Famco Danfoss Maneurop MT, Руководство пользователя

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Application guidelines
27
FRCC.PC.004.A9.22
System design recommendations
Cycle rate limit
Operating voltage range
Operating voltage and cycle
rate
There may be no more than 12 starts per hour
(6 when a soft start accessory is used). A higher
number reduces the service life of the motor-
compressor unit. If necessary, use an anti-short-
cycle timer in the control circuit. A time-out of six
minutes is recommended. The system must be designed in such a way to guarantee a minimum
compressor running time in order to provide
proper oil return and sufficient motor cooling
after starting.
Note that the oil return rate varies as a function of
the system design.
The operating voltage limits are shown in the
table on “Voltage application range” paragraph.
The voltage applied to the motor terminals must
always be within these table limits. The maximum
allowable voltage unbalance for 3-phase compressors is 2%. Voltage unbalance causes
high current draw on one or more phases, which
in turn leads to overheating and possible motor
damage.
Voltage unbalance is given by the formula:
|V
avg
- V
1-2
|+|V
avg
- V
1-3
|+|V
avg
- V
2-3
|
2 xV
avg
Vavg = Mean voltage of phases 1, 2 and 3
V1-2 = Voltage between phases 1 and 2
V1-3 = Voltage between phases 1 and 3
V2-3 = Voltage between phases 2 and 3.
% voltage unbalance:
x 100
Off-cycle migration
During system standstill and after pressure
equalization, refrigerant will condense in the
coldest part of the system. The compressor can
easily be the coldest spot, for example when it
is placed outside in low ambient temperatures.
After a while, the full system refrigerant charge
can condense in the compressor crankcase. A
large amount will dissolve in the compressor
oil until the oil is completely saturated with
refrigerant. If other system components are
located at a higher level, this process can be
even faster because gravity will assist the liquid
refrigerant to flow back to the compressor. When
the compressor is started, the pressure in the
crankcase decreases rapidly. At lower pressures the oil holds less refrigerant,
and as a result part of the refrigerant will violently
evaporate from the oil, causing the oil to foam.
This process is often called “boiling”.
The negative effects from migration on the
compressor are:
• oil dilution by liquid refrigerant
• oil foam, transported by refrigerant gas and
discharged into the system, causing loss of oil
and in extreme situations risk for oil slugging
• in extreme situations with high system
refrigerant charge, liquid slugging could occur
(liquid entering the compressor cylinders).
Refrigeration compressors are basically designed
as gas compressors. Depending on the
compressor design and operating conditions,
most compressors can also handle a limited
amount of liquid refrigerant. Maneurop® MT and
MTZ compressors have a large internal volume
and can therefore handle relatively large amounts
of liquid refrigerant without major problems.
However even when a compressor can handle
liquid refrigerant, this will not be favorable to its
service life. Liquid refrigerant can dilute the oil, wash oil out of bearings and result in high oil
carry over, resulting in loss of oil from the sump.
Good system design can limit the amount of
liquid refrigerant in the compressor, which will
have a positive effect on the compressor service
life.
Liquid refrigerant can enter a compressor in
different ways, with different effects on the
compressor.
Liquid refrigerant control
and charge limit
Liquid floodback during
operation
During normal and stable system operation,
refrigerant will leave the evaporator in a
superheated condition and enter the compressor
as a superheated vapor. Normal superheat values
at compressor suction are 9 to 54
o
F. However the
refrigerant leaving the evaporator can contain
an amount of liquid refrigerant due to different
reasons:
• wrong dimensioning, wrong setting or
malfunction of expansion device