Trane UH-PRC001-EN Formula, Determining Special Steam Capacities, Useful Data, Surplus Capacity

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 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 (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 SP-9, page 18.)

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 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

FromTable 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.

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