Application guidelines
38
FRCC.PC.014.A4.22
System design recommendations
Crankcase heater / sump heater:
When the
compressor is idle, the oil temperature in the
sump of the compressor must be maintained at
no lower than 18°F above the saturation tem-
perature of the refrigerant on the low-pressure
side. This requirement ensures that the liquid
refrigerant is not accumulating in the sump.
A crankcase heater is only effective if capable of
sustaining this level of temperature difference.
Tests must be conducted to ensure that the ap-
propriate oil temperature is maintained under
all ambient conditions (temperature and wind).
Provide separate electrical supply for the
heaters so that they remain energized even
when the machine is out of service (eg. seasonal
shut-down).
Refer to section “Crankcase heating function”
for details and settings of crankcase heating
function integrated in the drive.
Liquid line solenoid valve
(LLSV)
:
An LLSV
may be used to isolate the liquid charge on the
condenser side, thereby preventing against
charge transfer or excessive migration to the
compressor during off-cycles. When installed,
EXV ensures also this function. The quantity of
refrigerant on the low-pressure side of the sys-
tem can be further reduced by using a pump-
down cycle in association with the LLSV.
Pump-down cycle:
A pump-down cycle
represents one of the most effective ways to
protect against the off-cycle migration of liquid
refrigerant. Once the system has reached its set
point and is about to shut off, the LLSV on the
condenser outlet closes. The compressor then
pumps the majority of the refrigerant charge
into the condenser and receiver before the
system stops on the low pressure pump-down
switch. This step reduces the amount of charge
on the low side in order to prevent off-cycle
migration. The recommended low-pressure
pump-down switch setting is 21 psig below the
nominal evaporating pressure. It shall not be set
lower than 33 psig.
Liquid receiver:
Refrigerant charge optimis-
ation varies with compressor speed. To avoid
flash gas at low speed, a receiver may be
necessary.
Receiver dimensioning requires special at-
tention. The receiver shall be large enough to
contain part of the system refrigerant charge,
but shall not be too large, to avoid refrigerant
overcharging during maintenance operations.
Liquid floodback during
operation
Liquid floodback occurs when liquid refrigerant
returns to the compressor when it is running.
During normal operation, refrigerant leaves the
evaporator and enters the compressor as a su-
perheated vapour. The suction gas can still con-
tain liquid refrigerant for example with a wrong
dimensioning, a wrong setting or malfunction
of the expansion device or in case of evaporator
fan failure or blocked air filters. A continuous
liquid floodback will cause oil dilution and, in
extreme situations, lead to liquid slugging.
VSH scroll compressors can tolerate occasional
liquid floodback. However system design must
be such that repeated and excessive floodback
is not possible.
During operations, liquid floodback may be
detected by measuring either the oil sump
temperature or the discharge gas temperature.
If at any time during operations, the oil sump
temperature drops to within 18°F or less above
the saturated suction temperature, or should
the discharge gas temperature be less than
63°F above the saturated discharge tempera-
ture, this indicates liquid floodback. Repetitive
liquid floodback testing must be carried out
under TXV threshold operating conditions: a
high pressure ratio and minimum evaporator
load, along with the measurement of suction
superheat, oil sump temperature and discharge
gas temperature.
