Thermostatic Circulation System Prevents Freeze-Up in New Air Force Fire Trucks

Thermostatic Circulation System Prevents Freeze-Up in New Air Force Fire Trucks

An automatically actuated and monitored circulation system, controlled by a pair of thermostats, keeping the water in fire trucks from freezing despite exposure to temperatures as low as —65F. This circulation system, installed in the new Type 0-10 fire trucks (Fig. 1), designed for the U. S. Air Force by American-LaFrance, continuously circulates the fire-fighting water through a heat exchanger when the water temperature falls below a safe level.

Fig. 2. Schematic layout of the automatic circulating and warning system in the Type 0-10 crash fire truck. One thermostat, set at 60 degrees F., starts the circulating pumps which circulate the water in the truck's storage tank through an ethylene glycol heat exchanger. Tne second thermostat is wired to the truck's horn, which gives an audible warning when the temperature falls to 40 degrees F.

Included in this circulation system, illustrated in Fig. 2, are two circulation pumps which draw the cold water from the 500-gallon water tank, pump it through a heat exchanger, where it is warmed, and return it through the fire pump piping to the tank. The heat is supplied to the heat exchanger by a circulating liquid, usually ethylene glycol, which is brought to temperature by a thermostatically-controlled 90,000 BTU gasoline heater.

The circulating pumps are controlled by a Fenwal thermoswitch unit. The thermostat, set at 60° F., is installed in an exposed section of the water line to assure earliest possible contact with low ambient temperatures. To warn personnel that some malfunction has permitted the water temperature to fall to a dangerously low level, a second thermoswitch is installed in the circulating line. This thermostat, whose contacts are adjusted to close at 40° F., is wired to the vehicle’s horn, to warn operating personnel of a malfunction so that they can make repairs before freezing damage occurs.

Fig. 1. The Type 0-10 crash fire truck, recently developed for the U. S. Air Force, is designed to operate at temperatures as low as —65 degrees F. A thermostatically-controlled circulation and warning system automatically circulates fire fighting water through a heat exchanger when the water temperature drops to 60 degrees F. and sounds an audible alarm if water temperature falls to 40 degrees F.

These thermostatic controls must meet severe operating conditions inasmuch as these trucks may be used in either arctic or tropical climates. They are expected not to stray from their temperature settings despite repeated shock and vibration caused by driving the truck over rough ground and by operating heavy pumping and power equipment.

Major extinguishing equipment on the Type 0-10 fire truck includes a 200 GPM turret, which can be directed either manually or by hydraulic power; three 35 gpm ground sweep nozzles mounted near the front bumper; two under-truck nozzles fixed beneath the truck; and two 100-ft. handlines for water or carbon dioxide. A full variety of extinguishing agents is available for fighting virtually any type of fire: foam, fog-foam, water fog, and a straight water stream. High pressure CO. is available.

This crash-fire truck was developed to obtain maximum usefulness in worldwide, all-climate, full cross-country terrain conditions. It is completely enclosed, insulated and heated so as to be operational at ambient temperatures as low as —65F. Another unusual feature is that the vehicle, whose gross weight is 26,700 lb, is designed to be transported by air in the C-124 Globemaster.

Fig. 3. A cut-away of the thermoswitch unit installed in the Type 0-10 Air Force truck. Expansion or contraction of the outer shell causes the electrical contacts on the strut assembly to make and break. Electrical contacts are completely enclosed within the unit, thus protecting them from possible malfunction because of dirt, spray, or other contamination.

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