Introduction

The following main methods are used for capacity control of compressors:

This document explains the integrated mechanical capacity control. It is available for all Bitzer screw compressors and is implemented differently depending on the series. The lowest capacity step usually also functions as start unloading; if necessary, a certain pause time must be observed before restarting the compressor.

For schematic wiring diagrams for different control variants:

The following variants of mechanical capacity control are found in Bitzer screw compressors:

Overview capacity control for Bitzer screw compressors
CR: capacity regulator
Vi: internal volume ratio (volume suction side / volume discharge gas side)

Characterisation

Vi (internal volume ratio)

ECO port

in compressors

stepped capacity control
by control pistons

2 control pistons shift the suction side sealing contour in 2 steps (75 and 50%), low pressure drop

Vi changes with the capacity step, because the suction gas volume is reduced, but the discharge gas port is not.

fixed ECO port near discharge gas port

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

dual capacity control
by CR slider

stepless or 3- resp. 4-step slider control with Vi compensation, no pressure drop

Vi varies with capacity, because suction gas volume and discharge gas port / internal discharge port are modified.
For CR >75%, Vi is often constant, for CR <75%, it is reduced.

With HS.85, OS.85 and CSH.5 integrated in slider.
With CSH.6 and CSW fixed in the housing.

HS.85
OS.85
CS.65 .. 96

stepless capacity control
by double slider

combined CR and Vi sliders, controlled by CM-SW module

Vi is automatically adjusted:
At full load, CR and Vi slider move together, the discharge port is modified.
At part load, the sliders move independently, the Vi control range is reduced.

fixed in the housing

HS.95
OS.95 .. 105
CS.105

For the combination of capacity control and ECO operation: ECO operation at part load.

Also observe the following technical documents