Although you can do many classifications of boilers according to different criteria, you can say there are two general types of boilers: the fire-tube (smoke tubes) and the water tube (water pipe) and within the latter will differ from boilers boiler water and steam boilers, single step. Additionally, the furnaces can be classified into high and low pressure saturated steam or superheated.
Saturated steam is the one that has not been heated above the saturation temperature. Will be called if it was completely dry evaporated, or wet with a% moisture if it has been. The superheated steam will be one that, on the contrary, it has been heated after complete evaporation, changing its temperature to the same pressure. Being overheated can deliver or lose some of their energy without condensing, with the benefits that this entails for transport or use in turbines.
Shall mean those high-pressure boilers operating at a pressure above 1 bar. One advantage of using high-pressure boilers is to reduce the size of the boiler and steam pipes for the same heat-carrying capacity due to increased vapor density with pressure. This may be particularly important if consumers are vapor at some distance from the boiler. Moreover, the available energy in the vapor pressure increases, which is essential when used in a steam turbine.
Another common classification of boilers would be for the type of shot. The air needed for combustion is supplied to the boilers typically by fans. According to these fans act on the air supply, we say that the boilers are forced draft, induced or balanced. The first are those in which the fan, located at the entrance, enter the air in the boiler, and are therefore pressurized home. The second is that having the fan suck the gases out of combustion and send them to the chimney, being home so depressed. When both fans coexist boiler firing is called balanced, making the home work a little depression to prevent leakage of gases.
Returning to the general classification, fire-tube boilers are those in which combustion gases flow through tubes that are surrounded by water. Many small and medium-sized boilers in the industry are of this type. The combustion gases are cooled as they circulate through the tubes, transferring its heat to the water. Heat transfer is a function of the conductivity of the tube, the temperature difference between water and gas transfer surface, contact time, etc. .. A typical example would be this type of boilers in Figure 1-4 which shows a small home on the container with water, which in turn is passed longitudinally through the tubes of the combustion gases. The fire-tube boilers can designed with different steps of the flue pipes for the water container. The home is considered the first step and each set of tubes in the same way an additional step (Figure 1-5). Fire-tube boilers usually work up to 20 bars to a high of about 20 tons / hr.
Figure 1-4.Caldera pirotubular
Figure 1-5.Boiler Steps
The water-tube boilers are those in which the water circulates inside the tubes. These tubes are usually connected to two boilers (Figure 1-6). The upper boiler steam, which is separated from the existing water vapor in circulation, and lower water, also known as boiler sludge to settle them in it.
Figure 1-6.Package water tube boiler
In some cases the lower boiler is replaced by collectors, such as recovery boiler in Figure 1-7.
Figure 1-7.Recovery boiler water tube
The tubes connecting the two boilers are distributed so that some of them is on the hot side of the boiler - the boiler area in contact with the gases of combustion - and one on the cold side (Figure 1-8 ). The water from the hot side tube is partially evaporated so that the steam boiler rises to the top due to the lower density compared to water it. The cold water from the boiler circulates from top to bottom due to the higher density of water in this area, so that there is a natural circulation of the water body. This type boilers tend to operate up to pressures of 100 bar in the case of industrial boilers and 200 in the case of boilers for central heating, with a production of 500 tons / hr and 4000 tons / hr respectively.
Figure 1-8.Circulation in a water tube boiler
For pressures above those indicated from which the flow decreases rapidly because the densities of water and steam are similar, they use single-pass water tube boilers. In these boilers, circulation is forced by a pump system introduced by the water at one end and, after being heated, steam comes out as the other. Are able to work up to 350 bar pressure. These boilers are those of power plants so we will not stop more specific about them.
