In a joint report, the pair said, “Secondary loops offer the possibility of dramatically reducing refrigerant leakage from supermarket systems, which will prove beneficial to supermarket owners as well as to the environment.”

Higher Efficiency Needed

They did acknowledge that for the technology “to be competitive with existing direct-expansion systems, its operation must first be optimized to provide higher system efficiencies.”

Research conducted at Purdue’s Ray W. Herrick Laboratories looked toward improving those efficiencies. “One of the first studies conducted was to investigate the effect of heat transfer in the supply and return lines,” noted Horton and Groll.

“During the testing phase of the project, it was learned that one of the important areas of focus in optimizing the performance of the secondary-loop refrigeration system is to provide proper insulation on the supply and return lines of the thermofluid circulating loop.

“Insulating these lines is important because the secondary fluid is at a much lower temperature than the ambient, which will result in a large amount of unwanted heat infiltration.”

In one test, “The distribution lines and secondary fluid reservoir gained almost three times the amount of heat that was transferred in the air coil itself. Such losses must be dramatically reduced in order for secondary loops to be competitive with direct-expansion refrigeration systems, which are not as vulnerable to cycle inefficiency because of heat transfer in the line sets.”

Role of Heat Transfer

The researchers ran a simulation to eliminate the heat transfer in the circulating loop and discovered “a dramatic improvement in performance. When heat gain in the distribution lines is eliminated, the model shows an increase in heat transfer at the air coil to almost a factor of three, while a similar increase is also observed for the overall cycle coefficient of performance.

“The air coil cooling capacity is enhanced because the secondary fluid arrives at the air coil with a lower temperature than it would if heat transfer to the fluid in the supply line would occur.

“Similarly, system efficiency is increased because the vapor-compression cycle is not required to reject as much heat through the condenser when insulation on the distribution lines is appropriate.”

The researchers called their progress “a beginning at the problem of optimizing secondary-loop refrigeration.”

They listed several other major issues, including:

Development of a tool for determining appropriate air coil physical properties and dimensions, so that its performance is optimized for a particular choice of secondary thermofluid;

Evaluation of primary loop refrigerants in combination with available secondary thermofluids;

Determination of appropriate distribution pipe sizing, “as well as the economics associated with proper insulation of these lines”; and

Determination of an appropriate method to control the mass flow rate, “to maintain optimum system performance as frost forms and other operating characteristics change over time.”

When solutions are found in these areas, “It will be possible to revisit the traditionally used direct-expansion, vapor-compression cycle and make appropriate comparisons.”

Publication date: 10/16/2000