Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSSUPERHEATERHEADERS^jSATURATEDSTEAMo oo o o " INLET1DRAINhave U-shaped tubes. Others, such as the oneshown in figure 10-8, have W- shaped tubes.A few boilers of recent design have vertical,rather than horizontal, convection-type superheaters.In these boilers, the U-bend superheatertubes are installed almost vertically,with the U-bends near the top of the boiler; thetubes are approximately parallel to the mainbank of generating tubes and the water screentubes. Two superheater headers are near thebottom of the boiler, running horizontally fromthe front of the boiler to the rear.Superheater tubes are generally identified byloop number, name of tube bank, and number oftube within the bank. In the case of horizontalsuperheater tubes, as shown in figure 10-6, youcount from the bottom toward the top to get thetube number. In the case of vertical superheatertubes, you count from the front of the boilertoward the rear.DesuperheatersOn boilers with noncontrolled superheaters,all steam is superheated, but a small amount ofsteam is redirected through a desuperheaterline, the desuperheater can be located in eitherthe water drum or the steam drum, most generallythe desuperheater will be found in thesteam drum below the normal water level. Thepurpose of the desuperheater is to lower thesuper-heated steam temperature back to orclose to saturated steam temperature for theproper steam lubrication of the auxiliary machinery.The desuperheater is most generallyan "S" shaped tube bundle that is flanged to thesuperheater outlet on the inlet side and theauxiliary steam stop on the outlet side.Economizers38.23Figure 10-7. — Convection-type superheater(double-furnace boiler).an access space or cavity is provided in themiddle of the superheater tube bank. The cavity,which runs the full length and height of thesuperheater, greatly increases the accessibilityof the superheater for cleaning, maintenance,and repair. Some of the walk- in superheatersAn economizer is installed on practicallyevery boiler used in naval propulsion plants.The economizer is an arrangement of tubesinstalled in the uptake space from the furnace;thus the economizer tubes are heated by therising gases of combustion. All feed water flowsthrough the economizer tubes before enteringthe steam drum, and the feed water is warmedby heat which would otherwise be wasted ascombustion gases pass up the stack. In general,boilers operating at high pressures and temperatureshave larger economizer surfaces thanboilers operating at low pressure and temperatures.239