Page 19 - Reciprocating Compressors Operation Maintenance

6 Reciprocating Compressors: Operation and Maintenance

METHODS OF COMPRESSION

Four methods are used to compress gas. Two are in the intermittent
class, and two are in the continuous flow class. (These are descriptive,
not thermodynamic or duty classification terms.)

1. Trap consecutive quantities of gas in some type of enclosure, reduce
the volume (thus increasing the pressure), then push the compressed
gas out of the enclosure.
2. Trap consecutive quantities of gas in some type of enclosure, carry
it without volume change to the discharge opening, compress the
gas by backflow from the discharge system, then push the com-
pressed gas out of the enclosure.
3. Compress the gas by the mechanical action of rapidly rotating
impellers or bladed rotors that impart velocity and pressure to the
flowing gas, (Velocity is further converted into pressure in station-
ary diffusers or blades.)
4. Entrain the gas in a high velocity jet of the same or another gas
(usually, but not necessarily, steam) and convert the high velocity of
the mixture into pressure in a diffuser.

Compressors using methods 1 and 2 are in the intermittent class and
are known as positive displacement compressors. Those using method 3
are known as dynamic compressors. Compressors using method 4 are
known as ejectors and normally operate with an intake below atmospher-
ic pressure.
Compressors change mechanical energy into gas energy. This is in
accordance with the First Law of Thermodynamics, which states that
energy cannot be created or destroyed during a process (such as compres-
sion of a gas), although the process may change mechanical energy into
gas energy. Some of the energy is also converted into nonusable forms
such as heat losses.
Mechanical energy can be converted into gas energy in one of two ways:

1. By positive displacement of the gas into a smaller volume. Flow is
directly proportional to speed of the compressor, but the pressure
ratio is determined by pressure in the system into which the com-
pressor is pumping.