Water-mist systems offer protection for commercial cooking areas
A recent study by the Institute for Research in Construction (IRC) has found that water-mist fire suppression systems are effective for extinguishing accidental fires in the kitchens of hotels, restaurants, and fast-food outlets. According to the IRC, these fires are difficult to extinguish because they burn at a high temperature and reignite easily.
In the IRC project, researchers conducted tests in a mock-up restaurant cooking area to determine the effectiveness of three different water-mist systems in suppressing fires in a commercial propane-fired, deep-fat fryer with an overhead hood. The impact of different spray characteristics (discharge pressure, flow rate, spray angle, and droplet size) and nozzle locations was used to measure extinguishing time, fire and liquid oil temperatures, and the amount of water collected by the fryer.
The results showed that with continuous heating, the cooking oil (a commercially available vegetable oil) autoignited at temperatures ranging from 365 degrees C to 370 degrees C. A two-minute free-burning period was allowed before the fire suppression system was activated. During that time, the cooking-oil temperature increased to 400 degrees C, allowing the fire to become fully developed. This fire scenario posed a significant challenge for the suppression system.
During the tests, one of the water-mist systems, which had a low flow rate, was unable to extinguish the cooking-oil fire; however, the other two systems, which had relatively high flow rates, effectively did so. The time taken to extinguish the fires ranged from one to 15 seconds after discharge, depending on the discharge pressure, flow rate, and nozzle location.
The fire flared up momentarily for a short time–less than one second–with initial activation of the water-mist system. This potential impact should be taken into account in the design of a water-mist system for this application. Subsequently, the water-mist systems with relatively high flow rates extinguished the fire. The water-mist discharge did not disperse any burning droplets of cooking oil outside the fryer.
While the fire was being extinguished, the liquid cooking oil cooled down quickly. However, when the water-mist discharge was discontinued shortly after the fire had been put out, the cooking oil reignited on its own at a temperature of 300 degrees C– 65 degrees C lower than its original autoignition point. This reignition occurred because the composition of the oil had been altered during the free-burn and fire suppression periods. In the tests, reignition was prevented by maintaining the water discharge for a defined period of time so that the oil could be sufficiently cooled.
Based on the results of the project, a cost-effective water-mist fire suppression system for protecting commercial cooking areas can now be developed. For additional information, contact Dr. Zhigang Liu at (613) 990-5075, fax (613) 954-0483, e-mail
Source: Construction Innovation, IRC, 4:3, Spring/Summer 1999, 1.
