Document G6DaYkBo11nxBVDg8N6GBNEjr

Boilers and Unfired Pressure Vessels Fig 18-4 --The meter and control board of a boiler should be located on the operating floor, in a place where the operator can also watch the boiler fire and the water column Courtesy National Engineer Magazine Superheaters After the heat transfer med ium (water, steam, or other material) leaves the boiler unit itself, it can have its temper ature raised even more by passing through a superheater Superheaters must be made to withstand high temperatures and be equipped with safety valves, as specified in the ASME Code They should be designed so as to give as little reduction as possible m the pressure of the heat transfer medium Stop valves and connecting pipe should be lagged to increase efficiency and to keep personnel from coming into direct contact with the hot surfaces Superheater drains should be left open until a full flow of steam is circulating through the superheater tubes Manufacturer's op erating instructions must be rigidly adhered to, especially when starting up or shutting down the unit Cutting in too soon may cause the tubes to fail, which will not only wreck the equipment but can cause injury to operating personnel Detailed operation procedures are given in Section VII of the ASME Code Air preheaters The two types of com bustion air preheaters are regenerative and recouperative The regenerative type em ploys a set of moving grids, which revolve like spokes of a slow-moving wheel at ap proximately 4 rpm The recouperative type has no moving parts, air flows past the tubes In both types the air flows outside the tubes while flue gas flows inside Fires may occur in an air preheater immedi ately after lighting a boiler and during periods of low-load operation This condition can be detected by a sudden rise in air heater temperature 474