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UH-PRC001-EN12
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Procedure
Formula
The following formula is used to arrive at
final air temperature volume when 70 F
(Standard Air Basis) is known or vice
versa:
Cfm at final temp. =
Cfm at 70 F x
460 + Final temp.
460 + 70
Cfm at 70 F =
Cfm final air x
460 + 70
460 + Final temp.
Determining Special Steam Capacities
Where capacity of the unit under
standard conditions is known – 2 lbs.
steam, 60 F entering air – and it is
desired to know the capacity of this
same unit under different steam and air
conditions, follow instructions given in
Example 1 below.
Where a set of conditions is given – Btu,
temperature rise, final temperature, cfm,
steam pressure available, etc. Refer to
Example 2.
Useful Data
Btu
240
= Sq. ft. of radiation (EDR)
Sq. ft. of radiation =Lbs. of condensate
4 per hour
Btu
Latent heat
= Lbs. of water per hour
Cfm x Temp. rise x 1.085 = Btu.
Specific heat of air (70 F) = .241
Specific weight of one cubic foot of air
(70 F) =.075.
Cfm x 60 = cfh.
Therefore, .241 x .075 x 60 = 1.085
Example 1
Given: Model 60S Horizontal Propeller
Unit Heater with a rated capacity of
60,500 Btu at 2 lbs. steam with 60 F
entering air.
Desired: Capacity of this unit using
15 lbs. steam and 40 F entering air.
Under 40 F and across from 15 lbs. in
SP-2, page 14, we find the factor 1.34.
60,500 x 1.34 = 81,070 Btu per hour.
Equivalent Direct Radiation
If working in EDR, rather than Btu,
multiply the EDR by the same factor in
the same way. Thus:
252 (From Table 3) x 1.34 =
338 sq. ft. or
81,070
240
Determining Amount of Condensate
To determine the amount of condensate,
divide the capacity in BTU by the latent
heat of steam:
81,070
= 86 lbs per hour.
945
945 = Latent Heat of Steam at 15 psi.
(See Table SP-9, page 18.)
Determining Final Temperature
Final temperature of air leaving Trane
Unit Heaters may be determined as
follows:
Inlet Air Temp. + Btu rating = FinalTemp
Cfm at 70F x 1.085
Inlet Air Temp. 40F + 81,070 = FinalTemp
815(from table 3) x 1.085
Example 2:
Problem: A Trane Model S Horizontal
Unit Heater is desired to deliver 280,000
Btu per hour in a garage where
ventilation is required. Entering air
temperature is 40 F. Steam at 30 lbs.
pressure is available. Temperature to be
maintained is 70 F.
Solution: Under 40 F and across from 30
lbs. in Table SP-2 page 14, find the factor
1.51.
Equivalent capacity of required unit at
2 lbs. steam 60 F entering air is
Required Btu per hour = 280,00 =185,400
F 1.51 Btu
From Table SP-3, page 15, select a
Number 186S Unit Heater with a
capacity of 185,200 Btu per hour with
standard coil, and at 2 lbs. steam and 60
F entering air.
The capacity of this unit at 30 lbs. steam
and 40 F entering air is
1.51 x 185,200 = 279,650 Btu per hour.
Surplus Capacity
While the ventilation load is being
handled, any air entering the space over
the desired room temperature should be
considered in the heating calculations.
This surplus capacity can be found by
multiplying the difference between the
final temperature leaving the heater and
the room temperature to be maintained
by 1.085 and that by the cfm. That
amount of heat can be used to offset
heat losses.