ROTARY-SCREW AIR COMPRESSORS – Know Your Air Requirements

As noted earlier in this chapter, when you are moving toward selecting a rotary-screw (or any compressor type), you first need to know how much air you’ll need in cubic feet per minute (CFM) at the psi you need for your plant, your tools, and all ancillary equipment for now and for the future. You’ll want to build in excess volume available, as one statistic we’ve seen says, on average, more than 10 percent of all compressor capacity is lost through leaks, despite the best efforts of the plant to reduce wasted air.

Once you got the compressor size figured out (link; I hope the information here will help), you will want to review the up-front cost of compressors from a host of manufacturers. Check their mean time, between-failure rates, their parts and service costs, the life expectancy of the unit with the duty cycle you will require, and the particular operating costs.

Compressing air is expensive, and one compressor might provide lower up-front capital costs, yet end up being far more expensive in the longer term due to higher operating costs.

All factors having been considered, and certainly this is claimed by many of the manufacturers of the rotary-screw type of compressor, the rotary-screw compressor may surface as your best choice for industrial application.

• Sliding-vane compressors, where an eccentric cam (into which sealing vanes slide) rotates inside a housing.
• Liquid-piston type, in which a partially liquid-flooded case creates the equivalent of sliding vanes.
• Diaphragm compressors, in which a flexible diaphragm is pulsed inside a concave housing.

The two types of compressors that convert velocity into pressure are:
• Radial-flow compressors, generally called “centrifugal compressors”
• Axial-flow compressors, known as “axial compressors”

In centrifugal compressors, the gas enters the eye of the impeller, and the rotative force moves the fluid to the rim of each wheel or stage. Diffusers convert the velocity head into pressure, and return passages are then used to lead the gas to the compressor discharge or to the next impeller stage.

In axial compressors, flow occurs through a series of alternating rotating and stationary blades, and in a direction basically parallel to the compressor shaft. Each passage through the rotating blades increases the velocity of the fluid, and each passage through the stationary diffuser blades converts the velocity head into a pressure head.

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