Refrigeration / Freezers & Coolers / Display Cases

Secondary Systems in Supermarkets

September 5, 2011
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A supermarket refrigeration technician walking into a store will more than likely find a direct expansion (DX) system. Of all the different approaches to commercial refrigeration that have been tried, none is today more common than DX. Multicircuited, centralized, parallel compressor systems are in use in more stores than any other kind.

But as new stores are built and older systems in existing stores replaced, another approach called secondary refrigeration is gaining wider acceptance and is steadily increasing in numbers.

Over time, manufacturers have made improvements in the efficiency and operating costs of DX systems through use of electronics and other technologies. Today’s DXs are among the most efficient and environmentally friendly ever produced. Yet one of the challenges with such systems relates to controlling leaks of the HCFC and HFC refrigerants typically used. Even the tightest of such systems can have some leak issues.

Regulatory constraints and cost of F-gas refrigerants can then cause retailers to look at various options such as secondary.

Reducing Charge

In medium-temperature applications, secondary refrigeration takes the approach of substituting a chilled-fluid loop system for the multicircuited store piping systems typically used in traditional refrigeration. The only HFC refrigerant used by the secondary system is contained entirely within the store’s mechanical center and external condensers. Some secondary systems even go a step further by using fluid condensers.

This approach prevents the possibility of a HFC refrigerant leak in the sales area. Instead, a propylene glycol solution of 35 percent glycol and 65 percent water is circulated through the loop piping system to the individual heat exchangers in the cases and coolers.

The glycol for these applications is dyed blue to aid in troubleshooting.

Differences

Some ways in which medium-temperature secondary systems differ from traditional systems include:

• Loop vs. circuited piping;

• Engineering plastic and water-grade M-copper vs. AC&R L-copper piping;

• Lower piping pressures (20 psi – 60 psi vs. 55 psi – 280 psi);

• Fully flooded heat exchangers vs. traditional evaporators;

• Balance valves instead of thermal expansion valves in each case and cooler;

• The only TXVs in the secondary system are for the plate-to-plate heat exchanger vs. 50 or more in a typical DX system;

• Approximately 40 percent less charge per rack for secondary systems.

• Reduced copper and refrigerant needed for installation of secondary vs. DX. Also in installation of glycol secondary, there is no need for nitrogen or line evacuation.

• Leak detection is done by virtue of dyed glycol rather than a hand-held detector.

Different Components

On the secondary side, a number of components are used that are not typically part of a traditional system. Many of these components are located on a key piece of equipment called a pump station (Figure 1). As its name implies, the pump station is used to circulate coolant through the secondary side of the system. Along with pumps, the pump station also contains an expansion tank to provide room for the coolant when it expands under certain normal conditions.

A fill tank, also typically located on the pumping station, allows coolant to be added whenever necessary such as during maintenance or when repairs are needed.

Another device usually included is an air separator to remove air from the system since air impedes the operation of the pumps.

Other components of the system such as the plate-to-plate heat exchanger are located on a rack adjacent to the primary side compressors. Also sometimes referred to as a chiller, this heat exchanger is where heat is removed from the secondary side of the system and given off to the primary side. Return fluid from the cases and coolers enters the chiller and through the action of the plate-to-plate heat exchanger, gives up the heat it has absorbed from the finned tube-type heat exchangers in the cases and coolers.

These heat exchangers in the cases and coolers look almost identical to conventional evaporators. But the difference between them is that the coolant does not undergo a state change, whereas in a traditional DX evaporator, the refrigerant does change from a liquid to a vapor as it absorbs heat. In the secondary heat exchanger, the liquid coolant stays a liquid.

Instead of the TXVs used with traditional evaporators, balance valves are used with the secondary heat exchangers to control the flow of coolant to them. Each case is designed for a predetermined flow rate, or gallons per minute. The flow rate is inversely proportional to the temperature of the case. As flow goes up, the temperature goes down.

The balance valve is used to adjust the gallons per minute flowing into the case.

Low Temperature

In low-temperature applications, CO2 has been used effectively as a secondary coolant. In a similar way to how propylene glycol is pumped through medium-temperature secondary systems, CO2 is used for low temperature systems (Figure 2, page 12). In these systems, heat is absorbed in the display cases and freezers through coils similar to those used in DX systems.

But unlike HFC refrigerants in traditional systems, the CO2 does not completely evaporate in the coil and is instead returned to a liquid-vapor separator. The liquid portion of the CO2 in the separator is then pumped back to the display cases and freezers, and the vapor portion returns to the condenser-evaporator where it is condensed back into a liquid. This is the point where the heat absorbed in the display cases and freezers is transferred from the secondary side of the system to the primary side. In both types of applications, the glycol in a medium-temp system and the CO2 in a low-temp system are considered secondary coolants because the heat they absorb is transferred to a DX component of the system (referred to as the primary side) before ultimately being released to the condensers.

The difference between this use of DX and how it is used in traditional systems is that the DX component in a secondary system is mostly contained within the store’s mechanical center. (Systems using air-cooled condensers still typically have those on the roof.)

Because considerably less piping is required to connect the primary side of the system to the secondary side, less HFC refrigerant is required.

Cascade

Another type of low-temperature system also uses CO2 but in some ways works more like a traditional system than a secondary system. The low-temperature, DX cascade system uses CO2 as a DX refrigerant. But, in a similar manner to a secondary system, the low-temp cascade combines an upper cascade HFC component with a lower cascade CO2 component (Figure 3).

Like conventional secondary systems where the HFC refrigerant in those systems is contained entirely on the primary side, the only HFC refrigerant in cascade systems is in the upper cascade. The total HFC refrigerant charge and the potential for leaks, therefore, are reduced. This approach allows for reductions too in the amounts of copper used since CO2 requires small-diameter piping than is needed for HFC refrigerant. The way that the system works is that liquid CO2 absorbs heat in the display case through coils similar to those used in traditional DX systems, but specially designed for use with CO2. The CO2 completely evaporates in the coils and the suction gas returns to the compressors. A high-efficient suction-liquid heat exchanger (SLHE) is used to ensure that all of the liquid CO2 is evaporated before returning to the compressors. The CO2 is then compressed and discharge gas from the compressor is condensed in the heat exchanger (called the condenser-evaporator) by the upper cascade of the system that operates in a similar manner to the primary side of secondary system. Liquid CO2 is then sent to a CO2 receiver and the SLHE before being distributed back to the liquid supply piping.

Conclusion

Although DX refrigeration has been the mainstay of the industry, more and more contractors and customers are becoming more comfortable working with both conventional and cascade secondary systems. As regulatory pressures increase and more and more of these systems come into use, this trend is expected to continue.

Publication date: 09/05/2011

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