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INCORPORATING COLD CHAIN |
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R744 BOOSTER SYSTEMS IN COMMERCIAL REFRIGERATION
By Oliver Javerschek , application engineering and product performance , and Tobias Fuhrer , product manager reciprocating compressors , BITZER .
In recent years , transcritical CO 2 systems have largely become standard practice in commercial refrigeration , such as in supermarkets and medium-sized cold stores . The systems are increasingly becoming integrated system solutions for low and medium temperature applications as well as air conditioning and heating . Co-ordinated interplay and control of the individual system components pave the way for a stable , reliable operation mode for the entire year .
BITZER analyses countless systems in close co-operation with customers and partners . Building on tests in its laboratory and demonstration systems , BITZER supports its customers from planning and building new commercial refrigeration systems to retrofitting pre-existing ones . By equipping compressors with VARISTEP mechanical capacity control , it becomes possible to build efficient refrigeration systems with excellent capacity graduation in the smallest of spaces , as demonstrated by a few case studies below .
THE CHALLENGES OF REFRIGERATION IN SUPERMARKETS
Supermarkets are characterised by their highly changeable need for refrigeration depending on the time of day or customer footfall . Besides the well-known basic requirements such as observing the permissible operating conditions and ensuring functional oil management , the capacity control must be tailored to the
FIVE GLOBAL TRENDS AND HOW TO RESPOND
Supplied by Danfoss
Right now , things are changing like never before . Mega trends like electrification , digital communications , and the fight against climate change are creating both opportunities and challenges for the food sector worldwide .
Danfoss sees that first-hand working with hundreds of food retail chains in more than 60 countries . In particular , it reports five big , overlapping changes that are combining to shape today ’ s workload for supermarkets ’ teams … and five key ways to respond .
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application for compressors to operate smoothly . The purpose of capacity control in parallel compounding is to cover minimum loads in order to minimise on-off cycles , especially in the lead compressor , and achieve a high control accuracy ( CF ) with minimal capacity changes per step , so as to lower operating costs and increase the reliability of the system .
In recent years , rationalisations in systems and installations have , in practice , often led to conflicting requirements in terms of high efficiency , low system complexity and low investment costs . Common consequences include unfavourable system performance and poorer operating reliability , such as 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 needed for production , installation and commissioning
• a reduced number of filter and oil changes
The discharge gas temperature illustrates the effects on the lead compressor . A standard compound control system monitors the following variables independently of one another and features a safety cut-out for the high pressure , discharge gas temperature , suction gas superheat , oil level and motor temperature . The permitted discharge gas temperature
• Intense pressure on operating margins makes cost savings essential : The average profit margin for a large food retailer currently stands at just 1.7 %. This puts every operating cost under scrutiny – because it has a direct impact on competitiveness and profitability . What this means in practice : supermarket teams need creative ways to save operating costs – including new technology and partnerships . They might also need to implement the technology for new revenue streams .
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in particular depends on the pressure ratio , suction gas superheat , operating frequency , operating time and dynamics of the operation . A lower operating frequency and higher suction gas superheat affect the thermal load of the compressor and lower its application limit .
Highly unfavourable operating conditions for the lead compressor in the medium temperature stage are characterised by :
• Daytime operation with lots of onoff cycles in the lag compressors and unstable ( fluctuating ) operating conditions brought about by low control accuracy
• Night-time operation with low operating frequency and regular pump-down cycles with a high-pressure ratio and high suction gas temperatures
• Night-time operation with low operating frequency and a high number of on-off cycles , characterised by an active liquid injection before the lead compressor was shut down causing an excess of liquid refrigerant on the suction side during the restart delay
The conditions shown as examples indirectly affect the tribology of the compressor drive gear and can result in increased wear on the bearings .
An excessively low control range and significant load or capacity changes result in instability in the overall system , especially when the control range of the lead compressor is unable to compensate for the drops in capacity caused by other compressors switching on and off . Control accuracy is used to describe this relationship . It is the difference between the capacity of the lead compressor at maximum and minimum frequency , divided by the capacity of the subsequent compressor ( source : ASERCOM , see also www . bitzer . de / shared _ media / html / kt-600 /).
• Increasing food safety and reducing food loss are more important than ever : According to the UN Food and Agriculture Organization ( FAO ), food wastage accounts for 3.3 gigatonnes of greenhouse gas emissions . Avoiding food loss has always been essential to limit costs ; it ’ s now part of sustainability too . What this means in practice : retailers will need to keep their focus on maintaining safe refrigeration temperatures – and avoiding equipment breakdowns that waste time , money and food .
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Cold Link Africa
From closed door freezers to smart sensors and natural refrigerants , forward-thinking stores are setting a new standard for environmentally responsible supermarket cooling .
• Climate change regulation makes refrigerant choice critical : Initiatives like Europe ’ s F-gas regulations and US Climate Alliance are having a direct impact on refrigerant availability and cost worldwide . Choosing an alternative with low global warming potential has growing advantages . What this means in practice : choosing the right refrigerant brings advantages in cost , regulation , availability , environmental impact and sometimes tax .
• Utilising waste heat as a resource : More than half the world ’ s population now lives in cities , and the United Nations predicts that it will climb to 68 % by 2050 . This means urban neighbourhoods will need new ways of meeting energy challenges – such as China ’ s district heating systems . This can put retailers at the heart of the energy revolution . What this means in practice : in some Nordic countries , retail stores divert around 30 % of their excess capacity into heating homes in the surrounding area . This is a potential source of revenue .
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