TWO STAGE COMPRESSION AND COMPRESSOR EFFICIENCY

  In order to increase operating efficiency at low evaporating temperature, the compression can be done in two steps or stages. for two stage operation, the total compression ratio is the product of the compression ratio of each stage. In other words, for a total compression ratio of 16 to 1, the compression ratio of each stage might be 4 to 1; or compression ratios of 4 to 1 and 5 to 1 in separate stage will result in total compression ratio of 20 to 1.

  Two stage compression may be accomplished with the use of two compressor with the discharge of one pumping into the suction of the second, but because of the difficulty of maintaining proper oil levels in the two crankcases, it is more satisfactory to use one compressor with multiple cylinder. On Copeland two stage compressors, the ratio of low stage ti high stage displacement is 2 to 1. The greater volume of the low stage cylinder is necessary because of the difference in specific volume of the refrigerant vapor at low and interstage pressure. While the compression ratios of the two stage are seldom exactly equal, they will be approximately the same. A typical 6 cylinder two the compressor with its external manifold and desuperheating expansion valve is shown in figure, and a typical 3 cylinder two stage compressor with external manifold is shown

  Shows a comparison of five different volumetric efficiency curves. The threes straight lines are typical single stage curves one for an air conditioning compressor, one for a typical multi-purpose compressor, and one for a low temperature compressor. There are some variations in compressor design involved, but the primary difference in characteristics is due to clearance volume.

  The two vertical curved lines represent the comparative efficiency of a two stage compressor. Actually each separate stage would have a straight line characteristic similar to the single stage curves, but to enable comparison with single stage compressor, the overall volumetric efficiency has been computed on the basis of the total displacement of the compressor, not just the low stage displacement.