Sidestream Compressors

Sidestream compressors are used in refrigeration processes where, for economy, the refrigerant is flashed off at different pressure levels. Ideally, separate compressors could be used to successively compress the gas back up to the condensing pressure level. The pressure ratio for each stage is low enough to enable this to be done with only one or two impellers in each section. Because of this, compressors can be made with all sections in one casing so that mixing of the streams takes place internally.

The inlet conditions to the suction of an intermediate section are those obtained by the mixing of the side stream entering the nozzle and the gas discharged from the preceding section. In designing the compressor, the manufacturer will assume suction pressure and temperature at the section based on complete mixing of the streams with an allowance for the losses in total pressure between the side load flanges and the section inlet. The test conditions must be based on the inlet condition at the side load flanges, but the pressure loss between flange and section inlet has to be verified in order to establish machine performance. If these losses are significantly different, then this can change the validity of the test results and some adjustment to the test speed may be required.

The measurement of the actual interstage test conditions is also difficult due to turbulence and stratification of the gas at the measuring points. However, much depends on the mixing arrangement adopted, and most manufacturers are coming to the view that the two streams should be brought to the same velocity prior to mixing, if only to ease the problem of computation.

Test Arrangement for Sidestream Compressors

The easiest, but most expensive, arrangement is to test each section independently by installing one section at a time. This involves three separate tests followed by dismantling and reassembly after each test. Test inlet conditions can be accurately maintained, and the problems of installing instruments for measuring pressure profiles across inlet passages are overcome by the space available. Verifying the losses between side nozzle and inlet to the section is less certain as the flow arrangement is not exactly identical to design.

Another method of test is to test all sections simultaneously. In this method, the weight flow through each section is maintained in the same proportion. A separate gas cooler is required for each stream and the loop must have sufficient capacity to enable stable conditions to be obtained. In this system, the machine is tested as fully built but requires extensive shop test space to accommodate the multiple gas coolers and large testloop piping required, if there are several sidestreams.

Loop testing the sidestream compressor is probably the most difficult, if more than two sidestreams are involved. Reviewing the sidestream problem should give an insight to the various configurations and the test arrangement.

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