second-stage piston starts its downstroke, the air
the LPAC. The frame extension houses the
is drawn into the second-stage cylinder. Then it
crossheads and crosshead guides and is open to
is further compressed, followed by a cooling and
the atmosphere. It separates the cylinders, which
moisture removal process similar to the first stage.
are not oil lubricated, from the crankcase. Oil
wiper assemblies (seals) are located in the frame
High-Pressure Air Compressors
extension to scrape lubricating oil off the piston
rods when the compressor is in operation. Oil
The HPAC is a vertical, five-stage, recipro-
deflector plates are attached to the piston rods to
cating air compressor. It is driven by being directly
prevent any oil that creeps through the scrapers
connected to an electric motor. Refer to figures
from entering the cylinders. Oil that is scraped
7-25 and 7-26 as we describe the compressor.
from the piston rods drains back to the sump. Air
The subbase supports the compressor assem-
leakage along the piston rods is prevented by full
bly, the electric drive motor, and the coolers and
floating packing assemblies bolted to the under-
rack assembly. The crankcase is bolted directly
side of the cylinder blocks.
to the subbase and is made up of the frame and
During operation, ambient air is drawn into
frame extension. The frame houses the crankshaft
the first-stage cylinders through the inlet filter
and oil pump. The frame extension is open to the
silencers and inlet valves during the downstroke.
atmosphere and isolates the conventionally
When the piston reaches the bottom of its stroke,
lubricated frame from the oil-free cylinders. The
the inlet valve closes and traps the air in the
crosshead guides are machined in the frame
cylinder. When the piston moves upward, the
extension. A uniblock casting contains the first
trapped air is compressed and forced out of the
three-stage cylinders and is mounted on the frame
first-stage cylinders, through the first-stage cooler
extension (fig. 7-26). The cylinders are arranged
and the first-stage moisture separator. When the
Figure 7-25.--HPAC.
7-22
