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Cooler steam can reap hot benefits. But weigh all of the options, so you don't get burned
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In typical process plants, there is a high demand for saturated steam. However, process steam is usually superheated, or heated to a temperature above saturation. The amount by which the superheated temperature exceeds the saturated temperature is known as the degree of superheat.
Desuperheaters are used to bring the outlet degree of superheat closer to that of saturation. From very simple mechanical designs to highly complex and flexible systems, the range of desuperheater capabilities is vast. Desuperheated steam is more efficient in the transfer of thermal energy. It may also allow the use of thinner pipes, lighter flanges or less-expensive materials.
Regardless of the process, there is a desuperheater available to accommodate most requirements. With numerous styles and models available, evaluation of the actual needs of the process is crucial to ensure selection of the right equipment. Specifying conditions less stringent than the actual operating conditions will result in a unit that cannot handle all operating cases. Similarly, overspecifying the thermal load or process requirements is detrimental to efficient operation and will increase the price tag of both the desuperheater and its controls.
Evaluating the requirements
Turndown capability, pressure drop and outlet superheat play lead roles in desuperheater design and selection. In general, no single parameter is more important than the other - some processes demand steep turndown capability, while others rule out significant pressure drop or outlet superheat. For the optimum design, it is imperative for engineers to understand the nature of these parameters and their potential to influence a process.
Turndown represents the variability of the steam flowrate. Certain processes have a constant steam flow, so turndown is not an important design factor. Other applications, including power generation and food processing, require large disparities in steam flow. As a general rule, higher turndown requirements call for more-complex and more-expensive desuperheaters.
Turndown is calculated by dividing the outlet velocity of the maximum steam flow by the outlet velocity of the minimum-recommended steam flow. As the density of the outlet steam does not change with flowrate, turndown can also be calculated by dividing the maximum mass flowrate by the minimum mass flowrate....