115. The general construction of a centrifugal fan is shown in Figs. 43 and 44. It consists of a wheel a, which revolves swiftly within a circular casing c, which has a number of internal vanes, or blades b, b'. Air enters the casing at the central opening d and is expelled through the nozzle e. The only effect that can be produced by a fan is a greater pressure of air at the outlet than at the inlet. In consequence of the partial removal of the air from the central to the outer part of the fan, the pressure of the air at the center diminishes accordingly. If the inlet be open, the atmosphere will rush in and restore the pressure to the normal.

Fig. 43.

Fig. 44.

The distinction commonly made between exhaust fans and pressure fans depends wholly upon the manner in which they are employed. When the air duct is connected to the inlet of the fan, the pressure of the air which passes through it will always be below that of the atmosphere. The fan is then called an exhaust fan. On the contrary, when the air is forced through a duct which is attached to the outlet of the fan, the pressure of the air will always be above that of the atmosphere, and the fan is then called a pressure fan.

The construction of the fan is substantially the same in both cases, and the mode of operation is always the same.

116. The pressure of the blast depends chiefly upon the velocity of the wheel, measured at the rim.

The volume of the blast depends upon the amount of air that can be induced to flow into the wheel through the inlet.

The size of the inlet opening must be proportioned to suit the volume and velocity of the inflowing air; it bears no relation to the diameter of the wheel.

The diameter of the wheel should always be made large enough to secure the desired velocity at the rim with a moderate rotative speed. There is much less waste of power due to the eddying of the air between the blades in a large wheel than in a small one. Therefore, a given volume and pressure of blast may be produced much more economically by a large wheel rotating at a moderate speed than by a small wheel rotating at a higher speed. The small wheel must rotate faster to secure the same velocity of rim and thus generate the required pressure.

The wear and tear on belting, countershafts, and bearings will be greatest with the small wheel. The small wheel will also make more noise than a large one doing the same work.

The power required to drive a fan, in addition to that consumed in friction and internal waste, will be equal to the volume of air delivered from the nozzle, in cubic feet per minute, multiplied by the difference between the pressures at the inlet and the delivery in pounds per square foot.

For all moderate or low pressures, the blades of the wheel should be radial, and their length should not exceed one-fourth of the diameter of the wheel, and they may in many instances be made much shorter. The sides of the wheel should be enclosed, as shown, to prevent the air from escaping past the edges of the blades and thus forming useless eddies. The side plates p may be made somewhat conical, mainly for the purpose of stiffening the wheel. The casing should afford plenty of space into which the air may escape from the rim of the wheel, and it should not approach the sides of the wheel too closely, except at the rim of the inlet opening. The area of the cross-section at any point should not exceed the area of the discharge nozzle.

The outlet nozzle may be directed horizontally or at any desired angle, and care should always be taken in purchasing fans to have the outlet turned in a proper direction, so as to avoid all unnecessary turns or elbows in the connections.

The outlet nozzle should always extend at a tangent to the case, and should never extend radially.

When small fans are driven by horizontal belting, they should be mounted on a sliding bed so that the belts may be tightened by shifting the fan slightly.

Two fans should not deliver into the same conduit, unless one is to be used only as a reserve for the other. If the system is too large for one fan, it should be divided into smaller but independent systems.

Of course, two or more fans of equal power may be coupled, if they are driven by the same countershaft, or are otherwise provided with means for maintaining an equality of speed and pressure.

117. The propelling wheel, or disk fan, Fig.

45, is composed of a number of vanes b, which are attached to a shaft c and are set at an angle to the shaft as shown. They are rotated rapidly within the casing a, and while they whirl the passing air, the centrifugal force thus generated is largely wasted and the principal effect is exerted by the inclined faces of the vanes which press the air forwards.

Fig. 45.

This variety of fan applies power to very poor advantage, and is the most wasteful and inefficient form in use. It should never be employed for moving the main air-current in heating or ventilating operations.