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1.1* Scope. This standard includes minimum requirements for dry chemical fire-extinguishing systems that discharge dry chemical from fixed nozzles or hand hose lines by means of expellant gas. A.1.1 The dry chemical systems described in this standard are designed to discharge dry chemical from fixed nozzles and piping or from hose lines by means of an expellant gas. The intent of the standard is to present the design considerations applicable to these systems. It contains only the essential requirements and recommendations needed to make the standard workable in the hands of those skilled in this field. Because the flow of dry chemical (solid particles suspended in a gaseous medium) does not follow general hydraulic theories, most of the flow principles have been determined experimentally. The dry chemicals produced by various manufacturers usually are not identical in all characteristics, and each manufacturer designs equipment for use with a specific dry chemical. System design principles applicable to the products of one manufacturer are not applicable to the products of another manufacturer. As a result, it is not practical to include system design details as a part of this standard. It is now generally accepted that the flame-extinguishing properties of dry chemicals are due to the interaction of the particles, which stops the chain reaction that takes place in flame combustion. Dry chemicals vary in their flame-extinguishing effectiveness. Multipurpose dry chemical owes its effectiveness in extinguishing fires involving ordinary combustibles, such as wood and paper, to the formation of a glow-retarding coating over the combustible material. For additional information on dry chemicals and their extinguishing characteristics, see A.4.6.1. Portable dry chemical equipment is covered in NFPA 10, Standard for Portable Fire Extinguishers.
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