IOMM AGZB1 AGZ 010B through 034B 63
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 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.
Figure 19 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 chiller water 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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