Prior to the Savannah Sam's Club, the use of CO2 in the states has primarily been in industrial applications in conjunction with ammonia in cascade systems. CO2 (R-744) is being promoted especially in Europe because it is a low-cost refrigerant with far less global warming potential than HFCs. However, some in the industry have raised concerns about CO2 in regards to high discharge pressures and a low critical temperature.

It is such issues that those involved in the Savannah store project believe they have overcome.

THE LOW-TEMP SIDE

On the low temperature side, the equipment from Hill-Phoenix includes the company's Second Nature® Advanced LT refrigeration system that operates with CO2 as the heat transfer fluid.

"In this system," said Hill-Phoenix officials, "CO2 is utilized as a secondary refrigerant, which is pumped through the store to remove heat from display cases and walk-in freezers.

"Heat is absorbed in the display case through coils similar to those used in common direct expansion systems. The CO2 does not completely evaporate in the coil and is returned to the separator as a mixture of liquid and vapor. The liquid portion of the CO2 in the separator is available to be pumped to the cases and the vapor portion returns to the condenser-evaporator to be condensed back into a liquid. This is the point where the heat absorbed in the display cases is transferred from the secondary system to the primary system."

In this approach, Hill-Phoenix said CO2 "has excellent thermodynamic and transport properties and good material compatibility. The cooling effect based on latent heat capacity results in lower flow rates when compared to other low temperature secondary refrigerant fluids.

"Further, the use of CO2 allows a 39 percent reduction in the weight of installed copper pipe versus a comparable direct expansion installation. This is a result of the unique characteristics of CO2 when used as a heat transfer fluid."

The CO2 at the Savannah store is Coleman grade (99.99 percent pure) which officials said is available at industrial gas supply houses.

MEDIUM TEMPERATURE

These innovations are all taking place on the low-temperature side. Other innovations were also utilized in medium temperature applications. The Hill-Phoenix Second Nature medium temperature system uses a secondary fluid composed of 35 percent aqueous propylene glycol (or food grade anti-freeze) and 65 percent water.

"A low pressure ‘chilled water' loop eliminates the circulation of high pressure refrigerant throughout the sales floor," officials said. "The ability to control product temperature, not just the case temperature, is another advantage particularly as applied to medium temperature applications such as meat, seafood, and produce. Temperature is more stable because defrost time is equal to or less than other defrost methods and defrost temperatures are significantly lower; plus there is a reduced recovery time after defrost."

Since the medium temperature glycol/water "is not under high pressure, ABS plastic piping was used," officials said.

"The combined savings of ABS pipe and smaller CO2 copper pipe resulted in a 68 percent reduction in lineal footage of installed copper."

HFCS STILL IN MIX

HFCs are still part of the refrigerant equation. While glycol/water is used as the secondary fluid in medium temperature and CO2 is the secondary fluid in the low temperature side, R-404A is the primary refrigerant in the mechanical room at the rear of the store for both the medium and low sides.

It was noted, however, that one key to the secondary loop is the reduced charge of HFCs. In the new Savannah Sam's Club, there are 1,300 pounds (lbs.) of CO2 and about 1,700 lbs. of R-404A, whereas an all-HFC refrigeration system of the same size would need about 2,700 lbs. of HFC, according to officials.

The air conditioning system uses HFC-410A which itself is an innovation as many commercial air conditioning applications still use HCFC-22 even though it is being phased out for use in new equipment over the next few years.

SECONDARY LOOP

The use of the secondary loop in supermarkets continues to gain favor even though the majority of food stores still rely on direct expansion. Hill-Phoenix officials see a number of advantages of secondary loop.

"A key feature of our display cases operating in the Sam's Club is a high efficiency case design featuring medium temperature evaporator coils engineered for 25°F secondary coolant fluid. Typically, standard display cases are designed to operate with a fluid temperature of 15°-20°. High efficiency fan motors have been added to the cases, and where applicable, electronically commutated motors (ECMs)" have been used. Those were described as "solid state, variable torque, electronically controlled fan motors that reduce the energy demand on motors by 53 percent."

ECM motors were also included in the low temperature design.

The low temperature side is coupled to a heat reclaim system designed to recover 50 percent of a refrigeration system's waste heat and, in turn, using it to heat potable water in the store.

"By capturing the waste heat of Sam's Club's low temperature refrigeration system, the natural gas usage will be reduced," said Ernie Northern, Hill-Phoenix national accounts manager for Sam's Club and Wal-Mart.

In the mechanical room, internally compounded two-stage compressors are used on the low temperature side with evaporative cooling. "Using evaporative condenser technology in this application allowed system designers to lower the total system condensing temperature from a normal Savannah location design of 115° to 95°. The resulting 20° decrease in condensing temperature, combined with the use of the two-stage compressors, will help this store reduce demand on their electrical grid."

Hill-Phoenix officials maintained that technological advances have narrowed the front-end premium for secondary loop versus DX. There is a learning curve for service technicians, officials said, which also is being reduced.

Both Wal-Mart and Hill-Phoenix said they will be closely monitoring the secondary loop system at the new Savannah Sam's Club - Wal-Mart to see if the approach should be introduced in other Sam's Clubs as well as Super Wal-Marts; and Hill-Phoenix as part of its ongoing efforts to develop more supermarket systems that use the secondary loop approach - now with CO2 in the mix.

Publication date: 09/04/2006