One of the options available on a screw compressor is a side port which provides an entry into the gas spaces during the compression process. This feature allows gas to be compressed from the intermediate to the discharge pressure, avoiding the power requirement of the compression from the suction to the intermediate pressure. One of the arrangements that uses the side port forms what is often called an economizer, as in Figure 3.24. This configuration is similar to the subcoolers in Figures 3.5 and 3.6, except that the separate compressor for the flash gas is not necessary.
The economizer is a low cost method of achieving some of the benefits of twostage compression, but requiring only a single compressor. The efficiency is not as high, however, as two-stage compression with two separate compressors. Also there are intricacies in the performance characteristics when using the side port of a screw compressor. These characteristics will be explained in greater detail in Chapter 5, after the construction and principles of operation of the screw compressor have been explored.