7TH SEMESTER ,MECHANICAL ENGINEERING BRANCH WASTE HEAT RECOVERY DEBASHREE SENGUPTA 7TH SEMESTER ,MECHANICAL ENGINEERING BRANCH 0501227172

TOPICS OF DISCUSSION 1.INTRODUCTION 2.TYPE OF WASTE HEAT RECOVERY 3.WASTE HEAT RECOVERY DEVICES 4.ASSESMENT OF WASTE HEAT RECOVERY 5.CONCLUSION

INTRODUCTION WASTE HEAT Waste heat is heat, which is generated in a process by way of fuel combustion or chemical reaction, and then “dumped” into the environment even though it could still be reused for some useful and economic purpose. The essential quality of heat is not the amount but rather its “value”. The strategy of how to recover this heat depends in part on the temperature of the waste heat gases and the economics involved. Large quantity of hot flue gases is generated from Boilers, Kilns, Ovens and Furnaces. If some of this waste heat could be recovered, a considerable amount of primary fuel could be saved. The energy lost in waste gases cannot be fully recovered. However, much of the heat could be recovered and thus loss is minimized

WASTE HEAT SOURCE AND QUALITY S. No Source of Waste Heat Quality of Waste Heat 1 Heat in flue gases The higher the temperature, the greater the potential value for heat recovery 2 Heat in vapour streams As above but when condensed, latent heat also recoverable 3 Convective & radiant heat lost from exterior of equipment Low grade – if collected may be used for space heating or air preheats 4 Heat losses in cooling water Low grade – useful gains if heat is exchanged with incoming fresh water 5 Heat losses in providing chilled water or in the disposal of chilled water High grade if it can be utilized to reduce demand for refrigeration Low grade if refrigeration unit used as a form of Heat pump 6 Heat stored in products leaving the process Quality depends upon temperature 7 Heat in gaseous & liquid effluents leaving process Poor if heavily contaminated & thus requiring alloy heat exchanger

HIGH TEMPERATURE HEAT RECOVERY Types of Devices Temperature (0C) Nickel refining furnace 1370 – 1650 Aluminum refining furnace 650 –760 Zinc refining furnace 760 – 1100 Copper refining furnace 760 – 815 Steel heating furnace 925 – 1050 Copper reverberatory furnace 900 – 1100 Open hearth furnace 650 – 700 Cement kiln (Dry process) 620 – 730 Glass melting furnace 1000 – 1550 Hydrogen plants 650 – 1000 Solid waste incinerators Fume incinerators 650 – 1450

METAL REFINERY FURNACE

CEMENT KILN

MEDIUM TEMPERATURE HEAT RECOVERY Types of Devices Temperature (0C) Steam boiler exhaust 230 – 480 Gas turbine exhaust 370 – 540 Reciprocating engine exhaust 315 – 600 Reciprocating engine exhaust (turbo charged) 230 – 370 Heat treatment furnace 425 – 650 Drying & baking ovens 230 – 600 Catalytic crackers Annealing furnace cooling systems

GAS TURBINE PLANT

CATALYTIC CRACKER

STEAM BOILER PLANT

LOW TEMPERATURE HEAT RECOVERY Source Temperature 0C Process steam condensate 55-88 Cooling water from: Furnace doors 32-55 Bearings 32-88 Welding machines Injection molding machines Annealing furnaces 66-230 Forming dies 27-88 Air compressors 27-50 Pumps Internal combustion engines 66-120 Air conditioning and refrigeration condensers 32–43 Liquid still condensers Drying, baking and curing ovens 93-230 Hot processed liquids 32-232 Hot processed solids 93-232

OPEN HEARTH FURNACE

Commercial waste heat recovery Devices- RECUPERATORS-metallic radiation, convective, radiation/ convective hybrid, ceramic REGENARETORS HEAT WHEELS HEAT PIPE ECONOMIZER PLATE HEAT EXCHANGER

RECUPERATOR CERAMIC RECUPERATOR Heat exchange between flue gases and air through metallic/ceramic walls Ducts/tubes carry combustion air for preheating CERAMIC RECUPERATOR Less temperature limitations: Operation on gas side up to 1550 ◦C Operation on preheated air side to 815 ◦C New designs Air preheat temperatures <700◦ C Lower leakage rates Inlet air from atmosphere Outside ducting Tune plate Preheated air Centre tube plate Exhaust gas from process

METALLIC RADIATION RECUPERATORS Simplest recuperator Two metal tubes Less fuel is burned per furnace load Heat transfer mostly by radiation

CONVECTIVE AND HYBRID RECUPERATORS CONVECTIVE RECUPERATORS The hot gases are carried through a number of parallel small diameter tubes, while the incoming air that is 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 HYBRID RECUPERATORS Combinations of radiation & convection More effective heat transfer More expensive but less bulky than simple metallic radiation recuperators

RECUPERATOR

CONVECTIVE RECUPERATOR

REGENERATORS AND HEAT WHEELS Large capacities Glass and steel melting furnaces Heat transfer in old regenerators reduced by Dust & slagging on surfaces heat losses from the walls HEAT WHEELS Porous disk rotating between two side-by-side ducts Low to medium temperature waste heat recovery systems Heat transfer efficiency up to 85 %

HEAT PIPE PERFORMANCE AND ADVANTAGE Transfer up to 100 times more thermal energy than copper Three elements: - sealed container - capillary wick structure - working fluid Works with evaporation and condensation PERFORMANCE AND ADVANTAGE Lightweight and compact No need for mechanical maintenance, input power, cooling water and lubrication systems Lowers the fan horsepower requirement and increases the overall thermal efficiency of the system Can operate at 315 ◦C with 60% to 80% heat recovery Process to space heating- Transfers thermal energy from process exhaust for building heating

PLATE HEAT EXCHANGER Run around coil exchanger Parallel plates forming a thin flow pass Avoids high cost of heat exchange surfaces Corrugated plates to improve heat transfer When directions of hot and cold fluids are opposite, the arrangement is counter current Run around coil exchanger Heat transfer from hot to colder fluid via heat transfer fluid One coil in hot stream One coil in cold stream

PLATE HEAT EXCHANGER

WATER TUBE BOILER Water tube boiler: hot exhaust gases pass over parallel tubes with water Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from gas turbines, incinerators, etc pass over a number of parallel tubes that contain 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.

ECONOMIZER Used when the medium containing SHELL AND TUBE ECONOMIZERS Utilize the flue gas heat for pre-heating the boiler feed water 1% fuel savings if 60 ◦C rise of feed water 200 ◦C rise in combustion air temp SHELL AND TUBE ECONOMIZERS Used when the medium containing waste heat is a liquid or a vapor that heats another liquid Shell contains the tube bundle, and usually internal baffles to direct the fluid Vapor contained within the shell

ASSESMENT OF WASTE HEAT RECOVERY Saving money by recovering heat from hot waste water: Discharge of the waste water is 10000 kg/hr at 75◦C Preheat 10000 kg/hr of cold inlet water of 20◦C A heat recovery factor of 58% An operation of 5000 hours per year The annual heat saving (Q) is: Q = heat content in kCal V = the flow rate of the substance in m3/hr  = density of the flue gas in kg/m3 Cp = the specific heat of the substance in kCal/kg oC T = the temperature difference in oC Cp (Specific heat of flue gas) = 0.24 kCal/kg/oC Q = V x  x Cp x T Q = m x Cp x T x 

HEAT SAVING CALCULATION EXAMPLE m = 10000 kg/hr = 10000 x 5000 kg/yr = 50000000 kg/year Cp = 1 kCal/kg ◦C T = (75 – 20) ◦C = 55 ◦C  = Heat Recovery Factor = 58% or 0.58 CV of Oil = 10,200 kCal/kg Equivalent Oil Savings = 159500000 / 10200 = 156372 L Cost of Oil = 0.35 USD/L Monetary Savings = 54730 USD/Annum  Q = 50000000 x 1 x 55 x 0.58 = 1595000000 kCal/year

CONCLUSION Benefits of Waste Heat Recovery Benefits of ‘waste heat recovery’ can be broadly classified in two categories: Direct Benefits: Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected by reduction in the utility consumption & costs, and process cost. Indirect Benefits: a) Reduction in pollution: A number of toxic combustible wastes such as carbon monoxide gas, sour gas, carbon black off gases, oil sludge, Acrylonitrile and other plastic chemicals etc, releasing to atmosphere if/when burnt in the incinerators serves dual purpose i.e. recovers heat and reduces the environmental pollution levels. b) Reduction in equipment sizes: Waste heat recovery reduces the fuel consumption, which leads to reduction in the flue gas produced. This results in reduction in equipment sizes of all flue gas handling equipments such as fans, stacks, ducts, burners, etc. c) Reduction in auxiliary energy consumption: Reduction in equipment sizes gives additional benefits in the form of reduction in auxiliary energy consumption like electricity for fans, pumps etc..

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