Page 26 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 1 3

VACUUM

Vacuum is a type of pressure. A gas is said to be under vacuum when
its pressure is below atmospheric. There are two methods of stating this
pressure, only one of which is accurate in itself.
Vacuum is usually measured by a differential gauge that shows the dif-
ference in pressure between that of the system and atmospheric pressure.
This measurement is expressed, for example, as

Millimeters of Hg vacuum = (mm Hg Vac)
Inches of Hg vacuum = (in. Hg Vac)
Inches of water vacuum = (in. H 2O Vac)
Unless the barometric equivalent of atmospheric pressure is also
given, these expressions do not give an accurate specification of pres-
sure. See Figure 1-6.
Subtracting the vacuum reading from the atmospheric pressure will
give an accurate absolute pressure. This may be expressed as

Inches of Hg absolute = (in. Hg abs)
Millimeters of Hg absolute = (mm Hg Abs)
Pounds/sq in absolute = (psia)

The word absolute should never be omitted; otherwise, one is never
sure whether a vacuum is expressed in differential or absolute terms.
THEORY OF RECIPROCATING COMPRESSORS

Reciprocating compressors are the best known and most widely used
compressors of the positive displacement type. They operate on the same
principle as the old, familiar bicycle pump, that is, by means of a piston
in a cylinder. As the piston moves forward in the cylinder, it compresses
the air or gas into a smaller space, thus raising its pressure.
The basic reciprocating compression element is a single cylinder com-
pressing on one side of the piston (single-acting). A unit compressing on
both sides of the piston (double-acting) consists of two basic single-act-
ing elements operating in parallel in one casting. Most of the compres-
sors in use are of the double-acting type.
Figure 1-7 shows a cross section of another variant—a V-oriented,
two-stage, double-acting water-cooled compressor.