating at less than rated capacity. It is alwa advisable to measure the power input wherever pos.sible. This is especially important in estimates for industrial inst.a.llations where the motor machine load is normally a major portion of the cooling load ..

14 .Appliancu, bu..rinas machine.s, ckctrnic cquipTTWitLocatioo, rated wattage, steam or gas consumption, hooded or unhoodcd, exhaust air quanlity instatlw much

the window is shaded a.t a given time.

A large: ponic.m oft he s

13

:c '(c....,.,le

c;!..u........ A \.-+ wa..!l --vT

HI k1/""'t : "" '3 :"'11li'o.NL

As,.., o" L+ 'o-\L,.

c...+dcor- klo.:.. "!.t.oc :!>.K

4\ t- ('""":

\..., 0.!).. ... -c.\.1" 2.+ .. < 100 = 38.6, say 40%.

r--v--+- .... ... cc

,.. I(U Uta 11 SPIC(

i ___ _

11.41 l.tt II t.C' I

1.11( c u .... c

. '" { ltll(CIIO J 1.111

Fig. 14-Rcaction on solar heat (R) Refe 1 renee g ass

30 angle of incidence

18

Basis of Table 14 .

- -Solar Heat Gain through Reference Glass

.

.

. .

Tal;/( 11 pro\idl-s dat for 0, 10!'; '2.0 .. , 30, 40 .. ,

a.na- so l;ni t udcs, for each month of the year and for

each hour of the are for use with Table 14.

Example 3-'-Peak Sofa; Heat Gain.through

"Stopray" Glass

__ ivcn:

West exposure, 30 South latitude

6 mm laminated "Stopray" (Gold), in steel sash.

Find:

. Peak solar heat gain.

Solution:

By inspection of Table H the boldface value for peak

solar heat gain, occurring at 4:00 p.m. on january 21

= 550W/m1

Steel s:uh window correction '""' I fO. 85 (bottom Tabu H).

From Table 15, the factor .... 0.44

550 X 0 4-1

Solar heal gain = 0

""' 2135 W /m:

.85

21

i I I

II

,. I

SHADING DEVICES

The effectiveness of a shadwg droice depends on its

ab ili ty to keep solar heat from the conditioned space.

All sha ding devices reflect and absorb a major port

ion of the solar gai1, lc!l ving a small portion to be

transmitted. The outdoor shading devices are much

more effective than the i nside devices because all of

th e reflected solar heat is kept out and the abiorbed

heat is dissipated to the outdoor air. Inside devices

necessarily dissipate their absorbed heat within the

con dit ioned space and must also re fle c t the solar heat

back through the g l ass (Fig. 18) wherein some of it is

absorbed.

Absor.ptivify, reflectivity, tra.nsmissioiiity an d oyerall

solar factors for various internal shading devices in

combination with reference glass are sho wn in

table (16).

: vera II Solar Factor is defined as the ra tio of solar

heat transtt.ci in to the space through a particuJar

glass in combination with a shading device, both

under reference conditions. The overall solar factors

are for use with Tabl 14.

Example 4-Partia!ly Drawn Shades

Occasionally it is necessary to estimate the cooling load

in a building where the blinds are not to be fully drawn.

The procedure is illustrated in the following example:

Given:

\'Vest exposure, 30 South latitude

Steel sash window with reference glass (3 mm, dear)

and white 'Brclla' Holland blind on inside, t drawn.

Find:

Pca.k solar heat gain.

Solution:

By mspcrtion of Table 11, the bold value$ for peak

solar heat gain, occurring at 4:00p.m. on January 21

. -= 550 W/m'

Correction for steel sash = 1/0.85 (bottom Tobit H)

In this example,. i of the window is covered with the

venetian blind and tis not ; therefore, the solar htruction material is approximately

U.8t kJ/kr;.C, the thermal 1111----8 mm BLACKENED GLASS

llm [f 11 S mm BRJCKWALL

tr/.J.

I

A/C r---, } I ncn I

I 1 10m

I A/C I

L---...1 1

rAIC

l 25m

--

25m

t

I STOREY PLAN

..

-'.;

._:,;:

,-- ---- 25mm CEMENT I SAND SffiED

I 12mm CEMENT I SAI-D SCRESJ

=:. :,E;Y:"t .

0.8m r ti:l.

'fl,'

f-

tSm

:;:;

l.lm

7

..

.. ( "

1..

(- t ( .{ 0 \. (;, l

v ..

,.

jO

,

I

l

l

r

6.2

6.3

6.4

East facade (SF= 130 x

OTIV =

= Wfml

North facade (SF= 130 x

OTfV =

= Wfm2

\Yest facade (SF= 130 x

OTIV =

=

=

6.5 Overall for Whole Building

OTIV =

=

=

W/m2)

II

6 OTTV Calculations (Alternative B)

6.1 South facade (SF= 130 x 0. 74 Wfm1)

Aw/Af

49.5

507.5

684.4

162.0

952.7

2356.1

Uw/Uf

. 2.71

1.98

1.93

5.82

2.96

OTTV =

TDeq/DoT sc

10

10

10

5 0.67 X 0.61

5

92857.75

2356.1

0.47

- 39.4 W/m1

6.2 East facad (SF= 130.x 1 17 Wfm1)

Aw/Af Uw/Uf

17 s l... 1 ')

4liLt'S ' l

b4o 't ' , 1-.--?

qo ?H.\

BB+ I+ z. '\ lo

lilt.'

OTTV =

-

TDeqfb.T sc _I

-

10 f

lo

T 0

'? 0..$)7 'l(()

s D 4":}

'

ill?b(:)o'73,

'lliJ.tf

(Aw x Uw x TDeq) Af (Uf x b. T + SC x SF)

1341.45

10048.50

13208.92

11083.54

57175.34

24598.87 68258.88

92857.75

. I

(Aw x Uw x TDeq) Af (Uf x D. T + SC x SF)

l4A.,lS

')s.o+

1 h{ () . q i $

OTIV =

llo

= 0\ \wfm2

6.5 Overall for whole Building

(Aw x Uw x TDeq) Af(Uf X t::.T + sc X SF)

o'3>\ '

';\ b

\' '" -\

lt-b(.'\ l

'l....l "''\1. lt\al'\ 1

'\) '"\ '\ \ S'\

-

(Aw x Uw x TDeq) Af(Uf x nT + SC x SF)

'0\ \a

,,,.'t. ft1-

.

11oo;\

\C\ 1\ "'.. ]\'6oo>\

'\\tA..-

,

OTIV = q l

I 't

(

( I

004- 0 3- 10 ( 3)

ROOF INSULATION AND ROOF OTTV

5.1 Thermal Transmittance of Roof

5.1.1 Soler heat gain into a building through an uninsulated roof increases air temperature

indoor. In all buildings, directional radiation received on the roof can be one of the main

causes of thermal discomfort.

5.1.2 For an air-conditioned building, solar heat gain through the roof also constitutes

a substantial portion of the cooling load. From on-site solar radiation measurements taken

in Singapore, the intensity of radiation on a horizontal surface can be as much as 3 times

of that on a vertical surface.

The pw:pose o 1oof insulation is therefore two.folds: to conserve energy in airconditioned

buildings and to promote thermal comfort in non air-conditioned buildings.

In both eases, the building regulations require that the roof shall not have a thermal transmittance

or U-value greater than the values tabulated below:-

Maximum U-value for Roof

Max Thermal Transmittance (Wfm2 K)

Weight Group Weight Range

(kg/m2) Aironditioned Non aironditioned

Building Building

Light Under 50 0.5 0.8

Medium 50 to 250 0.8 1.1

Heavy Over 250 1.2 1.5

5.1.3 The U-value of a roof shall be calculated in accordance with the method set out in

Appendix I.

5.1.4. Where more than one type of roof is used, the average thermal transmittance for the

gross area of the roof should be determined from:

Ur

where

Ur the average thermal transmittance of the gross roof area (Wfm2 K)

Ur1, Ur11 : the respective thermal transmittance of different roof

sections (W/m2 K)

the respective area of different roof sections (m2)

V(l)

Similarly, the average weight of the roof should be culated as follows:

Wr =

where

Wr

AI1 X Wr1 +AI-: X Wr2 +- + Ain X Wrn

AI, +AI-:++ Ain

average weight of roof (kg/m l)

the respective weight of different roof sections {kg/ml)

5.2. O'ITV of Roof

v (2)

5.2.1. In the case of air-conditioned building, the concept of overall thermal transfer

value, or OTIV, is also applicable to its roof if the latter is provided with skylight. The

OTIV concept for r-:, .i takes into consideration three basic elements of heat gain, viz:

a) heat conduction through opaque roof;

b) heat conduction through skylight;

c) solar radiation through skylight.

The maximum permissible OTTV for roofs is set at 45 W/m2 which is the same as that for

walls.

5.2.2 To calculate the OTTV of a roof, e followipg basic formula shall be used:-

OTTV

where

OTIV

Ar

Ur

TDeq

As

Us

AT

=

(AI x Ur x TDeq) + (As x Us x A T) + {As x SC x SF)

Ao

overall thermal transfer value (W jm2)

opaque roof area (m2)

thermal transmittance of opaque roof area (W/m2 K)

equivalent temperature difference (K), see sub-para 5.2.2.1

skylight area (m2}

L.ermal transmittance of skylight area (W/m2 K)

temperature difference between exterior and interior design

conditions (5 K)

SC shading coefficient of skylight

SF solar factor (Wjm2 ), see sub-para 5.2.2.2

Ao gross area of roof (m2)

= AI+As

V{3)

(

r

5.2.2.1 Equivalent Temperature Difference

For the purpose of simplicity in OTIV calculation, the TDeq of different types of

roof constructions have been standardised as follows:-

Equivalent Temperature Difference for Roof

Roo Construction - Mass Per Unit Area TDeq

0 - 50 kg/m'l 24K

50 - 230 kg/m'l 20K

Over 230 kg/m'l 16 K

5.2.2.2 Solar Factor

For a given orientation and angle of slope, the Solar Factor is given by:

SF = 320 X ( '' (W/m'l) V(4)

Where CF is the correction factor with reference to the orientation of the roof and the

pitch angle of its skyligt and is given as follows:-

Solar Correction Factor for Roof

N NE E SE s sw w NW

0

0

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

5

0

1.00 1.01 1.02 1.02 1.00 1.02 1.02 1.01

10

1.01 1.03 1.04 1.03 1.01 1.03 1.04 1.03

15

1.01 1.03 1.05 1.03 1.01 1.03 1.05 1.03

20

1.00 1.03 . 1.06 1.03 1.01 1.03 1.06 1.03

25

0.98 1.02 1.06 1.03

- 0.99 1.03 1.06 1.02

30

0.96 1.01 1.05 1.01 0.97 1.01 1.05 1.01

35

0.93 0.98 1.03 0.99 0.94 0.99 1.03 0.98

40

0.90 0.96 1.01 0.96 0.91 0.96 1.01 0.96

45

0.86 0.92 0.98 0.92 0.87 0.93 0.98 0.92

50

0.81 0.89 0.95 0.89 0.83 0.89 0.95 0.89

55

0.77 0.84 0.91 0.85 0.78 0.85 0.91 0.84

60

0.71 0.79 0.86 0.80 0.73 0.80 0.86 0.79

65

0.66 0.74 0.81 0.75 0.67 0.75 0.81 0.74

l

The Correction Factors for other orientations and other pitch angles may be found

by interpolation.

For the purpose of the building regulations, any construction with a pitch angle

less than 70 shall be treated as a roof.

5.2.3 If a roof consists of different sections facing different orientations or pitched at

different angles, the OTTV for the whokroof shall be calculated as follows:-

OTTV _ Aot X OTTVt + Ao2 X OTIV2 + + Aon X OTfVn -

Ao1 + Ao2 + + AoD. V (5)

5.2.4 The gross area of a roof shall include all opaque roof areas and skylight areas, when

such surfaces are exposed to outdoor air and enclose an air-conditioned space.

5.2.5. When more than one type of material and/or skylight is used, the respective term or

terms shall be expanded into sub-elements as:

5.2.6 The OTTV requirement for roof applies to an air-conditioned building and is over

and above the Uvalue requirement.

5.2. 7 The OTTV of the roof should not be computed together with that of the walls.

Each component should be treated separately.

5.3 Submission Procedure

At the time of submission of building plans, the architect should provide the information

on roof insulation by:-

11

submitting a drawing showing the cross sections of typical parts of the roof

construction, giving details ,of the type and thickness of basic construction

materials, insulation and air space;

calculating the U-value of the roof assembly according to Appendix I; and

m if the building is air-conditioned, calculating the OTTV of the roof assembly.

5.4 Example

5.4.1 Sketches

E

50 m

skylight

opaqull roof

PLAN VIEW OF ROOF

N

+

1

l

I

I '

(

\

'

\

5.4.3

5.4.4

Area Calculation

Area of whole roof =

Area of two skylights Ar

1

=

Area of opaque area Ar2 =

Weighted A. -..rage Weight of Roof

W = A.r1 X Wr1 + Ar2 + W r2

Ar1 '! Ar2

= 950 X 33.7 + 50 X 35.2

1000

=

.

kg/m2

50 X 20 = ml

2x5x5 = ml

1000- = . ml

From V(2)

Since both roof components belong to the light weight group, the combined roof

also belongs to the light weight group.

5.4.5 Weighted Average U-value

u = 0.28 X 950 + 2.62 X 50

1000 From V(1)

This satisfi:: the U-value requirement of the building regulations for light weight

roof.

5.4.6 Roof OTTV

For north orientation at 25 pitch, SF= 320 x

OTTV =

X X +50 ( . X + 0.5 X 320 X

=

1000

= Wfml

1000

This also satisfies the OTIV requirement for roof.

27

iMulatqd tile roof------.

doublq- glal:d ----,

skylight

5.4.2. U-value Calculation

a) For Opaque Roof

R = 0.055. + 0.012 +.

= m2oK/'vl

u = 1..

R

= 1

= Wfml K

SECTION X- X

+ 0.075 +

Weight = 1890 X 0.012 + X 0.075 +

= kgfml

TDeq = OK

b) For skylight

R = 0.055 o.ooa

+-+ + +

=

u

1 = 1

:;::;

R

= Wfml oK

sc :;::; 0.5 (given)

Weight = X 0.008 + x 0.006

= kg/ml

12 mm til -----,

rq f 1 qc.trvq foil

f ac.ing air gap

100 mm air gap

75 mm fit:nglass

0.012

+

X 0.012

6mm grcry glass

6 mm c.l110r glass

26

.....,...._ . .... ...--..-oi, --- I -, o .... ,. , o '

y I

, f ; ... t. ,... I '

-- ' 004-03-11

3 ?,8{:, l