Steam: Uses and Basic Cycle PDF

Objective1 Describe steam, its uses and the basic steam cycle Producing steam, and controlling the equipment that produces and consumes it, are among the primary duties of Power Engineers. So why is steam important, and why do Power Engineers produce it? STEAM Steam is water that has been evaporat...

Objective1 Describe steam, its uses and the basic steam cycle Producing steam, and controlling the equipment that produces and consumes it, are among the primary duties of Power Engineers. So why is steam important, and why do Power Engineers produce it? STEAM Steam is water that has been evaporated by adding heat. At home, this is done in kettles and pots. In industry, boilers are used. At home, one kilogram of water may boil in an hour. In a single industrial site, 500 000 kilograms of water may be boiled in an hour, by a single boiler! Power Engineers need to understand the answer to the question “Why does industry use steam?” All industrial processes transfer energy. Electrical energy is used to drive machines that punch holes in manufactured parts, or stamp metal pieces into their final shapes. In these cases, energy is transferred to machinery that converts electrical energy to mechanical energy needed in metal forming processes. Many process, though, rely on the transfer of heat energy. One very effective way to do this is by heating a fluid and moving the hot fluid to where the heat is needed. A good heat transfer fluid should transfer a lot of heat with a small mass. Heat transfer fluids should also be non- toxic. Water (especially in the form of steam) is exactly that sort of fluid. Few heat transfer fluids compare favourably to steam. Finally, water has a unique property that makes it ideal for a wide variety of heat transfer applications: when converted to steam, it expands to several hundred times its volume. As a matter of fact, at 100 degrees Celsius, a unit mass of steam is about 1600 times the volume of a unit mass of water! This means that pressurized steam can expand a great deal, and while expanding it can perform a tremendous amount of work. Page 3 of 23 BOILERS A steam boiler is a closed container, partially filled with water, which evaporates into steam under pressure by the application of heat. This heat is Fig 1 usually obtained from the burning of a fuel, such as natural gas, fuel oil, wood, or coal, in a furnace. In some cases, electrical elements may be used to provide heat. Because they are enclosed, boilers not only generate steam, they generate highly pressurized steam. When water is heated in an open atmosphere, like in Figure 1, the water and steam will not increase in temperature to higher than 100°C. Because boilers are sealed, the pressure of the boiling water increases as steam is generated. This raises the temperature of the water and steam to greater than 100°C. Figure 1 – Boiling Water Many boilers take the steam they produce and add additional heat energy. The resulting steam is superheated. Superheated steam can be around 500 °C, and over 16000 kPa pressure! Page 4 of 23 USES OF STEAM Steam is a very effective heat transfer medium. A small amount of heat contains a tremendous amount of energy! This energy can be used to power turbines that drive electric generators, or other rotating machines like fans and pumps. Steam energy can also be used for the heat it provides. Steam for heat and mechanical power are used in various applications in the following industry sectors (and many others): • Thermal electric power generation • Sugar refining • Petrochemical processing • Textiles industry • Food and beverage processing • Pharmaceutical manufacturing • Building heating systems • Steel industry • Pulp and paper manufacturing • Mining and metal refining • Chemical production • Vehicle manufacturing • Fertilizer manufacturing Steam has some varied and fascinating uses. For example, in the food industry, steam is used to peel potatoes. Under the skin of a potato lies a thin film of moisture. In the food industry, large sealed pots of potatoes are pressurized and heated with steam to raise the temperature of this moisture film to above 100°C. Then, the steam pressure is suddenly released. When the pressure drops, the moisture film immediately boils, violently blowing the skin off the potatoes! Steam can be injected hundreds of meters below ground to heat heavy oil. This valuable oil is too thick to flow when cold. When the oil is hot, it can flow to the surface for processing into a variety of hydrocarbon products. The remaining steam underground assists oil recovery by providing a driving pressure. Steam is used in many industries to control humidity. Animal hides and pelts shrink when dry. The tanning industry keeps pelts and hides at their maximum size by keeping the plant environment humid. By keeping pelts and hides large, tanners receive maximum payout for their products. Steam is even used for cooling. Many facilities are equipped with absorption refrigeration cooling systems. These rely on steam as a heat energy source to drive refrigerant through a cooling cycle. Some food and beverage plants use steam for both sterilization and to drive refrigeration compressors. Page 5 of 23 These are just a few of the fascinating uses of steam that Power Engineers encounter throughout their careers. THE FUNDAMENTAL STEAM CYCLE Steam is vapourized water. When boilers add heat to water, the water boils and turns Steam Cycle to steam. The steam travels to machinery or a heat exchanger, and transfers away part of its heat energy. This transfer of heat causes the steam to condense back to water. Condensed “condensate.” steam Condensate is is called reused, because it is pure and still contains heat. To reuse the condensate, the water must be pumped back into the boiler, because the boiler operates under pressure. When in the boiler, the condensate again vapourizes, and the cycle repeats. Figure 2 shows the basic steam cycle. The steam flows under pressure from the boiler to a unit heater. In the unit heater, the steam gives off heat, causing it to condense. Figure 2 – Basic Steam Cycle The condensate naturally flows to the bottom of the unit heater because water is denser than steam. At the base of the heater is an outlet with a steam trap. The steam trap keeps steam in the heater, and allows only condensate to pass through. Without a steam outlet, the heater would fill up with condensate and would not provide heat. Without the steam trap, steam would flow through the heater unimpeded. It would not spend enough time in the heat exchanger to give up heat and condense. Instead, the steam would flow directly to the condensate tank, where it would vent to atmosphere, wasting heat, water, and chemicals. Page 6 of 23 Once the condensate is in the condensate tank, it is cool enough to pump back into the boiler for the steam Figure 3 – Power Plant Steam Cycle cycle to start over again. This basic system is similar to most process steam systems, except there stages, may be higher more steam pressures, and multiple steam-consuming processes operating simultaneously. Figure 3 shows the steam cycle in a more complex energy plant. Figure 4 – A Boiler in a Small Power Plant Figure 5 – A Large Thermal Generating Station Page 7 of 23