RECIPROCATING COMPRESSORS
17
SYSTEM DESIGN RECOMMENDATIONS
In refrigeration applications, the Li-
quid Line Solenoid Valve (LLSV) is hi-
ghly recommended. During the off-cy-
cle, the LLSV isolates the liquid charge
in the condensor side, thus preventing
against refrigerant transfer or exces-
sive migration of refrigerant into the
compressor. Furthermore, when using
LLSV in conjunction with the pump-
down cycle (especially in low-tempe-
rature applications), the quantity of
refrigerant in the low-pressure side of
the system will be reduced.
A pump-down cycle design is required
when evaporators are fitted with elec-
tric defrost heaters.
Crankcase heater
A crankcase heater protects against
the off-cycle migration of refrigerant
and proves effective if oil temperature
is maintained 10 K above the satura-
ted LP temperature of the refrigerant.
Tests must thereby be conducted to
ensure that the appropriate oil tempe-
rature is maintained under all ambient
conditions. A PTC crankcase heater
is required with all Maneurop®‚ NTZ
compressors. PTC crankcase heaters
are self-regulating.
Under extreme conditions such as low
ambient temperature at –15°C or lower
a belt type crankcase heater could be
used in addition to the PTC heater, al-
though this is not a preferred solution
for 1 and 2 cylinder compressors. The
belt crankcase heater must be positio-
ned on the compressor shell as close
as possible to the oil sump to ensure
good heat transfer to the oil. The be-
low illustrated mounting positions are
recommended:
Belt crankcase heaters are not self-re-
gulating. Control must be applied to
energise the belt heater once the com-
pressor has been stopped and then to
de-energise it while the compressor is
running. The belt heater must be ener-
gised 12 hours before restarting the
compressor following an extended
down period.
If the crankcase heater is not able to
maintain the oil temperature at 10 K
above the saturated LP temperature
of the refrigerant during off cycles or
if repetitive floodback is present a the
Liquid Line Solenoid Valve (LLSV) +
pump-down cycle is required, even-
tually in conjunction with a suction
accumulator.
Models
NTZ048 - NTZ068 - NTZ096 -
NTZ108 - NTZ136
Models
NTZ215 - NTZ271
Liquid line solenoid valve &
pump-down
Suction accumulator
A suction accumulator offers consi-
derable protection against refrigerant
floodback both at start-up and during
operation or after the defrost opera-
tion. This device also helps protect
against off-cycle migration by means
of providing additional internal free
volume to the low pressure side of the
system.
The suction accumulator must be se-
lected in accordance with the accumu-
lator manufacturer recommendations.
As a general rule, Danfoss Commercial
Compressors recommends to size the
accumulator for at least 50% of the to-
tal system charge. Tests however must
be conducted to determine the opti-
mal size.
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