Carrier AquaForce B Series Controls, Start-Up, Operation, Service And Troubleshooting Instructions page 65

BRAZED PLATE HEAT EXCHANGER
(ECONOMIZER)
MAIN EXV
1. Cable
2. Glass Seal
3. Motor Housing
4. Stepper Motor
5. Bearing
6. Lead Screw
7. Insert
8. Valve Piston
9. Valve Seat
10. Valve Port
Low Suction Pressure (SPMIN)
The EXV control tries to open up the EXV to increase the suction
pressure and come out of this mode. The SST setting to enter this
mode is dependent on the fluid type. With water the EXV enters
this mode if SST is less than SST_Freeze – 6.5°F (
normal discharge superheat or less than SST_Freeze – 18.75°F
(–28.2°C) in low DSH condition. It remains in this mode until
SST is greater than SST_Freeze + 2°F (1.1°C). SST_Freeze is
32°F (0°C) for water and freeze set point for brine.
Maximum Suction Pressure (SPMAX)
This mode is disabled for 300 sec after start. The EXV enters
this mode if the SST is greater than 55°F (13°C) and the circuit
is not in DP mode. The EXV closes down to regulate the SST
at about 53.2°F (11.8°C). If the SST is less than 52.3°F
(11.3°C) or the circuit is in DP mode, then the EXV returns to
the normal mode of operation.
ECONOMIZER RELIEF
SIGHT GLASS
Fig. 59 — Economizer Assembly
Fig. 60 — Cutaway Views of the Electronic Expansion Valve
21.4°C) in
–
ECONOMIZER FLOW TO COMPRESSOR
ECONOMIZER EXV
FILTER DRIER
MAIN FLOW TO EVAPORATOR
ECONOMIZER EXV CONTROL
The economizer EXV is controlled by the SIOB (J18-STPR2). An
economizer gas temperature thermistor (ECT) and an economizer
pressure transducer (EPT) are located in the line running from the
economizer assembly to the compressor. The economizer pressure
is converted to saturated temperature and is used to calculate econ-
omizer superheat. Economizer superheat equals economizer tem-
perature minus saturated economizer temperature. The control
system controls the economizer EXV to maintain the economizer
superheat setpoint, which is approximately 18°F (–7.8°C). The
economizer will start operation when circuit capacity is at 55% or
above. It will turn off at 45%.
EXV TROUBLESHOOTING PROCEDURE
There are two different economizer EXVs. Both of the econo-
mizer EXVs have a total of 2625 steps. There are three differ-
ent main EXVs, which all have a total of 3810 steps. The EXV
motor moves at 150 steps per second. Commanding the valve
65
LIQUID LINE SHUT OFF VALVE
a30-4241