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This section outlines a step-by-stepprocedure that may be
used to select a Trane single-zone air conditioner. The
sample selection is based on the following conditions:
- Summer outdoor design conditions — 95 DB/76 WB ambient temperature
- Summer room design conditions — 78 DB/65 WB
- Total cooling load — 430 MBh (35.8 tons)
- Sensible cooling load — 345 MBh (28.8 tons)
- Outdoor air ventilation load — 66.9 MBh
- Return air temperature — 80 DB/65 WB
Winter Design:
- Winter outdoor design conditions — 0 F
- Return air temperature — 70 F
- Total heating load — 475 MBh
- Winter outdoor air ventilation load — 133 MBh
Air Delivery Data:
- Supply fan cfm — 17,500 cfm
- External static pressure — 1.2 in wg
- Minimum outdoor air ventilation — 1,750 cfm
- Exhaust fan cfm — 12,000 cfm
- Return air duct negative static pressure — 0.65 in wg
Electrical Characteristics:
- Voltage/cycle/phase — 460/60/3
Unit Accessories:
- Gas fired heat exchanger — high heat module
- Throwaway filters
- Economizer
- Modulating 100 percent exhaust/ return fan
COOLING CAPACITY SELECTION
Step 1 — Nominal Unit Size Selection
A summation of the peak cooling load and the outside air ventilation load shows: 430 MBh + 66.9 MBh = 496.9 MBh required
unit capacity. From Table 26-1, a 50-ton unit capacity
with standard capacity evaporator coil at 80 DB/65 WB, 95 F outdoor air temperature
and 17,500 total supply cfm is 551 MBh total and 422
MBh sensible. Thus, a nominal 50-ton unit with standard capacity evaporator coil is selected.
Step 2 — Evaporator Coil Entering Conditions
Mixed air dry bulb temperature determination: Using the minimum percent of OA
(1,750 cfm ÷ 17,500 cfm = 10 percent), determine the mixture dry bulb to the evaporator. RADB + % OA (OADB - RADB) = 80 +
(0.10) (95 - 80) = 80 + 1.5 =
81.5 F Approximate wet bulb mixture temperature: RAWB + % OA (OAWB -
RAWB) = 65 + (0.10) (76 - 65) = 65 + 1.1 = 66.1 F
Step 3 — Determine Supply Fan Motor Heat Gain
Having selected a nominal 50-ton unit, the supply fan bhp can be calculated. The supply fan motor heat gain must be considered
in final determination of unit capacity.
Supply Air Fan
Determine unit total static pressure at design supply cfm:
External Static Pressure 1.2 inches
Evaporator Coil (Table
57-1) 0.25 inches
Return Duct Negative Static
Pressure 0.65 inches
Heat Exchanger (Table
57-1) 0.31 inches
Throwaway Filter (Table
57-1) 0.10 inches
Economizer w/Exhaust Fan (Table
57-1) 0.12 inches
Trane Roof Curb (Table
57-1) 0.13 inches
Unit Total Static Pressure 2.76 inches
Using total of 17,500 cfm and total static pressure of 2.76 inches, enter Table 41-1. Table 41-1 shows 15.3 bhp with 924
rpm.
From Chart 19-1 supply fan motor heat gain = 46.0 MBh. Selection
Procedure |
