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. Through forced convection, the water absorbs the heat of the product making it possible to cool the system by transferring heat to the outside. Such processes employ forced convection, a heat transfer method in which hot water is constantly passed across the heat transfer surface. It collects this heat with the same heat transfer efficiency as when imparting it in heating processes. In cooling processes, hot water absorbs the heat of the product again through convection. Hot water can be used as a medium for cooling products. Hot water is a powerful tool in such a case. To prevent this, the excess heat needs to be removed and the product must be properly cooled. If this additional heat is not removed properly, the overall temperature may rise too high and the product may be ruined. There are some heating processes in which the product itself generates heat (for example through an exothermic reaction) and becomes hotter than the heating medium. 
Hot water heating has different features from steam heating. Is there still any merit in hot water heating, which is inferior to steam heating in terms of uniformity and heat transfer rate? With the advent of vacuum steam heating systems, steam heating has become possible even below 100☌. Hot water heating is therefore less uniform and transfers heat slower compared to steam heating. In this process, the hot water transfers its heat to the product and its own temperature drops. In contrast, hot water transfers sensible heat through convection. Vacuum steam heating systems have been developed so that the advantages of saturated steam can be utilized for heating below 100☌.
It condenses at a constant temperature (the saturation temperature).įor these two reasons, steam heats faster and more evenly compared to hot water. When it condenses, it releases a large amount of latent heat. It transfers heat through condensation. As a heating medium, steam offers the following advantages: Herein, we will refer to saturated steam simply as steam. How Saturated Steam and Hot Water Transfer Heat A vacuum pump is necessary to precisely control steam at below 100☌.īecause of this, in the majority of cases, hot water is used to heat at less than 100☌ and steam is used for heating at over 100☌. 
To use liquid water at temperatures above 100☌, you must continuously apply pressure above the saturation level to the system.Ĭonversely, since the saturation temperature of steam at atmospheric pressure is 100☌, it is not possible to create saturated steam below 100☌ simply by reducing its pressure with a pressure reducing valve. So, how should you use these two heat sources differently?Īt atmospheric pressure, water boils at 100☌. They may look different, but both are, in fact, water. Differences between Steam and Hot Water Heatingīoth steam and hot water are commonly used as heating media in industrial applications.
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