Application guidelines
35
FRCC.PC.014.A4.22
System design recommendations
High and low pressure
protection
High pressure
Low pressure
Crankcase heating function
Electronic expansion valve
Pressure settings
According to EN378-2, a high-pressure (HP) safety
switch is required to shut down the compressor.
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-pres-
sure limit. If a discharge valve is used, the HP
switch must be connected to the service valve
gauge port, which must not be isolated. The lock-
out circuit or HP switch must be connected to the
CDS302 input 27.
A low-pressure (LP) safety switch must be
used. Deep vacuum operations of a scroll
compressor can cause internal electrical arcing
and scroll instability. VSH 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. Lock-out circuit or
LP switch or series with other safety devices (HP,
...) must be connected to CDS302 input 27.
A DC-hold current through the motor windings
can be used as an alternative to an external
crankcase heater to keep the compressor warm
when stopped.
For VSH088 and VSH117 this function is factory
preset to “enabled”. Go to parameter 28.3* in the
frequency converter for settings (factory presets
are done).
For VSH170, this function must not be used and is
factory preset to “disabled”. An external crankcase
heater is required, and surface sump heater type
should be preferred. Refer to accessory list for
code numbers.
With variable capacity systems, an electronic ex-
pansion valve (EXV) is one of the better solutions
to handle refrigerant mass flow variations. Ramp-
up and ramp-down settings, of both EXV and
compressor, must be done with great care.
Ramp-up of the EXV must be shorter than
the ramp-up of the compressor, to avoid any
low pressure operation on suction side of the
compressor. The EXV can also be opened, up
to a certain degree, before the start up of the
compressor.
Ramp-down of the EXV must be longer than the
ramp-down of the compressor, also to avoid low
pressure operation (except with pump-down).
EXV should be closed, and remained closed,
when the compressor is off, to avoid any liquid
refrigerant entering the compressor.
Pressure settings
R410A
Working pressure range high side
psig
195 - 645
Working pressure range low side
psig
33 - 168
Maximum high pressure safety switch setting
psig
652
Minimum low pressure safety switch setting *
psig
21
Minimum low pressure pump-down switch setting **
psig
33
*LP safety switch shall never be bypassed.
** Recommended pump-down switch settings: 21 psig below nominal evaporating temperature with minimum of 33 psig
