Operating conditions
Cycle rate limit
A low-pressure (LP) safety switch must be used.
Deep vacuum operations of a scroll compressor
can cause internal electrical arcing and scroll
instability. Danfoss WSH Scroll compressors
exhibit high volumetric efficiency and may draw
very low vacuum levels, which could induce such
a problem. The minimum low-pressure safety
switch (loss-of-charge safety switch) setting is
given in the following 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 below.
Low pressure
Danfoss recommends a restart delay timer to
limit compressor cycling. The timer prevents
reverse compressor rotation, which may occur
during brief power interruptions.
The system must be designed in a way that
guarantees a minimum compressor running time
of two 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.
There must be no more than 12 starts per hour, a
number higher than 12 reduces the service life of
the motor-compressor unit. A three-minute (180-
sec) time out is recommended.
Note that these two different low pressure
switches also require different settings. The
low pressure pump down switch setting must
always be within the operating envelope, for
example 2.3 bar for R410A. The compressor can
be operated full time under such condition. The
minimum low pressure safety switch setting
may be outside the normal operating envelope
and should only be reached in exceptional
(emergency) situations, for example 1.5 bar for
R410A.
Please contact Danfoss Technical Support for any deviation from this guidelines.
T
T
TH
TH
KA
A1
A2
A3
A1
180 s
A2
KA
~
~
Pressure settings
R410A
Working pressure range high side
bar (g)
13.5 - 44.5
Working pressure range low side
bar (g)
2.3 - 11.6
Maximum high pressure safety switch setting
bar (g)
45
Minimum low pressure safety switch setting *
bar (g)
1.5
Minimum low pressure pump-down switch setting **
bar (g)
2.3
* LP safety switch shall never be bypassed and shall have no time delay.
** Recommended pump-down switch settings: 1.5 bar below nominal evap. pressure with minimum of 2.3 bar(g)
20
FRCC.PC.028.A5.02
Application Guidelines