Volumetric efficiency of a reciprocating compressor

Volumetric efficiency (VE) of a reciprocating compressor is calculated by following equation by accounting 4% loss at suction and discharge valves  

Cp specific heat at constant pressure, btu/lb-°F or btu/lb mole- F. Cv specific heat at constant volume, btu/lb- F or btu/lb moIe- F. 

N compressor rotating speed, rpm. n polytropic exponent, dimensionless. 

Subscripts avg average, d compressor discharge, p polytropic process, s isentropic process or compressor suctioa t total. 

Jandjel, Selea the right compressor, Chemical Qigineering Progress, July, 2000, p.15 29. 

---

In the plant, we use the term unloading to indicate various ways of reducing the volumetric efficiency of a reciprocating compressor. There are two sorts of unloaders ... 

Volumetric efficiency of a reciprocating compressor is calculated using Eq. (17) with corrections mcntioiiied in section V.4. It represents the actual gas compression capacity to the piston dii lacemenL It is a function of comptession ratio, clearance, compressibility factor at suction and discharge, and isentropic exponent... 

This means that a substantial volume of gas remains trapped between the cylinder head and the piston before the travel of the piston is reversed. This volume of gas is called the starting volumetric clearance. It determines the volumetric efficiency of the reciprocating compressor. We will discuss this later. 

Because there is no clearance volume expansion, as in the reciprocating compressor, the volumetric efficiency is a function of the rotor slip. This is the internal leakage from the higher pressure to the lower pressure side, reducing potential volume capacity of the compressor. 

The best way to unload a reciprocating compressor is to increase the cylinder clearance. When the piston completes the end of its stroke, the space between the piston face and the cylinder head is called the clearance. Increasing the clearance reduces the scfd flow of gas to a compressor running at a constant speed and a fixed suction pressure. The effect of adjusting the cylinder clearance is proportional to the suction stroke volumetric efficiency (Evs). Evs is calculated as follows ... 

Volumetric efficiency A measure of the volume of gas moved per stroke in a reciprocating compressor. 

---