REFRIGERANTS
REFRIGERATION
R-744 booster systems for commercial refrigeration
Oliver Javerschek, application engineering and product performance, and Tobias Fuhrer, product manager reciprocating compressors explain in depth how Bitzer’s Varistep provides intelligent capacity control for effi ciency and reliability.
T
ranscritical carbon dioxide systems have become a mainstream solution in supermarkets and medium- sized cold stores. They are also increasingly being used
in integrated systems for low and medium temperature applications, as well as in air conditioning and heating. The key to optimising performance is ensuring eff ective
control of components and the way they interact with each other as part of the total system. This ensures stable, effi cient and reliable operation across the year, in all conditions. Working with customers, Bitzer has analysed thousands
of refrigeration systems and built a deep understanding of their operating dynamics. Combined with laboratory tests and demonstration systems, we are able to use this knowledge base to support customers in planning new commercial refrigeration systems and retrofi tting existing plants to deliver optimum performance and working life. A key element of system optimisation is the use of advanced mechanical capacity control on compressors. Varistep, developed by Bitzer, enables excellent capacity control in the smallest of spaces, and delivers outstanding effi ciency and reliability. The following examples show the power and eff ectiveness of this approach in diff erent applications.
The challenge of supermarket refrigeration Supermarkets are characterised by their highly variable need for refrigeration, depending on the time of day and customer footfall. In addition to meeting the basic requirements to maintain desired operating conditions and ensure eff ective oil management, capacity control must be tailored to the application to ensure compressors operate smoothly and effi ciently.
The purpose of capacity control in parallel compounding is
to cover minimum cooling loads in order to minimise on-off cycles, particularly in the lead compressor, and achieve high control accuracy (CF) with minimal capacity changes per step. This reduces operating costs and increases system reliability. In recent years, however, the often confl icting requirements
of improving effi ciency, reducing system complexity and lowering capital cost have resulted in less than optimal system designs and installation practices. The consequences include reduced performance and reliability, due to: ■ A reduced number of compressors and/or excessively large compressors per suction group;
■ Active liquid injection with regular subsequent injections in lieu of an external desuperheater for the low temperature stage;
■ Low temperature capacity control with on-off cycles instead of capacity control with a frequency inverter;
■ Even stricter heat recovery requirements; ■ Heat recovery systems without storage tanks on the hot water side;
■Less time for production, installation and commissioning; ■Reduction in the number of fi lter and oil changes.
Image 1: Simplifi ed depiction of the thermal application limits of a compressor for transcritical applications.
16 September 2023 •
www.acr-news.com
Discharge gas temperature illustrates the eff ects on the lead compressor. A standard compound control system monitors key variables independently of one another, with a safety cut- out for high pressure, discharge gas temperature, suction gas superheat, oil level and motor temperature. The permitted discharge gas temperature in particular depends on the pressure ratio, suction gas superheat, operating frequency, operating time and dynamics of operation. A lower operating frequency and higher suction gas superheat aff ect the thermal load of the compressor and lower its application limit.
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