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W ASTE H EAT Waste Heat" may be defined as "Heat that is rejected from a process, at a temperature enough above the ambient temperature to permit the plant engineer to recover additional value from it". Waste Heat Energy may broadly be categorised in following three ranges (In Degrees Fahrenheit) I. High Temperature Range Above 1200 II. Medium Temperature Range 450 to 1200 III. Low Temperature Range Below 450 SOURCES OF WASTE HEAT:The combustion of fuel produces flue gases in the high temperature range. The maximum theoretical temperature that we come across usually is 3000°F. Hereunder temperature of waste gases from a few of industrial processes is being given: Glass Melting Furnace 1800°F to 2800°F Steel Heating Furnace 1700°F to 1900°F Cement Kilns (Dry Process) 1250°F to 1350°F Heat Treatment Furnaces 800°F to 1200°F Gas Turbine Flue Gases 700°F to 1000°F Diesel Engine Exhaust 600°F to 1100°F Drying & Baking ovens 450°F to 1200°F Steam Boiler ovens 450°F to 900°F Hot Processed Solids 200°F to 400°F Process Steam Condensate 130°F to 190°F Air Compressors 80°F to 120°F HOW TO USE WASTE HEAT? The most appropriate means of utilising waste heat is by transferring the heat from one fluid stream to another (e.g. from flue gases to feed-water or combustion air), that is why the device that transfers the heat is termed as "Heat Exchanger". In few cases, exchangers are given a particular name to enable them to identify their specific application, such as Economiser, Recuperate, Air-Pre heater etc. Just for reference, high temperature gases from glass melting furnace can be used to generate steam in Waste Heat Boilers. This steam can be used for running a Back Pressure Steam Turbine to generate electricity. Exhaust steam from this can be used for process heating. Medium temperature exhaust gases can be used to preheat the feed or boiler make up water by using Economisers or to preheat the combustion air. Still further, low temperature gases may profitably be used for preheating feed stocks in industrial processes by using finned tubes or "Tube-in-Shell" heat exchangers or may be used for space heating. Waste Heat Page 1 of 3 HEAT RECOVERY SYSTEMS: PRINCIPLES: Economy is a crucial factor in designing any Steam Generator or Heat Recovery System. Like any other functioning entity, such systems also operate under the law of Diminishing Returns. Temperature of flue gases drops while heating the surface of boiler which in turn absorbs the heat. A level soon arrives beyond which it becomes uneconomical to absorb heat through this heating surface at a saturation temperature. This is the limit from where an Economiser or heater must start absorbing the heat. Operation Engineers should never forget the fact that, "there always is most economical temperature for flue gases above & below which steam production costs tend to increase". Any superfluous reduction in flue gas temperature would eventually increase the capital investment and other charges so much that these may offset any possible gain in efficiency. This stresses the necessity of selecting or designing any Heat Recovery Equipment very carefully. Otherwise any such addition may actually reduce the effectiveness of the already available heating surface. For smaller, low pressure boilers, an Economiser is a more economical proposition as compared to Air Heater. For outputs more than 50,000 Lbs steam/hr however an Air Heater becomes a batter choice. In nutshell, the temperature to which flue gas can be cooled while passing through equipment is determined by the following facts: 1. 2. 3. 4. Amount of heat than can be absorbed by such equipment Temperature of incoming feed water or combustion air. Dew point of flue gases. Exist temperature below which any gain is off-set by costly equipment. APPLICATION: The arrangement of heat recovery systems must consider economy of design as well as accessibility for their operation and maintenance. Waste Heat Page 2 of 3 SPECIFICATION OF WASTE HEAT RECOVERY EQUIPMENT: The equipment that is used to recover waste heat can range from something as simple as pipe or duct to something as complex as a Waste Heat Boiler. The authentic Data prepares ground for the design of equipment having optimum effectiveness and low operational & maintenance cost. Temperature of Waste Heat Fluid _____________________°C Flow rate of Waste Heat Fluid __________________ M³/Hr Chemical Composition of Waste Heat Fluid _______________________ Name of Waste Heat Fluid _______________________ Acceptable Pressure Drop of Waste Recovery Stream _______________________ Acceptable Temperature Drop of Waste Recovery Stream_____________________°C Temperature of Fluid to be Heated _____________________°C Flow Rate of Fluid to be Heated __________________ M³/Hr Chemical Composition of Fluid to be Heated _______________________ Name of the Fluid to be Heated _______________________ Acceptable Pressure Drop on Fluid to be Heated side ________________ Kg/Cm² Acceptable Temperature rise of Fluid to be Heated _____________________°C Waste Heat Available _______________Hours/Day _______________Days/Year _____________Months/Year Others _______________________ _______________________ Waste Heat Page 3 of 3