Page 24 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications \ 1

PRESSURE

Pressure is expressed as a force per unit of area exposed to the pres-
sure. Because weight is really the force of gravity on a mass of material,
the weight necessary to balance the pressure force is used as a measure.
Hence, as examples:

Pounds/sq in = (psi)
Pounds/sq ft = (Ib/sq ft)
Grams/sq cm = (gr/sq cm)
Kilograms/sq cm = (kg/sq cm)

Pressure is usually measured by a gauge that registers the difference
between the pressure in a vessel and the current atmospheric pressure.
Therefore, a gauge (psig) does not indicate the true total gas pressure,
To obtain the true pressure, or pressure above zero, it is necessary to
add the current atmospheric or barometric pressure, expressed in proper
units. This sum is the absolute pressure (psia). See Figure 1-6. For all
compressor calculations the absolute pressure is required.

Any Pressure Above Atmospheric

Atmospheric Pressure-Variable

S>
ol 1 > Any Pressure Below Atmospheric
[I - (.
0) Q),O
II

•t; *i- 0)
<
Perfect Vacuum-Absolute Zero of Pressure

FIGURE 1 -6. Relationship and terms used to describe pressure.