Compressor calculation

Since most compressors operate along a polytropic path approaching the adiabatic, compressor calculations are generally based on the adiabatic cui ve. 

The generalized compressibility charts may be used with values obtained in the use of Equations 2.7 and 2.8 to determine the compressibility of a wide range of gases. The charts were derived from experimental data and are a good source of information for use in compressor calculations [1]. 

The most important thing to remember in compressor calculations is that compressor flow is a volumetric value based on the flowing conditions of pressure, temperature, relative humidity (if moisture is present), and gas composition at the compressor inlet nozzle. The flow units aix inlet cubic feet per minute (icfm). 

A common method of stating flow is standard cubic feet per minute where the flowing conditions are referred to an arbitrary set of standard conditions. Unfortunately, standard conditions are anything but standard. Of the many used, two are more common. The ASME standard uses 68°F and 14.7 psia. The relative humidity is given as 36%. The other standard that is used by the gas transmission industry and the API Mechanical Equipment Standards is 60°F at 14.7 psia. As can be seen from this short discussion, a flow value must be carefully evaluated before it can be used in a compressor calculation. 

Used and compiled by permission Plain Talks on Air and Gas Compression, Fourth of Series, Worthington. Dresser-Rand Corporation. Also compiled by permission from Reciprocating Compressor Calculation Data, 1956. Dresser-Rand Corporation. 

Cooper-Bessemer Corp., Reciprocating Compressor Calculation Data, Section E, Mount Vernon, Ohio, April (1954). 

Avogadro states that equal volumes of all gases, under the same conditions of pressure and temperature, contain the same number of molecules. This law is very important and is applied in many compressor calculations. 

All gases deviate from the perfect or ideal gas laws to some degree. In some cases the deviation is rather extreme. It is necessary that these deviations be taken into account in many compressor calculations to prevent compressor and driver sizes being greatly in error. 

R. James, Compressor calculation procedures, in Encyclopedia of Chemical Processing and Design, Dekker, New York, Vol. 10, pp. 264-313. 

Related Calculations. When specific thermodynamic charts, namely, enthalpy-temperature, entropy-temperature, and enthalpy-entropy, are not available for a particular system, use the generalized enthalpy and entropy charts to perform expander-compressor calculations, as shown in this example. 

The constant temperature process is a case when n=l, which is equivalent to isothermal compression, the constant pressure process n = 0 and the constant volume process n = Generally, it is impractical to build sufficient heat transfer equipment into the design of most compressors to convey the bulk of the heat of compression. Therefore most machines tend to operate along a poly tropic path that approaches the adiabatic. Most compressor calculations are based on the adiabatic curve [3]. 

Input Data and Computer Output of Polytropic Compressor Calculations... 

DETERMINE WHETHER THE COMPRESSOR CALCULATION IS POLYTROPIC OR ADIABATIC. 

FORMAT (///,26X, POLYTROPIC COMPRESSOR CALCULATION, /, 78(1H )) CALL POLYTR... 

Several heat exchanger and compressor configurations were considered, since the thermal integration of the system has a significant impact on the efficiency. An optimal process configuration was selected, and the process flow diagrams were developed. As shown in Table 1, the efficiency of the optimal configuration is 75.2%. Compressor calculations showed that electricity required was... 

The Compressor operation is used to increase the pressure of an inlet gas stream. Depending on the information specified, the Compressor calculates ether a stream property (pressure or temperature) or compression efficiency. 

Obviously every industrial application of equations of state can not be covered in this section of the paper. Thus, for example, calculation of Joule-Thompson coefficients, heats of mixing, and the velocity of sound, and obtaining various other parameters required in compressor calculations, which are an integral part of industrial design, will not be discussed. 

As mentioned before, most compressor manufactures use polytropic method for compressor calculation. The polytropic efficiency, Ep, is defined by following equation, Eq. (5a). 

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