Fires in refrigerated structures are rare, but they do occur and can be costly. The major loss associated with a fire may be the product, whose value may be 5 to 10 times that of the building. It may at first seem that there would be negligible flammable material in a low-temperature warehouse, but for a certain 20,000 m3 (700,000 ft3) building, the estimated content of wood in the pallets was 100,000 kg (220,000 lb), 150,000 kg (330,000 lb) of cardboard cartons, and 7,000 kg (15,400 lb) of plastic wrapping material16. Many types of insulation, including many cellular plastic foams, will also burn.
The fundamental approach of the designer and operator should be to prevent a fire in the first place. Toole16 and Duiven and Twilt17 cite the causes of most fires in their surveys as attributable to:
• Heating cable at doors and defrost drain lines
• Faults in other electrical equipment, such as transformers, battery chargers, or solenoids
• Hot bitumen from roof work
• Poor housekeeping (rubbish fire started by a cigarette butt)
To the above list could be added fires starting in adjacent areas, such as the machine rooms or shops. Some fires may, unfortunately, be attributable to arson or sabatoge by a disgruntled employee or former employee. Many insurance underwriters require water sprinkler systems for refrigerated storage areaus, even m those spaces maintained at subfreezing temperatures. For such applications a dry-type sprinkler system, as illustrated in Fig. 13.10, may be used. Some insurance companies require the dry system at temperatures lower than about 7°C (45°F). In this type of system, the distribution pipes are filled with compressed air. If heat from a fire melts the fusible element in a sprinkler head, the air pressure drops. This drop in air pressure is interlocked with a temperature sensor at each of the sprinkler heads, and if both indicate abnormal temperatures, the control starts the pump and opens the valve in the water line. Earlier systems were actuated by the melting of the fusible plug alone, and an errant opening of the plug allowed water to fill the distribution system. The water would soon freeze in the pipes, and the entire system had to be dismantled.
All refrigerated buildings are equipped with insulation, and the combustibility characteristics of various insulations should affect the choice of material and/or how the insulating material is applied. Polyurethane and polystyrene can burn and emit smoke and noxious fumes. These materials now can be purchased with self-extinguishing additives, although in one experience the self-extinguishing sulfate was vaporized by the heat of the fire and the remaining polystyrene became a burning, molten mass. There are factors in addition to the flammability characteristics that influence the choice of insulating material, but the following are broad characterizations of some frequently used insulations:
• Polystyrene—combustible, but is treated with a flame retardant that is effective against small flames
• Cellular glass—resistant to combustion
More precise descriptions of the fire characteristics are available from the Material Safety Data Sheet for each of the insulation materials. The fact that a material may burn under certain conditions does not necessarily preclude its use. It may simply require that the material be enclosed or otherwise protected by a noncombustible material. Polystyrene may be used in floors, for example, because the insulation material is protected by the concrete. Cellular glass has excellent fire resistance, but tends to be heavier and more expensive than some of the other insulations. Insulated built-up panels now predominate in construction and have either metal or plastic skins. The metal is fire-resistant but is harder to clean than the plastic, which might melt in the event of a fire.