SUPERMARKET SYSTEMS AND EQUIPMENT
units to always operate in the best conditions. On the one hand, DC technology means to have more stable parameters and increased reliability with minimum ON/OFF cycles. On the other hand, electronic expansion valves ensure optimal superheat control and synergy with envelope control. These modulating components are optimised if working with advanced control and monitoring systems, that allow a full continuous control of the units with advanced algorithms, making it much easier to prevent failures. All in all, by using advanced technology, maximum energy efficiency at part loads as well as optimum food temperature and humidity control are achieved. Additionally, the fact that each cabinet is independent of the others gives another significant thermodynamic advantage over traditional systems: cabinets do not have to work at low evaporation temperature due to demand from the most critical cabinet but each evaporation temperature is optimized for each cabinet.
3. Natural refrigerants
The unique characteristics of natural refrigerants such as high working pressures of CO2 and flammability of propane lead to some adaptations at the time of propose a system. CO2
is a refrigerant well accepted from the market, with optimal heat transfer, for which technologies are constantly being improved. Lately, efforts have been focused on increasing efficiency in warm climates: the low critical temperature of CO2
(31.1°C) means that the
system works in transcritical mode when that temperature is reached, which has to be taken into account at the time of design the system. The first proposal for CO2
consists of single
circuits for both medium and low temperature cabinets. A chiller allows low temperature cabinets work in subcritical whereas medium temperature cabinets work in transcritical. The chiller turns on when ambient temperature is
higher than 20-25 °C, ensuring subcritical mode for low temperature cabinets. The second proposal for CO2
is done by double
stage circuits in each cabinet. The high stage works at high temperature (transcritical), in order to keep low stage working in subcritical mode. These two CO2
proposals are suitable for semi
plug-in units with water loop system. Propane is a highly efficient refrigerant that operates at standard working pressures. However, its high flammability leads to a cautious design of the system and the respect of the specific requirements for flammable refrigerants dictated by the standards.
Nowadays, the charge limit of 150 grams for commercial refrigerating appliances dictated by IEC 60335-2-89 makes its diffusion more difficult. The update of the standard that can be approved by the end of this year, increasing the charge limit of propane to 500 grams among other modifications, will facilitate the use of
propane in commercial refrigeration. For small loads, or in the case that the limit is increased to 500 grams, propane plug-in and semi plug-in units can be done with a conventional single circuit, both for medium and low temperature cabinets. Compliance of the components with 60079 (standard for explosive atmosphere) facilitates the approval of the IEC 60335-2-89.
For higher loads, it is possible to design a multi circuit system with common evaporator and condenser. The typical configuration nowadays is three circuits of 150 grams.
With all these options, the end user can decide which are the advantages that fit better with its own needs.
The use of natural refrigerants for self- contained cabinets demonstrate once again that high pressures, low efficiency in warm climates, flammability and reduction of refrigerant charge are not an issue if managed correctly.
www.acr-news.com
April 2020 31
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