APW stands for "air-pressurized water." APWs are large, silver extinguishers that are filled about two-thirds of the way with ordinary tap water, then pressurized with normal air. In essence, an APW is just a giant squirt gun.
APWs stand about 2 feet tall and weigh approximately 25 pounds when full.
APWs are designed for Class A (wood, paper, cloth) fires only.
Never use water to extinguish flammable liquid fires. Water is extremely ineffective at extinguishing this type of fire, and you may, in fact, spread the fire if you try to use water on it.
Never use water to extinguish an electrical fire. Water is a good conductor, and there is some concern for electrocution if you were to use water to extinguish an electrical fire. Electrical equipment must be unplugged and/or de-energized before using a water extinguisher on it.
APWs extinguish fire by taking away the "heat" element of the fire triangle.
It is important to remember, that electrical equipment must be disconnected from its electrical source before using a water extinguisher on it.