Waste heat recoveryDefinition:- Waste heat is heat generated in a process by way of fuel combustion or chemical reaction,Which is then dumped into the environment and not reused for useful and economicPurposes. The essential fact is not the amount of heat, but rather its value. The mechanismto recover the unused heat depends on the temperature of the waste heat gases and theeconomics involved.And the Process which is used to recover that heat is called Waste heat recovery. Waste heat recovery Unit (WHru)Awaste heat recovery unit(WHRU) is anenergy recoveryheat exchangerthat recovers heat from hot streams with potential high energy content, such as hotflue gasesfrom a diesel generator or steam fromcooling towersor evenwaste waterfrom different cooling processes such as insteel cooling. Dumped heat that can still be reused Value (quality) more important than quantity Waste heat recovery saves fuelWhy to use Waste Heat Recovery Unit (WHRU)?Large quantities of hot flue gases are generated from boilers, kilns, ovens and furnaces. Ifsome of the waste heat could be recovered then a considerable amount of primary fuel couldbe saved. The energy lost in waste gases cannot be fully recovered. However, much of theheat could be recovered and adopting the following measures as outlined in this chapter canminimize losses.

Waste Heat Recovery equipments:-Waste heat found in the exhaust gas of various processes or even from the exhaust stream of aconditioning unitcan be used to preheat the incoming gas. This is one of the basic methods for recovery of waste heat. Manysteel making plantsuse this process as an economic method to increase the production of the plant with lower fuel demand.There are many different commercial recovery units for the transferring of energy from hot medium space to lower one: Recuperators: In a recuperator, heat exchangetakes place between the flue gasesand the air through metallic orceramic walls. Ducts or tubes carry the air for combustion to be preheated,the other side contains the waste heatstream. A recuperator for recoveringwaste heat from flue gases is shown in Figure.

Metallic radiation recuperator The simplest configuration for a recuperator is the metallic radiation recuperator, whichconsists of two concentric lengths of metal tubing as shown in Figure below.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 is the energy, which does not have to be supplied by the fuel; consequently, less fuel is burned for a given furnace loading. The saving in fuel also means a decrease in combustion air and therefore, stack losses are decreased not only by lowering the stack gas temperatures but also by discharging smaller quantities of exhaust gas.The radiation recuperator gets its name from the fact that a substantial portion of the heat transfer from the hot gases to the surface of the inner tube takes place by radiative heat transfer. The cold air in the annuals,however, is almost transparent to infrared radiation so that only convection heat transfer takes place to the incoming air. As shown in the diagram, the two gas flows are usually parallel, although the configuration would be simpler and the heat transfer would be more efficient ifthe flows were opposed in direction (or counter flow). The reason for the use of parallel flow is that recuperators frequently serve the additional function of cooling the duct carrying away the exhaust gases and consequently extending its service life. Economizers: In the case of boiler systems, an economizer can be provided to utilize the flue gas heat for pre-heating the boiler feed water. On the other hand, in an air pre-heater, the waste heat is used to heat combustion air. In both thecases, there is a corresponding reduction in the fuel requirements of the boiler.For every 220 0C reduction in flue gas temperature by passing through an economizer or a pre-heater, there is 1% saving of fuel in the boiler. In other words, for every 60 0C rise in feed water temperature through an economizer, or 200 0C rise in combustion air temperature through an air pre-heater, there is 1% saving of fuel in the boiler. Waste Heat Recovery BoilersWaste heat boilers are ordinarily water tube boilers in which the hot exhaust gasesfrom gas turbines, incinerators, etc., pass over a number of parallel tubes containing water.The water is vaporized in the tubes and collected in a steam drum from which it is drawnout for use as heating or processing steam.Because the exhaust gases are usually in the medium temperature range and in orderto conserve space, a more compact boiler can be produced if the water tubes arefinned in order to increase the effective heat transfer area on the gas side. Figure 12 shows amud drum, a set of tubes over which the 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 at which steam is produced dependon the temperature of waste heat. The pressure of a pure vapor in the presence of its liquid isa function of the temperature of the liquid from which it is evaporated. The steam tablestabulate this relationship between saturation pressure and temperature. If the waste heat in theexhaust gases is insufficient for generating the required amount of process steam, auxiliaryburners, which burn fuel in the waste heat boiler or an after-burner in which the exhaust gasesflue are added. Waste heat boilers are built in capacities from 25 m3 almost 30,000 m3 /min.of exhaust gas. Thermo-compressorIn many cases, very low-pressure steam is reused as water after condensation for lack of anybetter option of reuse. In many cases it becomes feasible to compress this low-pressure steamby very high-pressure steam and reuse it as a medium pressure steam. The major energy insteam is in its latent heat value, and thus thermo compressing would give a big improvementin waste heat recovery. The thermo-compressor is a simple equipment with a nozzle where HP steam is acceleratedinto a high velocity fluid. This entrains the LP steam by momentum transfer and thenrecompresses in a divergent venturi. A figure of thermo compressor is shown in Figure 14.It is typically used in evaporators where the boiling steam is recompressed and used asheating steam.

Passive Air Preheaters:-Passive air preheaters are gas togas heat recovery devices for low to medium temperature applications where cross contamination between gas streams must be prevented. Applications include ovens, steam boilers, gas turbine exhaust, secondary recovery from furnaces, and recovery from conditioned air.Passive preheaters can be of two types the plate type and heat pipe. The plate type exchanger consists of multiple parallel plates that create separate channels for hot and cold gas streams. Hot and cold flows alternate between the plates and allow significant areas for heat transfer. These systems are less susceptible to contamination compared to heat wheels, but they are often bulkier, more costly, and more susceptible to fouling problems.The heat pipe heat exchanger consists of several pipes with sealed ends. Each pipe contains a capillary wick structure that facilitates movement of the working fluid between the hot and cold ends of the pipe. As shown in Figure below, hot gases pass over one end of the heat pipe, causing the working fluid Furnace Regenerator:-Regenerative furnaces consist of two brick checker work chambers through which hot and cold airflow alternately (Figure 6). As combustion exhausts pass through one chamber, the bricks absorb heat from the combustion gas and increase in temperature. The flow of air is then adjusted so that the incoming combustion air passes through the hot checker work, which transfers heat to the combustion air entering the furnace. Two chambers are used so that while one is absorbing heat from the exhaust gases, the other is transferring heat to the combustion air. The direction of airflow is altered about every 20 minutes. Regenerators are most frequently used with glass furnaces and coke ovens, and were historically used with steel open hearth furnaces, before these furnaces were replaced by more efficient designs. They are also used to preheat the hot blast provided to blast stoves used in iron making; however, regenerators in blast stoves are not a heat recovery application, but simply the means by which heat released from gas combustion is transferred to the hot blast air (see Section 4.3.1.2 Blast Furnace). Regenerator systems are specially suited for high temperature applications with dirty exhausts. One major disadvantage is the large size and capital costs, which are significantly greater than costs of recuperators.

Working principle of whru.

After combustion some flue gases are produced having temperature about 5000C which exits through the chimney. This will be great loss if the flue gases are not used properly. For this a combine cycle plant is introduced which use the flue gas as working fluid. Here damper is used to recover the waste heat. Figure:Flow diagram of Waste Heat Recovery Unit (WHRU)Damper blocks the chimney and passes the flue gas to boiler and produce steam. This steam is used to generate electricity at steam turbine. Since both of steam and gas turbines are used it is called combine cycle power plant (CCPP). Figure:Waste Heat Recovery Unit

The Working Principle can also be seen through these diagrams.

The Process can be viewed more clearly after production of gas in figure below

This process is used worldwide like in cement industry in NSP.This can be seen in figure below

Applications: Waste Heat of low temperature range (0-120C) could be used for the production ofbio-fuelby growing of algae farms or could be used ingreenhouses[4]or even used in Eco-industrial parks. Waste Heat of medium (120-650C) and high (>650C) temperature could be used for the generation of electricity or mechanical work via different capturing processes. Waste heat recovery system can also be used to fulfill refrigeration requirements of a trailer (for example). The configuration is easy as only a waste heat recovery boiler and absorption cooler is required. Furthermore only low pressures and temperatures needed to be handled.Advantages and disadvantages of waste heat recoveryAdvantages:- These systems have many benefits which could be direct or indirect. Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the costs of fuel and energy consumption needed for that process. Indirect benefits: 1. Reduction inPollution:Thermalandairpollution will dramatically decrease since lessflue gasesof high temperature are emitted from the plant since most of the energy is recycled.2. Reduction in the equipment sizes: As Fuel consumption reduces so the control and security equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes.3. Reduction in auxiliary energy consumption: Reduction in equipment sizes means another reduction in the energy fed to those systems like pumps, filters, fans...etc.Disadvantages:- Capital cost: The capital cost to implement a waste heat recovery system may outweigh the benefit gained in heat recovered. It is necessary to put a cost to the heat being offset. Quality of heat: Often waste heat is of low quality (temperature). It can be difficult to efficiently utilize the quantity of low quality heat contained in a waste heat medium. Heat exchangers tend to be larger to recover significant quantities which increases capital cost. Maintenance of Equipment: Additional equipment requires additional maintenance cost.

DEPARTMENT OF MECHANICAL ENGINEERING COURSE BSME

SUBJECT Fluid Mechanics Lab-ISUBMITTED TO Sir Abdul Rafi SUBMITTED BY Kashif Khan (334)-Asif Mahmood (330)-M.Gulsher (335) Ameer Hamza (364)-M.Laraib sherwani (346) DATE 25-12-2014