Текст 3. A simple power plant cycle
The steam boiler provides steam to a heat consumer, usually to power an engine. In a steam power plant a steam turbine is used for extracting the heat from the steam and turning it into work. The turbine usually drives a generator that turns the work from the turbine into electricity. The steam, used by the turbine, can be recycled by cooling it until it condensates into water and then return it as feedwater to the boiler. The condenser, where the steam is condensed, is a heat exchanger that typically uses water from a nearby sea or a river to cool the steam. In a typical power plant the pressure, at which the steam is produced, is high. But when the steam has been used to drive the turbine, the pressure has dropped drastically. A pump is therefore needed to get the pressure back up. Since the work needed to compress a fluid is about a hundred times less than the work needed to compress a gas, the pump is located after the condenser. The cycle that the described process forms, is called a Rankine cycle and is the basis of most modern steam power processes.
Текст 4. Carnot efficiency
When considering any heat process or power cycle it is necessary to review the Carnot efficiency that comes from the second law of thermodynamics. The Carnot efficiency equation gives the maximum thermal efficiency of a system undergoing a reversible power cycle while operating between two thermal reservoirs at different temperatures.
The maximum efficiency as a function of the steam exhaust temperature can be plotted by keeping the cooling water temperature constant. Assuming the temperature of the cooling water is around 20 ºC (a warm summer day), larger temperature difference leads to a higher thermal efficiency.
Although no practical heat process is fully reversible, many processes can be calculated precisely enough by approximating them as reversible processes.
To give a practical example of the use of this theory on steam boilers, consider the Rankine cycle. The temperature of the hot reservoir would then be the temperature of the steam produced in the boiler and the temperature of the cold reservoir would be the temperature of the cooling water drawn from a nearby river or lake.
Текст 5. Properties of water and steam
Water is a useful and cheap medium to use as a working fluid. When water is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. The force produced by this expansion is the source of power in all steam engines. It also makes the boiler a dangerous device that must be carefully treated.
The theoretical amount of heat that can be transferred from the combustion process to the working fluid in a boiler is equivalent to the change in its total heat content from its state at entering to that at exiting the boiler. In order to be able to select and design steam-and-power-generation equipment, it is necessary to thoroughly understand the properties of the working fluid, steam, the use of steam tables and the use of superheat. These fundamentals of steam generator will be briefly reviewed in this chapter. When phase changes of the water is discussed, only liquid-vapor and vapor-liquid phase changes are mentioned, since these are the phase changes the entire boiler technology is used.
Текст 6. Boiling of water
Water and steam are typically used as heat carriers in heating systems. Steam, the gas phase, water, results from adding sufficient heat to water to cause it to evaporate. This boiler process consists of three main steps: the first step is the adding of heat to the water that raises temperature up to the boiling point of water, also called preheating. The second step is continuing addition of heat to change the phase from water to steam, the actual evaporation, the third step is the heating of steam beyond the boiling temperature of water, known as superheating. The first step and the third steps are the part where heat addition causes a temperature rise but no phase change. When all the water has been evaporated, the steam is called dry saturated steam. If steam is heated beyond its saturation point, the temperature begins to rise again and the steam becomes superheated steam. Superheated steam is defined by its zero moisture content: it contains no water at all, only 100 % steam.
Evaporation
During the evaporation the enthalpy rises drastically. If water is evaporated at atmospheric pressure from saturated liquid to saturated vapor, the enthalpy rise needed is 2260 kJ/kg, from 430 kJ/kg (saturated water) to 2690 kJ/kg (saturated steam). When the water has reached the dry saturated steam condition, the steam contains a large amount of latent heat, corresponding to the heat that was led to the process under constant pressure and temperature. So despite pressure and temperature is the same for the liquid and the vapour, the amount of heat is much higher in vapour compared to the liquid.
Superheating
If the steam is heated beyond the dry saturated steam condition, the temperature begins to rise again and the properties of the steam start to resemble those of a perfect gas. Steam with higher temperature than that of saturated steam is called superheated steam. It contains no moisture and cannot condense until its temperature has been lowered to that of saturated steam at the same pressure. Superheating the steam is particularly useful for eliminating condensation in steam lines, decreasing the moisture in the turbine exhaust and increasing the efficiency (i.e. Carnot efficiency) of the power plant.
Effect of pressure on evaporation temperature
It is well known that water boils and evaporates at 100˚C under atmospheric pressure. By higher pressure, water evaporates at higher temperature – e.g. a pressure of 10 bar equals an evaporation temperature of 184ºC. The pressure and the corresponding temperature when a phase change occurs are called the saturation temperature and saturation pressure. During the evaporation process pressure and temperature are constant, but if the vaporization occurs in a closed vessel, the expansion that occurs due to the phase change of water into steam causes the pressure to rise and thus the boiling temperature rises.
When 22.12 Mpa is exceeded (the corresponding temperature is 374ºC), the line stops. The reason is that the border between gas phase and liquid phase is blurred out at that pressure. The point, where the different phases cease to exist, is called the critical point of water.