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19RT-PRC010-EN
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 in Table 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 in Table PAF-3 before
calculating the heating supply air
temperature.
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 = 46,000 Btu ÷ (1.085 x 17,500 cfm) =
2.42 F
Air temperature entering heating
module = 63.0 + 2.42 = 65.4 F