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The U.S. industrial sector annually consumes about 3 x 1013 MJ of primary energy for heat and power.' It has been estimated that 50% of this energy is released to the environment as waste heat. This heat may be classified according to temperature: high-temperature range, above 650'C ; medium-temperature range, between 650 and 230'C ; and low-temperature range, below 230'C.2 In the low range, a large part of the heat rejected is at temperatures below 80'C ; fact, in the energy rejected in the form of condensate, cooling water, and process water (at temperatures between 40 and 80'C) amounts to 2.85 x 1012 MJ/year, or roughly 3% of the U.S. annual energy consumption.

Although the thermodynamic availability of this reject heat is low, its large amount justifies studying means of recovering it for useful purposes. It may be used directly in some applications for space heating or for preheating process streams. In many instances, the temperature of the waste heat may not be high enough for its intended use, and it is necessary to upgrade its temperature. Several types of heat pumps may be used for this purpose.