Operating conditions
A low pressure (LP) safety switch is
recommended. MLZ/MLM scroll compressors
exhibit high volumetric efficiency and may draw
very low vacuum levels, which could induce scroll
instability and electrical arcing at the internal
cluster. The minimum low-pressure safety switch
setting is given in the above table. For systems
without pump-down, the LP safety switch
must either be a manual lockout device or an
automatic switch wired into an electrical lockout
circuit. The LP switch tolerance must not allow for
vacuum operations of the compressor. LP switch
settings for pump-down cycles with automatic
reset are also listed in the table above.
Low pressure
On/off cycling (cycle rate
limit)
Depending on the application, a number higher
than 12 starts per hour can reduce the service life
of the motor-compressor unit. A one-minute time
out is recommended.
The system must be designed in a way that
provides a minimum compressor running time
of 3 minutes so as to provide for sufficient motor
cooling after start-up along with proper oil
return. Note that the oil return may vary since it
depends upon system design.
Danfoss recommends a restart delay timer to
limit compressor cycling.
R22
R404A
Working pressure range high side
bar (g)
7.1 - 26
9.9 - 27.7
Working pressure range low side
bar (g)
0.7 - 6.2
1.0 - 6.6
Maximum high pressure safety switch setting
bar (g)
27.1
29
Minimum low pressure safety switch setting
bar (g)
0.5
0.7
Recommended pump-down switch settings
1.5 bar below nominal evaporating pressure
Minimum low pressure pump-down switch setting
bar (g)
0.6
0.9
LP safety switch shall never have time delay.
For MLZ/MLZ116, a high pressure switch is
required to shut down the compressor should
the discharge pressure exceed the values shown
in the table above.
The high-pressure switch can be set to lower
values depending on the application and
ambient conditions. The HP switch must either be
placed in a lockout circuit or consist of a manual
reset device to prevent cycling around the
high-pressure limit. If a discharge valve is used,
the HP switch must be connected to the service
valve gauge port, which must not be isolated.
High pressure
High and low pressure
protection
Defrost cycle
In refrigeration system applications, there are
different defrost methods, such as electric
heating defrost, hot gas bypass defrost,
reversible defrost etc. For the system which use
hot gas bypass or reversible defrost method,
suction accumulator is necessary as a result
of the possibility of a substantial quantity of
liquid refrigerant remaining in the evaporator,
this liquid refrigerant can then return to the
compressor, either flooding the sump or as
a dynamic liquid slug when the cycle switch
back to normal cooling operations. Sustained
and repeated liquid slugging and flooding can
seriously impair the oil’s ability to lubricate the
compressor bearings. In such cases a suction
accumulator is a must.
22
FRCC.PC.035.A5.02
Application Guidelines
Single compressors
