Significant work has been performed to qualify and quantify the effects of operating with wet gas in a centrifugal compressor system [1, 2]. Of particular interest is the sharp decrease in the isentropic efficiency of the machine when operating with process gas containing various liquid volume fractions. However, it is unknown how much of the performance losses are due to aerodynamic effects, such as blade profile and flow separation losses, rather than the basic thermodynamic effects of compressing a multiphase gas that has a higher density, integral wet-cooling, and contains small amounts of high-density droplets. Previous studies showed that the overall efficiency losses exceeded those expected from purely thermodynamic effects so aerodynamic effects have been principally blamed for the lower efficiency. However, no test data exists in the public domain that quantifies these losses and it is experimentally difficult to perform this type of testing in centrifugal compressor.
Therefore, a series of tests was performed on a reciprocating compressor with power and efficiency recorded through dynamic pressure measurements obtained inside the compression cylinder, torque measured on the shaft, and enthalpy rise measurements obtained outside the cylinders. Using this approach one can eliminate (or differentiate) the aerodynamic effects of wet gas compression, such as valve losses, thus allowing the direct determination of the thermodynamic losses of wet gas compression. Specifically, when there is multi-phase flow entering the machinery, there is the thermodynamic effect of how a mixture of water and air behaves when being compressed [from a process perspective] and the aerodynamic effect of moisture encountering the blades of a centrifugal compressor [performance loss] or the valve passages of a reciprocating compressor [pressure loss]. Directly instrumenting the internals of a reciprocating compressor cylinder allows the evaluation of the thermodynamic performance of multi-phase compression separate from any aerodynamic penalties.
This paper describes the tests performed in a reciprocating compressor open test loop operating with varying amounts of liquid volume fractions (LVFs) of water in the process gas (air). The data was reduced using Pressure-Volume card measurements inside and outside the cylinder, enthalpy rise, as well as torque to determine the impact of volume fraction on compression power and efficiency. Additionally, the valve losses, system efficiencies, and peak compression “spike” were evaluated in relations to the LVFs.