Selection Procedure
at 95°F is approximatly 1336 MBH total cooling and 969 MBH sensible cooling capacity.
Step 6 - Determine Leaving Air Temperature
Unit sensible heat capacity corrected for supply air fan motor heat = 969 MBH Sensible - 109.0 MBH Motor
Heat = 860 MBH.
Supply air dry bulb temperature difference =
Sensible MBH X 1000/1.085 x Supply
CFM
Sensible Btu = 860 MBH x 1000 ÷ (1.085 x 36000 CFM) = 22°F
Supply air dry bulb = 79.5 DB - 22 = 57.5°F Leaving the cooling coil
Supply air wet bulb temperature difference = (need in RTU catalog too)
Total MBH x 1000 ÷ 4.5 x Supply CFM
=
Unit enthalpy difference = 1336 MBH x 1000 ÷ (4.5 x 36000 CFM) = 8.25 Btu/lb.
Leaving enthalpy = h (ent WB) - h (diff). From Table 6, p. 37, p. 40 h (ent WB) =
30.9 Btu/lb.
Leaving enthalpy = 30.9 Btu/lb. - 8.25
Btu/lb. = 22.65 Btu/lb.
Supply air wet bulb = 54.0 Leaving the cooling coil.
Leaving air temperature = 57.5 DB/ 54.0 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
Supply air fan motor heat temperature rise = 109000 Btu ÷ (1.085 x 36000 CFM) = 2.8°F
Air temperature entering heating module = 63.0 + 2.8 = 65.8°F
Step 2 - Determine Total Winter Heating Load
Total winter heating load = peak heating load + ventilation load - supply fan motor heat = 720 + 288.6 - 109.0 = 899.6 MBH
Electric Heating System
Unit operating on 460/60/3 power supply.
From Table 29, p. 70, kw may be selected for a nominal 105 ton air handler "C" unit operating at
Table 28, p. 70 shows an air temperature rise of 23.2°F for 36000 CFM through the 265 kw heat module.
Unit supply temperature at design heating conditions = mixed air temperature + air temperature rise = 65.8°F + 23.2°F = 89.0°F.
Gas Heating System (Natural Gas)
From Table 27, p. 70 select the high heat module (1440 MBH output) to satisfy winter heating load of 899.6 MBH at unit CFM.
Table 27, p. 70 also shows an air temperature rise of 37.0°F for 36000 CFM through the heating module.
Unit supply temperature at design heating conditions = mixed air temperature + air temperature rise = 65.8°F + 37.0°F = 102.8°F.
Hot Water Heating System
Using a hot water supply temperature of 190°F and an entering
coil temperature of 65.8°F.
Subtract the mixed air temperature from the hot water temperature to determine the ITD (initial temperature difference).
ITD = 190°F - 65.8°F = 124.2°F.
Divide the winter heating load by ITD
=1008.6 MBH ÷ 124.2°F = 8.12 Q/ ITD.
From Table 30, p. 71, select the low heat module. By interpolation, a Q/ ITD of 8.12 can be obtained at a gpm of 41. Water pressure drop at 41 gpm is 0.34 ft. of water.
Heat module temperature rise is determined by:
Total Btu = 1.085 x CFM x Air temperature rise, °F 1008600 / 1.085 / 36000 = 25.8°F
Unit supply air temperature = mixed air temperature + air temperature rise = 65. 8 + 25.8 = 91.6°F.
Steam Heating System
Using a 15 psig steam supply. From Table 31, p. 71, the saturated temperature steam is 250°F. Subtract mixed air temperature from the steam temperature to determine ITD.
ITD = 250°F - 65.8°F = 184.2°F.
Divide winter heating load by ITD = 1008.6 MBH ÷ 184.2°F = 5.48 Q/ITD.
Table 31, p. 71, select the low heat module. The low heat module at 36000 cfm has a Q/ITD = 7.44
Heat module capacity, Q = ITD x Q/ ITD = 185°F x 7.44Q/ITD = 1376 MBH
Heat module air temperature rise is determined by:
Total Btu = 1.085 x CFM x Air temperature rise, °F 1376000 / 1.085 / 36000 = 35.2°F
Unit supply temperature at design conditions = mixed air temperature + air temperature rise = 65.8°F + 35.2°F = 100.1°F.
Air Delivery Procedure
Supply fan performance tables include internal resistance of air handler.
For total static pressure determination, system external static must be added to appropriate component static pressure drop cooling coil, filters, optional economizer, optional exhaust fan, optional heating system, optional cooling only extended casing).
Supply Fan Motor Sizing
The supply fan motor selected in the cooling capacity determination was
40.4BHP and 1097 RPM. Thus, a 40 HP supply fan motor is selected.
Enter Table 39, p. 77 to select the proper drive. For anair handler "C" with 40 HP motor, a drive letter A - 1100 RPM is selected.
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