13
RT-PRC007-EN
Selection
Procedure
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 SP-1, the supply air fan
motor heat for 7.27 bhp = 20.6 MBh.
Capacity = 321 + 18.23 + 20.6 =
359.8 MBh (30 tons)
Step 5
— Determine unit capacity:
From Table PD-4 unit capacity at 81.5 DB.
67 WB entering the evaporator, 12000
supply air cfm, 95 F entering the
condenser is 361 MBh (30.1 tons) 279
sensible MBh.
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.
From Table PD-1, the leaving air wet bulb
temperature corresponding to an
enthalpy of 24.9 Btu/lb = 57.5.
Leaving air temperatures =
61.7 DB/57.5 WB
b
2” Hi-efficiency throwaway filters.
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 18-2, 30-ton unit capacity at 80 DB/
67 WB, 95 F entering the condenser and
12,000 total peak supply cfm, is 30.0 tons.
Thus, a nominal 30-ton unit is selected.
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 PD-14)
High efficiency filter 2”
.09 inches
(Table PD-14)
Economizer
.076 inches
(Table PD-14)
Unit total static pressure
1.50 inches
Using total cfm of 12,000 and total
static pressure of 1.50 inches, enter Table
PD-12. Table PD-12 shows 7.27 bhp with
652 rpm.
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) (76-66) = 68 + 1 = 67 F.
Selection of Trane 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/66 WB.
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/66 WB.
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.
60 Hz
