OIL TEMPERATURE
.
To
maintain
a separation
of
refrigerant
and oil in the refrigerant
atmosphere, the oil must be heated to a temperature approx-
imately
40°F
to 50°F above saturated refrigerant (Evap.)
temperature.
During normal compressor operation, saturated refrigerant
evaporating temperature is less than leaving chilled water
temperature, and sufficient heat is added to the oil by the fric-
tion of shaft journals rotating in the bearings. With the com-
pressor “off”, the saturated refrigerant temperature rises, ap-
proaching ambient, and the oil must be heated to maintain
the required temperature difference.
Heat is applied to the oil when the compressor is “off” by
the operation of one or two electric resistance heaters located
in the oil sump. The heaters are wired, electrically, through
the oil pump contactor. When the contactor is energized, and
the oil pump operating, the circuit to the oil sump heater(s)
is open, and the heaters are “off”. When the compressor, and
oil pump are “off”, the contactor is de-energized and the
heater(s) energized.
To protect the compressor from operating with refrigerant
laden oil being delivered to the bearings, the
MicroTech
soft-
ware includes a protective control located under the Service
Setpoints key. Press number 5 of that key will show, “Low Oil
Delta=XXX”. The setting may be Service adjusted between
40°F and
60” F; and is factory set for
50°F.
If the oil temperature is not equal to, or above the Low Oil
Delta
setpoint
plus the saturated refrigerant (Evap.)
temperature, the unit will not start, but will display the state-
ment “Waiting Low Sump T”.
If the chiller is running and the oil temperature in the sump
falls below a temperature equal to the saturated refrigerant
(Evap.) temperature plus the Low Oil Delta setpoint, the chiller
will cycle “off”, and one press of the “Fault History” key will
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403
produce the display
“Now=Low
Sump
Temp”.
The two sensors that produce the actual calculated
temperature difference are the oil sump sensor (Number 10)
and the Evap. Saturated Refr. Temp. sensor (Number 02).
The Low Oil Delta setpoint also serves a second function.
It’s temperature setpoint plus
lOoF,
plus the saturated
refrigerant (Evap.) temperature is displayed as the
“Calc
Oil
SPT” temperature. This is the oil temperature that the com-
puter software will attempt to maintain when the chiller is “off”.
As described above, electric heat is applied to the oil when
the compressor is
“off”. That heat increases the oil
temperature to the calculated oil setpoint temperature and
more.
When the temperature increases to 4°F higher than the
calculated setpoint temperature, the solid-state output relay
number 5 de-energizes, opening the circuit to one sump
heater. As the oil cools down to the calculated setpoint
temperature, the relay re-energizes and additional heat is
again added to the oil. In this manner, the oil temperature is
maintained within a
4” F to
5”
F temperature of the calculated
setpoint during compressor “off” cycles.
An additional oil temperature protective control prevents
overheated oil from being delivered to compressor bearings.
This protection is a “High Oil Feed” temperature, and its value
is determined by measurement through the “Oil Feed Temp”
sensor, (number 09).
If the oil temperature at the oil feed sensor exceeds the
set-
point temperature when the chiller is running, or “off”, a fault
condition will occur.
Based upon all of the above, it will be desirable to set the
control setpoints while considering both compressor operating
and “off” conditions to prevent overlapping temperatures and
nuisance tripping.
_
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