Heat utilization equipment in commerce and industry includes ovens, furnaces, kilns, dehydrators, dryers and quench tanks.
In the NFPA’s Industrial Fire Hazards Handbook, Simmons (1990) identified the fire problems with heating equipment to be:
1. the possibility of igniting combustible materials stored nearby
2. fuel hazards resulting from unburned fuel or incomplete combustion
3. overheating leading to equipment failure
4. ignition of combustible solvents, solid materials or other products being processed.
These fire problems can be overcome through a combination of good housekeeping, proper controls and interlocks, operator training and testing, and cleaning and maintenance in an effective fire prevention programme. Detailed recommendations for the various categories of heat utilization equipment are set out in the NFPA’s Fire Protection Handbook (Cote 1991).These are summarized below.
Ovens and furnaces
Fires and explosions in ovens and furnaces typically result from the fuel used, from volatile substances provided by the material in the oven or by a combination of both. Many of these ovens or furnaces operate at 500 to 1,000 °C, which is well above the ignition temperature of most materials.
Ovens and furnaces require a range of controls and interlocks to ensure that unburned fuel gases or products of incomplete combustion cannot accumulate and be ignited. Typically, these hazards develop while firing up or during shut-down operations. Therefore, special training is required to ensure that operators always follow safety procedures. Non-combustible building construction, separation of other equipment and combustible materials and some form of automatic fire suppression are usually essential elements of a fire safety system to prevent spread should a fire start.
Kilns
Kilns are used to dry timber (Lataille 1990) and to process or “fire” clay products (Hrbacek 1984). Again, this high-temperature equipment represents a hazard to its surroundings. Proper separation design and good housekeeping are essential to prevent fire. Lumber kilns used for drying timber are additionally hazardous because the timber itself is a high fire load and is often heated close to its ignition temperature. It is essential that kilns be cleaned regularly to prevent a build-up of small pieces of wood and sawdust so that this does not come in contact with the heating equipment. Kilns made of fire-resistive construction material, fitted with automatic sprinklers and provided with high-quality ventilation/air circulation systems are preferred.
Dehydrators and dryers
This equipment is used to reduce the moisture content of agricultural products such as milk, eggs, grains, seeds and hay. The dryers may be direct-fired, in which case the productions of combustion contact the material being dried, or they may be indirect-fired. In each case, controls are required to shut off the heat supply in the event of excessive temperature or fire in the dryer, exhaust system or conveyor system or failure of air circulation fans. Again, adequate cleaning to prevent build-up of products that could ignite is required.
Quench tanks
The general principles of fire safety of quench tanks are identified by Ostrowski (1991) and Watts (1990). The process of quenching, or controlled cooling, occurs when a heated metal item is immersed in a tank of quenching oil. The process is undertaken to harden or temper the material through metallurgical change. Most quenching oils are mineral oils which are combustible. They must be chosen carefully for each application to ensure that the ignition temperature of the oil is above the operating temperature of the tank as the hot metal pieces are immersed.
It is critical that the oil does not overflow the sides of the tank. Therefore, liquid level controls and appropriate drains are essential. Partial immersion of hot items is the most common cause of quench tank fires. This can be prevented by appropriate material transfer or conveyor arrangements. Likewise, appropriate controls must be provided to avoid excessive oil temperatures and entry of water into the tank that can result in boil-over and major fire in and around the tank. Specific automatic fire extinguishing systems such as carbon dioxide or dry chemical are often used to protect the tank surface. Overhead, automatic sprinkler protection of the building is desirable. In some cases, special protection of operators who need to work close to the tank is also required. Often, water spray systems are provided for exposure protection for workers. Above all, proper training of workers in emergency response, including use of portable fire extinguishers, is essential.