Engineering manual - SAB 355 Screw compressor unit (including ATEX)
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004100 en 2020.10
Technical description
Functional description
The pilot valve, pos. 221, is actuated by the compressor discharge pressure P1 and is kept closed
by a built-in spring whose closing force is adjusted to the pressure shown on the valve plate. The
reference pressure of the pilot valve is the atmospheric pressure Po, and the valve is dimensioned
in such a way that the pressure P2" does not affect the opening pressure of the valve.
If the pressure P1 rises above the set opening value on the valve plate, the pilot valve, pos. 221,
will modulate open and raise the pressure P2" below the piston, pos. 5, in the main valve.
In the chamber above the piston, pos. 5, there is the same pressure P1 as under the valve cone,
pos. 2, since the piston rod, pos. 3, is pierced. When the main valve is closed, the pressure P2" in
the chamber, pos. 6, below the piston will be the same as the compressor discharge pressure P2
since the pressure P2" is equalised through the nozzle, pos. 7.
If the pilot valve, pos. 221, opens due to too a high pressure P1, gas will flow from the pilot valve
to the main valve through the P2" line. If there is more gas that can flow through the nozzle, pos.
7, the pressure will rise in the chamber, pos. 6, and modulate open the main valve.
Thus the way is open for a gas flow through the main valve from the compressor discharge side to
the suction side.
When the pressure P1 falls below the set pressure of the pilot valve again, the pilot valve will
close. The pressure P2" in the chamber, pos. 6, is equalised to the P2 pressure through the nozzle,
pos. 7, and the main valve closes in less than 50 seconds. The activation of the compressor pro-
tecting system will usually not result in valve leaks or in the need for regulation.
Fig. 36: System with open valve
4.1.27 Oil cooling system and general piping layout
General:
Min. oil temperature before start-up: 4°C (+40°F)
Min. oil temperature in running condition: 20°C (+68°F)
Liquid injection oil cooling: The compressor oil is cooled by direct contact with the refrigerant
injected through an optimised port location prior to the compressor discharge. Liquid feed ar-
rangements include isolation valves, strainer, solenoid valve, sight glass, a thermal expansion
temperature control valve and a pressure differential control valve. The temperature control
valve will maintain the temperature of the oil returning to the compressor between 54.4°C [130°
F] and 76.7°C [170°F].
Water-cooled oil cooling: The optional water-cooled oil cooler is either a plate and shell design
(Vahterus) with oil on the plate side or a plate type oil cooler (AlfaNova). The coolers are designed
for design pressure 28 bar on both sides. The oil temperature is maintained between 50°C [120°
F] and 60° C [140° F] for ammonia and halocarbons with a thermally controlled valve. The oil
Oil separator, condenserCondensing pressure
Compressor
Evaporating
pr
essure
P 0
1
P 2 ”
P 1
P 1
P 1
P 2
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