15
RT-PRC007-EN
Selection
Procedure
50 Hz
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 (35/24.4°C), 95°F (35°C) entering
air to condenser.
2
Summer room design conditions –
76 DB/66 WB (24.4/18.9°C).
3
Total peak cooling load – 270 MBh
(79 kW) (22.5 tons).
4
Total peak supply cfm – 10,000 cfm (4720
L/s).
5
External static pressure – 1.0 inches wc
(249 Pa).
6
Return air temperatures – 80 DB/66°F WB
(26.7/18.9°C).
7
Return air cfm – 3540 cfm (1671 L/s).
8
Outside air ventilation cfm and load –
1000 cfm and 15.19 MBh (1.27 tons or
4.45 kW) 472 L/s.
9
Unit accessories include:
a
Aluminized heat exchanger – high heat
module.
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: 22.5 tons + 1.27 tons = 23.77 (79
kW + 4.45 kW = 83.45) required unit
capacity. From Table PD-18, 25 ton (89
kW) unit capacity at 80 DB/67 WB (27/
19°C), 95
°
F entering the condenser and
10,000 total peak supply cfm (4720 L/s), is
YC/TC/TE*305.
Step 2
– Having selected the correct 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
1.24 inches
(310 Pa)
Heat exchanger
(Table PD-27)
.12 inches
(30 Pa)
High efficiency filter 2” (25 mm)
(Table PD-27)
.07 inches
(17 Pa)
Economizer
(Table PD-27)
.07 inches
(17 Pa)
Unit total static pressure
1.50 inches
(374 Pa)
Using total cfm of 10,000 (4720 L/s) and
total static pressure of 1.50 inches
(38 mm), enter Table PD-25. Table PD-25
shows 5.35 bhp (4 kW) with 616 rpm.
Step 3
– Determine evaporator coil
entering air conditions. Mixed air dry
bulb temperature determination.
Using the minimum percent of OA (1,000
cfm ÷ 10,000 cfm = 10 percent),
determine the mixture dry bulb to the
evaporator. RADB + % OA
(OADB - RADB) = 80 + (0.10) (95 - 80) = 80
+ 1.5 = 81.5°F [26.7 + 1.5 = 28°C).
Approximate wet bulb mixture
temperature:
RAWB + OA (OAWB - RAWB) =
66 + (0.10) (76-66) = 68 + 1 = 67°F.
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 Chart SP-1, the supply air fan
motor heat for 5.35 bhp = 15 MBh.
Capacity = 270 + 15 + 15 =
300 MBh (89 kW)
Step 5
– Determine unit capacity:
From Table PD-18 unit capacity at 81.5
DB/67 WB entering the evaporator,
10,000 supply air cfm, 95°F (35°C)
entering the condenser about 304 MBh
(89 kW) with 235 MBh (68.8 kW)
sensible.
Step 6
– Determine leaving air
temperature:
Unit sensible heat capacity, corrected for
supply air fan motor heat 235 - 15 = 220
MBh (64.4 kW).
Supply air dry bulb temperature
difference = 220 MBh ÷ (1.085 x
10,000 cfm) = 20.2°F (-6.6°C)
Supply air dry bulb: 81.5-20.2 = 61.3
(16.3°C)
Unit enthalpy difference = 305.6 ÷
(4.5 x 10,000) = 6.76
Btu/lb leaving enthalpy = h (ent WB)
= 31.62
Leaving enthalpy = 31.62 Btu/lb -
6.76 Btu/lb = 24.86 Btu/lb.
From Table PD-1, the leaving air wet bulb
temperature corresponding to an
enthalpy of 24.8 Btu/lb = 57.5.
Leaving air temperatures = 61.3 DB/57.5
WB (16.3/14.2°C).
