Page 109 - Reciprocating Compressors Operation Maintenance

96 Reciprocating Compressors: Operation and Maintenance

the inertia forces mutually cancel each other. Only couples are transmit-
ted to the foundation.
In the single cylinder and "Y" type compressor, the secondary forces
can neither be balanced nor transferred in a different direction. This force
must be compensated for by proper design of the foundation.

COUPLES

Couples are a result of the forces acting on individual cranks in a
multi-crank compressor. These forces are not in the same plane, but act
on each crank in a plane at right angles to the shaft axis. The forces pro-
duce a moment (force couple) that will try to rotate the shaft about its
center in a plane passing through the shaft axis.
This is the primary couple. The secondary couple, occurring at twice
crankshaft speed, cancels out.
The design of a balanced type compressor permits a complete balanc-
ing of the primary as well as the secondary forces without attachment of
counterweights. Because the cranks are adjacent to each other (see Fig-
ure 2-54), the arrangement permits a minimum moment arm, resulting in
very small primary and secondary couples.
FOUNDATIONS FOR RECIPROCATING COMPRESSOR

The reciprocating compressor, because of the alternating movement of
its pistons and various others parts, develops a shaking or inertia force that

FIGURE 2-54. Balanced-opposed cylinder arrangement with closely spaced
cranks minimizes moment unbalance (Source: Dresser-Rand, Painted Post,
New York).