Heating capacity selection:

1

Winter outdoor design conditions—5 F.

2

Total return air temperature — 72 F.

3

Winter outside air minimum ventilation load and cfm — 1,200 cfm and 87.2 MBh.

4

Peak heating load 225 MBh. Utilizing unit selection in the cooling capacity procedure.

Mixed air temperature = RADB + %O.A. (OADB - RADB) = 72 + (0.10) (0-72) = 64.8 F.

Supply air fan motor heat temperature rise = 20,600 BTU ÷ (1.085 x 12,000) cfm = 1.6 F.

Mixed air temperature entering heat module = 64.8 + 1.6 = 66.4 F.

Total winter heating load = peak heating + ventilation load - total fan motor heat = 225 + 87.2 - 20.6 = 291.6 MBh.

Electric Heating System

Unit operating on 480/60/3 power supply. From Table 21-1, kw may be selected for a nominal 30-ton unit operating on 480-volt power. The high heat module — 90 KW or 307 MBh will satisfy the winter heating load of 291.6 MBh.

Table 21-1 also shows an air temperature rise of 23.6 F for 12,000 cfm through the 90 kw heat module.

Unit supply temperature at design heating conditions = mixed air temperature + air temperature rise = 66.4 + 23.6 = 90 F.

Natural Gas Heating System

Assume natural gas supply — 1000 Btu/ft 3 . From Table 21-3, select the high heat module (486 MBh output) to satisfy 291.6 at unit cfm.

Table 21-3 also shows air temperature rise of 37.3 F for 12,000 cfm through heating module.

Unit supply temperature design heating conditions = mixed air temperature + air temperature rise = 66.4 + 37.3 = 103.7 F.

Air Delivery Procedure

Supply air fan bhp and rpm selection. Unit supply air fan performance shown n Table 22-1 includes pressure drops for dampers and casing losses. Static pressure drops of accessory components such as heating systems, and filters if used, must be added to external unit static pressure for total static pressure determination.

The supply air fan motor selected in the previous cooling capacity determination example was 7.27 bhp with 652 rpm. Thus, the supply fan motor selected is 7.5 hp.

To select the drive, enter Table 25-1 for a 30-ton unit. Select the appropriate drive for the applicable rpm range. Drive selection letter C with a range of 650 rpm, is required for 652 rpm. Where altitude is significantly above sea level, use Table 17-2 and 17-3, and Figure 17-1 for applicable correction factors.

Unit Electrical Requirements

Selection procedures for electrical requirements for wire sizing amps, maximum fuse sizing and dual element fuses are given in the electrical service selection of this catalog.

Unit Designation

After determining specific unit characteristics utilizing the selection procedure and additional job information, the complete unit model number can be developed. Use the model number nomenclature on page 9.

Figure 16-1 — Fan Motor Heat

NOTES:

1. Fan motor heat (MBh) includes 1.1 correction factor for motor efficiency.

2. Capacities shown. Table 12-1 are gross values; heat gain from evaporator fan motor must be included in unit capacity determination.