Alternatively, the Spirax Sarco online steam tables can be used. The necessary information can be obtained or interpolated from hard copy steam tables the relevant extracts are shown in Table 15.1.1 and Table 15.1.2. Therefore, in such arrangements the inlet pressure of the superheated steam should be kept constant. The set temperature of the desuperheated steam would typically be 3☌ above that at saturation. Consequently, the temperature of the steam is reduced.Ĭontrol of the amount of water to be added is usually achieved by measuring the temperature of the steam downstream of the desuperheater. As it enters the desuperheater, the cooling water evaporates by absorbing heat from the superheated steam. When the desuperheater is operational, a measured amount of water is added to the superheated steam via a mixing arrangement within the desuperheater. A typical direct contact desuperheating station is shown in Figure 15.1.3. For example, in the case of steam desuperheaters, water is used. In most cases, the cooling medium is the same fluid as the vapour to be desuperheated, but in the liquid state. Direct contact type - The medium used to cool the superheated steam comes into direct contact with it.Control of the superheated steam flow for this purpose is not normally practical and most systems adjust the flow of the cooling medium. The temperature of the desuperheated steam could be controlled by either the inlet superheated steam pressure or the flowrate of the cooling water. As the superheated steam passes through the heat exchanger, heat is lost from the steam, and gained by the cooling medium. Here the superheated steam is supplied to one side of the heat exchanger and a cooler medium is supplied to the other side. Examples of this type of desuperheater are shell and tube heat exchangers. A cooler liquid or gas may be employed as the cooling medium, for example, the surrounding air.
Indirect contact type - The medium used to cool the superheated steam does not come into direct contact with it.There are basically two broad types of desuperheater: This becomes significant if there are long lengths of pipe separating the point of generation and the point of use.ĭesuperheating is the process by which superheated steam is restored to its saturated state, or the superheat temperature is reduced.Most desuperheaters used to restore the saturated state produce discharge temperatures approaching saturation (typically to within 3☌ of the saturation temperature as a minimum).Designs for discharge temperatures in excess of 3☌ above saturation are also possible and often used. In plants that have superheated steam available for process use, it makes sense to distribute the superheated steam to remote points in the plant, as this will ensure that the steam remains dry. If the quantity of waste is sufficiently large, then superheated steam may be produced for power generation.Įxamples of this type of plant can be found in the papermaking and sugar refining industries. Sites also exist where large quantities of waste are used as fuel for the boiler. (More information on superheated steam can be found in Module 2.3). However, sites exist where superheated steam is raised for power generation, and it makes economic sense to desuperheat some of this steam from some point in the power generation cycle, and then use it for process applications. These disadvantages mean that superheated steam is generally undesirable for thermal process applications. The higher temperature of superheated steam may damage sensitive equipment.The higher temperatures of superheated steam may mean that higher rated, and hence more expensive equipment is required.Some processes (for example, distillation columns) perform less efficiently when supplied with superheated steam.The presence of high heat transfer coefficients associated with saturated steam leads to smaller and cheaper heat exchangers than those which utilise superheated steam. This greatly assists accurate sizing and control of heat transfer equipment. Once the superheated steam is cooled to saturation temperature, the heat transfer coefficient increases dramatically, and the temperature at which the steam condenses back into water is constant. This makes accurate sizing and control of heat transfer equipment difficult, and will also result in a larger and more expensive heat exchanger. The coefficient of heat transfer when using superheated steam as the heating medium is variable, low and difficult to quantify accurately.
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