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heater is uniformly tensioned along its entire length and
that the circumference of the heater element is in
complete contact with the compressor shell. It's
important that the clamp screw is torqued to the range
shown in Table 1 to ensure adequate contact and to
prevent heater burnout. Never apply power to the heater
in free air or before the heater is installed on the
compressor to prevent overheating and burnout.
WARNING
Crankcase heaters must be properly grounded to
reduce the potential of a fire or shock hazard.
Pump Down Cycle
If a pump down cycle is required by the system
designer, a onetime pump down at the end of the
cooling cycle is preferred over a recycling pump down.
Although not preferred, a recycling pump down cycle
can be used to minimize off-cycle refrigerant migration
to the compressor. The risk of a short cycling condition
that can lead to oil pump out, excessive contactor wear,
unnecessary energy use, and excessive low pressure
cut-out switch cycles make recycling pump down
undesirable. In lieu of the pump down cycles mentioned
above, simply closing a liquid line solenoid valve when
the compressor cycles off is a good, simple, and cost
effective method of minimizing off-cycle refrigerant
migration.
Pump Out Cycle
A pump out cycle has been successfully used by some
manufacturers of large rooftop units. After an extended
off period, a typical pump out cycle will energize the
compressor for up to one second followed by an off time
of 5 to 20 seconds. This cycle is usually repeated a
second time, the third time the compressor stays on for
the cooling cycle. If pump out cycle is employed, a
crankcase heater must be used if the circuit charge
amount exceeds the values listed in Table 2.
Reversing Valves
Since Copeland scroll compressors have very high
volumetric efficiency, their displacements are lower than
those
of
comparable
compressors.
Reversing valve sizing must be within the guidelines of
the valve manufacturer. Required pressure drop to
ensure valve shifting must be measured throughout the
operating range of the unit and compared to the valve
manufacturer's data. Low ambient heating conditions
© 2020 Emerson Climate Technologies, Inc.
capacity
reciprocating
with low flow rates and low pressure drop across the
valve can result in a valve not shifting. This can result in
a condition where the compressor appears to be not
pumping (i.e., balanced pressures). It can also result in
elevated compressor sound levels.
During a defrost cycle, when the reversing valve
abruptly changes the refrigerant flow direction, the
suction and discharge pressures will go outside of the
normal operating envelope. The sound that the
compressor makes during this transition period is
normal, and the duration of the sound will depend on the
coil volume, outdoor ambient temperature, and system
charge level. If the compressor is operating during a
defrost cycle the compressor should operate between
50-60Hz. The preferred method of mitigating defrost
sound is to shut down the compressor for 20 to 30
seconds when the reversing valve changes position
going into and coming out of the defrost cycle. This
technique allows the system pressures to reach
equilibrium without the compressor running. The
additional start-stop cycles do not exceed the
compressor design limits, but suction and discharge
tubing design should be evaluated.
The reversing valve solenoid should be wired so that the
valve does not reverse when the system is shut off by
the operating thermostat in the heating or cooling mode.
If the valve is allowed to reverse at system shutoff,
suction and discharge pressures are reversed to the
compressor. This results in pressures equalizing
through the compressor
compressor to slowly rotate backwards until the
pressures equalize. This condition does not affect
compressor durability but can cause unexpected sound
after the compressor is turned off.
Contaminant Control
Manufacturing processes have been designed to
minimize the introduction of solid or liquid contaminants.
Dehydration and purge processes ensure minimal
moisture levels in the compressor, and continuous
auditing of lubricant moisture levels ensures that
moisture
isn't
inadvertently
compressor. However, due to the manufacturing
processes, Copeland scroll compressors will contain a
miniscule amount of solid and liquid contaminants when
they leave the factory.
During unit assembly and field servicing, compressors
shouldn't be left open to the atmosphere for longer than
five minutes. It is generally accepted that system
moisture levels should be maintained below 50 ppm. A
8
AE4-1486 R1
which can cause the
introduced
into
the
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