Civil Engineering Reference
In-Depth Information
Screw compressors are oil-flooded compressors where oil is injected to seal
clearances, absorb the heat of compression, prevent metal contact between rotors,
and lubricate the bearings. Oil separation is performed immediately after compres-
sion. The oil and air mixture passes through the discharge valve to an oil reservoir,
or a combination of an oil reservoir and an air receiver. The air then passes through
a separator before flowing to the discharge line where most of the remaining oil is
removed. Oil temperature controls are installed in all units to keep the injected oil
within a range 54-71
C (130-160
F).
In screw compressors, water- or air-cooled heat exchangers are usually
installed; recoverable heat temperature can be 60-80
C (140-160
F).
11.5
The Control of Air Compressors
The output of compressors must be controlled to match the system demand.
The parameter to control is usually the discharge pressure. A range of variation
of about 10 % between discharge pressure and end user demand is generally
accepted.
1. Reciprocating compressors. Automatic start-and-stop control in which
compressors run at full load for a period and then are stopped can be used if
starting unloaders are applied and if it is not started too often.
Constant-speed control is performed by different methods (intake throttling,
external bypassing, inlet valve loading).
Variable-speed control, which is the ideal control for positive-displacement
compressors, can conveniently be achieved by installing variable-speed drives,
particularly a.c. drives for safety reasons. Variable-speed drives also provide a
soft-start feature with minimum starting current, thus avoiding damage to the
motor through frequent starting. This allows the compressor to stop completely
and start again more frequently than in installations where electric motors are fed
directly from the line;
2. Screw compressors. Automatic start-and-stop control can be used if an auto-
matic start unloading system is installed. To unload the compressor and thus
reduce the power required, convenient systems are those which include closing
the inlet valve, releasing oil reservoir pressure, and reducing oil flow to the
compressor. Variable-speed drives can also be used;
3. Centrifugal compressors. There are three main methods of controlling centrifu-
gal compressors: variable speed, blowoff of unwanted output, and intake
throttling.
Speed is varied by means of variable-speed drives, either electrical or steam or
gas turbine depending on the size of the system. Power decreases following
recirculation control can be used to release any excess air into the atmosphere or
