21
SYSTEM DESIGN RECOMMENDATIONS
Models
NTZ048 - NTZ068 - NTZ096 -
NTZ108 - NTZ136
Models
NTZ215 - NTZ271
Liquid line solenoid valve &
pump-down
In refrigeration applications, the Liq-
uid Line Solenoid Valve (LLSV) is highly
recommended. During the off -cycle,
the LLSV isolates the liquid charge in
the condensor side, thus preventing
refrigerant transfer or excessive migra-
tion of refrigerant into the compres-
sor. Furthermore, when using LLSV in
conjunction with the pump-down cy-
cle (especially in low-temperature ap-
plications), 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 fi tted with elec-
tric defrost heaters.
Suction accumulator
A suction accumulator off ers consid-
erable protection against refrigerant
fl oodback both at start-up and dur-
ing operation or after the defrost op-
eration. This device also helps protect
against off -cycle migration by provid-
ing 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.
Crankcase heater
A crankcase heater protects against
the off -cycle migration of refrigerant
and proves eff ective if oil temperature
is maintained 10 K above the saturat-
ed LP temperature of the refrigerant.
Tests must thereby be conducted to
ensure that the appropriate oil tem-
perature is maintained under all ambi-
ent 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 low-
er a belt type crankcase heater could
be used in addition to the PTC heater,
although this is not a preferred solu-
tion for 1 and 2 cylinder compressors.
The belt crankcase heater must be
positioned on the compressor shell as
close as possible to the oil sump to en-
sure good heat transfer to the oil. The
below illustrated mounting positions
are recommended:
Belt crankcase heaters are not self-
regulating. 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 en-
ergised 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 fl oodback is present a the
Liquid Line Solenoid Valve (LLSV) +
pump-down cycle is required, possibly
in conjunction with a suction accumu-
lator.
