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BEE Energy Textbook 2024 : Energy

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Mihir Prajapati
Nirma University (NU), Ahmedabad
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Question bank for Energy Managers & Energy Auditors Ch ap te r 2 .8 : W as te H e at Re co ve ry Part-I: Obje ctive typ e qu e s tio n s an d an s w e rs 1. 2. Major advantage of waste heat recovery in industry is: a) reduction in pollution b) increase in efficiency c) both a & b d) none of the above Heat recovery equipm ent will be m ost effective when the tem perature of flue gas is: a) 250 oC 3. 5. 6. c) 40 0 oC d) 280 oC The waste gases com ing out from gas turbine exhausts are of the order of: a) 370 -540 4. b) 20 0 oC b) 450 70 0 c) 70 0 -80 0 d) 250 -440 Recuperator is used m ainly as a waste heat recovery system in a _ _ _ _ . a) boiler b) billet Reheating Furnace c) com pressor d) none of the above Recuperator will be m ore efficient if the flow path of hot and cold fluids is in: a) co-current m ode b) counter current m ode c) cross current m ode d) none of the above The m ajor lim itation of m etallic recuperator is ------a) lim itation of handling CO x, NOx etc. b) lim itation of reduced life for handling tem perature m ore than 10 0 0 oC c) m anufacturing difficulty of the required design d) none of the above 7. Ceram ic recuperators can withstand tem peratures up to: a) 60 0 oC 8. 9. b) 130 0 oC c) 170 0 oC d) 950 oC Air preheater is not used as a waste heat recovery system in a_ _ _ _ . a) boiler b) billet Reheating Furnace c) heat treatm ent furnace d) com pressor Typical waste gases tem perature from glass m elting furnace a) 10 0 0 -1550 oC b) 80 0 -950 oC 2.8 Waste Heat - revised (table format) c) 650 750 oC d) 760 -815 oC 58 Question bank for Energy Managers & Energy Auditors 10 . 11. Regenerator is widely used in: a) reheating Furnaces b) heat treatm ent furnaces c) baking Ovens d) glass m elting furnaces In a low to m edium tem perature waste heat recovery system which of the device is m ost suitable a) econom iser 12. 13. b) heat wheels a) waste heat recovery boiler b) heat pum p c) heat wheels d) econom izer From a reciprocating engine exhausts (turbo charged), the tem perature of waste gases lies in the range of b) 60 0 C 16. 19. 20 . b) heat wheel c) heat pipe d) regenerator a) preheating the boiler feed water b) preheating the stock c) preheating the com bustion air d) preheating fuel A recuperator counter flow type for preheating air receives flue gases at 816 oC and exits at 371 oC. The air enters at 37.8 oC and is preheated to 260 oC. The LMTD is : b) 40 4 c) 435 d) 224 For every 6 oC rise in feed water tem perature through an econom iser the fuel savings in the boiler is of the order of a) 1% 18. d) none of the above Econom izer is provided to utilize the flue gas heat for _ _ _ a) 60 4 17. c) 80 0 -90 0 C Capillary wick is a part of a) heat pum p 15. d) recuperator Recovery of heat from dryer exhaust air is a typical application of: a) 230 -450 C 14. c) air preheater b) 1.5% c) 3% d) 2% Recovery of waste heat from hot fluid to fluid is called: a) therm o com pressor b) waste heat recovery boiler c) heat Pum p d) econom izer Therm o-com pressor is com m only used for a) com pressing hot air b) flash steam recovery c) distillation d) reverse com pression of CO 2 Pick up the odd one out: 2.8 Waste Heat - revised (table format) 59 Question bank for Energy Managers & Energy Auditors a) regenerator b) recuperator c) m etallic recuperator d) econom iser Part-II: Sh o rt qu e s tio n s an d an s w e rs 1. List som e of the direct and indirect benefits of waste heat recovery system s. Direct Benefits 1. Efficiency of process is increased 2. Reduction in process cost. Indirect Benefits: a. Reduction in pollution b. Reduction in equipm ent sizes c. 2. Reduction in auxiliary energy consum ption What are the m ajor points to be considered for developm ents of WHRS? Understanding the process is essential for developm ent of Waste Heat Recovery system . This can be accom plished by reviewing the process flow sheets, layout diagram s, piping isom etrics, electrical and instrum entation cable ducting etc. Detail review of the following docum ents will help in identifying: a) Sources and uses of waste heat b) Upset conditions occurring in the plant due to heat recovery c) Availability of space d) Any other constraint, such as dew point occurring in an equipm ents etc. After identifying source of waste heat and the possible use of it, the next step is to select suitable heat recovery system and equipm ents to recover and utilize the sam e. 3. Mention any three com m ercial waste heat recovery devices. 1. Recuperator 2. Econom izers 3. WHRSG 4. Heat pum ps 4. What is the principle of recuperators ? The recuperator is a waste heat recovery device or a heat exchanger between waste gases and the air to be pre-heated. Heat exchange takes place between the flue gases and the air through m etallic or ceram ic walls. 2.8 Waste Heat - revised (table format) 60 Question bank for Energy Managers & Energy Auditors 5. What is the advantage of ceram ic recuperators over m etallic recuperators ? The ceram ic recuperators can allow operation on gas side up to 130 0 oC , whereas m etallic recuperators can with stand up to 10 0 0 oC only. 6. What is the principle of regenerators ? The regenerator is a waste heat recovery device used to recover heat from waste gases to preheat the com bustion air. Regenerator consists of m ultiples of slightly separated m etal plates supported in a fram e attached to a slowly m oving rotor shaft, which is arranged edge on to the gas and air flow. As these plates pass progressively through the gas stream , they give up heat to the air before re-entering the hot stream , thus m aintaining the regenerative cycle. 7. Describe briefly about heat wheels ? A heat wheel is a sizable porous disk, fabricated with m aterial having a fairly high heat capacity, which rotates between two side-by-side ducts: one a cold gas duct, the other a hot gas duct. The axis of the disk is located parallel to, and on the partition between, the two ducts. As the disk slowly rotates, sensible heat (m oisture that contains latent heat) is transferred to the disk by the hot air and, as the disk rotates, from the disk to the cold air. The overall efficiency of sensible heat transfer for this kind of regenerator can be as high as 85 percent. 8. List som e of the m ajor applications of a heat wheel ? The m ain area of application of heat wheel is where heat exchange between large m asses of air having sm all tem perature differences is required. Heating and ventilation system s and recovery of heat from dryer exhaust air are typical applications. 9. What is a heat pipe ? A heat pipe can transfer up to 10 0 tim es m ore therm al energy than copper, the best known conductor. In other words, heat pipe is a therm al energy absorbing and transferring system and have no m oving parts and require m inim um m aintenance. 10 . List at least five applications of heat pipe ? Process to Space Heating Process to Process HVAC Applications m ake up air. Preheating of boiler com bustion air Recovery of Waste heat from furnaces Reheating of fresh air for hot air driers Recovery of waste heat from catalytic deodorizing equipm ent Reuse of Furnace waste heat as heat source for other oven 2.8 Waste Heat - revised (table format) 61 Question bank for Energy Managers & Energy Auditors 11. Cooling of closed room s with outside air Preheating of boiler feed water with waste heat recovery from flue gases in the heat pipe econom izers. Drying, curing and baking ovens Waste steam reclam ation Brick kilns (secondary recovery) Reverboratory furnaces (secondary recovery) Heating, ventilating and air-conditioning system s Explain with a neat sketch the function of an econom izer? Econom izer is provided to utilize the flue gas heat for pre-heating the boiler feed water. A schem atic diagram of the econom izer is shown in Figure. For every 22 0 C reduction in flue gas tem perature by passing through an econom izer or a pre-heater, there is 1% saving of fuel in the boiler. 12. What is the advantage of plate heat exchanger over shell and tube heat exchanger ? The heat recovery efficiency is higher for plate heat exchanger when com pared with shell and tube heat exchanger. 13. What is the principle of run around coil exchanger? The heat from hot fluid is transferred to the colder fluid via an interm ediate fluid known as the Heat Transfer Fluid. One coil of this closed loop is installed in the hot stream while the other is in the cold stream . Circulation of this fluid is m aintained by m eans of land circulating pum p. 14. When run around coil exchangers are preferred? Run around coils is preferred when the hot land cold fluids are located far away from each other and are not easily accessible. 2.8 Waste Heat - revised (table format) 62 Question bank for Energy Managers & Energy Auditors 15. What is the principle of heat pum p? By nature heat m ust flow spontaneously from a system at high tem perature to one at a lower tem perature. Heat pum p reverses the direction of spontaneous energy flow by the use of a therm odynam ic system . 16. Give three exam ples of low tem perature air to air heat recovery devices? a) Heat wheel b) Heat pipe c) Heat pum p 17. Why therm o com pressors are required? Therm o com pressors are required to reuse very low pressure steam , by com pressing it with very high pressure steam and reuse as a m edium pressure steam . 18. Give two exam ples of usage of heat pipe ? i) Process to Space Heating: The heat pipe heat exchanger transfers the therm al energy from process exhaust for building heating. The preheated air can be blended if required. The requirem ent of additional heating equipm ent to deliver heated m ake up air is drastically reduced or elim inated. ii) Process to Process: The heat pipe heat exchangers recover waste therm al energy from the process exhaust and transfer this energy to the incom ing process air. The incom ing air thus becom e warm and can be used for the sam e process/ other processes and reduces process energy consum ption. 19. Briefly explain the principle of therm o com pression . In m any cases, very low pressure steam is reused as water after condensation for lack of any better option of reuse. In m any cases it becom es feasible to com press this low pressure steam by very high pressure steam and reuse it as a m edium pressure steam . The m ajor energy in steam is in its latent heat value and thus therm o com pressing would give a large im provem ent in waste heat recovery. The therm o com pressor is sim ple equipm ent with a nozzle where HP steam is accelerated into a high velocity fluid. This entrains the LP steam by m om entum transfer and then recom presses in a divergent venturi. It is typically used in evaporators where the boiling steam is recom pressed and used as heating steam . 20 . Write short notes on Direct Contact Heat Exchanger. Low pressure steam m ay also be used to preheat the feed water or som e other fluid where m iscibility is acceptable. This principle is used in Direct Contact Heat Exchanger (DCHE) and finds wide use in a steam generating station. They essentially consist of a num ber of trays m ounted one over the other or packed beds. Steam is supplied below the packing while the cold water is sprayed at the top. The steam is com pletely condensed in the incom ing water thereby heating it. Typical application is in the deaerator of a steam generation station. A figure of DCHE is shown below. 2.8 Waste Heat - revised (table format) 63 Question bank for Energy Managers & Energy Auditors VENT COLD WATER IN L.P. STEAM HOT WATER Direct Contact Condenser Part-III: Lo n g typ e qu e s tio n s an d an s w e rs 1. Explain any three types of recuperators ? a) Metallic radiation recuperator: The sim plest configuration for a recuperator is the m etallic radiation recuperator, which consists of two concentric lengths of m etal tubing. The inner tube carries the hot exhaust gases while the external annulus carries the com bustion air from the atm osphere to the air inlets of the furnace burners. The hot gases are cooled by the incom ing com bustion air which now carries additional energy into the com bustion cham ber. The radiation recuperator gets its nam e 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. b) Convective recuperator: These are all shell and tube type recuperator and are generally m ore com pact and have a higher effectiveness than radiation recuperator, because of the larger heat transfer area m ade possible through the use of m ultiple tubes and m ultiple passes of the gases. The hot gases are carried through a num ber of parallel sm all diam eter tubes, while the incom ing air to be heated enters a shell surrounding the tubes and passes over the hot tubes one or m ore tim es in a direction norm al to their axes. c) Ceram ic Recuperator: The principal lim itation on the heat recovery of m etal recuperator is the reduced life of the liner at inlet 2.8 Waste Heat - revised (table format) 64 Question bank for Energy Managers & Energy Auditors tem peratures exceeding 110 0 oC. In order to overcom e the tem perature lim itations of m etal recuperator, ceram ic tube recuperator have been developed whose m aterials allow operation on the gas side to 1550 oC and on the preheated air side to 815 oC on a m ore or less practical basis. This recuperator has short silicon carbide tubes which can be joined by flexible seals located in the air headers. 2. What are waste heat recovery boilers? Explain the need and benefits? Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from gas turbines, incinerators, etc., pass over a num ber 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 . Because the exhaust gases are usually in the m edium tem perature range and in order to conserve space, a m ore com pact boiler can be produced if the water tubes are finned in order to increase the effective heat transfer area on the gas side. The pressure at which the steam is generated and the rate of steam production depends on the tem perature of waste heat. The pressure of a pure vapor in the presence of its liquid is a function of the tem perature of the liquid from which it is evaporated. If the waste heat in the exhaust gases is insufficient for generating the required am ount of process steam , auxiliary burners which burn fuel in the waste heat boiler or an after-burner in the exhaust gases flue are added. Waste heat boilers are built in capacities from 25 m 3 alm ost 30 ,0 0 0 m 3 / m in. of exhaust gas. Typical applications of waste heat boilers are to recover energy from the exhausts of gas turbines, reciprocating engines, incinerators, and furnaces. 3. Explain the principles of Heat pum p . Heat m ust flow spontaneously downhill , that is from a system at high tem perature to one at a lower tem perature. It is possible to reverse the direction of spontaneous energy flow by the use of a therm odynam ic system known as a heat pum p. This device consists of two heat exchangers, a com pressor and an expansion device. A liquid or a m ixture of liquid and vapor of a pure chem ical species flows through an evaporator, where it absorbs heat at low tem perature and, in doing so, is com pletely vaporized. The low tem perature vapor is com pressed by a com pressor, which requires external work. The work done on the vapor raises its pressure an d tem perature to a level where its energy becom es available for use. The vapor flows through a condenser where it gives up its energy as it condenses to a liquid. The liquid is then expanded through an expansion valve back to the evaporator where the cycle repeats. The heat pum p was developed as a space heating system where low tem perature energy from the am bient air, water, or earth is raised to heating system tem peratures by doing com pression work with an electric m otor-driven com pressor. The heat pum ps have the ability to upgrade heat to a value m ore than twice that of the energy consum ed by the device. The potential for application of heat pum p is growing and num ber of industries have been benefited by recovering low grade waste heat by upgrading it and using it in the m ain process stream . Heat pum p applications are m ost prom ising when both the heating and cooling capabilities can be used in com bination. One such exam ple of this is a plastics factory where chilled water from a heat is used to cool injection-m oulding m achines whilst the heat output from the heat pum p is used to provide factory or office heating. Other exam ples of heat pum p installation include product drying, m aintaining dry atm osphere for storage and drying com pressed air. 4. Explain the principle of operation of heat pipe. Discuss three exam ples of its industrial application. The heat pipe is a therm al energy absorbing and transferring system and has no m oving parts and hence requires m inim um m aintenance. The Heat Pipe com prises of three elem ents a sealed container, a capillary wick structure and a working fluid. The capillary wick structure is integrally fabricated into the interior surface of the container tube and sealed un der vacuum . Therm al energy applied to the external surface of the heat pipe is in equilibrium with its own vapour as the container tube is sealed under vacuum . Therm al energy applied to the external surface of the heat pipe causes the working fluid near 2.8 Waste Heat - revised (table format) 65 Question bank for Energy Managers & Energy Auditors the surface to evaporate instantaneously. Vapour thus form ed absorbs the latent heat of vapourisation and this part of the heat pipe becom es an evaporator region. The vapour then travels to the other end the pipe where the therm al energy is rem oved causing the vapour to condense into liquid again, thereby giving up the latent heat of the condensation. This part of the heat pipe works as the condenser region. The condensed liquid then flows back to the evaporated region. The heat pipe heat recovery system s are capable of operating at 315 oC. with 60 % to 80 % heat recovery capability. Industrial applications of heat pipe: The heat pipes are used in following industrial applications: a. Process to Space Heating: The heat pipe heat exchanger transfers the therm al energy from process exhaust for building heating. The preheated air can be blended if required. The requirem ent of additional heating equipm ent to deliver heated m ake up air is drastically reduced or elim inated. b. Process to Process: The heat pipe heat exchangers recover waste therm al energy from the process exhaust and transfer this energy to the incom ing process air. The incom ing air thus becom e warm and can be used for the sam e process/ other processes and reduces process energy consum ption. c. HVAC Applications: Cooling: Heat pipe heat exchanger precools the building m ake up air in sum m er and thus reduces the total tons of refrigeration, apart from the operational saving of the cooling system . Therm al energy is supply recovered from the cool exhaust and transferred to the hot supply m ake up air. 5. In a process, low pressure and high pressure steam is available. Describe how can this steam be reused in industry? Which equipm ent is used for recovery of this? Explain with a neat sketch the principle of operation of such system ? In m any cases, very low pressure steam is reused as water after condensation for lack of any better option of reuse. In m any cases it becom es feasible to com press this low pressure steam by very high pressure steam and reuse it as a m edium pressure steam . The m ajor energy in steam is its latent heat value and thus therm o com pressing would give a large im provem ent in waste heat recovery. The therm o com pressor is a sim ple equipm ent with a nozzle where HP steam is accelerated into a high velocity fluid. This entrains the LP steam by m om entum transfer and then recom presses in a divergent venture. A figure of therm o com pressor is shown below. DISCHARGE STEAM M.P. MOTIVE STEAM H.P. SUCTION STEAM L.P. Thermocompressor 2.8 Waste Heat - revised (table format) 66

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