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ANUAL
2.10.2 Head Pressure Controls-Air Cooled
Systems
2.10.2.1 -20°F Condenser Fan Speed
Used for outdoor or indoor installations where ambient
condenser air inlet temperatures may fall to -20°F, variable
speed condenser motor control is used to maintain head
pressure. The fan speed control is a continual modulation
of the motor’s speed. The condenser fan speed controller is
factory installed in the remote outdoor condenser electric box.
The fan speed controller requires no adjustment.
Wire the condenser in accordance with the electrical
schematic and appropriate codes. The fan speed controller
will automatically control the head pressure.
Factory setting: Switch contacts are set to close on a pressure
rise to 440 psig and open at 330 psig. Set point range is 200
to 610 psig. The differential is set at 110 psi and is adjustable.
2.10.2.2 -30°F Flooded Head Pressure Control
Used for outdoor or indoor installations where ambient
condenser air inlet temperatures may fall to -30°F, fl ooded
head pressure control is designed to maintain head pressure
during low ambient conditions. A head pressure control valve
and a receiver is used in the refrigeration circuit to back up
liquid refrigerant into the condenser coil. The head pressure
control valve is a 3-way modulating valve controlled by
discharge pressure.
When the A/C unit begins to operate, the discharge pressure
rises to 330 psig and the condenser fan is cycled on, as
described in Section 2.10.2.1. When ambient temperature
drops, the discharge pressure drops also. When the discharge
pressure drops, the head pressure control valve diverts
discharge gas away from the condenser to the receiver. Liquid
fl ow from the condenser is restricted, causing liquid to back
up in the condenser.
Flooding the condenser reduces the area available for heat
transfer. The desired result is to increase the pressure into the
condenser, maintaining a minimum discharge pressure during
low ambient operation thus ensuring proper condensing
temperature. The head pressure control valve requires no
adjustment.
While the fan is running, the fl ooded head pressure control
valve modulates the amount of discharge gas entering the
receiver. As the pressure increases, the valve diverts more
discharge gas to the condenser, allowing more liquid to fl ow
from the condenser to the receiver.
When using this method of head pressure regulation there
must be enough refrigerant in the system to ensure adequate
charge at the lowest expected ambient temperature in which
the system will be operating. A receiver is used to store the
extra refrigerant when the condenser is not using it.
NOTE
Systems utilizing air cooled condensers must not
have a refrigerant line pressure drop over 14 psig
across the condenser and the interconnecting piping
to the condenser sections. See the line sizing tables
in section 2.7.1.2 on page 9.
2.10.3 Head Pressure Controls-Water/Glycol
Cooled Systems
In a water/glycol condenser, condensing temperature is
maintained by the liquid fl owing through a regulating valve
and then into a liquid-cooled condenser. The regulating
valve opens to increase the liquid fl ow as the refrigerant
pressure rises (or closes as the refrigerant pressure falls).
The regulating valve is factory set for the correct condensing
temperature; however, it can be adjusted to increase or
decrease the condensing temperature.
Head pressure regulating valves are available in 2-way
or 3-way configurations. Refer to the piping diagram to
determine which type valve is provided. The location and
size of regulating valves varies with the size and model of
the A/C unit.
2.10.4 Humidifi er Adjustment
The humidifi er has an adjustable capacity potentiometer on
the humidifi er control circuit board. The potentiometer may
need to be fi eld adjusted if the humidifi er is not supplying
enough capacity for the current room conditions.
It is recommended that if the humidifi er capacity potentiometer
requires adjustment, the adjustment be made in small
increments and verifi ed. Refer to the humidifi er manual sent
with your unit for the capacity potentiometer location.
CAUTION
Adjusting the capacity potentiometer too high may
result in condensate forming in the system.
2.10.5 Thermal Expansion Valve
Mini-Space EC units employ a Thermal Expansion Valve (TEV).
The TEV maintains constant superheat of the refrigerant
vapor at the outlet of the evaporator by metering the fl ow of
refrigerant into the evaporator. Superheat is the difference
between the refrigerant vapor temperature and its saturation
temperature at a given suction pressure. By controlling
superheat, the TEV keeps nearly the entire evaporator surface
active while preventing liquid refrigerant from returning to
the compressor. As a standard, superheat is factory set at
12
–
15 °F and should not need adjustment.
