Page 460 - Mechanical Engineers' Handbook (Volume 4)

7 System Components 449

driven through an internal speed increaser. These machines are ideally suited for clean,
noncorrosive gases in moderate-pressure process or refrigeration cycles in the range of
3
0.236–1.89 m /sec (5 cfm). Multistage centrifugal compressors are built for direct connection
to high-speed drives or for use with an external speed increaser. Designs available from
suppliers generally provide for two to eight impellers per casing covering the range of 0.236–
3
11.8 m /sec (500–25,000 cfm), depending on the operating speed. A wide choice of mate-
rials and shaft seals to suit any gas composition, including dirty or corrosive process streams,
is available.
The centrifugal compressor has a more complex head-volume characteristic than recip-
rocating machines. Changing discharge pressure may cause relatively large changes in inlet
volume, as shown by the heavy line in Fig. 12a. Adjustment of variable inlet vanes or of a
diffuser ring allows the compressor to operate anywhere below the heavy line to conditions
imposed by the system. A variable-speed controller offers an alternative way to match the
compressor’s characteristics to the system load, as shown in the lower half of Fig. 12b. The
maximum head capability is fixed by the operating speed of the compressor. Both methods
have advantages: generally, variable inlet vanes or diffuser rings provide a wider range of
capacity reduction; variable speed usually is more efficient. Maximum efficiency and control
can be obtained by combining both methods of control.
The centrifugal compressor has a surge point—that is, a minimum-volume flow below
which stable operation cannot be maintained. The percentage of load at which the surge
point occurs depends on the number of impellers, design–pressure ratio, operating speed,
and variable inlet-vane setting. The system design and controls must keep the inlet volume
above this point by artificial loading, if necessary. This is accomplished with a bypass–valve
and gas recirculation. Combined with a variable inlet-vane setting, variable diffuser ring, or
variable speed control, the gas bypass allows stable operation down to zero load.

Compressor Operation
Provision for minimum-load operation is strongly recommended for all installations, because
there will be fluctuations in plant load. For chemical plants, this permits the refrigeration
system to be started up and thoroughly checked out independently of the chemical process.
Contrasts between the operating characteristics of the positive displacement compressor
and the centrifugal compressor are important considerations in plant design to achieve sat-
isfactory performance. Unlike positive displacement compressors, the centrifugal compressor
will not rebalance abnormally high system heads. The drive arrangement for the centrifugal
compressor must be selected with sufficient speed to meet the maximum head anticipated.
The relatively flat head characteristics of the centrifugal compressor necessitates different
control approaches than for positive displacement machines, particularly when parallel com-
pressors are utilized. These differences, which account for most of the troubles experienced
in centrifugal-compressor systems, cannot be overlooked in the design of a refrigeration
system.
A system that uses centrifugal compressors designed for high-pressure ratios and that
requires the compressors to start with high suction density existing during standby will have
high starting torque. If the driver does not have sufficient starting torque, the system must
have provisions to reduce the suction pressure at startup. This problem is particularly im-
portant when using single-shaft gas turbine engines or reduced-voltage starters on electric
drives. Split-shaft gas turbines are preferred for this reason.
Drive ratings that are affected by ambient temperatures, altitudes, etc., must be evaluated
at the actual operating conditions. Refrigeration installations normally require maximum
output at high ambient temperatures, a factor that must be considered when using drives
such as gas turbines and gas engines.