Techniques of Waste Heat Recovery
Waste heat recovery is being more used more and more often today as it is becoming more effective, due to increasing energy prices, technological developments and a decrease in equipment costs. Heat recovery can be classified into three main strategies: recycling energy back into the process, recovering the energy for other onsite use and using it to generate electricity in combined heat and power systems. Once the system has been made as efficient as possible before using waste heat recovery, passive strategies are used. These have no significant input for operation except for auxiliary equipment such as pumps and fans. The last case is active strategies, which require some input of energy to upgrade the heat to a higher temperature or electricity. These need industrial heat pumps and a combined heat and power system. High temperature energy recovery options
Heat exchangers are commonly used to transfer heat from combustion exhaust gases to air entering the furnace. By increasing the temperature of the gases entering less work is required by the fuel, the saves fuel that also means a decrease in combustion air. Below are some common heat exchanger devices. Recuperators
Recuperators are used to recover exhaust gas heat in medium to high temperature applications such as annealing ovens and melting furnaces. They can be made of metal (below 1000ºC) or ceramic (900-1500ºC) tubing and can be based on radiation, convection or a hybrid. The most basic is the simple radiation recuperator, which consists of two concentric tubes one carrying exhaust one carrying combustion gases. Although it would be more efficient to run them counter flow they are commonly parallel flow, this is because it helps cool the ducts carrying gases away increasing service life. The convective one passes the hot gases through a series of relatively small diameter tubes contained in a large shell. This increases the contact area and as the incoming combustion air is baffled around the tubes, it picks up heat from the waste gas. These are generally more compact and efficient than the simple radiation one. Finally the hybrid includes both in order to maximize the heat transfer effectiveness. ( a) radiation recuperator b) convection recuperator ) Regenerators
Regenerators can be fixed or rotary (heat wheel) and they operate by storing heat in a porous media, such as brick, and alternating the flow of hot and cold gases. The fixed ones consist of two checker work chambers and as the hot exhaust gases pass through one the bricks absorb heat. The flow of air is then adjusted so that the incoming cold air passes through the hot bricks and the hot air is now heating the other chamber. The disadvantages of the fixed regenerator are the large size and initial cost is much greater than a recuperator. Rotary regenerators use a rotating disk across two parallel flow ducts. Large temperature differences can cause problems due to differential expansion and large deformations. Another issue is preventing cross contamination between the two ducts, as contaminants can be transported in the wheels porous material. One advantage of the heat wheel is that it can recover moisture as well as heat from clean gas streams, making it particularly useful on air conditioning applications. They haven’t been widely implemented yet due to the cost.
(fixed regenerator heat wheel)
A heat pipe can transfer up to 100 times more thermal energy than copper, it absorbs and transfers heat with no moving parts and requires minimal maintenance. It consists of three parts, a sealed container, a capillary wick structure and a working fluid. Thermal energy applied causes the working fluid to evaporate and the vapor formed absorbs the latent heat. The vapor then travels to the other end of the pipe where it now condenses giving up the latent heat of the condensation. Heat pipes can be bundles together the make a heat pipe exchanger. It is a...
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