High And Low Pressure Protection; On/Off Cycling (Cycle Rate Limit) - Danfoss MLM Series Application Manuallines

Application Guidelines
High and low pressure
protection
Working pressure range high side
Working pressure range low side
Maximum high pressure safety switch setting
Minimum low pressure safety switch setting 
Recommended pump-down switch settings
Minimum low pressure pump-down switch setting
Maximum testing pressure
 LP safety switch shall never have time delay.
High pressure
Low pressure
On/off cycling (cycle rate
limit)
Operating conditions
bar (g)
bar (g)
bar (g)
bar (g)
bar (g)
bar(g)
MLZ/MLM 015-048 scroll compressors are equip-
ped with an internal pressure relief valve (IPRV),
for protection against blocked condenser and
fan failure conditions (IPRV setting 27-34 bar
differential pressure HP / LP). Still, a high pressure
(HP) safety switch is recommended.
MLZ/MLM058-068-076 scroll compressors are not
equipped with an internal pressure relief valve;
therefore a high pressure switch is required to
shut down the compressor should the discharge
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
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 2 minutes so as to provide for sufficient motor
cooling after start-up along with proper oil
R22 R404A
7.03 - 27.9 7.20 - 27.7
0.71 - 6.4 1.04 - 7.2
29.8 29.7
0.51 0.8
1.5 bar below nominal evaporating pressure
0.94 1.31
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.
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.
return. Note that the oil return may vary since it
depends upon system design.
Danfoss recommends a restart delay timer to
limit compressor cycling.
R134a R407A
4.91 - 22.1 6.0 - 25.6
0.64 - 4.0 0.9 - 6.0
23.6 26.8
0.45 0.7
0.85 0.9
31
FRCC.PC.015.A8.02
R407F
6.3 - 23.9
1.1 - 6.3
25.1
0.9
1.1
33