Page 229 - Air and Gas Drilling Manual

t
=
t
T
.
at
o
T at at = 40˚F + 459 67 Chapter 5: Shallow Well Drilling Applications 5-71
= 499 67 R
.
at
Thus, P g and T g become
P = P at = 1 695 lb/ft abs
2
,
g
T = T at = 499 67 R
o
.
g
Using Equation 4-11, the specific weight of the gas entering the compressor is
(1 695, ) (1 0. )
γ =
g
(53 36. ) (499 67. )
γ g = 0 0636 lb/ft 3
.
Note the above specific weight value can also be obtained from Figure D-4 in
Appendix D.
The approximate minimum volumetric flow rate for this example adjusted for
the actual atmospheric conditions at the drilling location can be determined by
equating the weight rate of flow of the air at the two atmospheric conditions. This
reduces to
 0 0763. 
q = 155
g
 0 0636. 
q g = 186 acfm

The above volumetric flow rate is the adjusted minimum value for the atmospheric
conditions at this drilling location (6,000 ft above sea level).
5.3.2 Injection Pressure and the Selection of Compressor Equipment
The selection process requires that the borehole requirements be compared to the
capabilities of the compressor units available. Specifically, these comparisons are
made between; a) the required borehole volumetric flow rate and the compressor
volumetric flow rate capability, b) the borehole injection pressure (using the
compressor volumetric flow rate) and the pressure capability of the compressor and,
c) the horsepower required by the compressor and the input capability of the prime
mover. For shallow drilling operations the selection of the appropriate compressor