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© 2015 Emerson Climate Technologies, Inc.
AE4-1331 R7
will be protected for some time while it trips the
protector. However, as refrigerant leaks out, the mass
flow and the amperage draw are reduced and the
scrolls will start to overheat. A low pressure control
is recommended for loss of charge protection for the
highest level of system protection. The low pressure
cut-out can provide protection against indoor blower
failure in cooling, outdoor fan failure in heating, closed
liquid or suction line service valves, or a blocked
liquid line screen, filter, orifice, or TXV. All of these
can starve the compressor of refrigerant and result in
compressor failure. The low pressure cut-out should
have a manual reset feature for the highest level of
system protection. If a compressor is allowed to cycle
after a fault is detected, there is a high probability
that the compressor will be damaged and the system
contaminated with debris from the failed compressor
and decomposed oil. If current monitoring of the
compressor is available, the system controller can
take advantage of the compressor TOD and internal
protector operation. The controller can lock out the
compressor if current draw is not coincident with the
contactor energizing, implying that the compressor
has shut off on its internal protector. This will prevent
unnecessary compressor cycling on a fault condition
until corrective action can be taken.
Heat Pump Protection
A low pressure control is highly recommended for
loss of charge protection and other system fault
conditions that may result in very low evaporating
temperatures. Even though these compressors have
internal discharge temperature protection, loss of
system charge will result in overheating and recycling
of the motor overload protector. Prolonged operation in
this manner could result in oil pump out and eventual
bearing failure. A cut out setting no lower than 20 psig
(1.4 bar) is recommended.
Operation near -25°F (–32°C) saturated suction
temperature is clearly outside the approved operating
envelope shown in
Figure 7
. However, heat pumps
in some geographical areas have to operate in this
range because of the low ambient temperatures. This
is acceptable as long as the condensing temperature
is not above 90°F (32°C) and the resulting discharge
temperature is below 275°F (135°C). Some liquid
floodback to the compressor under these conditions
can help keep the discharge temperature under
control.
Discharge Line Thermostat
Some systems, such as air-to-air heat pumps, may not
work with the above low pressure control arrangement.
A discharge line thermostat set to shut the compressor
off before the discharge temperature exceeds 260°F
(125°C) may have to be used to achieve the same
protection. Mount the discharge thermostat as close
as possible to the compressor discharge fitting
and insulate well. See
Figure 5
and
Table 4
for
recommended Emerson Climate Technologies part
numbers.
Air Conditioning Unit Protection
Air-conditioning-only units can be protected against
high discharge temperatures through a low pressure
control in the suction line. Testing has shown that
a cut out setting of not lower than 55 psig (3.8 bar)
will adequately protect the compressor against
overheating from the aforementioned loss of charge,
blower failure in a TXV system, etc. A higher level
of protection is achieved if the low pressure control
is set to cut out around 95 psig (6.7 bar) to prevent
evaporator coil icing. The cut in setting can be as high
as 180 psig (12.5 bar) to prevent rapid recycling in
case of refrigerant loss. If an electronic controller is
used, the system can be locked out after repeated low
pressure trips.
High Pressure Control
If a high pressure control is used with these compressors
the recommended maximum cut out setting is
650 psig (45 bar).
The high pressure control should
have a manual reset feature for the highest level of
system protection.
It is not recommended to use
the compressor to test the high pressure switch
function during the assembly line test.
Shut Down Device
All scrolls in this size range have one of several types
of shutdown solutions that nearly eliminate shut down
noise. Single phase ZP*K3 scrolls use a cam-type
device that separates the scrolls when they are driven
backwards as high-pressure gas equalizes from the
high side of the compressor to the low side during
shutdown. Three phase ZP*K3 scrolls use a fluid brake
to mitigate reverse rotation sound after shutdown. The
new ZP*K5 scrolls incorporate a scroll discharge port
check valve that prevents high pressure gas trapped
in the dome from returning through the scroll set after
shutdown. All of these shut down solutions allow the
scroll compressor to restart immediately even if the
system is not equalized, eliminating the need for a time
delay. Development testing should include a review of
the shutdown sound for acceptability in a particular
system. Please refer to the section on
Brief Power
Interruption
.
