Selection Procedure
Steam Heating System
Assume a 15 psig steam supply.
From Table 34-4, the saturated temperature steam is 250 F. Subtract mixed air
temperature from the steam temperature to determine
ITD. ITD = 250 F - 65.4 F = 185 F.
Divide winter heating load by ITD = 563 MBh ÷ 185 F = 3.04 Q/ITD.
From Table 34-3, select the high heat module. The high heat module at 17,500
cfm has a Q/ITD = 5.11.
Heat module capacity, Q = ITD x Q/ITD = 185 F x 5.11 Q/ITD = 945 MBh
Heat module air temperature rise = Total Btu
1.085 x Supply cfm
945 Btu ÷ (1.085 x 17,500 cfm) = 49.8 F.
Unit supply temperature at design conditions = mixed air temperature + air temperature
rise = 65.4 F + 49.8 F = 115 F.
AIR DELIVERY PROCEDURE
Supply fan performance tables include internal resistance of rooftop. For total static pressure determination, system external
static must be added to appropriate component static
pressure drop (evaporator coil, filters, optional
economizer, optional exhaust fan, optional heating system, optional cooling only
extended casing, optional roof curb).
Supply Fan Motor Sizing
The supply fan motor selected in the cooling capacity determination was 15.3 bhp and 924 rpm. Thus, a 20 hp supply fan
motor is selected. Enter Table 58-1 to select the
proper drive. For a 50-ton rooftop with 20 hp motor, a
drive number 9 — 900 rpm is selected.
Exhaust Fan Motor Sizing
The exhaust fan is selected based on total return system negative static pressure and exhaust fan cfm. Return system
negative static include return duct static and roof
curb static pressure drop.
Return duct static pressure = 0.65 inches
Trane roof curb (Table 57-1) = 0.12 inches
Total return system negative static pressure = 0.77 inches
Exhaust fan cfm = 12,000 cfm
From Table 59-1, the required bhp is 3.45 hp at 574 rpm. Thus, the exhaust fan motor selected is 5 hp.
To select a drive, enter Table 60-1 for a 5 hp motor for a 50 ton unit. Drive selection number 6 — 600 rpm.
Where altitudes are significantly above sea level, use Tables 21-2 and 21-3 and Figure 21-1 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 section of this
catalog.
Altitude Corrections
The rooftop performance tables and curves of this catalog are based on standard air (.075 lbs/ft). If the rooftop airflow
requirements are at other than standard conditions (sea
level), an air density correction is needed to project
accurate unit performance.
Figure 21-1 shows the air density ratio at various temperatures and elevations. Trane rooftops are designed to operate between
40 and 90 degrees Fahrenheit leaving air temperature.
The procedure to use when selecting a supply or exhaust fan on a rooftop for elevations and temperatures other than standard
is as follows:
1
First, determine the air density ratio
using Figure 21-1.
2
Divide the static pressure at the nonstandard condition by the air density ratio
to obtain the corrected static pressure.
3
Use the actual cfm and the corrected static pressure to determine the fan rpm and bhp from the rooftop performance tables or
curves.
4
The fan rpm is correct as selected.
5
Bhp must be multiplied by the air density ratio to obtain the actual operating
bhp.
In order to better illustrate this procedure, the following example is used:
Consider a 60-ton rooftop unit that is to deliver 18,000 actual cfm at 3-inches total static pressure (tsp), 55 F leaving air
temperature, at an elevation of 5,000 ft.
1
From Figure 21-1, the air density ratio is 0.86.
2
Tsp = 3.0-inches / 0.86 = 3.49 inches tsp.
3
From the performance tables: a 60-ton rooftop (without inlet vanes) will deliver 18,000 cfm at 3.49-inches tsp at 906 rpm and
21.25 bhp.
4
The rpm is correct as selected -906 rpm.
5
Bhp = 21.25 x 0.86 = 18.3 bhp actual.
Compressor MBh, SHR, and kw should be calculated at standard and then converted to actual using the correction factors
in Table 21-2. Apply these factors to the capacities
selected at standard cfm so as to correct for the
reduced mass flow rate across the condenser.
Heat selections other than gas heat will not be affected by altitude. Nominal gas capacity (output) should be multiplied by the
factors given in Table 21-3 before calculating the
heating supply air temperature. |
