Danfoss R22 - Cycle Rate Limit

Operating conditions
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 eciency 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.
Low pressure
Internal pressure relief valve
Cycle rate limit 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 sucient
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
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.
Note that these two dierent low pressure
switches also require dierent settings. The low
pressure pump down switch setting must always
be within the operating envelope, for example
13 psi for R22. 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 7 psi for R22.
The SY/SZ240 to SY/SZ380 incorporate an
internal relief valve set to open between the
internal high and low pressure sides of the
compressor when the pressure dierential
between the discharge and suction pressures
surpasses 450 to 551 psi.
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.
HP
LP
Relief valve
T T
TH
T
H
KA
A1
A2 A3A1
180 s
A2
KA
~
~
R22
psig
R407C
psig
R134a
psig
R404A/R507A
psig
Working pressure range high side 158 - 401 152 - 422 97 - 292 184 - 451
Working pressure range low side 20 - 100 15 - 92 8 - 56 29 - 106
Maximum high pressure safety switch setting 406 427 297 457
Minimum low pressure safety switch setting * 7 7 7 7
Minimum low pressure pump-down switch setting ** 18 14 7 26
*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.
31
FRCC.PC.003.A5.22
Application guidelines