A
PPLICATION
GUIDELINES
O
PERATING
CONDITIONS
A low pressure (LP) safety switch is recommended.
MLZ/MLM scroll compressors exhibit high volu-
metric efficiency and may draw very low vacuum
levels, which could induce scroll instability and
electrical arcing at the internal cluster. The mini-
mum 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 lock-
out device or an automatic switch wired into an
electrical lockout circuit. The LP switch tolerance
must not allow for vacuum operations of the com-
pressor. 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 pro-
vides a minimum compressor running time of 2
minutes so as to provide for sufficient motor cool-
ing 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
R134a
Working pressure range high side
PSI
102 - 405
104 - 395
71.2 - 321
Working pressure range low side
PSI
10.3 - 92.8
15.1 -104
9.3 - 58
Maximum high pressure safety switch setting
PSI
432
431
342
Minimum low pressure safety switch setting
1
PSI
7.4
11.6
6.5
Recommended pump-down switch settings
7.3 PSI below nominal evaporating pressure
Minimum low pressure pump-down switch setting
PSI
13.6
19.0
12.3
1
LP safety switch shall never have time delay.
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 464 PSI ± 58 differ-
ential 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
pressure exceed the values shown in the table
above.
The high-pressure switch can be set to lower val-
ues 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
24
FRCC.PC.016.A1.22
