Advertisement
and determine if the accumulator design is adequate,
please see the section entitled Excessive Liquid Flood
back Tests at the end of this bulletin. The accumulator
oil return orifice should be from .040 to .055 inches (1 –
1.4 mm) in diameter depending on compressor size and
compressor flood back results. A large-area protective
screen no finer than 30 x 30 mesh (0.6 mm openings)
is required to protect this small orifice from plugging.
Tests have shown that a small screen with a fine mesh
can easily become plugged causing oil starvation to the
compressor bearings.
Screens
The use of screens finer than 30 x 30 mesh (0.6mm
openings) anywhere in the system should not be
used with these compressors. 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.
Crankcase Heat - Single Phase
Crankcase heaters are not required on single phase
compressors when the system charge is not over the
120% limit shown in Table 5. A crankcase heater is
required for systems containing more than 120% of the
compressor refrigerant charge limit listed in Table 5.
This includes long line length systems where the extra
charge will increase the standard factory charge above
the 120% limit.
Experience has shown that compressors may fill with
liquid refrigerant under certain circumstances and
system configurations, notably after longer off cycles
when the compressor has cooled. This may cause
excessive start up clearing noise or the compressor
may lock up and trip on protector several times before
starting. The addition of a crankcase heater will reduce
customer noise and dimming light complaints since the
compressor will no longer have to clear out liquid during
start. Table 6 lists the crankcase heaters recommended
for the various models and voltages.
Crankcase Heat – Three-Phase
A crankcase heater is required for three-phase
compressors when the system charge exceeds the
compressor charge limit listed in Table 5 and an
accumulator cannot be piped to provide free liquid
drainage during the off cycle (See Figure 2 and Table
6).
© 2010 Emerson Climate Technologies
Printed in the U.S.A.
Pump down Cycle
A pump down cycle for control of refrigerant migration
is not recommended for scroll compressors of this size.
If a pump down cycle is used, a separate external
check valve must be added. The 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. The check
valve will in some cases leak more than reciprocating
compressor discharge reeds, normally used with pump
down, causing the scroll compressor to recycle more
frequently. Repeated short-cycling of this nature can
result in a low oil situation and consequent damage to
the compressor. The low-pressure control differential
has 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.
Minimum Run Time
There is no set answer to how often scroll compressors
can be started and stopped in an hour, since it is
highly dependent on system configuration. Other
than the considerations in the section on Brief Power
Interruptions, there is no minimum off time because
scroll compressors start unloaded, even if the
system has unbalanced pressures. The most critical
consideration is the minimum run time required
to return oil to the compressor after startup. To
establish the minimum run time obtain a sample
compressor equipped with a sight tube (available from
Emerson Climate Technologies) and install it in a system
with the longest connecting lines that are approved for
the system. The minimum on time becomes the time
required for oil lost during compressor startup to return
to the compressor sump and restore a minimal oil
level that will assure oil pick up through the crankshaft.
Cycling the compressor for a shorter period than this,
for instance to maintain very tight temperature control,
will result in progressive loss of oil and damage to the
compressor. See Application Engineering Bulletin 17-
1262 for more information on preventing compressor
short cycling.
Reversing Valves
Since Copeland Scroll compressors have very high
volumetric efficiency, their displacements are lower
than those of comparable capacity reciprocating
compressors. As a result, Emerson recommends that
the capacity rating on reversing valves be no more than
2 times the nominal capacity of the compressor with
which it will be used in order to ensure proper operation
of the reversing valve under all operating conditions.
3
AE4-1312 R2
Advertisement
