816Classifying Waste Heat Quality

For convenience, the total range of waste-heat temperatures, 80 to 3000°F, is broken down into three

Figure 8.1 Matching waste-heat sources and loads.

Low range

Waste heat in the high-temperature range is not only the highest quality but is the most useful, and costs less per unit to transfer than lower-quality heat. However, the equipment needed in the highest part of the range requires special engineering and special materials and thus requires a higher level of investment. All of the applications listed in Table 8.1 result from direct-fired processes. The waste heat in the high range is available to do work through the utilization of steam turbines or gas turbines and thus is a good source of energy for cogeneration plants.*

Table 8.2 gives the temperatures of waste gases primarily from direct-fired process equipment in the medium-temperature range. This is still in the temperature range in which work may be economically extracted using gas turbines in the range 15 to 30 psig or steam turbines at almost any desired pressure. It is an economic range for direct substitution of process heat since requirements for equipment are reduced from those in the high-temperature range.

The use of waste heat in the low-temperature range is more problematic. It is ordinarily not practical to extract work directly from the waste-heat source in this temperature range. Practical applications are generally for preheating liquids or gases. At the higher temperatures in this range air preheaters or economizers can be

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