Influencing Factors of Compressor External Pressure Ratio

What is the external pressure ratio of the compressor?

External Pressure Ratio (External Pressure Ratio), also known as actual Pressure Ratio or system Pressure Ratio, is defined as the Ratio of absolute exhaust Pressure of compressor to absolute intake Pressure, namelyP_dis/P_suc. It is different from the internal pressure ratio (based on the ideal ratio of the process in the cylinder) and directly reflects the pressure lifting capacity of the compressor under actual working conditions.

Main Influencing factors

• inlet and exhaust pressure: the decrease of suction pressure or the increase of exhaust pressure all lead to the increase of external pressure ratio; System resistance, pipeline pressure drop, condensation/evaporation temperature change are common incentives.
• Gas Properties: gas molecular weight and ratio of specific heat (k value) affect compressibility, thus affecting isentropic efficiency and actual pressure ratio.
• Compressor speed and volume flow: Under the fixed structure, the change of rotating speed will change the internal leakage and flow loss, indirectly affecting the effective external pressure ratio.
• Isentropic efficiency: the lower the efficiency, the smaller the external pressure ratio that can be achieved under the same power consumption; The decrease in efficiency is often due to wear, overheating or deviation from the design conditions.

Engineering Significance

external pressure ratio is an important parameter for evaluating the load of compressor matching system, predicting energy consumption and reliability. Too high external pressure ratio is easy to cause overheating, lubrication failure or surge; Too low may indicate system leakage or valve failure. Comprehensive diagnosis should be combined with isentropic efficiency and actual intake and exhaust pressure.