Emerson VSG & VSSG Software Manual page 33

· Oil Separator Heater Temperature – This con-
trol limit determines when the oil separator
heater is energized. A decrease in oil separator
temperature below the ON setpoint energizes
the oil separator heater. On an increase in oil
separator temperature above the OFF setpoint,
the oil heater is de-energized.
· Oil Pump Restart – To determine the ON and
OFF values for the pressure ratios, take the ab-
solute discharge pressure (PSIA), and divide it
by the absolute suction pressure (PSIA). If the
pressure ratio is below the ON setpoint value,
the oil pump will restart and stay on until the
pressure ratio increases above the OFF setpoint.
This enables a high stage compressor with a part
time oil pump to temporarily operate under
conditions requiring a full time oil pump.
EXAMPLE: To calculate the OFF value, if the
absolute discharge pressure is 200 PSIA and the
desired absolute suction pressure of the cut-out
point is 67 PSIA, the discharge pressure is divided
by the suction pressure. The result is a OFF value
of approximately 3.0. This would then be entered
for the OFF pressure. Now determine the ON value,
take the absolute discharge pressure (200 PSIA) and
divide this by the desired absolute suction pressure
(71 PSIA). This results in a ON valve of 2.8.
· Capacity Slide Adjustment Range – This control
limit determines the capacity range the Capac-
ity Slide Adjustment factor will be active. The
factor will be active from 0% capacity and will be
deactivated when the OFF setpoint is reached.
On a decrease in capacity below the ON set
point, the factor will be active.
· Minimum Run Capacity – The Minimum Run
Capacity is the minimum capacity the com-
pressor will be allowed to run at. When the
compressor is started, it will be loaded to the
Minimum Run Capacity control setpoint minus
5%. This is done to prevent the capacity control
from hunting if the load is not great enough to
keep the compressor capacity at the Minimum
Run Capacity setpoint. On a call for unloading,
the compressor will unload until it reaches the
Minimum Run Capacity control setpoint. It will
remain there until the suction pressure reduces
and the compressor cycles off on the Suction
Pressure On/Off control setpoint.
· Volume Slide Adjustment Factor – This value is
normally zero (0) and will not require changing.
However, if the system operating conditions
show the volume ratio is not at the optimum
value for the system, this value can be adjusted
up or down to permit the most efficient position-
ing of the volume slide valves. To determine the
value to enter, first ensure that the system is in
a steady operating state and place the volume
side in manual. NOTE: The volume position set-
ting and increase the volume side position slowly
until the lowest amperage level is achieved. If the
amperage level rises instead of falls, decrease the
volume slide position until the lowest amperage
is achieved.
Record the differential from the original position.
This will then be adjustment factor to enter for
our system. NOTE: Negative numbers can be
used for this setting. This setpoint is active on a
drop in capacity below the Capacity Slide Adjust-
ment Range cut-in (ON) setting. The adjustment
is disabled when capacity rises above the Adjust-
ment Range OFF setting. Normally, the "ON-Off"
set points should both be set at 100%, so the
volume side adjustment factor will be applied
over the full range of 0 to 100%
· Current Transformer Ratio – The value entered
must agree with the Current Transformer Ratio
on the current transformer being used. The
current transformer is mounted in the compres-
sor motor conduit box. The ratio is stated as
the ratio of measured current to a nominal full
scale current in the secondary of 5 amps; only
the first of these is entered. For example, if the
ratio reads 250/5, enter 250.
· Low Suction Pressure Load Limit – Active in
Discharge Pressure Capacity Control mode only.
These setpoints limit the compressor from load-
ing at low suction pressure conditions. They
override the discharge pressure capacity control
setpoints. When the OFF setpoint is reached
(at or below setpoint), the compressor will not
be allowed to load any further. If the suction
pressure continues to fall, the capacity of the
compressor will decrease when the suction
pressure is at or below the ON set point. It will
stop decreasing when the suction pressure rises
to a point that is just below the ON set point.
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