McQuay AWS, Operating Manual

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36 KOMAC00607-09EN
The target value is adjusted as needed to maintain discharge superheat within a range
from 27 degrees F to 45 degrees F. As the discharge superheat approaches 27
degrees F, the suction superheat target is adjusted up. As the discharge superheat
approaches 45 degrees F, the suction superheat target is adjusted down. The control will
apply a 0.9 degrees F maximum reset every 5 minutes to the base target.
Maximum Operating Pressures
The EXV control maintains the evaporator pressure in the range defined by the
maximum operating pressure.
If the leaving water temperature is higher than 68°F at startup or if the pressure becomes
higher than 50.8 psi during normal operations, then a pressostatic (constant pressure)
control will be started to keep the compressor in the envelope.
Maximum operating pressure is 50.8 psi. It switches back to normal operation as soon as
the suction superheat drops below 7.2°F suction superheat.
Response to Compressor Capacity Change
The logic will consider transition from 50% to 60% and from 60% to 50% as special
conditions. When a transition is entered the valve opening will change to adapt to the
new capacity, this new calculated position will be kept for 60 seconds. The valve
opening will be increased during 50% to 60% transition and decreased in 60% to 50%
transition.
Purpose of this logic is to limit liquid flood back when changing from 50% to 60% if the
capacity increases above 60% due to slides movement.
Manual Control
The EXV position can be set manually. Manual control can only be selected when the
EXV state is Pressure or Superheat control. At any other time, the EXV control set
point is forced to auto.
When EXV control is set to manual, the EXV position is equal to the manual EXV
position setting. If set to manual when the circuit state transitions from run to another
state, the control setting is automatically set back to auto. If EXV control is changed
from manual back to auto while the circuit state remains run, the EXV state goes back to
the normal operations if possible or to pressure control to limit maximum operating
pressure.
Transitions Between Control States
Whenever EXV control changes between Startup Operation, Normal Operation, or
Manual Control, the transition is smoothed by gradually changing the EXV position
rather than changing all at once. This transition prevents the circuit from becoming
unstable and resulting in a shutdown due to alarm trip.
Economizer Control
The economizer is activated when a circuit is in a run state and capacity exceeds 95%.
It turns off when the load drops below 60% or the circuit is no longer in a run state.
Liquid Injection
Liquid injection is activated when the circuit is in a run state and the discharge
temperature rises above the Liquid Injection Activation set point.
Liquid injection is turned off when the discharge temperature decreases below the
activation set point by a differential of 10-degrees C.