A compressor may be thought of as a high pressure pump for a compressible fluid. By ''high pressure'' is meant conditions under which the compressible properties of the fluid (gas) must be considered, which normally occur when the pressure changes by as much as 30% or more. For ''low pressures'' (i.e., smaller pressure changes), a fan or blower may be an appropriate ''pump''

Figure 8-6 Effect of suction speed on stable operating window due to recirculation. (Numbers on the curves are the values of the suction specific speed, Nss.) (From Raymer, 1993.)

for a gas. Fan operation can be analyzed by using the incompressible flow equations, because the relative pressure difference and hence the relative density change are normally small. Like pumps, compressors may be either positive displacement or centrifugal, the former being suitable for relatively high pressures and low flow rates whereas the latter are designed for higher flow rates but lower pressures. The major distinction in the governing equations, however, depends upon the conditions of operation, i.e., whether the system is isothermal or adiabatic. The following analyses assume that the gas is adequately described by the ideal gas law. This assumption can be modified, however, by an appropriate compressibility correction factor, as necessary. For an ideal (frictionless) compression, the work of compression is given by the Bernoulli equation, which reduces to

The energy balance equation for the gas can be written

p1 P

which says that the work of compression plus the energy dissipated due to friction and any heat transferred into the gas during compression all go to increasing the enthalpy of the gas. Assuming ideal gas properties, the density is

The compression work cannot be evaluated from Eq. (8-15) using Eq. (8-17) unless the operating condition or temperature is specified. We will consider two cases: isothermal compression and adiabatic compression.

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