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STEAM: A HEAT TRANSFER FLUID

Steam provides a means of transporting controllable amounts of energy from a central boiler room, where it can be efficiently and economically generated, to the point of use. For many reasons, steam is one of the most widely used commodities for conveying heat energy. Its use is popular throughout industry for a broad range of tasks from mechanical power production to space heating and process applications. This is why some consider steam to be the energy fluid [1].

The ability of steam to retain a large amount of energy on a per weight basis (1000–1200 btu/lb) makes it ideal for use as an energy transport medium. Since most of the heat energy contained in steam is in the form of latent heat, large quantities of energy can be transferred efficiently at constant temperature, which is useful in many process heating applications.

The use of steam has come a long way from its traditional associations with nineteenth-century locomotives and the Industrial Revolution. Steam today is an integral and essential part of modern technology. Without the use of steam, our food, beverage, textile, chemical, medical, power, heating, and transportation industries would be crippled.

WHY STEAM?

Water can exist in the form of a solid (ice), a liquid (water), or a gas (steam). In this book, our attention will concentrate on the liquid and gas phases, and the equipment required to facilitate the change from one phase to the other. If heat energy is added to water, its temperature rises until a value is reached at which the water can no longer exist as a liquid. We call this the “saturation” point. Further addition of energy will cause some of the water to boil as steam. This evaporation requires relatively large amounts of energy, and while it is being added, the water and the steam are both at the same temperature. As the steam is formed in a closed vessel, it develops pressure allowing it to flow anywhere to a lower pressure, that is, through piping and distant equipment. Likewise, if we allow the steam to cool it will release the energy that was added to evaporate it. These boiling, transfer, and condensing events provide a simple mechanism to transfer energy from one place to another, hence the basis of a steam system. Interestingly, steam is colorless; the white color often seen when steam discharges to the atmosphere is from the condensed water vapor in the steam.

Steam Is Safe and Flexible

Water is plentiful and inexpensive. It is nonhazardous to health and environmentally sound. In its gaseous form, it is a safe and efficient energy carrier. Steam can hold five to six times as much energy as an equivalent mass of water. It can be generated at high pressures to give high steam temperatures. The higher the pressure, the higher the temperature, so it’s potential to do work is greater. Modern boilers are compact and efficient in their design, using multiple passes and efficient burner technology to transfer a very high proportion of the energy contained in the fuel to the water, with minimum emissions. The boiler fuel may be chosen from a variety of options, including, natural gas, LP gas, oil, solid fuels, alternative fuels, and electricity, which makes the steam boiler an economical and environmentally sound option amongst the choices available for providing heat energy. Highly effective heat recovery systems can significantly reduce exhaust gas and water discharge energy losses creating an overall efficiency of the steam system approaching 85 percent. Boiler plants can be centralized or installed at the point of use. Sizes range from a few pounds of steam to thousands of pounds of steam per hour. Steam is one of the most widely used media to convey heat over distances. Because steam flows in response to the pressure drop along the pipe line, expensive circulating pumps are not needed. Not only is steam an excellent carrier of heat, it is also sterile and thus popular for process use in the food, pharmaceutical, and health industries. Other industries within which steam is used range from huge petrochemical and bio fuel plants to small local laundries. Further uses include the production of paper, plastics, textiles, beverages, food, metal, and rubber. Steam is also used extensively for power generation, humidification, and space heating.

Steam is also intrinsically safe – it cannot cause sparks and presents no fire risk. Many chemical plants and refineries utilize steam fire-extinguishing systems. It is ideal for use as a heat transfer media in hazardous areas or explosive atmospheres.

Steam Is Easy to Control

Because of the direct relationship between the pressure and temperature of saturated steam, simply controlling the steam pressure one can control the temperature of the steam and the process material being heated. Furthermore, the total amount of energy input to a process stream is directly related to the steam mass flow heating that process. Modern steam controls like pressure reducing valves and flow control valves are designed to respond very rapidly to process inputs. Therefore, today steam pressure and flow can be precisely regulated to add heat energy to a process. Industrial processes that have tight heating tolerances are well suited for process steam use.

The heat transfer properties of saturated steam are high and the required heat transfer area is relatively small. This enables the use of compact heat transfer equipment, which reduces installation costs and takes up less space in the plant. Most steam controls are able to interface with modern networked instrumentation and control systems to allow centralized control, as the case of a Building/Energy Management System or Process Computers. With proper maintenance, a steam plant will last for many years, and many aspects of the system are easy to monitor on an automatic basis.

In contrast, hot water and hot oils have a lower potential to carry heat energy per pound. Consequently large amounts of water or oil must be pumped around the system to satisfy process or space heating requirements. However, hot water is popular for general space heating applications and for low-temperature processes (up to 200 F) where some temperature variations can be tolerated. Furthermore, thermal fluids, such as mineral oils, may be used where high temperatures (up to 700 F) are required, but where high steam pressure is undesirable. An example would include using hot oil as the heating source in certain chemical process reactors. Figure 1.1 shows that saturated and superheated steam can provide a heating range from 200 to >1000 F.

FIGURE 1.1 Heat transfer fluids useful temperature ranges.

THE CONCEPT OF STEAM FORMATION: BOILING

Perhaps the best way of visualizing the formation of steam in a boiler is to compare what happens in a pot of water being heated on a kitchen stove. See Figure 1.2. Addition of heat to the water in the pot raises its temperature, until a point where the temperature reaches the boiling point but does not boil yet. Thus far the heat added by the stove burner is called sensible heat. Sensible heat is the energy added or removed from a substance that corresponds to a temperature change only. Water that enters a boiler and heated up to the boiling point temperature, without boiling occurring, is considered to have a gain in sensible heat only.

FIGURE 1.2 A pot of boiling water: The simplest boiler. Markus Schweiss / Wikimedia Commons / CC BY-SA 3.0.

To allow the hot water molecules to change from the liquid state to a gaseous state, a large amount of additional energy must be added. This amount of heat energy is called the latent heat of vaporization. Latent heat is the heat added to a substance that does not cause a change in temperature, rather, creates a change in state. Latent heat is added when ice is changed to water and again when water is changed to steam. Figure 1.3 shows the relationship between temperature and energy content of water and steam [2, 3]. The curve shows two forms of heat gain, latent and sensible heat. Latent heat tends to be much greater than sensible heat. For instance heating water in a pot, open to the atmosphere, from 33 to 212 F requires only 179 btu, but changing the 212 F water to 212 F steam requires about 971 btu. The latent heat value is important because it not only is the amount of energy required to change water to a gas (steam), but is the same amount of energy steam will yield when it condenses in process equipment (i.e., heat exchanger). Notice, as sensible heat is added to steam above the Saturation Temperature (boiling point), the steam becomes superheated steam.

FIGURE 1.3 Water and steam enthalpy curve [2, 3].

Pressure and Boiling

The pot of water on the stove mentioned above is subject to “atmospheric pressure.”

This is simply the pressure exerted on all things, in all directions by the earth’s atmosphere. The pressure exerted by the atmosphere at sea level, happens to be 14.7 psi (pounds per square inch) or 1 Bar. Water subject to atmospheric pressure will boil at 212 F. The boiling of water in a pot on the stove is quite easy to visualize; however, we must consider the effects of pressure to really understand steam formation in a boiler. If we placed a pressure gauge in the pot of water on the stove, open to atmosphere, it would read 0 pounds per square inch gauge or psig. Therefore, any pressure shown on a pressure gauge mounted in the pressure vessel of a boiler, is in...