Fans and compressors

The operation of a motor at a rated load may be for an unlimited period to reach thermal equilibrium (Figure 3.1) and possible applictilions are pumps, blowers, fans and compressors. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Rotating machines subject to imbalance caused by turbulent or unbalanced media flow include pumps, fans, and compressors. A good machine design for these units incorporates the dynamic forces of the gas or liquid in stabilizing the rotating element. The combination of these forces and the stiffness of the rotor-support system (i.e., bearing and bearing pedestals) determine the vibration level. Rotor-support stiffness is important... 

Application ranges of fans and compressors are indicated on Figures 7.20 and 7.21. Some of these categories of equipment now will be discussed in some detail. 

The theory of dimensionless groups of Section 7.2, Basic Relations, also applies to fans and compressors with rotating elements, for example, Eqs. (7.8)-(7.10) which relate flow rate, head, power, speed, density, and diameter. Equivalent information is embodied in Table 7.5. The concept of specific speed, Eqs. (7.11) and (7.12), also is pertinent. In Figures 7.21 and 7.25 it is the basis for identifying suitable operating ranges of various types of compressors. 

Fans and compressors are gas movers, analogous to pumps as liquid movers. The term "gas mover" will be used when discussing the characteristics of faiK and compressors collectively. The geometry of gas movers is somewhat similar to that of pumps however, operating parameters and safety considerations are quite different. Fans and compressors must handle gases that are compressible and extremely sensitive to temperature and pressure changes, while pumps handle liquids that are relatively insensitive to pressure and temperature changes and can be considered incompressible. 

Mufflers, also called silencers or attenuators by their manufacturers, are located in pipes, ducts, and passageways used to convey gas, liquid, or solid materials to or from a machine, process, or area for the purpose of reducing the propagation of noise from the source to the receiver. Mufflers are commonly used at the intake and/or exhaust of engines, fans, and compressors and at the outlets of hi -pressure gas vents. 

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