Testing Guidelines; Pump-Down Cycle; Compressor Hi-Pot Testing; Scroll Compressor Functional Check - Emerson ZB Copeland Scroll Product Manual

ZB
Series
Application Guide

4.3 Pump-Down Cycle

A pump-down cycle for control of refrigerant migration may be used
instead of, or in conjunction with, a crankcase heater when the
compressor is located so that cold air blowing over the compressor
makes the crankcase heater ineffective. A separate external check
valve must be added to the discharge line if pump-down is used.
The built-in scroll discharge check valve is designed to stop extended
reverse rotation and prevent high pressure gas from leaking rapidly
into the low side after shut off. High side leak-back through the check
valve may exceed amounts typically found in reciprocating compressors
with reed valves. This can cause the compressor to recycle more frequently.
Repeated short-cycling of this nature can result in low compressor oil
and consequent damage to the compressor. The recommended external
check valve will prevent the frequent recycling due to leak-back. The low
pressure control cut-in and cut-out settings have to be reviewed since
a relatively large volume of gas will re-expand from the high side of the
compressor into the low side on shut down. Emerson recommends that
the cut out setting of the pump-down pressure control be set no more
than a few degrees of equivalent saturated pressure below the lowest
expected normal operating pressure. It is not necessary to pump-down
into nearly a vacuum to remove all liquid refrigerant for the low side.
To achieve a fairly wide control differential the cut in setting should be
set a few degrees of equivalent saturated pressure below the lowest
expected temperature of the medium that is cooled. Copeland Scroll
compressors trap a considerable volume of high pressure gas between
the muffler plate and the top cap. When the compressor shuts down the
trapped gas will expand back into the suction side of the system. This
frequently causes a pulse of pressure to propagate down the suction
line and can cause the low pressure switch to reset. The compressor
must not be allowed to short cycle which may result in oil pump out.
The electrical circuitry should be arranged so that compressor restart
is triggered by demand from the thermostat rather than a reset low
pressure switch. Setting a wider differential between the cutout and cut
in pressures of a low pressure switch may solve the short cycling problem
but may also result in unacceptable temperature swings in the cooled
space. If short cycling cannot be avoided, using a 3 minute time delay
will limit the cycling of the compressor to an acceptable level.
4.4 Filter Screens In System
The use of screens finer than 30 x 30 mesh (0.6 mm openings) anywhere
in the system is not recommended. Field experience has shown that finer
mesh screens used to protect thermal expansion valves, capillary tubes,
or accumulators can become temporarily or permanently plugged with
normal system debris and block the flow of either oil or refrigerant to the
compressor. Such blockage can result in compressor failure.

5. Testing Guidelines

5.1 Compressor Hi-Pot Testing

Refrigeration scroll compressors are configured with the motor in the
bottom of the shell. Unlike most other hermetic compressors, the motor
of a scroll compressor can be immersed in refrigerant when liquid is
present in the shell. Hi- Pot tests with liquid refrigerant in the shell can
show higher levels of current leakage due to the higher electrical
conductivity of liquid refrigerant vs. refrigerant vapor and oil. This
phenomenon can occur with any compressor when the motor is
immersed in refrigerant and does not present any safety issue. To lower
the current leakage reading, operate the system for a brief period of time
redistributing the refrigerant to a more normal configuration and test
again. Under no circumstances should the Hipot test be performed while
the compressor is in vacuum.

5.2 Scroll Compressor Functional Check

A functional compressor test with the suction service valve closed to
check how low the compressor will pull suction pressure is not a good
indication of how well a compressor is performing. Such a test will
almost certainly damage a scroll compressor. The following diagnostic
procedure should be used to evaluate whether a Copeland Scroll
compressor is working properly.
Proper voltage to the unit should be verified.
1.
2. The normal checks of motor winding continuity and short to ground
should be made to determine if the inherent overload motor
protector has opened or if an internal motor short or ground fault
has developed. If the protector has opened, the compressor must
be allowed to cool sufficiently to allow it to reset.
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