IOMM ACZ1 ACZ 010A through 039A
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Optional Controls
SpeedTrol Head Pressure Control
The SpeedTrol method of head pressure control operates in conjunction with FanTrol by modulating
the motor speed on system #1 fan in response to condenser pressure. By reducing the speed of the
last fan as the condensing pressure falls, the unit can operate to 0°F (-18°C) ambient air temperature.
The SpeedTrol fan motor is a single-phase, 230/460 volt, thermally protected motor specially
designed for variable speed operation. The solid-state speed control is mounted in the unit control
panel and is connected to a Schrader fitting on the liquid line. The control is factory-set to start
modulating fan speed at 230 psig, and it will maintain a minimum condensing pressure of 170 to 180
psig. Minimum starting voltage for SpeedTrol motors is 120 volts.
A low ambient timer function is included in the microprocessor. When the solenoid valve and lead
compressor are energized by the controller, the low pressure cutout control is bypassed and the
compressor is allowed to start with the low pressure control open.
After about 2-3/4 minutes, the time delay will open and the low pressure cutout function is again
operable. If the system has not built up enough evaporator pressure to close the low pressure setting,
the compressor will stop.
Due to the vertical condenser design, it is recommended that the unit be oriented so that prevailing
winds blow parallel to the unit length, thus minimizing effects on minimum ambient operation. If it is
not practical to orient the unit in this manner, a wind deflector should be constructed.
Hot Gas Bypass
Hot gas bypass is a system for maintaining evaporator pressure at or above a minimum value. The
purpose for doing this is to keep the velocity of the refrigerant as it passes through the evaporator
high enough for proper oil return to the compressor when cooling load conditions are light. It also
maintains continuous operation of the chiller at light load conditions.
The field installed hot gas solenoid valve should be wired to open whenever the liquid line solenoid
valve is energized. This can be accomplished by wiring the hot gas solenoid (SV5) in parallel with
the liquid line solenoid at terminals 14 and 16. The pressure-regulating valve is factory-set to begin
opening at 58 PSIG (32°F for R-22) when the air-charged bulb is in an 80°F ambient temperature.
The bulb can be mounted anywhere as long as it senses a fairly constant temperature at various load
conditions. The compressor suction line is one such mounting location. It is generally in the 50°F to
60°F range.
The chart below (Figure 14) indicates that when the bulb is sensing 50°F to 60°F temperatures, the
valve will begin opening at 54 PSIG. This setting can be changed as indicated above, by changing
the pressure setting, remove the cap on the bulb and turn the adjustment screw clockwise. To lower
the setting, turn the screw counterclockwise. Do not force the adjustment beyond the range it is
designed for, as this will damage the adjustment assembly.
The regulating valve opening point can be determined by slowly reducing the system load (or
increasing the required evaporator temperature setting indicated on the unit thermostat), while
observing the suction pressure. When the bypass valve starts to open, the refrigerant line on the
evaporator side of the valve will begin to feel warm to the touch.
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