Page 118 - Air and Gas Drilling Manual

4-4 Air and Gas Drilling Manual
Dynamic compressors are generally applied to industrial operations where the
volumetric flow rate and pressure ratio requirements are relatively constant.
4.2 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
conditions for dry air is a temperature of 60°F (which is 459.67 + 60 = 519.67˚R)
and a pressure of 14.696 psia (760 mm, or 29.92 inches Hg). The equation of state
for the perfect gas can be written as
P = R u T
γ m (4-1)
w
2
where P is the pressure (lb/ft abs),
3
γ is the specific weight (lb/ft ),
R u is the universal gas constant (which is 1,545.4 ft-lb/lb-mole-˚R),
T is the temperature (˚R),
m w is the mole weight of the gas (lb/lb-mole).
3
Thus, using Equation 4-1, the specific weight γ, or the weight of 1 ft of dry air is
.
(
14 696 144) ( 28 96)
.
γ =
.(
.
1545 4 519 67)
3
.
γ = 0 0763 lb/ft
where m = 28.96 lb/lb-mole (for dry air). Thus, a dry cubic foot of air at the API
w
Mechanical Equipment Standards standard atmospheric condition weighs 0.0763
3
pounds (or a specific weight of 0.0763 lb/ft ) [2, 3, 4].
Other organizations within the United States and regions around the world have
established slightly different standard atmospheric conditions. For example, ASME
standard atmosphere is a temperature of 68°F, a pressure of 14.7 psia, and a relative
humidity of 36 percent. The United Kingdom uses a standard atmosphere with a
temperature of 60°F and a pressure of 30.00 inches of Hg. Most Continental
European countries use a standard atmosphere with a temperature of 15°C (59°F) and
pressure of a bar or 750 mm of Hg (14.5 psia) [5].
When selecting and sizing compressors, care should be taken in determining
which standard has been used to rate a compressor under consideration, particularly if
the compressor has been produced abroad. All further discussions in this section
will utilize only the API Mechanical Equipment Standards standard atmospheric
conditions.
Compressors are rated by their maximum volumetric flow rate input and their
maximum pressure output. The volumetric flow rate input ratings are usually
specified in units of standard cubic feet per minute (scfm) and the maximum
pressure output ratings are specified in units of psig (at a specified standard sea level
condition, e.g., API, ASME, etc). The scfm volumetric flow rate refers to the