REFRIGERANTS
Image 3: System with optimised capacity control. Orange: Operating conditions, black: Compressor in stop mode. Blue: The compressor covers the minimum capacity after conversion to Varistep, negligible operating condition fl uctuations.
Covering partial load conditions all the way to minimum load, such as by the lead compressor, guarantees a stepless mass fl ow, stable suction and high pressures with stable suction gas temperatures.
In this way it becomes possible to eff ectively prevent reduced system effi ciency, potential wet operation, reduced oil return from the system, fl uctuating control circuits and unfavourable operating conditions for compressors. It makes sense to regulate refrigerant mass fl ow in a stable, controlled manner. This involves controlling system components in a way that is precisely tailored to the circumstances of the system, delivering a stable, controlled refrigerant mass fl ow with no fl uctuating control circuits. In this context, it is important to take into account the number of compressor start-stop cycles. For compressors without frequency inverters, six starts per hour and ten minutes minimum time between two starts is recommended. High cycle rates are a challenge for the compressor motor (thermomechanical) and the drive gear (mechanical). In contrast, when operating with a frequency inverter, high
cycle rates easily lead to mechanical stress on the drive gear. When operating exclusively at low frequency and with periodic shutdown cycles, there is a risk of insuffi cient lubrication. When evaluating operating systems, it is important to take into account that higher cycle rates normally occur at night time or outside normal opening hours. Generally speaking, cycle rates of >120 per day can be considered critical, and cycle rates of >160 per day can be considered wholly excessive. Operation that is as stepless as possible with low compressor cycle times promotes good oil return. This prevents cold oil mixed with refrigerant suddenly being returned from evaporators to compressors after defrost, or under high refrigerating capacity. Moreover, stable operating conditions allow the oil barrier in oil level switches to settle and align with the level in the drive gear. The following measurements demonstrate the desirability of stepless minimum load coverage during compressor operation. Before being converted to stepless capacity control with
Varistep and IQ Module, the lead compressor was operated with a frequency inverter. In this case, the necessary refrigerating capacity was far lower than expected. The consequences for the lead compressor were cycle rates of
18 September 2023 •
www.acr-news.com
between 120 and 160 per day in winter operation, long shut-off periods and operation with cyclical pressure and temperature fl uctuations. This is illustrated in the visualisation of the operating conditions in the application limit. Use of a Varistep capacity control and IQ Module made it possible to stabilise operation and largely avoid the start-stop cycles of the lead compressor. This is also shown by the time- weighted mean operating condition values. For example, the average evaporation temperature is 2K higher and the average high pressure is 3 bar lower – with a consequent improvement in energy effi ciency.
Retrofi tting refrigerated display cabinets Exceptional care is needed when refrigerated display cabinets
are retrofi tted with glass doors. Most systems have considerable capacity reserves. Retrofi tting glass doors lowers the required refrigerating capacity by around 40 to 50%, depending on the temperature class and evaporation temperature. This exacerbates the deviation between the installed and necessary refrigerating capacity, which has a signifi cant impact on the part load behaviour of the system. Cycle rates will rise rapidly. If a lead compressor has high cycle rates, it is necessary to check whether installation of glass doors can be combined with a conversion of the compound system. Suitable measures would include: Selection of a lead compressor with a smaller displacement volume, and installation of a secondary compressor with stepped capacity control.
A look ahead Compound CO2
systems have become commonplace in
supermarkets. Practical experience backed by extensive data analyses shows that some system designs are only partially able to meet the diff erent load requirements for various reasons. The goal must be to try and reconcile the often competing
requirements for high effi ciency, low complexity and low investment cost. Ensuring control accuracy and coverage of minimum load conditions play a key role. As demonstrated, mechanical capacity control of compressors can improve both signifi cantly. In addition we can expect systems to be optimised further in the future. For our part, Bitzer continues to harness its considerable worldwide expertise and technical resources to keep pushing the boundaries forward.
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