Economizer Exv Control - Carrier AQUAFORCE 30XA080-500 Controls, Start-Up, Operation, Service And Troubleshooting Instructions

The discharge superheat leaving the compressor is main-
tained between approximately 18° and 25° F (10° and 14° C),
or less. Because EXV status is communicated to the Main Base
Board (MBB) and is controlled by the EXV modules, it is pos-
sible to track the valve position. The unit is then protected
against loss of charge and a faulty valve. During initial start-up,
the EXV is fully closed. After an initialization period, valve
position is tracked by the EXV module by constantly monitor-
ing the amount of valve movement.
DX (Direct Expansion) COOLER MAIN EXV CON-
TROL — Each circuit has a thermistor located in a well in the
discharge of the compressor (DGT) and another one located in
the compressor motor cavity (SGT). Thermistors are also locat-
ed in a well in the water inlet and outlet nozzles. Apart from
thermistors, each circuit also has a discharge and suction pres-
sure transducer. Discharge and suction pressures as measured
by the transducers are converted to saturated temperatures. The
main control logic for the EXV uses approach temperature in
the cooler (Water Inlet Temperature – Saturated Suction Tem-
perature) and suction superheat (Suction Temperature – Satu-
rated Suction Temperature) to control the position of the EXV.
The EXV module controls the position of the electronic expan-
sion valve stepper motor to maintain a minimum approach and
a suction superheat set point.
The EXV control logic has several overrides, which are also
used to control the position of the EXV.
• Maximum Operating Pressure (MOP)
• High Discharge Gas Temperature
• Low Discharge Superheat
MOP — The EXV is also used to limit cooler saturated suction
temperature to 62 F (16.6 C). This makes it possible for the
BRAZE PLATE
HEAT EXCHANGER
*Economizer EXV has sight glass
on opposite side of valve shown.
MAIN EXV
*SIGHT GLASS
MAIN FLOW
TO COOLER
Fig. 46 — Economizer Assembly
chiller to start at higher cooler fluid temperatures without over-
loading the compressor. This is commonly referred to as MOP
(maximum operating pressure). If the SST (Saturated Suction
Temperature) is equal to or greater than the MOP set point then
the MBB will try to control the EXV position to maintain the
MOP set point.
High Discharge Gas Temperature — The EXV is also used to
limit the discharge gas temperature to 200 F (93.3 C). This
makes it possible for the chiller to operate at low load and high
ambient temperature without overloading the compressor. If
the DGT is approaching the High DGT limit then the control
will try to control the EXV position to bring the DGT down by
opening the EXV.
Low Discharge Superheat — The EXV is also used to limit
the discharge superheat (DSH) to 18° F (10° C). This makes it
possible for the chiller to operate at high load and low ambient
temperature without overloading the compressor. If the DSH
goes below 18° F (10° C) the MBB will try to control the EXV
position to bring the discharge superheat up by closing the
EXV valve.
The suction superheat entering the compressor is main-
tained between approximately 15° and 25° F (8.3° and 14° C)
while maintaining a constant approach in the cooler. Because
EXV status is communicated to the Main Base Board (MBB)
and is controlled by the EXV modules, it is possible to track the
valve position. During initial start-up, the EXV is fully closed.
After an initialization period, valve position is tracked by the
EXV module by constantly monitoring the amount of valve
movement.
ECONOMIZER
RELIEF
ECONOMIZER
FLOW TO
COMPRESSOR
ECONOMIZER
RELIEF
ECONOMIZER
EXV
75
LIQUID LINE
SHUT OFF
VALVE
FILTER DRIER
a30-4468