Altronic DE-2500 - Primary and Secondary Control Actions; Primary and Secondary Control Inputs;Actions

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22.7 The DE-2500 controls the compressor throughput based upon the PRIMARY control input; an
analog value selected from channels 30, 31, or 32 which would typically be engine manifold
pressure, suction pressure or discharge pressure respectively. Additionally, the DE-2500
controller allows for up to 14 additional SECONDARY CONTROL setpoints for compressor
throughput control or other supplemental control functions.
For each of the 14 analog inputs and engine RPM, both a low and a high CONTROL setpoint
is provided. These CONTROL setpoints are in addition to the safety shutdown setpoints. The
violation of a CONTROL setpoint (below a low or above a high setpoint value) causes a
SECONDARY CONTROL action to occur. The SECONDARY CONTROL actions which can
be programmed using Terminal Software version 1.8 or lower are shown below. For units being
programmed from Terminal software 2.0 or higher see section 22.8.
A) INHIBIT LOADING OF PRIMARY CONTROL
B) FORCE UNLOADING OF PRIMARY CONTROL
C) ACTIVATE SECONDARY CONTROL OUTPUTS
For the 13 analog input parameters NOT assigned to the PRIMARY CONTROL FUNCTION,
the control setpoints can be individually assigned to INHIBIT LOADING or FORCE
UNLOADING. When the INHIBIT LOADING selection is made, any violation of the setpoint
value will automatically prevent any additional loading of the compressor, regardless of the
status of the PRIMARY control loop. One typical application of the LOAD INHIBIT function for
a secondary control setpoint would be a permissive to load signal based upon the temperature
of the oil being used for oil injection on a screw compressor. By assigning a LOAD INHIBIT
function to the oil temperature low control setpoint, the screw compressor will not be loaded
until the oil has reached the required minimum temperature.
Because each of the monitored analog channels has these two control setpoints available,
many fairly elaborate control options are possible. For example, the DE-2500 system can be
configured in a manner to take advantage of varying ambient operating conditions. An example
of this would be to limit the compressor throughput of a reciprocating compressor on the basis
of a high discharge gas temperature. In some cases, the compressor gas cooling system is
not capable of maintaining an acceptable discharge temperature at the highest compressor
throughput of the machine on the hottest days. This same cooling system can maintain an
acceptable discharge temperature at maximum throughput during normal weather conditions.
By setting a load inhibit function on high discharge temperature, which is somewhat higher than
normal at maximum throughput on cooler days, but well below the safety shutdown setpoint,
the primary control setpoint can be selected to take advantage of the cooler weather by running
at maximum throughput whenever possible. The secondary control LOAD INHIBIT function
permits this without the risk of overheating the compressor on the hottest days without
requiring a change to the PRIMARY CONTROL setpoint based on the weather.
The FORCE UNLOADING function can be used in conjunction with the LOAD INHIBIT or
independently. When a control setpoint which has been assigned the FORCE UNLOAD
function is violated, the throughput of the compressor will be reduced. When conditions return
to normal, the programmed control routine will resume. This function is intended to prevent the
transient overloading of compressor, or engine components, by forcing the unloading of the
system before the conditions can cause equipment damage or equipment shutdown due to the
safety shutdown setpoints being violated.