Page 116 - Air and Gas Drilling Manual

4-2 Air and Gas Drilling Manual

Figure 4-1: Compressor classification [2].
Each of the two general classes of compressors and their subclasses have certain
advantages and disadvantages regarding their respective volumetric flow rate
capabilities and overall compression pressure ratios. Figure 4-2 shows the typical
application range in volumetric flow rates (actual cfm) and compression pressure
ratios for most important compressor classes and subclasses [2].
In general, positive displacement compressors are best suited for handling high-
pressure ratios (i.e., up to approximately 200), but this can be accomplished with
3
only moderate volumetric flow rate magnitudes (i.e., up to about 10 actual cfm).
Dynamic compressors are best suited for handling large volumetric flow rates (i.e.,
6
up to about 10 actual cfm), but with only moderate pressure ratios (i.e., up to
approximately 20).
Figure 4-3 gives the general performance curves for various positive displacement
and dynamic compressors [2]. The positive displacement compressors, particularly
the multistage reciprocating compressors, are very insensitive to downstream back
pressure changes. These compressors will produce their rated volumetric flow rate
even when the pressure ratio approaches the design limit of the machine. Rotary
compressors are fixed pressure ratio machines and are generally insensitive to
downstream back pressure ratio changes as long as the output pressures required are
below the maximum design pressures of the machines (i.e., problems with slippage).
Dynamic compressors are quite sensitive to pressure ratio changes. The volumetric
flow rates will change drastically with rather small changes in the downstream back
pressure (relative to the pressure ratio around which the machine has been designed).
Thus, positive displacement compressors are normally applied to industrial
operations where volumetric flow rates are critical and pressure ratios are variable.