Experiments on a Fan with Varying Discharge-opening.

Revolutions nearly constant.

The fan wheel was 23 inches in diameter, 65% inches wide at its periphery, and had an inlet of 12% inches in diameter on either side, which was partially obstructed by the pulleys, which were 5 9/16 inches in diameter. It had eight blades, each of an area of 45.49 square inches,

The discharge of air was through a conical tin tube with sides tapered at an angle of 3% degrees. The actual area of opening was 7% greater than given in the tables, to compensate for the vena contracta.

In the last experiment, 89.5 sq. in. represents the actual area of the mouth of the blower less a deduction for a narrow strip of wood placed across it for the purpose of holding the pressure-gauge. In calculating the volume of air discharged in the last experiment the value of vena contracta is taken at.80.

Experiments were undertaken for the purpose of showing the results obtained by running the same fan at different speeds with the discharge-opening the same throughout the series.

The discharge-pipe was a conical tube 81⁄2 inches inside diameter at the end, having an area of 56.74, which is 7% larger than 53 sq. inches; therefore 53 square inches, equal to .368 square feet, is called the area of discharge, as that is the practical area by which the volume of air is computed.

Experiments on a Fan with Constant Discharge-opening and Varying Speed.-The first four columns are given by Mr. Snell, the others are calculated by the author.

Mr. Snell has not found any practical difference between the efficiencies of blowers with curved blades and those with straight radial ones. From these experiments, says Mr. Snell, it appears that we may expect to receive back 65% to 75% of the power expended, and no more.

The great amount of power often used to run a fan is not due to the fan itself, but to the method of selecting, erecting, and piping it.

(For opinions on the relative merits of fans and positive rotary blowers. see discussion of Mr. Snell's paper. Trans. A. S. M. E., ix. 66, etc.)

Comparative Efficiency of Fans and Positive Blowers.~ (H. M. Howe, Trans. A. I. M. E., x. 482.)-Experiments with fans and positive (Baker) blowers working at moderately low pressures, under 20 ounces, show that they work more efficiently at a given pressure when delivering large volumes (i.e., when working nearly up to their maximum capacity) than when delivering comparatively small volumes. Therefore, when great vari ations in the quantity and pressure of blast required are liable to arise, the highest efficiency would be obtained by having a number of blowers, always driving them up to their full capacity, and regulating the amount of blast by altering the number of blowers at work, instead of having one or two very large blowers and regulating the amount of blast by the speed of the blowers.

There appears to be little difference between the efficiency of fans and of Baker blowers when each works under favorable conditions as regards quantity of work, and when each is in good order.

For a given speed of fan, any diminution in the size of the blast-orifice de creases the consumption of power and at the same time raises the pressure of the blast; but it increases the consumption of power per unit of orifice for a given pressure of blast. When the orifice has been reduced to the normal size for any given fan, further diminishing it causes but slight elevation of the blast pressure; and, when the orifice becomes comparatively small, further diminishing it causes no sensible elevation of the blast pressure, which remains practically constant, even when the orifice is entirely closed.

Many of the failures of fans have been due to too low speed, to too small pulleys, to improper fastening of belts, or to the belts being too nearly ver tical; in brief, to bad mechanical arrangement, rather than to inherent de fects in the principles of the machine.

If several fans are used, it is probably essential to high efficiency to provide a separate blast-pipe for each (at least if the fans are of different size or speed), while any number of positive blowers may deliver into the same pipe without lowering their efficiency.

Capacity of Fans and Blowers.

The following tables show the guaranteed air-supply and air-removal of leading forms of blowers and exhaust fans. The figures given are often exceeded in practice, especially when the blowers and fans are driven at higher speeds than stated. The ratings, particularly of the blowers, are below those generally given in catalogues, but it was the desire to present only conservative and assured practice. (A. R. Wolff on Ventilation.) QUANTITY OF AIR SUPPLIED TO BUILDINGS BY BLOWERS OF VARIOUS SIZES.

If the resistance exceeds the pressure of one ounce per square inch, of above table, the capacity of the blower will be correspondingly decreased, or power increased, and allowance for this must be made when the distributing ducts are small, of excessive length, and contain many contractions and bends.

QUANTITY OF AIR MOVED BY AN APPROVED FORM OF EXHAUST FAN, THE

FAN DISCHARGING DIRECTLY FROM ROOM INTO THE ATMOSPHERE.

The capacity of exhaust fans here stated, and the horse-power to drive them, are for free exhaust from room into atmosphere. The capacity decreases and the horse-power increases materially as the resistance, resulting from lengths, smallness and bends of ducts, enters as a factor. The difference in pressures in the two tables is the main cause of variation in the respective records. The fan referred to in the second table could not be used with as high a resistance as one ounce per square inch, the rated resistance of the blowers.

Caution in Regard to Use of Fan and Blower Tables.Many engineers report that manufacturers' tables overrate the capacity of their fans and underestimate the horse-power required to drive them. In some cases the complaints may be due to restricted air outlets, long and crooked pipes, slipping of belts, too small engines, etc.

CENTRIFUGAL FANS.

Flow of Air through an Orifice.

VELOCITY, VOLUME, AND H.P. REQUIRED WHEN AIR UNDER GIVEN PRESSURE IN OUNCES PER SQ. IN. IS ALLOWED TO ESCAPE INTO THE ATMOSPHERE. (B. F. Sturtevant Co.)

The headings of the 2d and 3d columns in the above table have been abridged from the original, which read as follows: Velocity of dry air, 50° F., escaping into the atmosphere through any shaped orifice in any pipe or reservoir in which the given pressure is maintained. Volume of air in cubic feet which may be discharged in one minute through an orifice having an effective area of discharge of one square inch. The 5th column, not in the original, has been calculated by the author. The figures represent the horse-power theoretically required to move 1000 cu. ft. of air of the given pressures through an orifice, without allowance for the work of compression or for friction or other losses of the fan. These losses may amount to from 60% to 100% of the given horse-power.

The change in density which results from a change in pressure has been taken into account in the calculations of the table. The volume of air at a given velocity discharged through an orifice depends upon its shape, and is always less than that measured by its full area. For a given effective area the volume is proportional to the velocity. The power required to move air through an orifice is measured by the product of the velocity and the total resisting pressure. This power for a given orifice varies as the cube of the velocity. For a given volume it varies as the square of the velocity. In the movement of air by means of a fan there are unavoidable resistances which, in proportion to their amount, increase the actual power considerably above the amount here given.

For any size of centrifugal fan there exists a certain maximum area over which a given pressure may be maintained, dependent upon and proportional to the speed at which it is operated. If this area, known as its "capacity area, or square inches of blast, be increased, the pressure is lowered (the volume being increased), but if decreased the pressure remains constant. The revolutions of a given fan necessary to maintain a given pressure under these conditions are given in the table on p. 519, which is based upon the abve table. The pressure produced by a given fan and its effective capacity area being known, its nominal capacity and the horsepower required, without allowance for frictional losses, may be determined from the table above.

In practice the outlet of a fan greatly exceeds the capacity area; hence the volume moved and the horse-power required are in excess of the amounts determined as above.

Steel-plate Full Housing Fans. (Buffalo Forge Co.) Capacities in cubic feet of air per minute. (See also table on p. 525.)

Size,

in.

Revolutions per Minute.

100 150 200 250 300 350 400 450 500 550 600

50

60

70

1650 2475 3300 4125 4950 5775 6600, 7425 8250 9075 9900 2480 3720 4960 6200 7440 8680 9920 11160 12400 13640 14880 4500 6750 9000 11250 13500 15750 18000 20250 22500

The above table relates to common cupolas under ordinary conditions and to forges of medium size. The diameter of cupola given opposite each size blower is the greatest which is recommended; in cases where there is a surplus of power one size larger blower may be used to advantage. The melting capacity per hour is based upon an average of tests on some of the best cupolas found, and is reliable in cases where the cupola is well constructed and carefully operated. The blast-pressure required in wind-box is the maximum under ordinary conditions when coal is used as fuel. When coke is employed the pressure may be lower.

The cupola pressures given are those in the wind-box, while the basis pressure for forges is 4 ounces in the tuyere pipe. The corresponding revolutions of fan given are in each case sufficient to maintain these pressures at the fan outlet when the temperature is 50°. The actual speed must be higher than this by an amount proportional to the resistance of pipes and the increase of temperature, and can only be determined by a knowledge of the existing conditions.

(For other data concerning Cupolas see Foundry Practice.)