Posted By: Shraddha Raut

June 18, 2016
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Overview

The 3 systems of circulation adopted in boiler are: i) Natural circulation system ii) Controlled circulation system iii) Combined circulation system Natural Circulation System Water delivered to a steam generator from feed heaters is at a temperature well below the saturation value corresponding to that pressure. Entering first, the economizer, it is heated to much nearer the saturation temperature. From economizer the water enters the drum and thus joins the circulatory system. Water entering the drum flows down through the downcomer and enters bottom of the water wall tubes. In the water wall tubes a part of the water is converted to steam and the mixture flows back to the drum. In the drum, the steam is separated. Remaining water mixes with the incoming water from the economizer, and the cycle is repeated. The circulation, in this case, takes place on the thermo-siphon principle. The downcomers contain relatively cold water, whereas the water wall tubes contain a steam water mixture, whose density is comparatively less. This density difference is the driving force, for the mixture. Circulation takes place at such a rate that the driving force and frictional resistance are balanced. As the pressure increases, the difference in density between water and steam reduces (See Fig. 2). Thus the hydrostatic head available will not be able to overcome the frictional resistance for a flow corresponding to the minimum requirement of cooling of water wall tubes. Therefore natural circulation is limited to boiler with drum operating pressure around 175 kg/cm2. Controlled Circulation System Beyond 175 kg/cm2 of pressure, circulation is to be assisted with mechanical pumps, to overcome frictional losses. To regulate the flow through various tubes, orifice plates are used. This system is applicable in the high sub-critical regions (say 200 kg /cm2) Combined Circulation System Beyond the critical pressure, phase transformation is absent, and hence a once through system is adopted. However, it has been found that even at supercritical pressures, it is advantageous to recirculate the water through the furnace tubes at low loads. This protects the furnace tubes and simplifies the start-up procedure. A typical operating pressure for such a system is 260 Kg/cm2

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