Ozone treatment of extract air from a restaurant kitchen with heat recovery

Ozone generation equipment was installed to reduce the cost of cleaning the fat out of the extract ducting system in a fast-food restaurant. The reduced fat content would also help to protect the air handling unit, which comprised recovery of heat from the extract air. However, there is a health respect for ozone, which is a highly reactive irritant. Chemical reactions may also lead to the production of nitrogen dioxide, aldehydes and other volatile organic compounds. Concentrations of the compounds mentioned were measured both by passive and active sampling. Measurements were made outdoors and indoors as well as in the supply, extract and exhaust air streams. With ozone generation the ozone concentration in the extract and exhaust air was higher than 450 μg/m3. However, no increase of the ozone concentration in the supply or indoor air was observed. There were no observations of any formaldehyde generation. Neither were there any signs of nitrogen dioxide generation influencing the supply or indoor air, although the exhaust air concentration was higher than the extract air concentration. The measurement results point to incomplete chemical reactions still occurring in the exhaust air. This indicates that the length of ducting available and the air speed did not leave enough time to complete the chemical reactions to break down the different contaminants. However, in this situation with a coil heat exchanger this is of no significance, but if a rotary heat exchanger was to be used, the design mustallow enough reaction time at maximum flow.

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