REFRIGERATION REFRIGERANTS In image 1, dashed line 3 shows the maximum permitted


discharge gas temperature (t max.) for an operating frequency of 25Hz with suction gas superheat of 30K. Highly unfavourable operating conditions for the lead compressor in the medium temperature stage are characterised by: ■ Daytime operation with multiple on-off cycles in the sec- ondary compressors and unstable (fl uctuating) operating conditions caused by low control accuracy;


■ Night-time operation with low operating frequency and reg- ular 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 indirectly aff ect the tribology of the compressor drive gear, and can result in increased bearing wear. An excessively low control range and signifi cant load or capacity changes result in instability in the overall system, particularly 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 diff erence between the capacity of the lead compressor at maximum and minimum frequency, divided by the capacity of the subsequent compressor.


Improved capacity control accuracy with Varistep Supermarket refrigeration systems are generally designed


with a primary focus on low capital cost and small dimensions, illustrated 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 fi xed-speed reciprocating


compressors have also been added to the compound system. A mechanical capacity control system has not been fi tted. The minimum capacity of the system in winter is 19.1kW, and its maximum capacity in summer is 88kW. A fl ash gas bypass system with the following operating conditions was selected for the design: Summer: Operation mode:


Evaporation temperature


Gas cooler outlet temperature High pressure


Winter: Operation mode:


Evaporation temperature Condensing temperature


Image 2: Operation of a refrigeration system with three compressors and low control accuracy on an average winter’s day.


Constant switches between minimum and maximum lead compressor operating frequency, frequent secondary compressor cycle rates, strongly fl uctuating 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 behaviour of such a system during opening hours on a typical winter’s day at our latitudes is depicted in image 2. In the fi rst step, the frequency range of the lead compressor


Transcritical -8°C 40°C


98.6 bar


Subcritical -8°C 15°C


(The low temperature stage is not included to keep the examples simple.)


The control accuracy of the system is just 36% in summer and 34% in winter. These values clearly need optimising. In this case, the control accuracy (CF) does not allow for good coverage of the refrigerating capacity required by the evaporator.


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is expanded to the standard range. In this way, the control accuracy (CF) can be improved to 36 and 55 relatively easily. At this point, it is necessary to check whether the thermal load is still permissible for the lead compressor, given the suction gas superheating and operating conditions. Likewise, the motor power reserve needs to be inspected if the frequency-controlled motor has star wiring. An eff ective way to improve control accuracy is to retrofi t a secondary compressor with a Varistep mechanical capacity control system (CR100% or CR50%). Keeping the initial situation the same, the second compressor in the compound system is equipped with the Varistep mechanical capacity control system. This optimises control accuracy to 109% in summer and 107% in winter, as a result of avoiding capacity jumps between the lead compressor and subsequent compressors. It is necessary here to check whether the compressor rack controller is able to energise another capacity-controlled compressor. Use of Varistep capacity control opens up many more options


for improving performance. In this example, the compound system could be planned even more benefi cially. The lead compressor 4HTE-15K-40P is equipped with Varistep for virtually stepless capacity control. The integrated IQ Module makes it easy to operate. Subsequent compressors are a 4FTE-20K-40P with stepped capacity control (50% /100%) and a fi xed-speed 4FTE-20K-40P. This signifi cantly increases part load effi ciency. The minimum capacity in winter is now 4.4 kW and maximum capacity in summer is 88.5 kW. The control accuracy has been optimised to 122% in summer and winter. This combination of compressors makes it possible to realise the largest capacity control range with just three compressors (see image 3).


Continued on page 18 www.acr-news.com • September 2023 17


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