Comparing air compressors: Which one is best for your plant?

Knowing operating characteristics and physical limitations will help you select the best machine for your plant.

By Niff Ambrosino

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Most industrial air compressors are supplied as self-contained packages that include drive motor, inlet filter, mechanical and electrical controls and various optional accessories. Depending on the type and size of air compressor, the package might be mounted on an air receiver. Air compressors are classified either as positive-displacement or dynamic.

Positive-displacement compressors

Air is drawn into an enclosed chamber where the volume is reduced by mechanical means, causing the pressure within to rise and forcing the air into the system. A simple example of a positive-displacement compressor is the hand pump for inflating tires. The operating principle for positive-displacement compressors is either reciprocating or rotary.

Reciprocating type

A reciprocating compressor uses a moving piston in an enclosed cylinder. In a single-acting design, compression takes place on just one side of the piston and produces air on only one stroke per revolution. Double-acting compressors develop compression on both sides of the piston and produce air on two strokes per crankshaft revolution. This results in almost twice the capacity of a single-acting design of identical bore and stroke. In either case, the compressor might be air or water-cooled, lubricated or non-lubricated.

Single-stage reciprocating compressors have one or more cylinders connected in parallel to compress air from atmospheric pressure to the final discharge pressure in one step. Most single-stage compressors are designed for a maximum discharge pressure of 100 psig.

Multi-stage reciprocating compressors, on the other hand, have two or more cylinders connected in series. Each stage adds some degree of compression. For example, in a two-stage unit, air is compressed from atmospheric pressure to an intermediate pressure in the first stage, cooled by an intercooler and raised to the discharge pressure in the second cylinder.

Multi-stage reciprocating compressors are more efficient, run cooler and have longer life than single- stage compressors, all because the intercooler(s) remove the heat of compression. While not typical for plant air, some special-application, two-stage compressors can deliver 250 psig or more.

Single-acting reciprocating compressors are commonly air-cooled, have one or two stages and available to 150 hp. However, in most industrial applications, the maximum size is generally 30 hp. A measure of operating efficiency is called specific power and is the kW input to produce 100 cfm, or kW/100 cfm. For a single-stage, single-acting compressor, the specific power is approximately 24 kW/100 cfm at 100 psig. Typical specific power for a two-stage, single-acting compressor at 100 psig is 19 kW/100 cfm to 21 kW/100 cfm.

Double-acting compressors are generally water-cooled and range in size from 25 hp (single-stage) to 500 hp. Common sizes for two-stage industrial applications range between 75 hp and 250 hp. A two-stage double-acting reciprocating compressor is the most energy-efficient air compressor. Typical specific power at 100 psig is approximately 15 kW/100 cfm to 16 kW/100 cfm. Double-acting compressors have a higher initial price, more expensive installation and higher maintenance costs than other types of compressors.

Rotary compressors

The lubricated rotary-screw air compressor is the most widely used design for industrial applications. It’s characterized by low vibration, a simple installation with minimal maintenance in broad ranges of capacity and pressure. A rotary-screw air end consists of two close-clearance helical rotors turning in synchronous mesh. The male rotor has four helical lobes and the female rotor has five or six mating grooves. In a lubricated rotary-screw compressor, the male rotor drives the female rotor.

Ambient air is drawn through a suction port into a space between the spinning rotors, which then force the air into the decreasing inter-lobe cavity until it reaches the discharge port at the opposite end of the rotor. Oil injected into the rotor housing lubricates the moving parts, removes heat and seals the clearances to prevent back slippage of the compressed air. The air/lubricant mixture discharges into the lubricant reservoir, which also serves as a separator that relies on directional and velocity changes. A coalescing-type filter reduces the final lubricant-to-air concentration to 3 ppm to 5 ppm.

Operating at too low a system pressure (65 psig to 75 psig) increases the velocity across the separator, which leads to greater lubricant carryover. To prevent carryover at startup and when system pressure is too low, a minimum-pressure device is used to maintain internal compressor pressure above the manufacturer’s minimum.

The lubricant separated from the air stream circulates through a cooler and filter before being injected back into the air end. The temperature of the lubricant at the injection port needs to be high enough to prevent condensation from forming in the lubricant. Air-cooled units use a thermostatic valve to maintain an injection temperature of 150°F to 170°F. Water-cooled packages use a water-flow-regulating valve or thermostatic valve, or both.

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