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H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289 types are mainly used to improve the efficiency of boiler systems, whereas the other types are commonly used in thermal power plants and large processing units to recover waste heat from the flue gas. Having mentioned that, Vandagriff [21] investigates, economisers that are used for low-temperature heat recovery namely as deep economisers are also available that are made out of advanced materials such as Teflon, carbon and stainless-steel tubes and can withstand the acidic condensate deposition on the surface of the heat exchanger. Glass-tubed economisers are on the hand used for gas to gas heat re- covery and for low to medium temperature applications [22]. 2.3. Waste heat boilers Waste heat boilers consists of several water tubes that are placed in parallel to each other and in the direction of the heat leaving the system. The system is suitable to recover heat from medium – high temperature exhaust gases and is used to generate steam as an output. The steam can then be used for power generation or directed back to the system for energy recovery [23]. For example, as J + G [24] reports, in a coal power plant the heat generated from the combustion process after leaving the combustion chamber has a temperature of up to 1000 °C. The use of a waste heat boiler in this case allows the recovery and utilisation of the heat of the flue gas to vaporise a fluid and produce steam that can be used for energy generation through turbines and generators. The pressure and the rate of steam production mainly depends on the temperature of the waste heat. If the waste heat is not sufficient for the system to produce the required amount of steam, an auxiliary burner unit or an after burner in the exhaust gases can be added to the system to compensate for that [25]. As Turner [26] reports, waste heat boilers can also be coupled with other waste heat recovery equipment such as afterburners, preheaters and finned-tubed evaporators to improve efficiency by preheating the feed water and produce superheated steam if required (see Fig. 4). 2.4. Air preheaters Air preheaters are mainly used for exhaust-to-air heat recovery and for low to medium temperature applications. This system is particularly useful where cross contamination in the process must be prevented. Such applications can include gas turbine exhausts and heat recovery from furnaces, ovens, and steam boilers [27]. Air preheating can be based on two different designs, the plate type and the heat pipe type. The plate type consists of parallel plates that are placed perpendicular towards the incoming cold air inlet. Hot exhaust air is fed into the channels between the plates, transferring heat to the plates and creating hot channels, through which the cold air is passed. The heat pipe type on the other hand consists of a bundle of several sealed pipes placed in parallel to each other in a container. The con- tainer is split into two sections accommodating cold and hot air, inlet and outlet. The pipes inside the container accommodate a working fluid which when faced with the hot waste gas at one end of the pipes, evaporates and moves towards the other end of the pipe where cold air is passing [28]. This results in heat being absorbed at the hot section of the pipe, which is transferred to the cold section, heating the cold moving air over the pipes. The working fluid then condenses and moves towards the hot section of the pipe, repeating the cycle [29]. As Nicholson [30] explain, there are mainly three commonly used types of air preheaters which are classified as regenerators, including rotary regenerators, run around coil, and recuperative. These technol- ogies all function with the same principle as air preheaters, however, have different configurations and used for different purposes (see Fig. 5). 2.4.1. Recuperators Recuperators are a form of heat exchanger units that are usually made out of metallic or ceramic materials, depending on their appli- cation, and they are used to recover waste exhaust gases at medium to high temperature [32]. In this technology, the hot exhaust gases are passed through a series of metal tubes or ducts that carry the inlet air from atmosphere. This result in the recuperator preheating the inlet gas which then re-enters the system. The energy that is now available in the system can therefore be described as the energy which does not have to be supplied by the fuel, meaning that a decrease in energy demand and production costs is achieved [33]. Metallic recuperators are used for applications with low – medium temperatures, while heat recovery in high temperature application is better suited to ceramic recuperators. Recuperators can be said to mainly transfer heat to the inlet gas based on convection, radiation or a combination of radiation and convection. A radiation recuperator consists of metallic tubes around the inner shelf where hot exhaust gases pass through. The cold incoming air is then fed to the tubes around the hot shelf and heat is radiated to the wall of the tubes (see Fig. 6). The tubes transfer the heat to the cold air, which is then delivered to the furnace burners. On the other hand, the convective recuperator exchanges heat by passing hot exhaust gases through relatively small diameter tubes that are placed in a larger shelf. The cold air is passed through the large shelf, picking up heat from the small hot tubes inside the shelf that is heated by the waste gas. A combination of radiant and convective recuperators provides another possibility which can maximise heat transfer effectiveness. In this technology, hot exhaust gas is fed into a larger shelf and then split into smaller diameter tubes. Cold air is fed into and around the shelf, and this results in a quantitative improvement in heat transfer [35] (see Fig. 7). 2.4.2. Regenerators Regenerators transfer heat from the hot gas duct to the cold gas duct through storing the waste heat in a high heat capacity material. The Fig. 4. Schematic of a waste heat boiler incorporating parallel water tubes [26]. 271PDF Image | Waste Heat Recovery Technologies and Applications
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