Application guidelines
26
FRCC.PC.004.A9.22
System design recommendations
Low pressure
A low pressure safety switch is recommended to avoid compressor operation at too lower suction
pressures.
Low ambient temperature
operation
At low ambient temperatures, the condensing
temperature and condensing pressure in air
cooled condensers will decrease.
This low pressure may be insufficient to supply
enough liquid refrigerant to the evaporator. As a
result the evaporator temperature will strongly
decrease with the risk of frosting. At compressor
start-up, the compressor can pull a deep vacuum
and it can be switched off by the low pressure
protection. Depending on the low pressure
switch setting and delay timer, short cycling can
occur. To avoid these problems, several solutions
are possible, based on reducing condenser
capacity:
• Indoor location of condensers
• Liquid flooding of condensers (note: this
solution requires extra refrigerant charge, which
can introduce other problems. A non-return valve
in the discharge line is required and special care
should be taken when designing the discharge
line.)
• Reduce air flow to condensers.
Other problems can also occur when the
compressor is operating at low ambient
temperature. During shut down periods, liquid
refrigerant can migrate to a cold compressor.
For such conditions a 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 control &
charge limits" for more details.
Oil charge and oil separator
Filter driers
Operating limits
High pressure
In most installations the initial compressor oil
charge will be sufficient. In installations with line
runs exceeding 66 ft, 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.
For new installations with MTZ 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.
A high pressure safety switch is required to stop
the compressor, should the discharge pressure
exceed the values shown in the table below. The
high pressure switch can be set to lower values
depending on the application and ambient
conditions. The HP switch must either be in a
lockout circuit, or be a manual reset device to
prevent compressor cycling around the high
pressure limit. When a discharge valve is used,
the HP switch must be connected to the service
valve gauge port, which cannot be isolated.
MT
MTZ
R22
160P
R417A
160PZ
R407A
160PZ
R407C
160PZ
R407F
160PZ
R134a
160PZ
R404A / R507
160PZ
Test pressure low side
psig
360
360
360
360
360
360
360
Working pressure range high side
psig
158 - 402
135 - 370
168 - 374
181 - 426
175 - 348
114 - 328
191 - 402
Working pressure range low side
psig
15 - 100
8 - 82
8 - 86
21 - 95
14 - 91
1 - 68
15 - 104
*Relief valve opening pressure difference
psig
435
435
435
435
435
435
435
*Relief valve closing pressure difference
psig
115
115
115
115
115
115
115
* Relief valve fitted on 2 and 4 cyl.