Specifying a Compressor

In specifying a compressor it is necessary to choose the basic type, the number of stages of compression, and the horsepower required. In order 

10 do this the volume of gas, suction and discharge pressure, suction tem perature, and gas specific gravity must be known. 

F = an allowance for interstage pressure drop = 1.00 for single-stage compression 1,08 for two-stage compression 1.10 for three-stage compression Qg = flow rate, MMsefd 

Once the required horsepower and number of stages are estimated, a choice of compressor type can be made from the considerations included earlier. Some example selections are included in Table 10-1, The selections listed in this table are meant as common types that would normally be specified for the given conditions. It must be emphasized that the.se are not recommendations that should be accepted without consideration of the advantages and disadvantages listed earlier. In addition, local foundation conditions, type of drivers available, cost of fuel, availability of spaie 

Procedure for More Accurate Determination of Horsepower and Number of Stages 

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The isentropic efficiency r)c can be found, or at least estimated, from charts such as those described in the following section. To find the power needed from the motor or turbine driving the compressor, the figure found from equation 9.7 should be divided by the mechanical efficiency, J]m- For a good estimate, 0.9 can be used for this. However, when specifying a compressor from a supplier, the figure from equation 9.7 is best used unaltered. 

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