Cycle Rate Limit - Danfoss R134a Application Manuallines

Application Guidelines
Internal pressure relief valve
Low pressure

Cycle rate limit

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AB237986441643en-010801
Operating conditions
The SY240 to SY380 incorporate 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 31 to 38 bar.
This safety feature prevents the compressor from
developing dangerously high pressures should
the high pressure cutout, for whatever reason, fail
to shut down the compressor.
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 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
Working pressure range high side
Working pressure range low side
Maximum high pressure safety switch setting
Minimum low pressure safety switch setting *
Minimum low pressure pump-down switch setting **
*LP safety switch shall never be bypassed and shall have no time delay.
** Recommended pump-down switch settings: 1.5 bar (R22, R407C, R404A) or 1 bar (R134a) below nominal
evaporating 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
1.3 bar for R22. The compressor can be operated
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 (6 when a resistor soft-start accessory is
introduced); a number higher than 12 reduces
the service life of the motor-compressor unit. If
HP
Relief valve
LP
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.
R22 R407C R134a R404A/R507A
bar (g) bar (g) bar (g)
10.9 - 27.7 10.5 - 29.1 6.7 - 20.2
1.4 - 6.9 1.1 - 6.4 0.6 - 3.9
28 29.5 20.5
0.5 0.5 0.5
1.3 1.0 0.5
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 0.5 bar for R22.
necessary, place an anti-short-cycle timer in the
control circuit, connected as shown in the wiring
diagram section "Suggested wiring diagrams
logic". A three-minute (180-sec) time out is
recommended.
Please contact Danfoss Technical Support for any
deviation from this guidelines.
A1
TH
T T
A2
R513A
bar (g) bar(g)
12.7 - 31.1 5.12 - 20.87
2 - 7.3 0.83 - 4.26
31.5 22.27
0.5 0.5
1.8 0.6
~
KA
A2 A1 A3
TH
180 s
KA
~