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Blowers






Blowers may be divided into two types: 1) rotary, and 2) centrifugal. A common type of rotary blower is the Roots two-lobe blower. Two double-lobe impellers mounted on parallel shafts connected by gears rotate in opposite directions and at the same speed. The impellers are machined to afford only a small clearance between them and between the casing band impellers. As the lobes revolve, air is drawn into the space between the impellers and the casing, where it is trapped, pushed toward the discharge, and expelled. The air is trapped and discharged in volumes equal to the space between the impellers and casing, and the operation is repeated four times for each rotation of the shaft.

In order to change the volume rate of flow or volume Capacity of the blower, the blower speed is changed. The pressure developed by the blower will be whatever is necessary to force the air through the piping system. The volume of air delivered by the blower will not change appreciably with variations in resistance to flow. Thus, the blower is called a positive-displacement blower.

Note that at a speed of 600 rpm an increase in pressure from 2 to 3 psi increases the power required by 1.5 times, but the capacity remains fairly constant. Care should be taken in operating any positive-displacement blower. A safety


valve or limit valve should be placed on the discharge line to prevent the discharge pressure becoming excessive in case the outlet is fully closed. The limit valve will prevent overloading the discharge line and the driving motor. The advantages of the rotary blower are: 1) simple construction, 2) positive air movement, 3) economy of operation and low maintenance.

Centrifugal blowers and compressors operate on the same principles as centrifugal pumps and resemble to a marked degree the closed-impeller centrifugal pumps. A single- stage single-suction blower is capable of delivering 15, 000 cfm against a pressure of 3 psi. The casing or housing is constructed of heavy steel plate, and the impeller is an aluminum-alloy casting. If care is taken in providing the proper drive motor, the overload characteristics of the centrifugal blower will cause no trouble.

For volumes greater than those that can be handled by the single-stage single-suction blower, a single-stage double-suction blower is used. This blower is capable of supplying 26, 000 cfm of air at 60° F and atmospheric pressure against a 54-in. water column or 2 psi.

 


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