A high-pressure (HP) safety switch is required to
shut down the compressor should the discharge
pressure exceed the values shown in the table
section “System pressure test”. 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 and low pressure
protection
High pressure
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 CH 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
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 2.15 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.
Pressure settings
R410A
Working pressure range high side
bar (g)
16.2 - 41.6
Working pressure range low side
bar (g)
2.15 - 10.7
Maximum high pressure safety switch setting
bar (g)
46.1
Minimum low pressure safety switch setting *
bar (g)
1.5
Minimum low pressure pump-down switch setting **
bar (g)
2.15
* 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 2.15 bar(g)
Cycle rate limit
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 2 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.
Please contact Danfoss Technical Support for any
deviation from these guidelines.
T
T
TH
TH
KA
A1
A2
A3
A1
180 s
A2
KA
~
~
Operating conditions
21
FRCC.PC.026.A4.02
Application Guidelines