RECIPROCATING COMPRESSORS

 RECIPROCATING COMPRESSORS 

 RECIPROCATING COMPRESSORS 

 Reciprocating compressors are considered for applications where the inlet gas rate is about 3,000 cu.ft/min or less. They are favored for low-flow, high-pressure services. 

 The maximum compression ratio per stage is usually about 3:1 to 4:1. Higher compression ratios can result in reduced volumetric and mechanical efficiencies and greater mechanical stress. The outlet temperature also limits the compression ratio. The mechanical design limit is usually 350 to 400 °F, but lower temperatures may be required depending on the gas properties. In addition, the pressure rise per stage is often limited by compressor valve design considerations to 1,000 psi or less. 

  1 shows approximate application ranges for reciprocating compressors. Maximum discharge pressures range from more than 60,000 psia for small machines, to about 3,000 psia for larger units. 

 Volumetric piston displacement 

 

The volumetric efficiency is principally a function of the clearance volume within the compressor. Gas within the clearance pockets expands and contracts with each stroke, thereby reducing the net compressor throughput. Volumetric efficiency is affected to a lesser extent by slippage past piston rings and valves, and other minor losses. 

 

A reciprocating machine operating at constant speed will compress essentially the same inlet acfm, regardless of the gas composition, suction conditions or discharge pressure. This, of course, applies within the design limits of the compressor and drive. Thus, at fixed speed, the inlet acfm remain essentially unchanged, even when compressing against a variable discharge pressure. The inlet acfm is essentially proportional to rotating speed. 

 Due to fairly high maintenance requirements for reciprocating compressors, it is often desirable to either provide two half-capacity machines in parallel, or a full-capacity spare compressor. This is good practice for continuous process applications where a compressor shutdown would in -turn shut down other major equipment.