Quantcast Section 2. Compressor Controls

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1 CAPACITY CONTROL. Capacity control is usually achieved by one of three
methods: automatic stop/start, constant speed, and variable speed. Automatic
start/stop is primarily limited to electric motor-driven units. Constant
speed can be applied to all types of compressors. Automatic start/stop and
constant speed control are often combined in a dual control to meet differing
demand periods. The variable control is used where the driver is capable of
fluctuating with the demand, such as with gas- and steam-driven engines. The
method of control used is usually associated with the type of compressor plant
being controlled.
1.1 Constant Speed Control.  Under constant speed control, the load on the
compressor changes,being fully loaded for a certain period of time and then
varying between partially and fully loaded during another period of time.
There are several ways used to achieve constant speed control.
1.1.1 Reciprocating Compressor.  Three methods of control are used for
reciprocating compressors: automatic start/stop, constant speed, and variable
speed. Automatic start/stop is usually limited to electric motor-driven
units. Constant speed is applicable to all types of compressors and drivers.
When variable speed is used, the driver can operate at speeds commensurate
with the demand. Inlet Valve Unloader.  There are several methods of unloading the
compressor.  One system holds the inlet valves open mechanically during both
the suction and compression strokes, thereby preventing the air from being
compressed. The unloader (figure 5-3) is located above the inlet valve so
that the yoke fingers are almost touching the valve. When the air receiver
pressure rises to the preset unloading pressure, a pressure switch operates a
solenoid unloader valve which operates and sends air receiver pressure to the
inlet valve unloader. The pressure from the air receiver acting on the
diaphragm of the inlet valve unloader forces the yoke fingers against the
inlet valve, holding it open. The intake air is pushed back out of the inlet
valve on the compression stroke so that no compression takes place.
(a) Five-Step Capacity Control.
(1) Figure 5-4 shows a typical airflow diagram of a five-step
capacity control system applied to a two-stage, four-cylinder, double action
reciprocating  compressor. Assuming that the compressor is required to
maintain a pressure of 93 to 100 psi, the pressure switches would be set to
load and unload as follows: air pressure switch (APS) No. 1, load at 93 psi,
unload at 97 psi;APS No. 2, load at 94 psi, unload at 98 psi; APS No. 3, load
at 95 psi, unload at 99 psi; and APS No. 4, load at 96 psi, unload at 100
psi. As the receiver pressure reaches the high setting of each pressure
switch, 25 percent of the compressor capacity unloads. As receiver pressure
falls to the low setting of each switch, 25 percent of the compressor capacity
loads. APS No. 1 therefore unloads 25 percent of the compressor capacity at
97 psi and loads 25 percent at 93 psi.  As receiver pressure fluctuates
between 93 and 100 psi, the compressor capacity varies in five steps: full,
75 percent, 50 percent, 25 percent, and zero capacity.


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