Read the text and look for answers to these questions:
What is steam used for?
What is the purpose of steam-generating equipment?
What is the basic aggregate of the steam-generating equipment?
How many types of steam-generating unit do you know?
What parts does the steam-generating unit with natural circulation consist of?
What parts is boiler composed of?
What are the functions of induced-draft fan and a forced-draft one?
What is the difference between the steam generating unit with natural circulation and the once-through steam-generating unit?
The Steam Generating Units
Steam is used either for space heating, in manufacturing processes, or for power generation. Except for hydroelectric power plants, practically all the central stations generating capacity is in the form of steam turbines.
The purpose of steam-generating equipment is to convert the energy of fuel into heat energy carried by steam, so that it may be employed either in an engine in the form of mechanical energy or used, without conversion, as heat. Where the heat energy of the steam is to be converted into mechanical energy it is necessary to generate the steam at a greater pressure than that of the atmosphere.
The basic aggregate of the steam-generating equipment at thermal power station is the steam-generating unit. There are two major types of steam-generating units - the steam-generating unit with natural circulation and the once - through steam-generating unit
The steam-generating unit with natural circulation consists of a furnace in which the fuel is burned, a boiler, and a superheater in which high-pressure steam is generated, an economizer and an air heater in which the loss of the energy due to combustion of the fuel is reduced to a minimum. A steam boiler is a closed metallic vessel in which steam is generated from water by means of the application of heat.
The boiler is composed of a drum, in which a water level is maintained at about the mid-point, so as to permit separation of the steam from the water, and a bank of lubes, connected to the drum in such a manner as to permit water to circulate from the drum through the tubes and back to the drum. The hot products of combustion from the furnace flow across the boiler lubes and evaporate part of the water in the tubes. The furnace walls are composed of tubes which are also connected to the boiler drum to form very effective steam-generating surfaces.
Since the gaseous products of combustion leaving the boiler tube bank are at a relatively high temperature and their discharge to the chimney would result in a large loss in energy, an economizer may be used to recover part of the energy in these gases. The economizer is a bank of tubes through which the boiler feedwater is pumped on its way to the boiler drum.
A reduction in gas temperature may be made by passing the products of combustion through an air heater which is a heat exchanger cooled by the air required for combustion. This air is supplied to the air heater at normal room temperature and may leave the air heater at 250-420 ° C, thus returning to the furnace energy that would otherwise be wasted up the chimney. The products of combustion are usually cooled in the air heater to an exit temperature of 120-150°C, after which they may be passed through a dust collector which will remove objectionable dust and thence through an induced-draft fan to the chimney. The function of the induced-draft fan is to pull the gases through the heat-transfer surface of the boiler, superheater, economizer and air heater and to maintain a pressure in the furnace that is slightly less than atmospheric pressure. A force-draft fan forces the combustion air to flow through the air heater, duct work, and burner into the furnace.
The one-through generating units are also widely used in thermal engineering. The difference between the steam-generating unit with natural circulation and the once-through steam-generating unit is that the latter has no drum, and water from the economizer passes directly to the heat-transfer surfaces. In these water is completely evaporated and steam passes to the steam-heaters. Thus, water passes through the whole aggregate without multiple circulation, that is, straight; therefore, such steam-generating units are called the once-through generating units. The absence of the drum reduces the cost of the steam-generating unit. The feed water in the once-through generating units should be very clean. The once-through generating units are widely used at high pressures. At the pressure more than 225 atmosphere, only the once-through generating units are used.