|
Type 6970 Desuperheaters serve a wide range of applications. In a combined pressure reducing, desuperheating station where flow rates vary widely, this unit, with adequate controls, provides dependable operation with turndown ratios as high as 50 to 1 and greater, depending on exact operating conditi ons.
Type 6970 Desuperheaters are recommended for use where sufficient high pressure steam is available to provide the atomizing steam supply. The most frequent application would be in combination reducingdesuperheating stations. The minimium atomizing steam pressure ratio required is 1.4 times the absolute steam pressure through the desuperheater with a minimum atomizing steam pressure of 50 psig. The amount required is constant. (See Table 2).
Construction: In the Type 6970, the water preheating and distributing device is installed in a short pipe section with weld neck flanged ends (Type C) or butt weld ends (Type A). This unit can also be mounted on a blind flange for insertion through a nozzle connection (Type D). The various mounting arrangements are shown on the reverse side. It is recommended that the unit be mounted so that the atomizing steam and water inlet pipes enter the unit from the bottom as shown.
This desuperheater uses a steam atomizing device, operating on the jet principle, to entrain cooling water, preheat, and discharge the atomized water into the superheated steam flow.
 Operation: Ejector-type steam atomizing desuperheaters utilize steam at higher than line pressure to atomize water. In the Type 6970, the ejector action is used to entrain condensate from the pipeline. This is an important SK innovation and a feature of this type unit.
Few problems are encountered in operating desuperheaters at normal pipeline velocities. However, SK research has proved conclusively that at low pipeline velocities encountered at 1/50 up to 1/4 of normal flow, unvaporized liquid will "settle out" of a horizontal stream. When it is desired to approach saturation temperature within 10° F, it becomes impossible to completely vaporize the liquid. Thus, while superheated steam is flowing through the pipeline, water accumulates in the bottom of the line. Since this keeps temperature from being reached, a control valve will continue to supply or "pump" excess water into the line while attempting to maintain the control temperature.
Type 6970 overcomes these complications by recycling excess water back into the atomizing device. The water added through the control valve is therefore limited to the amount required for desuperheating. As indicated in Fig. 2, high pressure steam enters through the ejector steam nozzle which is precisely designed for each application. This steam entrains the mixture of fresh and excess cooling water through the water inlet line and atomizes this water, which is discharged into the superheated steam line at saturation temperature. The preheating reduces the time required to evaporate the liquid, and the consequent small particle size and turbulent stream improves heat transfer. At low flows the return line entrains excess water. At high flows, where no excess water is required, the unit operates as a steam atomizing desuperheater.
|
