Components used in Waste heat recovery systems
There are many different heat recovery technologies available which are used for capturing and recovering the waste heat and they mainly consist of energy recovery heat exchangers in the form of a waste heat recovery unit. These units mainly comprise common waste heat recovery systems such as air preheaters including recuperator, regenerators, including furnace regenerators and rotary regenerators or beat wheels and run around coil. regenerative and mcuperative burners, heat pipe heat exchangers, plate heat exchangers, economisers, waste heat boilers and direct electrical conversion devices. These units all work by the principle to capture, recover and exchange heat with a potential energy content in a process.
1. 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. The function of the air preheater is to increase the temperature of air before it enters the furnace.
● Air preheater is used are constructed in two different designs, the plate type and the heat pipe type. It is generally placed after economizer. So the flue gases passes through economizer and then to the air preheater.
● The plate type consists of parallel plates or tube that are arranged perpendicular towards the incoming hot gases on one side and air on the other side. 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. It preheat the air which supplied to the furnace.
● Preheated air accelerate the combustion and increase the efficiency.
● The heat pipe type on the other hand consists of a bundle of several scaled cylindrical pipes placed in parallel to each other in a container. The container is split into two parts cold end and hot end provided with air inlet and outlet. The pipes inside the container accommodate a working fluid which when comes in indirect contact with the hot waste flue gas at one end of the pipes, evaporates and moves towards the other end of the pipe where cold air is passing. This results in exchange of bent between them i.cheat being absorbed at the hot side of the pipe, which is transferred to the cold side, 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.
2. Recuperators
● It is a type of heat exchanger units which are manufacture from metallic or ceramic materials, according their application, and they are used to recover waste exhaust gases at medium to high temperature,
● The hot exhaust gases are passed through a series of metal tubes or ducts that carry the inlet air from atmosphere. A recuperator for recovering waste heat from flue gases. It is the metallic radiation recuperator, which consists of two concentric lengths of metal tubing. The inner tube carries the hot exhaust gases while the external annulus carries the combustion air from the atmosphere to the air inlets of the furnace burners. The hot gases are cooled by the incoming combustion air which now carries additional energy into the combustion chamber.
● This result in the recuperator preheating the inlet air 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.
● 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
● The shell and tube type or convective recuperator, the hot gases are passes through a number of parallel small diameter tubes, while the incoming air to be heated enters a shell surrounding the tubes and passes over the hot tubes one or more times in a direction normal to their axes.
● The baffled are provided at intermediate place to allow the gas to pass over them increase, the heat exchanger is termed a two-pass recuperator or a three-pass recuperator according to number of buffle. Although baffling increases both the cost of the exchanger and the pressure drop in the combustion air path, it increases the effectiveness of heat exchange. Shell tube type recuperators are generally more compact and have a higher effectiveness than radiation recuperato because of the larger heat transfer area made posible through the use of multiple tubes and multiple passer ot the gases.
● A combination of radiant and convective recuperator A provides another possibility which can maximise he transfer effectiveness. In this technology, hot exha 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.
3. Economiser :
● In case of boiler system Economizer is a device whose Function is toutilizes the waste heat for pre heating feed water to boiler.
● These are classified in two type
(i) independent type
(ii) integral type
● On the other hand, in an air pre-heater, the waste heat is used to heat combustion air.
● In both the cases, there is a corresponding reduction in the fuel requirements of the boiler. For every 22°C reduction in flue gas temperature by passing through an economiser or a pre-heater, there is 1% saving of fuel in the boiler. In other words, for every 6°C rise in feed water temperature through an economiser, or 20°C rise in combustion air temperature through an air pre-heater, there is 1% saving of fuel in the boiler.
● Green's economizer which is of independent type.
● It consist of large number of pipe placed in the flue gas path. These tubes are connected to two horizontal tube or pipe one at top and other at bottom.
● It is located in the path of flue gases between the boiler and chimney.
● The feed water enter from bottom in vertical tube and flows upward as the water gain the heat from flue gas, it became hot and so density decreases.
● Then the water flow out through the top horizontal pipe. On the vertical tube 'H' shape scraper is provided.
● The scraper moves up and down by chain and pulley arrangement the function of scraper is to remove soot deposited on tube otherwise it reduces the efficiency of economizer.
● The blow of cock is also provided at back end of vertical pipe for removal of sediment deposit in bottom header (tubes).
● By pass arrangement is also provided for flue gases and feed water may be put in action when necessary. when economizer is in use damper D1, and D2, open and D3, is closed.
● By closing D1, and D2, damper and opening damper D3 the flue gases are diverted directly to the chimney.
Advantages
(i) Evaporative capacity of boiler increase
(ii) Overall efficiency of the plant increase.
(iii) Reduces thermal stresses as temperature difference is decrease.
4. Super Heater
● Function : Super heater is a device used to increase the temperature of steam (wet or dry) above saturation temperature at constant pressure using waste heat of flue gases.
● Super heater are located in the path of flue gases so that the heat of flue gases is utilized. These are convective type and Radiant type super heater
● It consist of two heater made of steel to which a 'U steel tube attached. These tubes are arranged in bundles. The steam from boiler enter and discharges through header as shown by arrow, also shows the arrangement of steam pipe when the wet steam enter directly in main pipe the valve C, B are closed and R is open
● When steam passes to superheater the valve A is closed and valve C and B are open. The function of damper F is to controlled the path of gases operated by handwheel.
5. Shell and Tube Heat Exchanger:
● When the medium containing waste heat is a liquid or a vapor which heats another liquid, then the shell and tube heat exchanger must be used since both paths must be scaled to contain the pressures of their respective fluids.
● The shell contains the number of the arranged parallel to each other supported in backing plate provided with internal battles intermediate position to direct the Quid in the shell over the tubes in multiple passes. The shell is inherently wenker thun the tubes so that the higher-pressure fluid is circulated in the tubes while the lower pressure fluid flows through the shell. When vapor contains the waste heat, it usually condenses giving up its latent heat to the liquid being heated. In this application, the vapor is almost invariably contained within the shell. If the reverse is attempted the condensation of vapors within small. diameter parallel tubes causes flow in stabilities. Tube and shell heat exchangers are available in a wide range of standard sizes with many combinations of materials for the tubes and shells.
6. Waste Heat Boilers
● Waste heat boilers, such as the two pass boile are water tube boilers that use medium to high temperature exhaust gases to generate steam, if the water tubes are finned in order to increase the effective heat transfer area on the gas side The hot exhaust gases from gas or steam turbines, etc., pass over number of parallel tubes containing water. The water is vaporized in the tubes and collected in a steam drum from which it is drawn off for use as heating or processing steam.
● The boiler consist of a mud drum, a set of tubes over which hot gases make a double pass, and a steam drum which collects the steam generated above the water surface. The pressure at which the steam is generated and the rate of steam production depends on the temperature of waste heat. The pressure of a pure vapor in the presence of its liquid is a function of the temperature of the liquid from which it is evaporated.
● In cases where the waste heat is not sufficient for producing desired levels of steam, auxiliary burners or an after burner can be added to attain higher steam output. The steam can be used for process heating or for power generation. Generation of superheated steam will require addition of an external super heater to the system. Waste heat boilers are are available in a variety of capacities from 26 m.cu/min almost 31,000 m cu / min. of exhaust gas.
7. Waste Heat recovery steam generator plant:
● The construction of waste heat recovery system for steam generator plant is a complex in design. It consists of several heat recovery division such as an evaporator, super heater, economiser and steam drum, which are very large in size.
● The system contains an evaporator part and a steam drum for converting water to steam.the evaporator is located between the economiser and the superheater with the steam drum on top of it.In this the super heater is placed in the path of the hottest flue gas upstream of the evaporator and the economiser is placed downstream of the evaporator in coolest gas.
● In the evaporator, the steam for the turbine is generated which is then delivered to the steam drum and the super heater
● In the steam drum the steam and water mixture is separated from the saturated steam as the feed water is delivered to the evaporator. The steam is separated in two steps through a combination of gravity and mechanical work before it gets delivered to the super heater. The steam is then superheated as its temperature is increased beyond the saturation point.
● The economiser on the other hand, preheats the feed water to the evaporator, thus improving the efficiency of steam generation. The steam generated in the process is then sent to a thermodynamic cycle such to generate power and improve the efficiency of the plant.
