Selection Procedure
Selection ofTrane commercial air conditioners is divided into five basic areas:
1
Cooling capacity
2
Heating capacity
3
Air delivery
4
Unit electrical requirements
5
Unit designation
Factors Used In Unit Cooling Selection:
1
Summer design conditions — 95 DB/
76 WB, 95 F entering air to condenser.
2
Summer room design conditions — 76 DB/66WB.
3
Total peak cooling load — 321 MBh (27.75 tons).
4
Total peak supply cfm — 12,000 cfm.
5
External static pressure — 1.0 inches.
6
Return air temperatures — 80 DB/66WB.
7
Return air cfm — 4250 cfm.
8
Outside air ventilation cfm and load — 1200 cfm and 18.23 MBh (1.52 tons).
9
Unit accessories include:
a
Aluminized heat exchanger — high heat module.
b
2”
c
Exhaust fan.
d
Economizer cycle.
Step 1 — A summation of the peak cooling load and the outside air ventilation load shows: 27.75 tons + 1.52 tons = 29.27 required unit capacity. From Table
Step 2 — Having selected a nominal 30- ton unit, the supply fan and exhaust fan motor bhp must be determined.
Supply Air Fan:
Determine unit static pressure at design supply cfm:
External static pressure | 1.20 inches |
Heat exchanger | .14 inches |
(Table |
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High efficiency filter 2” | .09 inches |
(Table |
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Economizer | .076 inches |
(Table |
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Unit total static pressure | 1.50 inches |
Using total cfm of 12,000 and total static pressure of 1.50 inches, enterTable
Step 3 — Determine evaporator coil entering air conditions. Mixed air dry bulb temperature determination.
Using the minimum percent of OA (1,200 cfm ÷ 12,000 cfm = 10 percent), determine the mixture dry bulb to the evaporator. RADB + %OA (OADB - RADB) = 80 + (0.10) (95 - 80) = 80 + 1.5 = 81.5F
Approximate wet bulb mixture temperature:
RAWB + OA (OAWB - RAWB) = 66 + (0.10)
A psychrometric chart can be used to more accurately determine the mixture temperature to the evaporator coil.
Step 4 — Determine total required unit cooling capacity:
Required capacity = total peak load + O.A. load + supply air fan motor heat.
From Figure
Capacity = 321 + 18.23 + 20.6 = 359.8 MBh (30 tons)
Step 5 — Determine unit capacity:
FromTable
Step 6 — Determine leaving air temperature:
Unit sensible heat capacity, corrected for supply air fan motor heat 279 - 20.6 = 258.4 MBh.
Supply air dry bulb temperature difference = 258.4 MBh ÷ (1.085 x 12,000 cfm) = 19.8 F.
Supply air dry bulb: 81.5 - 19.8 = 61.7.
Unit enthalpy difference = 361 ÷ (4.5 x 12,000) = 6.7
Btu/lb leaving enthalpy = h (ent WB) = 31.62
Leaving enthalpy = 31.62 Btu/lb - 6.7 Btu/lb = 24.9 Btu/lb.
FromTable
Leaving air temperatures = 61.7 DB/57.5WB
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