In this assignment , an office building in Montreal area requires the estimation of the size of equipment’s to provide cooling for a single story building . And then, the cooling load has been calculated using the Cooling Load Temperature Difference / Solar Cooling Load/ Cooling Load Factor ( CLTD/SCL/CLF) method. The cooling load are approximated corresponding to the four items of heat gain.
1) Conduction through surface, e.g. walls, windows, roofs.
2) Solar heat through fenestration.
3) Internal heat from lights, people and equipment’s.
4) Ventilation.
During calculation the building has been divided into five different zones. Sensible and latent heat gain in different section have been calculated in steps due to different orientation of different zones.
Location: Montreal
Type of building Office
Number of floors 1
Floor area: 64. Ft x 80. Ft = 5120 ft2
Floor-to-floor Height : 15 ft
Window area: 20 % of the wall area
Windows: double glazed
Wall: U= 0.2 Btu/h.ft2.F, Wall number
13 as per table 33A*, pg 28.46
Windows: U= 0.055 Btu/h.ft2.F, Roof number
13 as per table 31, pg 28.42
Recommended ventilation: ½ air change per hour (min) or
20 cfm/person, 7 person/1000
Montreal ( 45° N 73° 45’W) *
Outdoor design dry-bulb: 90 °F
Outdoor design wet-bulb: 75 °F
Indoor design Dry-bulb: 78 °F
Relative humidity f : 50%
Wind velocity : 7½ mph
* Reference: 1997 ASHARE Handbook - Fundamentals Table-2, pp.26.25.
* Throughout the calculation for the Montreal has been considered to be on 40° N latitude.
The whole building envelope is divided into 5 zones having 20 rooms of equal areas (It’s assumed the two corners in each zone as a one room with the same situation with the other rooms on that zone). The central zone heat gain conduction through the walls are zero due to DT=0 (There isn’t exterior walls in central zone)
The following assumptions were made during the course of calculation:
1) Occupancy hours for people from 0900 to 1700 hrs.
2) 2 persons per room.
3) Light remains on from 0900 to 1700 hrs.
4) 1536 watts of lights per room.
5) 200 watts for computer per room.
6) Roof number is 13 as per Table 31t page‑28.42 and U value for roof is
0.055 Btu/h.ft2 .°F.
7) Wall number is 13 as per Table 33A, page‑28.46 and U value for wall is 0.2 Btu/h.ft2.°F.
8) Window is Uncoadted double glazed and U value is 0.55 which is
found from Table 11, page‑29.8
9) Values of Cooling Load Temperature Difference (CLTD) for roof are taken from Table30, Page‑28.42
10) Values of Cooling Load Temperature Difference (CLTD) for wall
are taken from Table32, Page‑28.45
11) Values of Cooling Load Temperature Difference (CLTD) for glass
are taken from Table34, Page‑28.49
12) Values of Shading Coefficient (SC) are taken from Table 11,
page‑29.25
13) Values of Solar Cooling Load (SCL) are taken from Table 36,
page‑28.50, zone Type C
14) Values of Cooling Load Factor (CLF) for lighting are taken from
Table38 based on lights on for 8 hours, Page‑28.52, zone Type C
15) Values of Cooling Load Factor (CLF) for people are taken from
Table37 based on 8 hours in space , Page‑28.51, zone Type C
16) Rates of Sensible Heat Gain (SHG) and Latent Heat Gain (LHG) from people are taken from Table 3, page‑28.8,
17) It is assumed that there will be one Computer in each room also assumed that heat gain will be 680 Btu/hr
Source: ASHRAE Handbook‑ 1997 Fundamentals
|
Zone Number |
Number of Walls per room |
Wall Side |
Number of Room |
Area of wall (ft2) |
Area of Window (ft 2) |
Area of Roof (ft 2) |
|
1 |
1 |
N |
3 |
192 |
48 |
256 |
|
2 |
1 |
E |
4 |
192 |
48 |
256 |
|
3 |
1 |
S |
3 |
192 |
48 |
256 |
|
4 |
1 |
W |
4 |
192 |
48 |
256 |
|
5 |
- |
Interior |
6 |
- |
48 |
256 |
· Roof area each room is 256 ft2 ( 16 ´ 16 ).
· Window area for each wall is 48 ft² which is 20 % of gross wall area of 240 sq.ft (16 ´ 15´0.2=48 ft²).
· Net area for each wall is 192 sq. ft , which is gross wall area minus window area (240 ft²-48 ft²=192 ft²)
2.0 CLTD/SCL/CLF CALCULATION PROCEDURE :
To calculate aspace cooling load using the CLTD/SCL/CLF convention, the basic heat gain calculation concepts of solar radiation, total heat gain through exterior walls and roofs (heat gain through interior surface is zero due to DT=0 ) and heat gain through infiltration, ventilation, occupants, lighting and equipment.
The CLTD/SCL/CLF method is used to approximate the cooling load corresponding to the first four modes of heat gain ( conductive heat gain through surface such as windows, walls and roofs; solar heat gain through fenestration ; and internal heat gain from lights, people and equipment’s) and the cooling load from infiltration and ventilation.
The acronyms are defined as follows:
CLTD - Cooling Load Temperature Difference
SCL - Solar Cooling Load
CLF - Cooling Load Factor
The following sections give details of how the CLTD/SCL/CLF technique is used. The sources of the space cooling load, forms of equation to use in the calculations, appropriate references, tables.
2.1.1 Exterior Roofs, Walls and Windows:
This method is used to calculate Heat gain was converted to cooling load using the room transfer functions for the rooms with light, medium and heavy thermal characteristics. The inside air temperature is 78 ° F .
q = UA(CLTD)........................................... (1)
q = cooling load, Btu/h
U = Coefficient of heat transfer roof or wall or glass,
Btu/hr.ft².°F
A = area of roof, wall or glass, ft2
CLTD = cooling load temperature difference °F
2.1.2 Solar Load through glass:
q = A ( SC ) ( SCL ) .................................(2)
SC = Shading coefficient; Table 11, Chapter 27.
SCL = Solar cooling load factor Table 36,chapter 28
2.2.1 People :
qsensible = N ( sensible heat gain) CLF ................... (3)
qlatent = N ( Latent heat gain)..................................(4)
N = number of people in space from best available source.
Sensible and latent heat gain from occupancy, Table-3-28
CLF = cooling load factor, by hour of occupancy, Table-37
Note: CLF= 1.0 with high density or 24-h occupancy and /or if cooling off at night or during weekends.
2.2.2 Lights:
qel = 3.4 W. Ful . Fsa . (CLF)...................................(5)
W = watts input from electric plans or lighting fixture data.
Ful = lighting use factor, from section 1, as appropriate.
Fsa = special allowance factor, from section 1, as appropriate.
CLF = cooling load factor, by hour of occupancy, Table 37
Note: CLF = 1.0 with 24-h light usage and/or if cooling off at night or
during weekends.
Power:
qp = 2545 P .EF .CLF .................................................(6)
P = horse power rating from electrical plans or manufacturer’s data
EF = efficiency factors from and arrangements to suit circumstances.
CLF = cooling load factor, by hour of occupancy, Table 38
Note: CLF = 1.0 with 24-h light operation and/or if cooling off at night or
during weekends.
2.2.3 Appliances:
qsensible = qis . Fu . Fr CLF................................ ( 7 )
qlatent = qil . Fu ………………………………. (8)
qis , qil = sensible and latent heat gain from appliances- Table 5 through 9 ,
or manufacturer’s data.
Fu , Fr = usage factors, radiation load factors from the general principles section
CLF = cooling loads factor, by scheduled hours and hooded or not; Tables
37 and 39.
Note 1: CLF = 1.0 with 24-h appliance operation and/or if cooling off at night or during weekends.
Note 2: Set latent load = 0 if appliance under exhaust hood.
2.3. Ventilation and Infiltration Air :
qsensible = 1.10 Q (to - ti ) ............................ ( 9)
qlatent = 4840 Q ( Wo - Wi ) .........................( 12)
qtotal = 4.5 Q (ho - hi ) ...............................(11)
Q = ventilation cfm from ASHARE Standard 62; infiltration from
Chapter 25 .
to , ti = outside, inside air temperature , ° F .
Wo , Wi = outside , inside air humidity ratio, lb water/lb dry air
ho , hi = outside , inside air enthalpy , Btu/lb dry air.
3.1.Calculation of corrected CLTD:
CLTDc = CLTD + (78- tr) + (tm- 85)
tr = inside design temperature = 78 o F
tm = mean outdoor temperature
= maximum outdoor temperature - (daily range/2)
= 90-(20/2) = 90-10 = 82
Therefore CLTDc = CLTD + (82 - 85) = CLTD - 5
Outdoor temperature for Montreal=90 °F (db)
Indoor temperature = 78° F
Indoor Relative Humidity = 50 %
Range = 20
Wind velocity = 7.5 mph
8 hrs of working Þ 9.00 to 17.00 hrs .
3.2 Calculation for Ventilation :
Recomended ventilation is 20 cfm/person; 7 person/1000 sq.ft.
As 2 persons are taken in each room therefore - 2 ´ 20 = 40 cfm for each room.
qsensible = 60 ´ Q´r ´ CR ´ Dt
= 60 hr/min´ Q ft³/min ´ (0.075) Ibm/ft³ ´(0.24) Btu/Ib.°F ´Dt (°F)
= 1.08 ´Q´Dt
r = Air density (0.075 Ibm/ft3)
Cp = Specific heat of air (0.24 Btu/Ib.F)
Q = Air flow rate in cfm.(40 cfm)
Dt = Indoor minus Outdoor temperature difference. (to - ti)
Now
q sensible = 44´ (to - ti)
to = 90 °F , ti= 78 °F
qsensible= 44´(90-78)= 528 Btu/hr
or :
qsensible = r (Ibm/ft3 ) ´ cfm (ft3 /min) ´ (hx – hi) (Btu/Im.da) ´ 60(hr/min)
From Psychrometric Chart (Sea Level):
hi=30 Btu/Ibm.da , hx=32.8 Btu/Ibm.da
qsensible = 0.075(Ibm/ft3 )´40(ft3 /min) ´ (32.8 – 32) (Btu/Im.da) ´60 (hr/min)
qsensible = 504 Btu/hr
qlatent = 4840´ Q ´ r ´ Dw
where :
Dw = Humidity ratio of indoor air minus humidity ratio of outdoor air. lbm water/ lbm dry air.
Q=40 cfm
From Psychometric chart Chapter 6 page .6.15:
Indoor - For 78° F dry bulb temperature (dbt) at 50% Relative Humidity Wi = 0.0102 Ibm water/Ibm dry air
Outdoor - For 90 ° F dry bulb temperature (db) at 50% Relative Humidity
Wo = 0.0152 Ibm water/Ibm dry air
Dw =( wo - wi) = 0.0152 - 0.0102 = 5´10-3 Ibm water/Ibm dry air
therefore qlatent = 4840´40´5´10-3
qlatent = 968 Btu/hr
or :
qlatent = r (Ibm/ft3 ) ´ cfm (ft3 /min) ´ (ho – hx) (Btu/Im.da) ´60 (hr/min)
From Psychrometric Chart (Sea Level):
ho=38.6 Btu/Ibm.da , hx=32.8 Btu/Ibm.da
qlatent = 0.075(Ibm/ft3 ) ´ 40(ft3 /min) ´ (32.8 – 32) (Btu/Im.da) ´60(hr/min)
qlatent = 1044 Btu/hr
3.3. Lighting Load (per room):
q ( Btu/hr) = 3.4 (Btu/hr.wat) ´ wattage ´ fb ´ CLF
where ;
fb = Floursent ballast factor =1.3
Therefore ;
q = 3.4 Btu/(hr.wat) ´1536 Wat ´1.3´0.72(CLF)
q = 4888 Btu/hr .
* The Value of q = 4888 Btu/hr is calculated at 8.00 hrs.
Please Refer Table-11 for other calculation.
3.4. Appliances :
For each room:
Number of Computer in each room = 1
Therefore, q = 1´3.4 Btu/(hr.wat) ´200 wat=680 Btu/hr
Kitchen (in central zone) :
Water cooler = 5970 Btu/hr
2 Microwaves = 1360 * 2 = 2720 Btu/hr.
2 Coffee maker=2´3580=7160 Btu/hr (sen.)
=2´1540=3080 Btu/hour (lat.)
Total Sensible load due to Kitchen= 5970+ 2720+7160= 15850 Btu/hr
Total latent load due to Kitchen =3080 Btu/hr
Office Services (in central zone):
2 Small Copier = 2´1570=3140 Btu/hr
2 Type Writer = 2´230=460 Btu/hr
2 Letter quality printer = 2´1000=2000 Btu/hr
2 computer =2´680=1360 Btu/hr
Therefore, Total head load (Sensible) due to office services is:
= 3140+460+2000+1360 = 6960 Btu/hr.
Refer Table-12 for other calculation
Results:
Sensible and the latent space loads for each zone at 10:00, 12:00, 14:00,16:00 hours, Table - 19
Maximum sensible and latent space loads for the building, Table – 20 (at 15:00 hr.)
Sensible and the latent fresh air loads for the building at 10:00, 12:00, 14:00, 16:00, Table - 21
Maximum sensible and latent loads(total space and fresh air loads) for the building, Table- 22
*The highlighted area in each table shows the Maximum Loads.
CONCLUSIONS :
Cooling Loads for Individual Zones are calculated. Also the cooling loads for room in the zones are calculated. The maximum loads of the building is found at 17.00 hours. The maximum space loads which required for the building are 224987 Btu/hr sensible , 11080 Btu/hr latent.
Also the maximum total loads for the building (included space and fresh air loads) are 235067 Btu/hr “sensible” , 28880 Btu/hr “latent” .
As it has been shown, the maximum loads of the building is found at “17 hours” (The highlighted area show that).
REFERENCES:
1. ASHRAE HANDBOOK- FUNDAMENTALS, 1997 “ The American society of Heating, Refrigerating and Air Conditioning”.