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 . Coordinated interplay and control of the individual system components pave the way for a stable , reliable operation mode for the entire year .
TECHNICAL
R744 booster systems in commercial refrigeration
Authors : Oliver Javerschek , Application Engineering and Product Performance , BITZER 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 . Coordinated 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 cooperation 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 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
All images by : BITZER
• 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 cutout for the 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 the operation . A lower operating frequency and higher suction gas superheat can affect the thermal load of the compressor and lower its application limit . In image
Image 1 : Simplified depiction of the thermal application limits of a compressor for transcritical applications
1 , dashed line 3 shows the maximum permitted discharge gas temperature ( t max .) for an operating frequency of 25 Hz with suction gas superheat of 30 K . 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 /).
CASE STUDIES
Supermarket refrigeration systems are built with a primary focus on low investment costs and small system dimensions , as shown in the following example . The lead compressor 4JTE-15K- 40P is equipped with a frequency inverter that can operate the compressor at between 30 Hz and 60 Hz . Two additional 4FTE-20K-40P fixedspeed reciprocating compressors have also been added to the compound system .
A mechanical capacity control system has not been fitted . The minimum capacity of the system in winter is 19.1 kW and its maximum capacity in summer is 88 kW . A flash gas bypass system with the following operating conditions was selected for the design :
Summer : Operation mode : Evaporation temperature -8 ° C
Gas cooler outlet temperature
High pressure
Winter : Operation mode : Evaporation temperature Condensing temperature
Transcritical
40 ° C 98.6 bar
Subcritical -8 ° C 15 ° C
The low temperature stage has not been included in order to keep the following examples simple . The control accuracy of this system configuration is just 36 % in summer and 34 % in winter .
These values are in need of optimisation . In this case , the control accuracy ( CF ) does not allow for good coverage of the refrigerating capacity required by the evaporator . Constant switches between minimum and maximum lead compressor operating frequency , frequent lag compressor cycle rates , strongly fluctuating suction , medium and high pressures and cyclical operation with high and low suction gas superheat are all typical symptoms of a system operating with low control accuracy . The operating behaviour of such a system during opening hours on a typical winter ’ s day at our latitudes is depicted in image 2 . ( See image 2 )
In the first step , the frequency range of the lead compressor is expanded to the standard range . This way , the control accuracy ( CF ) can be improved to 36 and 55 in a relatively quick and easy manner . At this point , it is necessary to check whether the thermal load is still
Image 2 : Operation of a refrigeration system with three compressors and low control accuracy on an average winter ’ s day
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