The efficiency of the heaters used in connection with forced blast varies greatly, depending upon the temperature of the entering air, its velocity between the pipes, the temperature to which it is raised and the steam pressure carried in the heater. The general method in which the heater is made up is also an important factor.

In designing a heater of this kind, care must be taken that the free area between the pipes is not contracted to such an extent that an excessive velocity will be required to pass the given quantity of air through it. In ordinary work it is customary to assume a velocity of 800 to 1000 feet per minute; higher velocities call for a greater pressure on the fan which is not desirable in ventilating work.

In the heaters shown, about .4 of the total area is free for the passage of air; that is, a heater 5 feet wide and 6 feet high would have a total area of 5 X 6 = 30 square feet, and a free area between the pipes of 30 X .4 = 12 square feet. The depth or number of rows of pipe does not effect the free area although the friction is increased and additional work is thrown upon the fan. The efficiency in any given heater will be increased by increasing the velocity of the air through it, but the final temperature will be diminished, that is, a larger quantity of air will be heated to a lower temperature in the second case and while the total heat given off is greater, the air quantity increases more rapidly than the heat quantity which causes a drop in temperature.

Increasing the number of rows of pipe in a heater with a constant air quantity increases the final temperature of the air but diminishes the efficiency of the heater, because the average differ ence in temperature between the air and steam is less. Increasing the steam pressure in the heater (and consequently its temperature) increases both the final temperature of the air and the efficiency of the heater. Table I has been prepared from different tests and may be used as a guide in computing probable results under ordinary working conditions. In this table it is assumed that the air enters the heater at a temperature of 10 degrees below zero and passes between the pipes with a velocity of 800 feet per minute. Column 1 gives the number of rows of pipe in the heater and columns 2, 3 and 4 the final temperature of the air for different steam pressures. Columns 5, 6 and 7 give the corresponding efficiency of the heater.

For example. Air passing through a heater 10 pipes deep and carrying 20 pounds pressure will be raised to a temperature of 90 degrees and the heater will have an efficiency of 1650 B.T.U. per square foot of surface per hour. When the air is taken in at zero we may add 10 to the final temperatures given in the table, although theoretically it would be slightly less; in this case we must take the efficiency corresponding to the final temperature after the 10 degress have been added.

 TABLE I.Temp. of entering air 10° below zero.Velocity of air between the pipes 800 feet per minute. Rows of pipe deep. Temp. to which the air will be raised from 10° below 0. Efficiency of the heating surface in B. T. U., per square foot per hour. Steam Pressure in Heater. Steam Pressure in Heater. 5 lbs. 20 lbs. 60 lbs. 5 lbs. 20 lbs. 60 lbs. 4 30 35 45 1600 1800 2000 6 50 55 65 1600 1800 2000 8 65 70 85 1500 1650 1850 10 80 . 90 105 1500 1650 1850 12 95 105 125 1500 1650 1850 14 105 120 140 1400 1500 1700 16 120 130 150 1400 1500 1700 18 130 140 160 1300 1400 1600 20 140 150 170 1300 1400 1600

For a velocity of 1000 feet, multiply the temperatures given in the table by .95 and the efficiencies by 1.13.

Example. How many square feet of radiation will be required to raise 600,000 cubic feet of air per hour from 10 below zero to 80 degrees, with a velocity through the heater of 800 feet per minute and a steam pressure of 5 pounds? What must be the total area of the beater front and how many rows of pipes must it have?

Referring back to our formula for heat required for ventilation, we have

600,000 X 90 = 981,818 B. T. U. required. 55

Referring to table I we find that for the above conditions a heater 10 pipes deep is required, and that an efficiency of 1500

B. T. U. will be obtained. Then 981,818 = 654 square feet of surface required, 600,000 = 10,000 cubic of air per minute, and

10,800 = 12.5 square feet of free area required through the heater. If we assume .4 of the total heater front to be free for

12.5 the passage of air, then -- = 31 the required total area.

.4

For convenience in estimating the approximate dimensions of a heater, the following table is given. The standard heaters made by different manufacturers vary somewhat, but the dimensions given below represent average practice. Column 3 gives the square feet of heating surface in a single row of pipes of the dimensions given in columns 1 and 2, and column 4 gives the free area between the pipes.

TYPICAL HEATING INSTALLATION SHOWING SECTIONAL BOILER AND RADIATOR.