Safety devices and system control

Safety devices

Each individual compressor must be secured with pressure switch(es) to prevent the permitted high pressure from being exceeded and with a protection device as well as oil pressure and oil level monitoring. In particular when operating at the thermal application limit, a discharge gas temperature sensor (accessory) is additionally recommended. As protection against increased refrigerant solution in the oil at standstill, for customary applications an oil heater (accessory) is required. A temperature-dependent control of the oil heater allows the energy requirement to be clearly reduced.

The Bitzer compressor module CM-RC (CM-RC-01 or CM-RC-02 with extension boards CM-IO-A or CM-IO-B) can secure the compressor very reliably and integrate many functions, and the electrical installation is less complicated. Securing and operating integrated functions:

• Activation of additional fan, refrigerant injection, oil heater, start unloading, switching times of the compressor contactors during start
• Activation of the capacity control for adjusting the cooling or heating capacity to the actual demand (via Modbus or set point signal) – thus guaranteeing maximum compressor availability and efficiency
• Active monitoring of the operating parameters (application limits)
• Communication via the BEST Software: Feedback via compressor operation
• Communication with system controller
• Control of the OLM-IQ (Oil level controller OLM-IQ)

Detailed information, assembly information and information about the electrical connection of the CM-RC:

• KT-230: Technical information Compressor module CM-RC-01 for reciprocating compressors
• KT-240: Technical Information Compressor module CM-RC-02 for reciprocating compressors
• KT-241: Technical Information Extension board CM-IO-A for CM-RC-02
• KT-242: Technical Information Extension board CM-IO-B for CM-RC-02

To secure the overall system against the pressure being exceeded, the specifications of the standard safety regulations and standards (e.g. EN378, ISO5149) must be observed. To monitor the minimum permitted suction pressure, a common low pressure limiter set to a lower pressure level than the suction pressure control is usually sufficient. It only serves as protection against a possible failure or a fault in the system control.

Information on compressor protection devices:

• CT-120: Protection devices for BITZER compressors

Information on oil monitorings:

• AT-170: Oil monitoring for BITZER products – overview

System control

A compound system is controlled either as a function of the suction pressure or of a defined process temperature. For systems having a relatively high number of evaporators, a suction pressure control adjusted to the demand is customary. To this end, the manufacturers of regulators offer a wide range of products. The control algorithm should guarantee at least the following:

• Demand-dependent switch-on and switch-off of individual compressors and their capacity control (finely tuned mechanical or speed control), as a function of a defined suction pressure with a narrow bandwidth. In doing so, the design criteria described above should be taken into account (Selection of compressor and accessory).
• Securing the max. cycling rate and minimum running time of the compressors. This is possible, for example, through a time limit from start to start as well as a minimum running time secured by the control algorithm, which may only be fallen short of during safety cut-outs.
• Automatic change of the lead compressor for uniformly distributed operating hours of the compressors. This ensures a balanced load and minimum wear.
• When the lead compressor is switched off in controlled fashion (via the min. suction pressure), you must ensure that the solenoid valves in the liquid line or the electronic expansion valves can only be activated following a possible time delay of the lead compressor. This is also true when all compressors are shut down by a safety device (e.g. high pressure switch-off). This avoids flooding of the evaporators with liquid.
• Adjustment of the suction pressure to demand, for example automatic increase at reduced load
• Reasonable control of the condensing temperature (Further components of the refrigeration system)

Information on the operation of reciprocating compressors with frequency inverters:

• KT-420: BITZER reciprocating compressors with external frequency inverters