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ENERGY SAVING


this way, after some trial and error, it will be possible to find your preferred settings, as I have done.


In the table below are my ‘magic


numbers’. These PID settings are suitable for all stepper and other kinds of expansion valves in CO2


applications, for both medium and low temperature evaporators. What about defrosts? How are these


managed? How many defrosts should be performed in a day? By optimising these settings, even more energy can be saved. On the other hand, defrosting may be hard to optimise with just some general settings, as the load on the evaporators is always different; this is where the “skip” or “smart” defrost functions come into play. With these functions enabled, the defrost frequency is based on the coil temperature or the running time of the evaporator: in other words, a defrost is performed only if it is needed. If we can run with just one defrost a day instead of more, this for sure will save energy. Remember that defrosting is one of the biggest energy consumers in a supermarket application, so the better settings, the more energy can be saved. After looking at the cabinet / cold room


controllers, we come to the compressor rack controller. Certainly the best way to save energy in a refrigeration installation is to have as few compressors running as possible, while at the same time fulfilling the temperature request from the cabinets / cold rooms. I often see compressors that switch on/off frequently, and this is often “simply” due to the desire to avoid temperature fluctuations. Do we really need to have a constant rack temperature? Can the fluctuations actually be seen on the cabinets? My experience is this: “No, we don’t”. If the rack control temperature can be kept within 1-2°C from the set point, all of the cabinets / cold rooms will remain inside the temperature limits. And by allowing these “small” fluctuations, less compressors need to operate / start /stop, meaning higher energy savings. Let’s then look at the control settings for the compressors, especially the operating set point, differential and timings. An example of settings for a CO2 rack with 4 medium temperature compressors could be:  Control type: dead band;  Compressor set point: 26 bars (-6.8°C);  Dead band: 2 bars around the set point (1 bar on each side) before starting to control the capacity, except for inverter compressors that modulate capacity at all times, accord- ing to the request;


 Time delay of at least 60 to 90 sec between www.acr-news.com


starts of different compressors;


 Time delay of 30 sec before switching off compressors.


In this way, there will generally be slow control around the set point, with the compressors starting 2-3 times per hour, or maybe even less, instead of 6-8 times. When on the other hand the temperature is below the set point, the compressors need to be switched off faster, as it is wasteful to run a compressor when below the set point. The dead band settings for the low temperature line are actually the same as those for the medium temperature line. The only difference is the set point value. An example off suction pressure with fluctuation but at the same time stable control temperature in the cabinet. If P+I control is preferred for the rack, this is also possible. The key to good P+I control is the differential. The smaller the differential, the faster the compressors are controlled. So when using P+I, this needs to be kept clearly in mind, also when setting the compressors on/ off times.


If using the same rack with 4 medium- temperature compressors, my suggestion for P+I control would be:  Control type: P+I;  Compressor set point: 26 bars (-6.8°C);  Differential: 6 bars;  Integral time: 600 sec;  Time delay of 60 sec between starts of dif- ferent compressors;


 Time delay of 30 sec before switching off compressors.


My experience with both these control settings is very good, and I can say that no matter which you choose, the end result will be “good” control and energy savings. All of these suggestions on where to look for energy savings in a supermarket application can also be applied to all kinds of refrigeration applications: when trying to save energy and optimise HVAC applications, you can always start by looking at compressor control and electronic expansion valve control. My main message is to take time to look at the control settings – this is a way to save energy and save money.


January 2021 15


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