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Step 4 — Determine Total Required Cooling Capacity
Required capacity = Total peak load + OA load + supply air fan motor heat. Required capacity = 430 + 66.9 + 46.0 = 543
MBh (45.2 tons) Step 5 — Determine Unit
Capacity From Table 26-1, unit capacity at 81.5
DB/66.1 WB entering the evaporator, 17,500 supply air cfm, 95 F outdoor ambient,
is 561 MBh (45.8 tons) with 426 MBh sensible.
Step 6 — Determine Leaving Air Temperature Unit sensible heat capacity
corrected for supply air fan motor heat = 426 MBh -46
MBh = 380 MBh. Supply air
dry bulb temperature difference = Sensible Btu
= 1.085 x Supply
cfm 380 MBh ÷ (1.085 x 17,500 cfm) = 20.0 F Supply air dry bulb =
81.5 DB - 20.0 = 61.5 F Unit enthalpy difference =
Total Btu = 4.5 x Supply cfm 561 MBh ÷ (4.5 x 17,500 cfm)
= 7.12 Btu/lb Leaving
enthalpy = h(ent WB) -h( diff). From Table 21-1 h(ent
WB) = 30.9 Btu/lb Leaving
enthalpy = 30.9 Btu/lb - 7.12 Btu/lb = 23.78 Btu/lb
Supply air wet bulb = 55.9 Leaving
air temperature = 61.5 DB/55.9 WB HEATING CAPACITY SELECTION Step 1
— Determine Air Temperature Entering Heating
Module Mixed air temperature = RADB + % OA (OADB - RADB) = 70 + (0.10) (0 - 70) = 63 F Selection Procedure Chart 19-1 — Fan Motor Heat
0 5 10 15 20 25 30 35 40 0
10 20 30 40 50 60 70
80 90 100 110 120 STANDARD MOTOR HIGH EFFICIENCY MOTOR FAN MOTOR HEAT - MBH
MOTOR BRAKE HORSE POWER
Supply air fan motor heat temperaturerise = 46,000 Btu ÷
(1.085 x 17,500 cfm) = 2.42 F
Air temperature entering heating module = 63.0 + 2.42 = 65.4 F
Step 2 — Determine Total Winter Heating Load
Total winter heating load = peak heating load + ventilation load - supply fan motor heat = 475 + 133 - 46.0 = 562 MBh
Electric Heating System
Unit operating on 460/60/3 power supply.
From Table 35-3, kw may be selected for a nominal 50-ton unit operating 460-volt power. The 170 kw heat module (580.1
MBh) will satisfy the winter heating load of 563 MBh.
Table 35-1 shows an air temperature rise of 30.6 F for 17,500 cfm through the 170 kw heat module.
Unit supply temperature at design heating conditions = mixed air temperature +
air temperature rise = 65.4 F + 30.6 F = 96.0 F.
Gas Heating System (Natural Gas)
From Table 34-1 select the high heat module (697 MBh output) to satisfy winter heating load of 563 MBh at unit cfm.
Table 34-1 also shows an air temperature rise of 36.0 F for 17,500 cfm
through the heating module.
Unit supply temperature at design heating conditions = mixed air temperature +
air temperature rise = 65.4 F + 36.0 F = 101.4 F.
Hot Water Heating
Assume a hot water supply temperature of 190 F. Subtract the mixed air temperature
from the hot water temperature to determine the ITD
(initial temperature difference).
ITD = 190 F - 65.4 F = 125 F. Divide the winter heating load by ITD = 563 MBh ÷ 125 F = 4.50 Q/ITD.
From Table 36-1, select the low heat module. By interpolation, a Q/ITD of 4.50 can be obtained at a gpm at 25.7.
Water pressure drop at 25.7 gpm is 0.57 ft. of water. Heat module temperature
rise is determined by:
Total Btu = delta T 1.085 x Supply cfm 563,000 =
29.7 F (1.085 x 17,500)
Unit supply air temperature = mixed air temperature + air temperature rise = 65.4 + 29.7 = 95 F |
