A low charge direct expansion evaporator system configured to use less ammonia

A US-based meat processor operates a cold storage facility that uses just 1 kg approx ammonia per kW.
West Liberty Foods, a meat processor, was founded in 1996 in Iowa. The company uses four cold storage facilities located in West Liberty and Mount Pleasant (Iowa), Tremonton (Utah), and, since 2014, in Bolinbrook (Illinois).

The Bolinbrook cold storage facility, run by Liberty Cold Storage, uses a lot less ammonia than in the older plants. It only uses 3.4 tonnes of ammonia to refrigerate West Liberty's meats and an array of produce, desserts and other third-party foods.

This facility has recently been enlarged. It underwent a 11,427m2 expansion in early 2018. It now encompasses 23 537m2 and its ammonia charge supports a capacity of approximately 3263 kW, with a ratio of only 1.04kg of ammonia/kW.

This amount of ammonia is under the threshold (4.54 tonnes) above which cold storage plants need to abide by strict regulations mandated by the Environmental Protection Agency.

Liberty Cold Storage succeeded in maintaining a low charge of ammonia thanks to the employment of a direct expansion (DX) evaporator for both medium-temperature and low-temperature applications.

DX evaporators, which require much less ammonia than an overfeed system, have been used for medium-temperature applications but not for low-temperatures. However, the company Colmac Coil, based in the state of Washington, came up with a design  to make them work with low temperatures.

This system, called "Advanced DX system" or "ADX system", could have used around 362 kilograms less ammonia if it did not incorporate thermosyphon oil cooling, but according to an ADX designer, this alternative would have been costly.

The ADX system primarily reduces the ammonia charge by dint of the patented design of the evaporator. According to Bruce Nelson, president of Colmac Coil, the internal surface of the evaporator tubes has a “wicking structure,” which produces “sufficient capillary pressure to cause liquid ammonia to completely coat the inside of the tubes,” he said. “Otherwise, the liquid falls to the bottom of the pipe, with incomplete wetting.”

The ADX designer likened the grooves inside the tubes to that of a rifle barrel, which enables the ammonia to rotate 360 degrees through the length of a tube like a spinning bullet, enhancing the heat transfer.

The other part of the evaporator system that lowers the charge is a distributor that optimises flow of ammonia throughout the tubes (circuits).

The technology behind this evaporator is actually not new, but Colmac Coil found a way to harness it differently in the ADX unit to reduce the amount of ammonia needed, noted Watters. According to Nelson, the ADX evaporator decreases the ammonia charge in an overfeed system by 30-50 times.

According to Tim Cox, vice president at Liberty Cold Storage, by reducing the ammonia charge, the ADX system is simpler to manage than a traditional system.

To enhance the safety of the system, most of the 32 evaporators used at Liberty Cold Storage are housed in penthouse enclosures on the roof of the building, blowing cold air through ducts into the cooling area (the exception is the dock area and a small cooler, which use hanging evaporators). In this way, they resemble low-charge packaged units - except that the latter contain all of the refrigeration equipment, not just the evaporators.

In terms of cost, Tim Cow saw “an advantage on price” with the ADX system being a little less expensive (in equipment plus installation) than a liquid-overfeed system. According to Nelson, what is helping to drive adoption of the ADX system is its lower cost – 2% to 5% less than that of a traditional overfeed system.

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