INCORPORATING COLD CHAIN
TECHNICAL permissible for the lead compressor , given the suction gas superheating and operating conditions . Likewise , the motor power reserve needs to be inspected if the frequencycontrolled motor has star wiring .
An effective way to improve control accuracy is to retrofit a lag 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 from BITZER . This optimises the control accuracy to 109 per cent in summer and 107 per cent in winter , because it avoids capacity jumps between the lead compressor and the subsequent compressors . It is necessary here to check whether the compressor rack controller is able to energise another capacitycontrolled compressor .
However , the use of VARISTEP capacity control opens up many more options than this alone . In the example in question , for instance , the compound system could be planned even more beneficially . The lead compressor 4HTE-15K-40P is equipped with VARISTEP for virtually stepless capacity control . The integrated IQ MODULE makes it easy to operate . The subsequent compressors are a 4FTE-20K-40P with stepped capacity control ( 50 % / 100 %) and a fixed-speed 4FTE-20K- 40P . This means that the part load efficiency of the system increases significantly thanks to VARISTEP capacity control . The minimum capacity in winter is now 4.4 kW and its 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 3ab ).
Coverage of partial load conditions all the way to minimum load , such as by the lead compressor , guarantees a stepless mass flow , stable suction and high pressures and stable suction gas temperatures . As such , it becomes possible to effectively prevent reduced system efficiency , potential wet operation , reduced oil return from the system , fluctuating control circuits and unfavourable operating conditions for compressors . This
3a makes it prudent to control the refrigerant mass flow in a stable , controlled manner : Control of the system components that is tailored to the circumstances of the system and not excessively dynamic results in a stable , controlled refrigerant mass flow with no fluctuating control circuits .
The number of compressor start-stop cycles must be taken into account in this context . For compressors without frequency inverters , six starts per hour and at least ten minutes between two starts are recommended . High cycle rates are a challenge for the compressor motor ( thermomechanical ) and the drive gear ( mechanical ). In contrast , high cycle rates can easily lead to mechanical stress on the drive
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 .
Image 3 ab : System with optimised capacity control . Orange : Operating conditions , black : Compressor in stop mode . Blue : The compressor covers the minimum capacity after conversion to VARISTEP , with negligible operating condition fluctuations
3b
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