Page 138 - Air and Gas Drilling Manual

4-24 Air and Gas Drilling Manual
the top of the cylinder. This clearance volume results in a volumetric efficiency
term, ε , that is unique to the reciprocating compressor. This volumetric efficiency
v
is only applicable to the first stage of the reciprocating compressor. The expression
for the volumetric efficiency, ε , of a reciprocating compressor can be approximated
v
as
  1 

ε v =  1 − cr  s k − 1    (4-38)
    

where c is the clearance volume ratio for the compressor model. The clearance
volume ratio is the clearance volume divided by the sweep volume of the first stage
piston. The range of values for the clearance volume term c is from 0.06 to 0.12.
Figure 4-16 gives a plot of the term in the brackets for various values of c. The
ordinate of Figure 4-16 is given in terms of ε /0.96. This allows the figure to be
v
readily used for equation constants other than 0.96 [1].

Figure 4-16: Volumetric efficiency term for reciprocating compressor [1].
The mechanical efficiency term, ε , is used with the volumetric efficiency term
m
to determine the input power requirement. This mechanical efficiency term is a
measure of the friction losses in the mechanical application of the prime mover
power to the compressor. These losses are due to the friction in the bearings and
linkages in the compressor system. The values of mechanical efficiency for typical
reciprocating compressor systems can vary from about 0.84 to 0.99.