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21.0 AUTOMATIC COMPRESSOR LOAD CONTROL
21.1 The DE-2500 control system is designed to control the throughput or gas transmission rate of
engine or motor driven compressors. The throughput or actual volume of gas compressed by
the machine per unit of time can be controlled by two different means. The two means are: A)
to vary the rate at which compression events occur, SPEED CONTROL or B) to vary the
volume of gas compressed per event, CAPACITY CONTROL. For both reciprocating and screw
compressors, the most efficient type of throughput control is to change the repetition rate of
the compressor action, that is the cycles per unit time or SPEED CONTROL. This parameter
is normally described in terms of the operating RPM of the prime mover. The control of the
prime mover speed to change the compressor throughput is almost 100% efficient with respect
to the work required by the compressor itself for the increased throughput. For a given set of
operating conditions; suction pressure, discharge pressure, gas temperature and gas
composition; the effort required to move each cubic foot of gas (a fixed mass) remains
constant. Each stroke of the reciprocating compressor or each turn of the screw compressor
(unless modified physically) captures the same amount of gas at the input end (suction). Each
compressor cycle moves the same mass of gas to the output end (discharge). This action
requires the same torque for each cycle from the prime mover. As the rate (prime mover RPM)
of compressor events (each requiring a fixed amount of Torque) is increased, so is the
horsepower (RPM X TORQUE). At the same time, the volume of gas compressed is increasing
at the same rate. In this case, the horsepower and the throughput increase together in direct
proportion with very little lost work. This is why SPEED CONTROL is generally the first step in
managing compressor throughput.
A factor which limits the effectiveness of speed control in compressor throughput management,
is the RPM range of the prime mover available with adequate torque. In the case of gas
engines in particular, the prime mover efficiency is significantly affected by the operating RPM.
Most gas engines, for example, can carry their rated torque load with acceptable performance
in only the upper 1/2 or 1/3 of their rated RPM range. For this reason, it is necessary to select
a MINIMUM RPM and a MAXIMUM RPM to define the range in which the DE-2500 will control
the speed of an engine/ compressor system. After a startup sequence MANUAL or AUTO
START, the speed of the prime mover must reach the MINIMUM LOAD RPM for load control
to begin. After the MINIMUM LOAD RPM is reached, the CAPACITY CONTROL OUTPUT will
be used to begin increasing compressor throughput per cycle using CAPACITY CONTROL to
satisfy the PRIMARY CONTROL setpoint. In a screw compressor application the amount of
gas moved per turn of the screw (work per cycle) is typically controlled by using a slide or turn
valve (internal stepless bypass). In reciprocating compressor applications an external bypass
is often used. For more detail on applications see section 22. The PRIMARY CONTROL input
parameter is selected during the programming of the unit as either CHANNEL 30, 31, or 32.
The input value of the assigned parameter (30,31,32) is compared to the setpoint and the
CAPACITY OUTPUT varied to maintain this value within a user selected DEADBAND in a
closed loop fashion, at a rate determined by user adjustable tuning values. In a typical
application, channel 30 would be engine intake manifold pressure, channel 31 would be
compressor suction pressure, and channel 32 would be compressor discharge pressure. The
DE-2500 system will continue to increase the CAPACITY OUTPUT to its maximum value in
an attempt to reach the PRIMARY CONTROL setpoint. If the PRIMARY CONTROL setpoint
is not reached by the time that the CAPACITY CONTROL has reached 100%, then the DE-
2500 controller will begin to increase the RPM CONTROL OUTPUT to the governor.
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