These units provide steam for heating, power, or process. They canbe located outdoors if desired and require a minimum of space. A relativelyconstant steam temperature over a wide load range can beobtained, and an air preheater or economizer can be adapted readilyto the unit if economics so dictate.Figure 2.33 shows a unit designed to produce 200,000 lb/h of steamat 625 psi and 750°F. It is equipped with waterwalls, a radiant superheater,an economizer, a tubular air preheater, and a mechanical dustcollector. It is fired by a continuous-ash-discharge spreader stokerand auxiliary burners. It is designed to burn pine and gum bark, oil,natural gas, or coal. The boiler is supported from overhead steelcolumns as shown, and its expansion is downward.The VU-60 boiler (Fig. 2.34) is a pressurized furnace unit usingbare waterwall tubes with floor tubes covered as shown. A front-firingarrangement is shown, although tangential firing is available, ifdesired. It is designed to burn oil, gas, and waste heating fuels andcan deliver over 1 million lb of steam per hour.As an alternative to horizontal burners, tangential firing systems areused in some boiler designs. In this design, both fuel and combustionair are projected from the corners of the furnace along a line tangentto a small circle (Fig. 2.35). A rotary motion is imparted to the flame
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