Traditional refrigeration systems use a hot-gas defrost cycle to melt ice off the evaporator coil. As the coil heats and the ice defrosts, heat and moisture are reintroduced into the space. Defrosted water that remains on the coil creates an environment for potential microbial growth. Also, ice can form on floors, creating hazardous working conditions. Systems must be defrosted and sanitized on a regular basis, particularly in high humidity areas, and during this cycle, manufacturing must be halted.
Dry desiccant systems continuously remove moisture from the air using a corrugated ceramic composite, impregnated with desiccant, formed into a wheel. Generally, silica gel is used as the composite. Moist process air, which has a high vapor pressure, passes through the upper portion of the rotating desiccant wheel. The desiccant, which has a low vapor pressure, absorbs the moisture. The dry process airstream then passes through the conventional refrigeration coil, where the temperature is lowered to design conditions.
The dry desiccant wheel is effective only until it is saturated. Once it is, a scavenger hot airstream is forced through the desiccant wheel to remove moisture. After it is regenerated, the desiccant is cooled to lower its vapor pressure, then rotated back into the moist airstream, where the dehumidification cycle repeats.
Liquid Desiccant OperationLiquid desiccant technology has been in use for many years, primarily in process applications requiring dehumidification and humidity control. In some applications, it may be more energy efficient than traditional defrost or dry desiccant systems.
A liquid desiccant system is able to capture and remove airborne pathogens from the airstream. Growing concern about food-borne disease along with recent processing facility shut downs and associated financial penalties make this attribute an important consideration.
The principal of liquid desiccant operation is similar to that of dry desiccant systems; the difference is the evaporator coil (figure 1). In a liquid desiccant system continuous spray of liquid desiccant is pumped over a specially designed evaporator coil. The air to be conditioned is moved across the coil and into the conditioned space by means of a fan. Because the coil never freezes and defrost cycles are not required, the leaving-air conditions remain constant.
Liquid desiccant systems can be used with any type of refrigerant or refrigeration system. Mediums include salt (sodium chloride, calcium chloride or lithium chloride) and propylene glycol. Calcium and lithium chloride use is not permitted with edible food product. With sodium chloride, temperatures are limited to approximately -6?F (-21.1?C), but its use is permissible around edible food product. The biggest problem with inorganic salt solutions is that they are highly corrosive. Thus, there is the potential for increased maintenance costs.
- Is approved by the United States Department of Agriculture (USDA) for use in food plants.
- Does not have corrosive characteristics.
- Is antimicrobial.
- Is effective as a food preservative.
- Is more effective than the inorganic salts at lower temperatures.
The temperature and concentration of the desiccant liquid determines the leaving dry bulb and dewpoint.
In a glycol system, the concentrator portion monitors and adjusts the desiccant's concentration levels by removing excess moisture. Glycol is circulated through the concentrator, which heats the solution to about 150?F (65.6?C). In addition to rejecting the moisture, the heating process neutralizes any microorganisms in the solution. A heat exchanger between the evaporator and concentrator minimizes energy usage.
Recent tests were performed by Daniel Amsterdam, Ph.D., professor of microbiology at the State University of New York at Buffalo. Samples of Listeria monocytogenes, Salmonella B and Yersinia enterocolitica were exposed to three glycol solutions under conditions that simulated in-use operations. These contaminants can be found in food and beverage production, storage and distribution as well as the pharmaceutical industry. Each species of bacteria was cultured in the glycol solution at a range of temperatures.