Computer control for refrigeration systems has become extremely popular in the fruit storage industry.

The primary driving force behind the purchase of most systems is the energy payback analysis. While the payback analysis and the savings associated with energy conservation are certainly of interest, attention must be given to other issues.

Energy can be saved in a number of areas in the typical refrigeration system. I will review the basics of the refrigeration system and relate the operation of the system to the savings in energy that can be realized. Here are three areas which can provide the bulk of the opportunities for energy savings:

1. Compressor discharge pressure - for a given suction pressure, reducing the compressor operating discharge pressure will have two complementing effects. As the discharge pressure is reduced the capacity of the compressor increases. At the same time, the horsepower required to operate the compressor decreases. Obviously, it is advantageous to operate the compressor at the lowest possible discharge pressure.
   
   
2. Compressor suction pressure - for a given discharge pressure, increasing the suction pressure will also have two effects. These effects are not complementary as in discharge pressure but still have the desired results. As the suction pressure increases, the compressor capacity increases and the horsepower increases. The number in which you, as a refrigeration facility operator, are interested is the tons of refrigeration per horsepower. As the suction increases, the tons of refrigeration per horsepower increases. i.e., you get more refrigeration for the same horsepower dollars.
   
   
3. Fan cycling - during harvest a large amount of air is required to bring the product down to temperature in a reasonable amount of time. As the field heat is removed from the fruit and the atmosphere in CA is reduced, the heat that the refrigeration system must handle decreases radically. At that time most refrigeration systems have much more air in the rooms than the basic refrigeration process requires. Since the fans in many facilities can be a substantial portion of the total horsepower, we can reduce the amount of fan heat that we are putting into the room and save energy by either turning off all the fans for a portion of the day, turning off a portion of the total number of fans or reduce the speed of the fans with a variable frequency drive.

A Model System

Let's look at a model system and how we can optimize the operation of the refrigeration system:

• The plant has 10 CA rooms with 2,000 bins per CA for a total of 20,000 bins in CA. Each CA has 15 hp in fans. There is common storage with 15,000 bins of fruit. This common storage has 96 hp in fans. The compressor and condenser match required refrigeration load.
  
  
• This facility has an annual operating cost of $106,725 per year without computer control. If we implement a reasonable compressor room control strategy that allows the discharge pressure to float with the wet bulb temperature and the suction pressure to reset depending on the current needs of the facility, we see an operating cost of $94,775 per year.
  
  
• If we also fan cycle at an optimum rate, we can further reduce the annual operating costs to $85,257. This shows a total savings of energy dollars of $21,468 per year.

The Reality

You're sold on the concept, you buy the system and now you have to operate it. You immediately rush out and magically turn your best fork lift mechanic into your refrigeration operator. You send him to the Postharvest Conference to learn all he needs to operate your new refrigeration system. He learns the basics discussed above.

The Problems

1. Your operator did not realize that you can get discharge pressure too low. Discharge pressure that is too low can cause poor feed to some evaporators and irregular and insufficient cooling for some screw compressors. Oil can carry over from some compressors causing heat exchange surface fouling and an increase in the required condenser fan horsepower in excess of the compressor horsepower saved. In addition you can have low defrost water temperatures.
   
   
2. He also did not realize that you can get the suction pressure too high. High suction pressure can cause slow room pull down; an inability to maintain storage room temperatures. Oil carry over from some compressors causing heat exchange surface fouling.
   
   
3. Now he is really going to save you some big money. He devises a fan cycling scheme that is way off the map. Whether he is fan cycling by the on-off method, alternate fan method or variable frequency drives, the results are still the same. Excessive fan cycling can cause an increase in shrinkage due to depressed humidity levels in the room; poor or irregular temperatures in the fruit and poor air circulation in parts of the room.

The Results

You are having troubles getting the temperatures in your rooms down. After two weeks of struggling, your operator discovers that he is running the suction pressure too high. He reduces the suction pressure to catch up. He knows he can get some of the lost capacity back by reducing discharge pressure, so he really goes after that. Later that week his rooms stop declining in temperature. He discovers that the defrost water has gotten too cold and your rooms are all iced up. Three weeks into the program and you are finally getting your rooms down to temperature. You can start your Mexico export qualification now. Another thirty days into the storage season, your load is reduced enough and the ambient temperatures are low enough that you can finally make use of the really low discharge pressure operation. The discharge pressure is so low that one of your screw compressors will not cool the oil properly and the compressor quits. Suction pressure rises and temperatures increase. Your operator gets this back under control. He is now realizing that extremes in suction pressures and discharge pressures may not be all good.

But not to worry. Your operator knows he can make this up to you by saving a lot with fan cycling. Those rooms without benefit of variable frequency drives (they were expensive so you only put a few in this year) he fan cycles twenty three hours per day OFF and one hour per day ON. Those rooms with VFD he runs at 20%. Now we're making real progress. Unfortunately, you discover when you open the rooms to pack the apples that the excessive fan cycling has caused a little more shrinkage. Nothing big, just 1.5% more than you would expect. Oh, by the way! Also in the room with the VFDs, the air didn't make it to some of the corners of the rooms and those apples just didn't make it at all. They are now culls.

Economic Summary

While computer control can be an excellent tool that can give you opportunities to save energy and provide superior monitoring for improved quality control, it still must be done intelligently and with complete knowledge of your system.