Wet compression with a twin screw compressor prototype

Abstract

The main objective of this study was to develop a quick model for an oil free twin screw compressor. The computational time of the model was to be kept as low as possible so that the model could be used for quick estimations and optimizations of the process. This was done by using a simple geometry focusing on the volume curve and port sizes, that can easily be changed by scaling the process as needed. The whole model was done in Matlab, with thermodynamic properties of the ammonia water mixture implemented by Rattner and Garimella. The model was validated by comparing general trends seen in oil free twin screw compressors using different working fluids.
The quick computational time allows for many different features to be examined in a short time to get a general idea of how the compressor would react to them. While using models with more detailed geometries one test can take hours compared to minutes with the model of this study. From the model search it was concluded that for this level of model a finite volume method would be the best option for this study. Various methods were explored to solve the governing equations, counting a few integrated solvers in Matlab and some numerical procedures for ODEs. The method that was chosen as the best for this system was the Euler method. All methods had in common was that the step size needed to be very small, for smaller geometries they needed to be even smaller in some sections of the compressor. To account for that the model was adjusted to accept variable steps. The variable steps allowed for smaller steps around the beginning and end of compression and thus lowering computational time. The main output of the model is the efficiency of the compressor and the Coefficient Of Performance (COP) of the cycle.
The model was tested for various operating conditions to determine how it, and the compressor, would respond. The main result is that the vapor quality should be kept rather low, around 40%, to achieve the highest efficiencies. The performance is also better with higher rotational speeds, however higher speeds also mean more mas flow through the whole system and increased mechanical losses. This means that up to a certain point the rotational speed can be increased until it starts decreasing the efficiency. The ideal ammonia concentration is around 33-35wt%, at higher or lower values the coefficient of performance decreases.
As some preliminary work for future experiments the experimental cycle was set up using pumps and heat exchangers to simulate the compressor. This included verifying the calibrations of sensors, confirming that correct data was read from sensor to LabView and getting the system leakproof. This cycle was also implemented in Matlab to be tested. From those tests the conclusion is that after going through the separator and the vapor and liquid streams being mixed back together the temperature decreases around 60 degrees usually, with a small increase in vapor quality around 2-3%.