Engineering manual - SAB 355 Screw compressor unit (including ATEX)
004100 en 2020.10
49/156
Technical description
When cleaning the oil filter during operation, please note that the pressure is higher when the
vent valve/plug is opened, (see Fig. 52). Especially in connection with HFC/HCFC plants there
may be a lot of oil foam.
Note: During operation at least one or both discharge valve(s) must be open to avoid hydraulic
pressure in the filter housing.
4.1.15 Compressor hydraulic system
The compressor hydraulic system moves the movable slide valve (MSV) to load and unload the
compressor, See Fig. 26 and Fig. 27. It also moves the movable slide stop (MSS) to increase or
decrease the compressor’s volume ratio (Vi).
The hydraulic cylinder located at the inlet end of the compressor serves a dual purpose. It is sepa-
rated by a fixed bulkhead into two sections. The movable slide valve (MSV) section is to the left of
the bulkhead and the movable slide stop (MSS) to the right. Both sections are considered double-
acting hydraulic cylinders as oil pressure moves the pistons in either direction.
Both sections are controlled by double-acting, four-way solenoid valves which are actuated when
a signal from the appropriate microprocessor output energises the solenoid valve. Valves V1, V2,
SC1, SC3 and SC4 are always open.
Single-acting mode, high-stage (T
C
> +15°°C)
• Close the valve at SC2
• Open the valve at BP (by-pass)
High-stage compressor loading
The compressor loads when the MSV solenoid valve, YY2, is energised, and oil flows from the un-
loading side of the cylinder out of the port, SC1, through the valve ports, A and T, to the compres-
sor suction side. Simultaneously, the discharge pressure loads the slide valve.
High-stage compressor unloading
The compressor unloads when the MSV solenoid valve, YY1, is energised, and oil flows from the
oil manifold through the valve ports, P and A, to the cylinder port, SC1, and enters the unloading
side of the cylinder. Simultaneously, gas on the loading side of the cylinder is vented through the
port, SC2, and the valve, BP, to the compressor suction side.
Note: To control the rate of loading and unloading, change cycle time, proportional band and
dead band setpoints with the control system. If additional control is needed, throttle SC2 or BP.
Double-acting mode, booster (T
C
< +15°°C)
• Open the valve at SC2
• Close the valve at BP (by-pass)
Booster compressor loading
The compressor loads when the MSV solenoid valve, YY2, is energised, and oil flows from the oil
manifold through the valve ports, P and B, to the cylinder port, SC2, and enters the loading side
of the cylinder. Simultaneously, oil contained in the unloading side of the cylinder flows out of cyl-
inder port, SC1, through the valve ports, A and T, to the compressor suction side.
Booster compressor unloading. The compressor unloads when the MSV solenoid valve, YY1,
is energised, and oil flows from the oil manifold through the valve ports, P and A, to the cylinder
port, SC1, and enters the unloading side of the cylinder. Simultaneously, oil contained in the load-
ing side of the cylinder flows out of compressor port SC2, through the valve ports, B and T, to the
compressor suction side.
Note: To control the rate of loading and unloading, change cycle time, proportional band and
dead band set points with the control system. If additional control is needed, throttle SC2 or BP.
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