The advantages of waste heat recovery are it can reduce facility’s annual fuel bills, reduce plant emissions and improve productivity.In process heating, using waste heat will displace a portion of the fuel or electricity that would otherwise be purchased.
As a result, the specific steam consumption of turbine will also come down and consequently the overall efficiency of the whole system is improved.
Chapter 1: INTRODUCTIONWaste heat is heat which is generated in a process by fuel combustion or chemical reaction, and then “dumped” into the environment even though it could still be reused for some useful and economic purpose.
Waste heat recovery is always a good idea when: The temperature of the waste heat is hotter than the input requirements of the process.
The fuel savings achieved are greater than the capital and operational costs of the waste heat recovery equipment.
This can be accomplished by reviewing the process flow sheets, layout diagrams, piping isometrics, electrical and instrumentation cable ducting etc.
Detail review of these documents will help in identifying:a) Sources and uses of waste heatb) Upset conditions occurring in the plant due to heat recoveryc) Availability of spaced) Any other constraint, such as dew point occurring in an equipments etc.Obviously, the higher the temperature, the higher the quality or value of the waste steam.According to a recent Department of Energy (DOE) report, with stack temperatures of 1,000o F (538o C), the heat carried away is likely to be the single biggest loss in the process.After identifying source of waste heat and the possible use of it, the next step is to select suitable heat recovery system and equipments to recover and utilize the same.(Reay, 1979)The basic technique of the waste heat recovery is to capture the waste heat steams and, utilizing a heat exchanger, transfer that heat to another medium to put back into the process.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.The value of the waste heat steam is determined primarily by its temperature.It is widely held that any waste heat stream (air or liquid) of at least 500o F (260o C) is a viable source for recovery.Above 1,800o F (982o C), stack losses will consume at least 50 percent of the total fuel input to the process.Equipment used for waste heat recovery: The variety of equipment available for waste heat recovery includes recuperators, regenerators and waste heat and exhaust gas boilers/steam generators.