8 1 Introduction

8.1.1 Definitions

Waste heat, in the most general sense, is the energy associated with the waste streams of air, exhaust gases, and/or liquids that leave the boundaries of a plant or building and enter the environment. It is implicit that these streams eventually mix with the atmospheric air or the groundwater and that the energy, in these streams, becomes unavailable as useful energy. The absorption of waste energy by the environment is often termed thermal pollution.

In a more restricted definition, and one that will be used in this chapter, waste heat is that energy which is rejected from a process at a temperature high enough above the ambient temperature to permit the economic recovery of some fraction of that energy for useful purposes.

8.1.2 Benefits

The principal reason for attempting to recover waste heat is economic. All waste heat that is successfully recovered directly substitutes for purchased energy and therefore reduces the consumption of and the cost of that energy. A second potential benefit is realized when waste-heat substitution results in smaller capacity requirements for energy conversion equipment. Thus the use of waste-heat recovery can reduce capital costs in new installations. A good example is when waste heat is recovered from ventilation exhaust air to preheat the outside air entering a building. The waste-heat recovery reduces the requirement for space-heating energy. This permits a reduction in the capacity of the furnaces or boilers used for heating the plant. The initial cost of the heating equipment will be less and the overhead costs will be reduced. Savings in capital expenditures for the primary conversion devices can be great enough to completely offset the cost of the heat-recovery system. Reduction in capital costs cannot be realized in retrofit installations unless the associated primary energy conversion device has reached the end of their useful lives and are due for replacement.

A third benefit may accrue in a very special case. As an example, when an incinerator is installed to decompose solid, liquid, gaseous or vaporous pollutants, the cost of operation may be significantly reduced through waste-heat recovery from the incinerator exhaust gases.

Finally, in every case of waste-heat recovery, a gratuitous benefit is derived: that of reducing thermal pollution of the environment by an amount exactly equal to the energy recovered, at no direct cost to the recoverer.

8.1.3 Potential for Waste-Heat Recovery in Industry

It had been estimated1 that of the total energy consumed by all sectors of the U.S. economy in 1973, that fully 50% was discharged as waste heat to the environment. Some of this waste is unavoidable. The second law of thermodynamics prohibits 100% efficiency in energy conversion except for limiting cases which are practically and economically unachievable. Ross and Williams,2 in reporting the results of their second-law analysis of U.S. energy consumption, estimated that in 1975, economical waste-heat recovery could have saved our country 7% of the energy consumed by industry, or 1.82 x 1016 Btus (1.82 quads.)

Roger Sant3 estimated that in 1978 industrial heat recovery could have resulted in a national fuel savings of 0.3%, or 2.65 x 1016 Btus (2.65 quads). However, his study included only industrial furnace recuperators.* In terms of individual plants in energy-intensive industries, this percentage can be greater by more than an order of magnitude.

The Annual Energy Review 19914 presents data to show that although U.S. manufacturing energy intensity increased by an average of 26.7% during the period 1980 to 1988, the manufacturing sector's energy use efficiency, for all manufacturing, increased by an average of 25.1%. In reviewing the Annual Energy Reviews over the years, it becomes quite clear that during periods of rising fuel

*Recuperators are heat exchangers that recover waste heat from the stacks of furnaces to preheat the combustion air. Section 8.4.2 subjects this device to more detailed scrutiny.

prices energy efficiency increases, while in periods of declining fuel prices energy efficiency gains are eroded. Although the average gain in energy use efficiency, in the 7-year period mentioned above, is indeed impressive, several industrial groups accomplished much less than the average or made no improvements at all during that time. As economic conditions change to favor investments in waste-heat recovery there will be further large gains made in energy use efficiency throughout industry.

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