Foam Fixer Hydrodynamics and Design Calculation

Air-mechanical foam, generated with foam mixer is used to extinguish such specific fires as burning of petrochemicals, glycerin, paraffin and other media. The essential part of a foam mixer is a dispenser that provides the required ratio of water flow and foam concentrate in a mixed stream. Dosing systems divided into two following types based on the method of foam concentrate extraction and the type of tank storage: the dosing system with foam concentrate extraction from the storage tank with hydrostatic pressure; and dispenser tanks, where the foam concentrate is stored inside or outside the flexible inner container and is displaced by feeding a part of the water flow into the tank from the high-pressure line. Diaphragm dispensers are used in dosing systems of the second type. A fixed “washer” is installed on the water line which is aimed at creating pressure difference across dispenser, sufficient to displace foam concentrate from the tank through the calibrated orifice of the second diaphragm. This article deals with the development of a calculation model of a diaphragm dispenser for foam mixers, which provides for the consistency of mixing ratio in the specified range of water flow. Initial equations for the calculation model are Bernoulli equations for the water flow and foam concentrate; water balance equation and empirical dependences of diaphragm discharge coefficients and hydraulic friction from Reynolds number and configuration of a dispenser flow part. Based on the general system of equations, a methodology to calculate basic dimensions of the dispenser, namely, diameters of metering orifices, which provide the specified ratio of water flow and foam concentrate, is proposed. The methodology is illustrated by a numerical example.