New technology promises in creased efficiency

Store Equipment & Design, March, 2000 by Marc Sandofsky

Superheat Sensor Technology may yield savings for refrigeration and HVAC systems

Improper superheat control can lead to thermal expansion valve (TXV) hunting, evaporators not being properly fed and a significant reduction in cooling capacity. Recently, Super S.E.E.R. Systems, Toronto, came up with a novel superheat control that the company calls a better solution.

CLOSED LOOP DRAWBACKS

"The closed loop feedback system control with the TXV bulb sensing the temperature of the suction gas at the evaporator outlet and controlling the flow of liquid refrigerant into the evaporator is fine in theory," said Super S.E.E.R. president Charles Gregory. "It can have a great many problems in actual operation, though." Gregory gave the following examples:

* Unequal loading of multi-circuit evaporators, with the least loaded circuit dictating the control of the TXV, can result in more gas and less liquid, thereby significantly reducing system capacity and efficiency.

* Laminar flow in evaporator tubes can lead to a lag in TXV bulbs' sensing refrigerant temperature because of oil coating the inside of the suction line. This is known as the "barrier effect."

* Continuously changing temperatures of the liquid refrigerant supplied to the TXV further complicates the equation. As the liquid temperature changes, so does the mass flow rate through the TXV.

According to Gregory, one solution is to implement Super S.E.E.R.'s new patented superheat sensor and to insulate all the liquid lines in refrigerated areas. "This will provide consistent superheat control under 5[degrees]F and fully flooded evaporator coils," he said. "It will also provide energy efficience."

The Super S.E.E.R. superheat sensor consists of two concentric copper tubes. "The outer tube is larger and sealed to the inner tube," Gregory explained. "This forms an annular space between the tubes. The inner tube is closed at its midpoint and is perforated to allow flow into and out of the annular space.

"Suction gas then flows into the first inner chamber, out through the perforations into the annular chamber, through the perforations into the outlet chamber of the inner tube, into the suction line and finally into the compressor."

The purpose of this flow path is to mix and turbulate the liquid refrigerant component with the superheated vapor portion, Gregory said. This causes instant vaporization of any liquid. "The turbulence generated in the chamber also averages the temperature of the suction gas from multi-circuit coils, achieving, stable and accurate superheat control," he added.

Gregory also claims to have improved upon the liquid to-suction heat exchangers often incorporated into refrigerated cases to improve efficiency. "The conventional thinking was that a bare copper liquid line was advantageous since it produced further sub-cooling of the liquid," he said. "In actual operation, though, it was simply passing the heat from the liquid into the refrigerated case, which increased case loads and caused an energy loss."

Super S.E.E.R.'s solution is to insulate the liquid line and any components that are exposed to the cold supermarket case environment. "This causes the suction gas temperature to increase up to 35[degrees]F across the heat exchanger," Gregory said.

This same superheat sensor technology also works well in HVAC applications, according to Consolidated Energy Solutions (CES), Toronto "In HVAC systems, the superheat is typically set at 10[degrees]F to 15[degrees]F, then rarely checked again, even during maintenance," said Bryan Elliott, director of engineering at CES. "As the superheat setting increases, so does the amount of inactive coil that's filled with gas. This reduces system capacity."

Ray Mierins, director of sales and marketing at CES, pointed out that the superheat sensor can be extremely valuable in dual-path dehumidification HVAC systems, particularly on the dedicated outside air coil path where coil conditions can fluctuate drastically in a short period of time and accurate TXV control is imperative.

"With the new sensor, superheat can now be set to between 3[degrees]F and 5[degrees]F with no chance of liquid being passed to the compressor," Mierins said. "This results in improved HVAC system efficiency, capacity, energy savings and overall control."