Page 104 - Air and gas Drilling Field Guide 3rd Edition

5.2 Standard Units 95

140

120
Percent Pressure Ratio Or Head 100 DESIGN POINT

80
Positive Displacement Compressors
Axial-Flow Compressors
60 Centrifugal Compressors

50 60 70 80 90 100 110 120
Percent Flow

FIGURE 5-3. General performance curves for various compressor types [2].

3
only moderate volumetric flow rate magnitudes (i.e., up to about 2 10 actual cfm
3
or 10 actual liters/sec). Dynamic compressors are best suited for handling large vol-
6 5
umetric flow rates (i.e., up to about 10 actual cfm or 5 10 actual liters/sec), but
with only moderate pressure ratios (i.e., up to approximately 20).
Figure 5-3 gives general performance curves for various positive displacement and
dynamic compressors [2]. Positive displacement compressors, particularly the multi-
stage 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 maxi-
mum design pressures of the machines (i.e., problems with slippage). Dynamic com-
pressors are quite sensitive to pressure ratio changes. The volumetric flow rates will
change drastically with rather small changes in the downstream back pressure (rela-
tive to the pressure ratio around which the machine has been designed). Thus, posi-
tive displacement compressors are normally applied to industrial operations where
volumetric flow rates are critical and pressure ratios are variable.
Dynamic compressors are generally applied to industrial operations where the
volumetric flow rate and pressure ratio requirements are relatively constant.

5.2 STANDARD UNITS
In the United States, a unit of air or any gas is referenced to the standard cubic
foot of dry air. The API Mechanical Equipment Standards standard atmospheric