Operating conditions
Note that these two different low pressure
switches require different setting. 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.
A low-pressure (LP) safety switch must be used.
Deep vacuum operation of a scroll compressor
can cause internal electrical arcing and scroll
instability. Danfoss PSH scroll compressor exhibit
high volumetric efficiency and may draw very
low vacuum levels, which could induce such
a problem. The minimum low pressure 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 compressors.
LP switch setting for pump down cycles with
automatic reset are also listed in the table below.
Low pressure
Pressure settings bar(g)
R410A
Working pressure range high side
13.5-44.5
Working pressure range low side
1.7-15.5
Maximum high pressure safety switch setting
45
Minimum low pressure safety switch setting*
1.5
Minimum low pressure pump-down switch setting**
1.7
*LP safety switch shall never be bypassed and shall have no time delay.
**Recommended pump-down switch setting: 1.5 bar below nominal evaporating pressure with minimum of 2.3 bar(g).
Danfoss PSH requires liquid injection to maintain
sufficiently low discharge gas temperature in the
operating envelope. The PSH compressors are
provided with a liquid injection connection.
Refrigerant in liquid phase is directly injected
in the scroll set. The Liquid Injection Valve
is activated according to the discharge
temperature.
Liquid injection is active for when discharge
temperature exceed 121°C and will regulate to
keep constant this discharge temperature.
The discharge temperature sensor must use
PT1000. The discharge temperature sensor
must have a quick response, be attached on the
surface of discharge pipe within 40mm away
from compressor discharge fitting, the sensor
should be fixed by a metallic collar, have a good
thermal contact by adding thermal paste and be
insulated from ambient disturbance by thermal
wrap.
A minimum 5K subcooling is necessary to ensure
correct liquid injection.
Liquid Injection
In(P)
Pc
Pi
Po
h (kJ/kg)
LIV
Sol V
Discharge T sensor
Control
Condenser
Filter drier
EXV
Evaporator
19
FRCC.PC.022.A4.02
Application Guidelines
