Trane RT-PRC010-EN manual Selection Procedure, From Figure PAF-1,the air density ratio is

Selection Procedure

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 PAF-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 inTable PAF-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 inTable PAF-3 before calculating the heating supply air temperature.

HEATING CAPACITY SELECTION

Step 1 — Determine AirTemperature Entering Heating Module

Mixed air temperature = RADB + % OA (OADB - RADB) = 70 + (0.10) (0 - 70) = 63

F

Supply air fan motor heat temperature rise = 46,000 Btu ÷ (1.085 x 17,500 cfm) = 2.42 F

Air temperature entering heating module = 63.0 + 2.42 = 65.4 F

RT-PRC010-EN 19