Application guidelines
26
FRCC.PC.021.A5.22
Operating conditions
High pressure
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
can not be isolated.
Internal pressure relief valve
High pressure safety switch test
Low pressure
The PSH077 incorporates an internal relief valve
set to open between the internal high and low
pressure sides of the compressor when the
pressure differential between the discharge and
suction pressures surpasses 615 to 711 psig. This
safety feature prevents the compressor from
developing dangerously high pressures should
the high pressure cut-out, for whatever reason,
fail to shut down the compressor.
High-pressure safety switch shall be tested on
the unit and this test may be in conflict with OCS
reliability protection in the case of HP switch
setting is higher than “Pdisch_limit” (663 psi(a)).
During this test, the alarm A36 and consecutively
the opening of M1-M2 switch shall not be
considered to validate the HP switch setting.
HP
LP
Relief valve
Deep vacuum operations of a scroll compressor
can cause internal electrical arcing and scroll
instability. Danfoss PSH Scroll compressors
exhibit high volumetric efficiency and may draw
very low vacuum levels, which could induce
such a problem. Low back pressure protection
is integrated in OCS function, so no additional
protection is needed.
LP switch settings for pump-down cycles with
automatic reset are listed in the table below.
The low pressure pump down switch setting
must always be within the operating envelope,
for example 25 psig for R410A. The compressor
can be operated full time under such condition.
Pressure settings
R410A
Working pressure range high side
psig
84 - 648
Working pressure range low side
psig
25 - 194
Maximum high pressure safety switch setting
psig
669
Minimum low pressure pump-down switch setting *
psig
25
* Recommended pump-down switch settings: 21 psig below nominal evap. pressure with minimum of 25 psig