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Operation
GENERAL
The basic operating principles of all centrifugal com-
pressors are the same, regardless of the size or capac-
ity of the compressor or the number of impellers used.
The following paragraphs describe the path of the re-
frigerant gas flow through a Series M Turbomaster Com-
pressor and the effect upon the gas as it passes from
the suction to the discharge connection(s) of the com-
pressor.
Primary Compressor Gas Flow (Figure 3)
The gas enters the compressor through the suction inlet
connection, and passes through the pre-rotation vanes
to the inlet throat of the first stage impeller.
The high rotative speed of the impellers causes the suc-
tion gas to be drawn into the first stage impeller and is
discharged from the blade tips at high velocity and in-
creased temperature and pressure. The discharge space,
which is formed by the diffuser and the housing, is shaped
to convert some of the velocity energy into pressure
rise before directing the gas from the discharge of the
first stage impeller into the inlet of the second stage
impeller.
From the second stage impeller, the gas passes through
the remaining impellers at high velocity and steadily in-
creasing temperature and pressure. The gas is eventu-
ally discharged into the annular compressor discharge
space, at the oil reservoir end, where it leaves the com-
pressor and enters the discharge line.
A centrifugal compressor does not move a definite quan-
tity of gas under all operating conditions as does a posi-
tive displacement compressor. The ability to compress
is determined by a performance curve which can be
depicted by various parameters, but most frequently head
and flow are used.
14
SECTION 3
OPERATION
Balance Piston (Figure 4)
Since suction gas enters the first stage impeller (1) at
suction pressure and leaves the high stage impeller (2)
at discharge pressure, the pressure differential becomes
progressively greater between the inlet and the discharge
of each impeller.
The inlet areas of the impellers are only affected by
suction pressure of that stage while the sides of the
impeller are subjected to the higher impeller discharge
pressure of that stage resulting in a net force pushing
toward the impeller inlet.
Because of this pressure difference, an axial thrust
force exists which is toward the suction end of the
compressor.
To maintain the axial thrust within design limits, a bal-
ance piston (3) may be used as a part of the last stage
impeller. Gas leakage past the balance piston is regu-
lated by a "floating" seal ring (4).
The balance piston (3) serves as a rotating partition at
the end of the discharge gas space. The pressure on
the outboard side of this piston is equalized to a lower
stage pressure through one or more internal (5) or ex-
ternal gas equalizing lines (See Figure 5, Item 6). The
compressor diffuser geometry will determine whether
the venting is done internally or externally.
The balance piston (3) is sized so that the pressure dif-
ferential across it reduces the total axial thrust of the
rotor assembly. The balance piston seal (4) is designed
to regulate the leakage into the chamber (6). Elevation
of the pressure differential across the balance piston
above the design will result in increased axial thrust
load toward the suction end of the compressor. The
thrust bearing (7) serves to physically or mechanically
position the shaft (8) and impellers (1 and 2) in the cen-
ter of the diffuser(s) while the thrust load minimizes the
rotor assembly (1, 2 and 3) from shifting axially during
surging and load changes. Extreme elevation of the bal-
ance piston pressure is an indication of a problem that
can result in failure of the thrust bearing (7).
FORM 220.11-NM2 (602)
YORK INTERNATIONAL
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