Book - School of Science and Technology

334 Combustion and chimneysMaterials for chimneys and flues (gas fired boilers)For each unit of heat released, combustion of natural gas will produce between one and ahalf and two and a half times as much water vapour as will combustion of oil or coal.Hence, condensation is likely to occur in gas flues and chimneys, particularly on start-up.Masonry constructed chimneys should be lined with an impervious and acid-resistingmaterial such as glazed stoneware or refactory lining. It is desirable to keep the chimneywarm to assist draught, either by enclosing it in brickwork or concrete, or by insulation.Connecting flues between such boilers and vertical chimneys are usually of double-wallstainless steel.Special methods for flue gas disposal (Natural gas)Apart from the conventional chimneys mentioned, a number of special methods for fluegas disposal have been developed, principally by the various research bodies within thegas industry. Some of these have been adapted for use with equipment burning thelightest grades of fuel oil but such applications apply only on a domestic scale.Reference should be made to BS 5440 Part 1 1990 that describes the requirements,locations, clearances, etc., applicable to the installation of such flues for gas appliancesup to 60 kW.Fan diluted draughtIn this system, one form of which is shown in Figure 12.13, the combustion products,together with a quantity of diluting air, are exhausted to the atmosphere by a fan. By thismeans it is possible to dispense with a chimney entirely, for a gas fired boiler plant, bydischarging through the wall of the boilerhouse. The diluting air quantity must be such asDiluting airinletBoiler room ventilatorFan failure deviceOutletCombustionair inletFanBoilers with built-indraught divertersCombustionair inletFigure 12.13 Fan diluted draught