Page 147 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 133

1. Large passage area and good flow dynamics for low throttling effect
(pressure drop)
2. Low mass of the moving parts for low impact energy
3. Quick response to low differential pressure
4. Small outside dimensions to allow for low clearance volume
5. Low noise level
6. High reliability factor and long life
7. Ease of maintaining and servicing

Bask Function of a Compressor Valve

A compressor valve regulates the cycle of operation in a compressor
cylinder. Automatic compressor valves are pressure activated, and their
normal movement is controlled by the compression cycle.
The valves are opened solely by the difference in pressure across the
valve; no mechanical device is used. The best illustration of a compres-
sor valve cycle is obtained by correlating the piston movement to the
pressure volume diagram.
To visualize the sequence of events, we align a schematic drawing of a
horizontal single-acting reciprocating air compressor (top Figure 3-8)
directly above its piston-velocity graph (center Figure 3-8) and its cylin-
der-pressure graph (bottom Figure 3-8). P 5 represents inlet pressure and
P 2 represents discharge pressure.
The piston is shown at its top dead center, momentarily motionless at
the end of its compression stroke, Point D in the pV-diagram. At this
moment, the discharge valve has just closed and the suction valve has not
yet opened.

Gas Intake

When the piston starts moving to the right on the suction stroke, the
small amount of gas remaining in the cylinder, its clearance volume, is
expanded from P 2 to Pj and lower. The resulting slight underpressure per-
mits the suction pressure, P p to push open the suction valve, and gas from
the suction plenum is drawn into the cylinder (point E in the pV-diagram).
As the piston nears the end of its suction stroke, its deceleration
reduces the gas speed through the open valve, and in a properly designed
valve, the spring load closes the valve at the moment the piston reaches
its bottom dead center (point A in the pV-diagram.)