Selection 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 = |
| 460 + 70 | |
Cfm final air x | |||
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
Btu240 = Sq. ft. of radiation (EDR)
Sq. ft. of radiation =Lbs. of condensate
4 | per hour |
Btu | = Lbs. of water per hour |
Latent heat |
Cfm xTemp. 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
Equivalent Direct Radiation
If working in EDR, rather than Btu, multiply the EDR by the same factor in the same way.Thus:
252 (FromTable 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.
(SeeTable
Determining FinalTemperature
Final temperature of air leavingTrane Unit Heaters may be determined as follows:
Inlet AirTemp. + | Btu rating | = FinalTemp |
Cfm at 70F x 1.085 | ||
Inlet AirTemp. 40F + | 81,070 | = FinalTemp |
815(from table 3) x 1.085
Example 2:
Problem: ATrane 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. inTable
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 |
FromTable
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.
12 |