Application with Bitzer reciprocating compressors

Start unloading equalizes the pressure between the high and low pressure sides. The back pressure is thus lower, the starting torque is reduced to the required value, and the motor can start as easily as possible. This can be particularly useful with star-delta start and short on-off cycles (short pause time) of the compressor.

Many Bitzer reciprocating compressors are available with integrated start unloading. It essentially consists of a bypass between the discharge and suction gas sides, which is controlled by a solenoid valve. Ideally, this opens 1 .. 2 s before the start, the compressor then delivers from the outlet directly into the inlet with a small pressure difference. When the motor has reached nominal speed, the solenoid valve is closed, thereby closing the bypass, and the compressor is in normal operation.

For most Bitzer reciprocating compressors, the following applies: If a cylinder bank is equipped with start unloading, it is no longer available for capacity control. A 6-cylinder compressor can therefore be equipped with max. 3 CRII capacity regulators or with start unloading and 2 CRII capacity regulators. The capacity control range is reduced accordingly.
An exception are R744 compressors: Here the capacity regulators are designed as bypass and can therefore also be used for start unloading.

For compressors for which no integrated start unloading is available, an external start unloading can be set up: For an unloaded start, a solenoid valve is opened in the external bypass line. The compressor delivers discharge gas via the bypass line directly to the suction gas chamber or into the suction gas line. A check valve in the discharge gas line prevents discharge gas from flowing back out of the condenser. In normal operation, the solenoid valve and thus the bypass line are closed, and the compressor delivers at normal load.

For further information on mounting and retrofitting the start unloading see the respective Operating Instructions and:

• KT-110: Start unloading (SU) for BITZER reciprocating compressors

Information in the Bitzer Software

If a specific compressor is selected in the Bitzer Software, the tab "Technical Data" indicates if start unloading is available:

Setting in the Best Software

When using the compressor module CM-RC, the module also controls the start unloading if necessary. For details:

• KT-230: Technical information Compressor module CM-RC-01 for reciprocating compressors
• KT-240: Technical Information Compressor module CM-RC-02 for reciprocating compressors

Schematic wiring diagram

With reciprocating compressors, start unloading replaces one of the capacity regulators (exception: R744 compressors, see above). A compressor with start unloading is depicted like this, for example: