HS.53 .. 74, OS.53 .. 74

The capacity of these compressors is controlled in several steps with the help of hydraulically actuated control pistons. They engage directly in the profile area and reduce the displacement. The control pistons are precisely adapted to the contour of the rotor housing; in full load operation, they are in absolutely positive contact - with the face flange (axial pistons) or profile space (radial piston). This makes the housing particularly stable, and the gaps between rotors and housing remain within narrow limits even at high temperatures and pressures.

Control pistons

For part load operation, the pistons move one after the other to the rear position, releasing amply dimensioned openings between profile space and suction side. The suction gas is conveyed directly into a profile area with reduced suction volume. This reduces the active profile volume resp. the active rotor length and thus also the displacement resp. capacity. The control is designed for two steps, so that the compressor capacity can be adapted very precisely to the load condition of the system by switching the 1 or 2 solenoid valves intermittently.

Due to the direct hydraulic actuation of the control pistons, no additional components are required for protection against liquid slugging or for start unloading. The pistons open whenever the pressure in the compression chamber is above their actuating pressure - this is usually the oil or condensing pressure. This ensures automatic start unloading as well as protection against severe over-compression.

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View from above of compressor OSN74 with 2 control pistons and the associated solenoid valves
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Constructional design of the capacity control or start unloading for OS.74 compressors. The picture shows one of the two control pistons and the associated solenoid valve. For OS.53 and OS.7441 compressors, the functional principle is similar, but the overflow opening to the suction side is located on the periphery of the male rotor.

Solenoid valve de-energised = port is open = capacity reduced.
Solenoid valve energised = port is closed = capacity increased.

Stepped capacity control

The following figure shows the control with a simplified view of the unwound outer surfaces of the rotors.

Stepped capacity control for OS.A53 or OS.A74 compressor with 2 control pistons (CR1, CR2) and fixed economiser port, unwound view.
SL: suction gas line
DL: discharge gas line
dashed line: end of the suction process
ECO may only be used ≥75%.
OS.A53: CR1 is optional.
OS.A74: CR1 and CR2 are standard.
The capacity steps given here are the nominal ones. The effective capacity steps depend on the operating conditions.

The capacity steps given here are the nominal ones. The effective capacity steps depend on the operating conditions.

In actual operation, the respective application limits must also be observed (see BITZER SOFTWARE).

Start unloading

At standstill, the control pistons open automatically, so that the compressor starts unloaded.

Control with solenoid valves

The arrangement of the solenoid valves on the discharge flange can be found in the dimensional drawing of the respective compressor, e.g. in the BITZER SOFTWARE. Which solenoid valves must be energised and de-energised for the desired capacity step is shown in the respective operating instructions:

Switch solenoid valves CR1 and CR2 in the correct order and with a time delay of min. 5 s. Operation with economiser is only possible at ≥75%.

Switch solenoid valves CR1 and CR2 in the correct order and with a time delay of min. 5 s. Operation with economiser is only possible at ≥75%.