Operating Characteristics of Reciprocating Air Compressors

Quincy QRDT-10A 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.

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

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.

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