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plying the square of the diameter, or the square inches of blast area, as the case may require, by the accompanying factors, in which W is the width of blade and D the diameter of wheel.
Selection of Fans. While the capacity of an encased fan varies directly with its speed, the power required varies as the cube of the speed; hence, as a rule, it will be found preferable to use a comparatively large fan and run it at a moderate speed.
This is particularly true where the motive power is electricity, gas, or water, but, in the case of an engine-driven fan, does not amount to much when the exhaust steam is utilized in heating the air. There are, however, other reasons why a large fan is preferable, particularly in public buildings and schools, as a lower speed may be maintained to move the same volume of air, thus reducing the vibration to a minimum. In mill work, where the hum of other machinery makes the vibration of the fan inaudible, it is customary to use comparatively small fans, producing high velocities. Should it be found preferable to use a type different from the ordinary encased fan, there are two alternatives: the disk, or propeller, fan and the cone fan. The former may be relied on under certain conditions, but when used in connection with a ventilating system, blowing air through pipes and over heating coils, it is not able to deliver air against any appreciable resistance. In selecting centrifugal fans, small wheels operated at high speed should be avoided. Large wheels waste less power and have the advantage of possessing a large reserve capacity, that may be brought into use by increasing the speed.
Efficiency of Fans.-The efficiency of a fan is obtained by dividing the power supplied by that usefully employed in moving the air. The efficiency varies with the pressure. against which the fan works, the highest efficiency commonly being found under a back pressure of about 1 oz. Under ordinary working conditions an efficiency of 50 per cent. is as high as can be expected. The extreme limit of possible efficiency is about 75 per cent.
Condensation in Heaters.-The amount of steam condensed in hot-blast heaters varies with the velocity of the air and the number of coils over which the air passes. The deeper the heater is, the higher will be the temperature of the air and the less the condensation per square foot; also, the higher the velocity of the air passing through the heater, the greater will be the amount of condensation.
Under the conditions usually obtaining in practice, with an outside temperature of 20°, it would be necessary to make provision for about 14 lb. of water of condensation per sq. ft. of surface per hr.
It has been found, in practice, that as condensation takes place more rapidly on the first section of the heater, there is a tendency toward such a reduction of pressure therein as to cause the water of condensation to be held back in the first section by the greater steam pressure in the others. To obviate this difficulty, it has frequently been found necessary to increase the area of the steam inlet openings to the first section, or, in some cases, to make the first section entirely independent of the others.
Value of Heating Surface. The first section of a hot-blast heater does about 40 per cent. of the work; the second, about 25 per cent.; the third, about 15; the fourth, about 10; the fifth, about 6; and the sixth section, about 4 per cent. Beyond this, the increase in temperature is slight. The average value of the heating surface for a velocity of 600 feet through coils, when one section is used, is about 8 B. T. U. per degree difference between the steam in the pipes and the incoming air, dropping to 7.3 B. T. U. with the addition of a second section; 6.6 B.T.U., for the addition of a third; 6 B.T.U., for the addition of a fourth; and 5.5 B. T. U., for the addition of a fifth section; after which the heat emitted remains constant at about the last-named amount. Thus, with a heater having four sections, with an air velocity of 600 ft. through the heater, with steam at 215°, and air at 10°, a difference of 205°, we would have 205 X 6 = 1,230 B. T. U. per hr. per sq. ft. of pipe surface.
Heaters made up of pipes properly arranged will give, roughly, 1,500 to 2,500 B. T. U. per hr. per sq. ft. of surface during zero weather, according to the steam pressure, the arrangement of the pipes, and the velocity of the air flowing between them.
Number of Rows of Pipes.
The approximate amount of heat transmitted, in B. T. U., per square foot of heating surface per degree difference in temperature between that of the steam and air entering the hot-blast heater is given in the following table:
HEAT TRANSMISSION IN HOT-BLAST HEATER.
Velocity of Air in Feet per Minute.
600 720 960 1,200 1,800 2,400 3,600
Heat Emitted per Square Foot of Surface per
Heaters made up of ordinary cast-iron indirect pin-type radiators will not give off much more than half the heat per square foot of extended or rated surface that is given off per square foot of prime surface in regular 1" pipe heaters; therefore, fan radiators for ordinary work should be rated to give off 1,000 to 1,500 B. T. U. per sq. ft. per hr., according to the steam pressure and velocity of the air flowing between the sections.
Size of Heater Compared With Direct Radiating Surface Required. A heater used in connection with a fan that takes its air supply from inside the building should give off 1,200 to
1,500 B. T. U. per sq. ft. of heating surface per hr. This is a cheap way to heat large mills and factories.
Size of Heaters.-In any system, to maintain rooms at the standard temperature of 70°, there must be supplied an amount of heat sufficient to offset the transmission losses through the outside walls, windows, and roof, plus the accidental leakage, and outflow of air through the vent flues.
To compute heat losses and size of heaters, it is convenient, and sufficiently accurate for ordinary work, to consider 1 sq. ft. of glass surface as equivalent in heat-transmitting power to 4 sq. ft. of wall. Reduce the exposed wall and glass surface to equivalent glass surface by dividing the wall surface by 4 and add to this the actual glass surface. Compute the equivalent glass surface (E. G. S.) of each wall in this manner, then multiply by the following factors to compensate for the effect of winds: north, 1.25; south, 1.05; east, 1.15; west 1.25. Add to this the E. G. S. of roof, which is approximately equal to one-tenth of the roof area, and multiply the total E. G. S. by 85, the latter number representing the number of B. T. U. transmitted per hour per square foot of glass with a temperature of 70° inside and zero outside. The product is the number of B. T. U. transmitted per hour.
When the air supply is taken from the outside, the leakage of air from the building is commonly assumed to be equal to the air supply, the accidental leakage or ingress of air due to winds having been allowed for by the factors used in connection with the exposure.
With an inside temperature of 70° in zero weather, each cubic foot of air escaping from the building carries with it about 14 B. T. U. The volume escaping per hour, multiplied by 14, gives the heat loss by leakage. Adding the losses of heat by transmission and by leakage gives the total, which, divided by the heat emitted per square foot of heating surface, gives the size of the heater required.
It has been found, in practice, that with a steam pressure of, say, 80 ib. and air taken from outdoors, factories may be warmed successfully with heaters based on 1 lin. ft. of 1" pipe to every 125 to 150 cu. ft. of space.
If the air is used over and over, i. e., has internal circulation only, ratios of 1 to every 150 to 175 cu. ft. of space, according to the conditions, may be used. Even greater ratios are permissible with very large buildings.
Heaters will not condense more than two-thirds as much exhaust as live steam; this fact must be borne in mind.
In churches, where the heating is intermittent and the air change more frequent than in manufacturing buildings, an allowance of 1 lin. ft. of 1" pipe to every 60 to 90 cu. ft. of space represents common practice.
In schools heated entirely by the blower system, each standard 50-pupil room of about 11,000 cu. ft. of space should have at least 300 ft. of 1" pipe provided in the heater, giving a ratio of about 1 to 40. The large allowance for schoolrooms is due to the frequent air change, which must never be overlooked in proportioning heaters.
The free area between the heater pipes should be 50 per cent. greater than that of the fan outlet to insure a maximum velocity of 1,500 to 1,800 ft. per min.
When the total B. T. U. required is approximately 100 times the volume of air, in cubic feet per minute, as is frequently the case, a short method of determining the size of the heater required is to divide the volume of air per minute by 15 for low-pressure steam, or by 23 for steam at 80 lb.
For heating and ventilating schools, churches, and public buildings, an area between the pipes that will permit an air velocity of from 900 to 1,200 ft. per min. will be found satisfactory. In factory heating an air velocity of from 1,500 to 1,800 ft. per min. is not excessive.
Sizes of Steam Pipes to Supply Heaters.-Since the water of condensation from heaters is generally trapped to a receiver and not returned directly to the boiler, a considerable drop in the pressure of the steam in passing through the supply pipe may be assumed, and on that account the pipe may be made much smaller than would be necessary with a gravity return to the boiler. When the heater is within 100 ft. of the boiler, the pipe sizes are given in the table headed "Trap-Discharge Pipes for Heaters."