System design recommendations
Oil charge and oil
separator
Filter driers
Suction pressure control
Suction line heat
exchanger
In most installations the initial compressor oil
charge will be sufficient. In installations with line
runs exceeding 20 m, or with many oil traps or an
oil separator, additional oil may be required. In
installations with the risk of slow oil return such
as in multiple evaporator or multiple condenser
installations, an oil separator is recommended.
Also refer to the section "Oil charge and oil level".
For new installations with NTZ compressors
Danfoss recommends using the Danfoss DML
100% molecular sieve, solid core filter drier.
Molecular sieve filter driers with loose beads
from third party suppliers shall be avoided.
For servicing of existing installations where acid
formation is present the Danfoss DCL solid core
filter driers containing activated alumina are
recommended.
The drier is to be oversized rather than
undersized. When selecting a drier, always
take into account its capacity (water content
capacity), the system refrigerating capacity and
the system refrigerant charge.
An MOP-type expansion valve or suction
pressure regulator (i.e. Danfoss KVL) must be
used to limit suction pressure at a maximum of
4 bar relative (-5°C). Do not apply both of these
devices in combination with one another.
When compressors are mounted onto a rack
for a multi-evaporator system (i.e. supermarket)
or when evaporators operate at different
temperatures, use pressure regulators (Danfoss
KVP) without an MOP expansion valve.
A suction line heat exchanger is recommended
for low temperature applications, better
performance and efficiency are expected.
However in hot location this may cause high
suction gas superheat which can result in too
high discharge temperature.
Low ambient temperature
operation
At low ambient temperatures, the condensing
temperature and condensing pressure in air
cooled condensors will decrease. This low
pressure may be insufficient to supply enough
liquid refrigerant to the evaporator. As a result
the evaporating temperature will decrease,
leading to low capacity and eventual poor oil
return. At start-up, the compressor will pull into
vacuum and it will be switched off by the low
pressure protection. Depending on how the low
pressure switch and delay timer are set, short
cycling can occur. To avoid these problems,
several solutions are possible, based on reducing
condenser capacity:
In air-cooled machines, cycling the fans with
a head pressure controller will ensure that the
fans remain off until the condensing pressure
has reached a satisfactory level. Variable speed
fans can also be used to control the condensing
pressure. In water-cooled units, the same can
be performed using a water regulator valve
that is also operated by head pressure, thereby
ensuring that the water valve does not open until
the condensing pressure reaches a satisfactory
level.
The minimum condensing pressure must be
set at the minimum saturated condensing
temperature shown in the application envelopes.
Under very low ambient conditions, in which
testing has revealed that the above procedures
might not ensure satisfactory condensing and
suction pressures, the use of a head pressure
control valve is recommended. Note: This
solution requires extra refrigerant charge, which
can introduce other problems. A non-return
valve in the discharge line is recommended and
special care should be taken when designing the
discharge line.
When the compressor is located in a low ambient
temperature environment, increased refrigerant
migration will occur during shut down periods.
For such conditions an extra belt-type crankcase
heater is strongly recommended.
Note that with 100% suction gas cooled motors,
Maneurop® compressors can be externally
insulated. Refer to section "Liquid refrigerant
migration & charge limits" for more details.
19
FRCC.PC.046.A4.02
Application Guidelines
