Whenever a high-pressure fluid stream

or simply stream HP

is depressurized

a potential for mechanical energy recovery exists. On the other hand

pressurizing a low-pressure fluid stream

or simply stream LP

always requires mechanical energy. In principle

a work exchanger (WE)may be applied to any high-pressure processing systems where it is also desirable to recover mechanical energy to offset the energy requirement for pressurization of low pressure streams. Because the gas streams in the chemical process often appear with big flux and high pressure

there are great potential to recover work in the process industry. But

it is difficult to forecast and appraise the work recovery result between two gas streams because gas is compressible. For the two kinds of structure of gas-gas WE

the reciprocating machine and the centrifugal machine

their thermodynamic processes were analyzed and the equations of work recovery efficiencies were proposed. The in-depth simplified work recovery efficiency equations of the work transfer process in two WE structures were presented through reasonable assumption and were compared finally. The work recovery efficiency equation of centrifugal machine was also extra influenced by initial volume flow ratio than the reciprocating machine. In order to see what pure influence of the stream phase state on the work recovery efficiency

the influence of initial volume flow ratio was expected to be neglected. So the reciprocating machine was chosen to take as example and its performance of work exchange between two streams with different phase states were compared. Two WE structures corresponded to different work recovery efficiency equations. The simplified work recovery efficiency equation of reciprocating machine followed a power function of compression ratio. Its work recovery efficiency decreased with increasing compression ratio

while for centrifugal structure

the work recovery efficiency was relevant to not only pressure ratio but also initial volume flow ratio. Furthermore

because the centrifugal structure did not involve minimal pressure differential

a centrifugal machine could achieve higher LP compression than a reciprocating machine. The present work compared the work recovery efficiency between the streams with different phase states. The result showed that with the same compression ratio and initial volume flow ratio

liquid-liquid reciprocating work exchanger had the highest work recovery efficiency value. Its ideal value was 100%. The work recovery efficiency values of gas-liquid WE and liquid-gas WE decreased successively

while the value of gas-gas WE was the lowest. The work recovery efficiency equation of centrifugal machine had also extra influence by initial volume flow ratio than the reciprocating machine.