18
SYSTEM DESIGN RECOMMENDATIONS
Operating limits
NTZ – R404A / R507A
Working pressure range, high side (bar gauge) 13.2 – 27.7
Working pressure range, low side (bar gauge) 0.1 – 3.3
Minimum low pressure safety
switch setting
(bar gauge)
0
Minimum low pressure pump-down
switch setting
(bar gauge) 0.1
Relief valve opening pressure di erence (bar) 30
Relief valve closing pressure di erence (bar) 8
Low ambient temperature
operation
At low ambient temperatures, the con-
densing temperature and condensing
pressure in air cooled condensors will
decrease. This low pressure may be
insu cient to supply enough liquid
refrigerant to the evaporator. As a re-
sult the evaporating temperature will
decrease, leading to low capacity and
eventual poor oil return. At start-up,
the compressor will pull into vacuum
and it will be switched o by the low
pressure protection. Depending on
how the low pressure switch and delay
timer are set, short cycling can occur.
To avoid these problems, several solu-
tions are possible, based on reducing
condenser capacity:
z
Liquid ooding of condensors (note:
this solution requires extra refrig-
erant charge. A non-return valve
in the discharge line is required
and special care should be taken
when designing the discharge line.)
z
Reduce air ow to condensors
z
Alternatively the condensor may be
installed indoor.
When the compressor is located in a
low ambient temperature environ-
ment, increased refrigerant migration
will occur during shut down periods.
For such conditions an extra belt-type
crankcase heater is strongly recom-
mended.
Note that with 100% suction gas
cooled motors, Maneurop® compres-
sors can be externally insulated. Refer
to section "Liquid refrigerant migra-
tion & charge limits" for more details.
Low pressure A low-pressure (LP) safety switch
must also be used; deep vacuum op-
erations will result in failure. The mini-
mum LP safety switch (loss of charge
switch) setting is 0 relative bar (0 bar
g). For systems without pump-down
feature, the LP safety switch must be
either a manual lockout device or an
automatic LP safety switch wired into
an electrical lockout circuit. LP switch
tolerance must not allow for vacuum
operation of the compressor. LP safety
switch settings for pump-down cycles
with automatic reset are listed in the
following table.
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 below.
This switch can be set at lower values
depending on the application and
ambient conditions. It must be either
located in a lockout circuit or associ-
ated with 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 serv-
ice valve gauge port, which cannot be
isolated.
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