"Traditional designs utilize a single water-cooled condensing unit located in a centralized mechanical room connected to the refrigeration load by individual liquid suction lines and selected according to application of each particular object of refrigeration."

Leyzarovich went on to note that such configurations often have "significantly oversized capacity and compressor horsepower."

He said another disadvantage comes when there might be "difficulty in controlling refrigerant loss from each individual condensing unit, since it is very expensive to equip each unit with leak indicator devices."

One of the biggest drawbacks, he said, is that compressors end up spread out with a compressor for each display case, a compressor for each walk-in cooler and freezer, compressors for the prep rooms, etc. "This means quite a lot of refrigerant piping, control valves, receivers, electrical components, and water-cooled condensers."

A Solution

Leyzarovich said, "Engineering analysis indicates that most of these disadvantages can be eliminated by utilizing a centralized refrigeration rack with a group of parallel compressors such as is used in typical supermarket designs."

In the Pittsburgh project, the engineer said a "mini" parallel system was created by using six scroll compressors with 34 hp total. Four of the compressors were medium temp and two were low temp. All used HFC-404A refrigerant. The compressors had vapor injection ports, "which operate at a higher pressure than the main suction," he said.

Also employed were a split manifold for medium and low temperature and a brazed plate heat exchanger. Liquid from the receiver is cooled to 55 degrees F using the heat exchanger. A controller is used for the suction grouping, circuits, and sensor control applications.

Circuits are controlled, said Leyzarovich, by using "a unique method of pressure/temperature control of very small refrigeration loads for food service equipment."

One result of this approach is "precise temperature and humidity control required for individual load and thus enhanced product integrity." Further, he said, energy consumption was reduced about 25 percent from the previous configuration.

As set up, "the low temperature system operates with mechanical subcooling provided by the medium temperature systems, thus reducing energy consumption and eliminating flashing of liquid refrigerant as it enters the thermostatic expansion valve in cases where there is significant lift on vertical liquid lines."

Previously the refrigeration arrangement encompassed about 308 square feet of space. The new design checks in at 160 square feet, Leyzarovich said.

With most mechanicals in one area, "unpleasant noise generated by individual condensing units located near the conference rooms and halls is eliminated."

The refrigerant charge was said to be reduced from a previous 400 pounds to about 300 pounds. Further, he said, "any refrigerant leaks are detected by a level control device installed in a common receiver."

All the temperature control is monitored via a personal computer in an on-site technician's office.

"This new design concept allows for additional capacity to be factored in at a minimum cost, thus allowing for any future coolers, freezers, or cases to be added with little additional cost, other than the cost of the unit," Leyzarovich said.

Publication date: 01/10/2005