Refrigeration in a supermarket is an interesting and onerous problem for the energy engineer because the solution requires both theoretical and practical knowledge of thermodynamics. To begin with, in order to keep food stored at healthful temperatures, heat must constantly be removed.

But remove just a bit too much heat and there are costly results: Food freezes slowly and must be discarded. Frozen meat slowly thaws to a condition where it spoils very rapidly. Frozen foods can be kept colder than necessary, but the cost of doing so is needlessly high.

At the other end of the thermometric scale, fresh foods not kept cold enough often don't stay fresh long enough to sell. And frozen foods that thaw in the store are a total loss.

Stressed Refrigeration

Complicating the refrigeration problem is the environment in which cooling takes place. A supermarket is not a carefully controlled laboratory, but a working environment where cases are constantly stocked to keep them full.

Customers are constantly opening case doors, admitting warm air to the refrigerated interior. Even in store cases that do not have doors, every time a hand enters the cold zone, heat enters with it. All of these activities put heat stress on the refrigeration system.

Before electronic controls came along, simple thermostats were used to open and close solenoid valves that delivered liquid refrigerant to the fixtures in a store. At the fixture, another valve (a thermostatic expansion valve), again activated by temperature, metered the refrigerant into a heat-absorbing evaporator. For some circuits, mechanical evaporator pressure regulation valves were used.

Electronic controls make possible more repeatable control and allow remote system setting and performance monitoring. They also allow a good deal of remote troubleshooting.

Heat from the case is carried away with the refrigerant (charged now into a gas) back to the compressors in the machine room. From there it is pumped to a condenser where it changes state to liquid so that the whole process can occur again.

This system is called liquid side control because the flow of liquid through the expansion valve in turn controls the refrigeration process.

Thermostatic expansion valves have to be set for a superheat that is based on the load capacity of the individual fixture. But cases and cooler boxes are not always stocked to the same level. It has been seen that staff and customer activity continuously change the load of the refrigeration system. So the setting is done to meet average conditions.

Smart Case Controller

Smart case control uses solid state sensors and an electronic controller in each case. An electronic expansion valve supplies just enough liquid refrigerant to meet the load on a case.

Software algorithms in the controller exactly match the superheat to the load in the case in order to most efficiently regulate the flow of refrigerant through the valve. This means no over-cooling takes place as with a purely mechanical system. An adaptive superheat algorithm (such as in the AKC 164 Smart Case Controller made by Danfoss) continuously changes the superheat in response to the load changes. If warm food is put into the case, within six seconds the superheat is changed to match the new load.

Liquid side adaptive superheat control has shown savings of more than nine percent in tests audited by public utility companies. Supermarket chain owners have received utility rebates based on these savings, in addition to the direct reduction of their utility bills.

Systems using adaptive liquid side control also achieve much lower head pressure, the pressure the system has to run to keep refrigerant moving in the system. Savings here are directly proportional to saved energy expense.

Other benefits of case control include a less expensive installation, since less cable and electrical labor are required in a case control story. There are also fewer components on the refrigeration machinery, reducing initial cost of equipment. There is less product loss because of the built-in alarms. There is also easier troubleshooting both on and off site.

Robinson is principal writer and editor for Technical Communications at Danfoss Inc., Air Conditioning & Refrigeration Division. For more information, visit www.acr.danfoss.com.

Publication date: 09/06/2004