Page 44 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 31

Crosshead Design

In addition to being water-cooled, heavy duty reciprocating compres-
sors are designed with a separate crosshead to guide the piston in the
cylinder bore instead of depending on the piston skirt to do the guiding.

• Allows the use of a narrow piston and larger valve area for greater
efficiency.
• Permits a longer stroke and greater capacity.
«Separates crankcase from cylinder, allowing control of oil carryover
into the cylinder.
• Gives greater stability to piston, eliminating piston "slap" and reduc-
ing ring wear.
• Permits stronger piston design and higher operating pressures.
• Makes possible a piston design which allows "pumping" or compres-
sion at both ends of the stroke. "Double-acting" results in twice the
capacity for every revolution, as opposed to the single-acting design.

The heavy duty, water-cooled, crosshead type reciprocating compres-
sor has a relatively slow rotative speed (180-900 rpm). These machines
are conservatively designed for long service life at low maintenance. It is
not unusual for this type of machine to be in operation for 50 to 55 years
after installation.
Because they are conservatively designed with crossheads and water-
cooled cylinders, they are heavier, more expensive to manufacture, and
more expensive to install than the other type compressors.
They are also the most efficient of all compressors on a BHP/CFM
basis, particularly at part loads, due to their ability to be controlled effec-
tively at part loads. All other types of compressors are compared to the
heavy duty, water-cooled compressor when efficiency is discussed.
AUTOMATIC VALVES

The reciprocating compressor uses automatic spring-loaded valves that
open only when the proper differential pressure exists across the valve.
Inlet valves open when the pressure in the cylinder is slightly below the
intake pressure. Discharge valves open when the pressure in the cylinder
is slightly above the discharge pressure.
Figure 1-18 shows a typical ideal pressure-volume (pV) diagram (ide-
alized indicator card) of a single-stage compressor with corresponding
compressor piston locations.