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Engineering manual - UniSAB III 1.10.8
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Compressor control and surveillance

Vi ratio regulation

UniSAB III calculates the Vi ratio according to refrigerant and the actual pressure ratio. The Vi ratio is
shown in the

/Capacity

picture. There is no Vi slide and therefore no Vi regulation, but the relief valve will

be activated if the compressor is running in Vi conditions which is significantly less than the configured
fixed Vi ratio. This works in two steps:

•

If speed is 100% the relief valve will be unloaded when the calculated Vi ratio is less than 65%
of the fixed Vi ratio. This is to protect the compressor.

•

If speed is less than 95% the relief valve can be unloaded if the calculated Vi ratio is less than
80% of the fixed Vi ratio. This is to optimise efficiency. To be activated this function must be con-
figured, i.e. the configuration point

VI comp. plug valve

must be set to Yes.

The compressor can be delivered with a number of fixed Vi ratios which can be configured, see subsec-
tion

6.4.3 Compressor block, VI comp. plug valve.

3.3.22

SAB 193 H, 233 H, 283 H and 355 H

The H type compressor is the high-pressure version of the standard screw range, SAB 193 S to 355 S.
The compressors are equipped with capacity slide and automatic Vi-control.

If the compressor is equipped with a frequency converter (VSD), see subsection

6.4.1 Drive, 6: Rotatune

for additional set-up of UniSAB III.

If the compressor is equipped with oil pump, the oil pump is used for maintaining a minimum oil pressure
level during operation and for returning the capacity slide to minimum position before the compressor is
started:

During operation, the oil pump is started and stopped in accordance with the compressor differential
pressure (discharge pressure - suction pressure). The difference is set in the picture

Oil/Pressure/Con-

trol

where

Set point 1

and

Set point 2

will appear.

Set point 1

is the pump start pressure and

Set point 2

is the pump stop pressure. Therefore,

Set point 1

must be set for the lowest pressure level.

If the compressor differential pressure is below

Set point 1

during operation, the pump will start immedi-

ately. When the compressor differential pressure has been higher than

Set point 2

for 60 seconds, the

pump will stop.

Note:

For software version 1.10.7, the following applies: Multisab will not work for SAB 193 H – SAB

355 H with fixed speed, which require the

Capacity zero

parameter set to the default value of 60%. It is

not possible to operate in Multisab sequence with a

Capacity zero

parameter set that high.

See subsection

6.4.3 Compressor block

11: Capacity zero

and subsection

6.3.1 Multisab

Parallel offset

for more information.

Starting sequence

When the compressor has received starting permission, UniSAB III will check if the capacity slide is in
minimum position. If the capacity slide is higher than a 5% position, the solenoid valve for decreasing ca-
pacity slide position is activated, and, if equipped with oil pump, the oil pump is started for maximum 30
seconds. If the capacity slide is still higher than a 5% position, an

Oil log error

shutdown will be issued,

and the compressor will not start. If the capacity slide is lower than 5%, the compressor is allowed to
start.

Before start-up, UniSAB III will also check the oil level and temperature in the oil separator. If the oil level
is too low, an

Oil system error

alarm will be issued and compressor state will be set to

Pause

. No signal

from the oil level switch during operation will result in shutdown after a delay of 30 seconds.

If the oil temperature is lower than the set limits a

Low oil separator temp

alarm or shutdown will be is-