826 Conceptual Design of the Waste HeatRecovery System

Prior to equipment design and before a detailed economic analyses is performed, it is necessary to develop one or more conceptual designs which can serve as a model for the future engineering work. This approach is illustrated by the analyses done in Sections 8.2.4 and 8.2.5 for the two waste-heat streams. An excellent reference text which is useful for the design of waste-heat recovery systems is Hodge's Analysis and Design of Energy Systems.14

Stack-Gas Stream

Clearly, recuperation is the most promising candidate for heat recovery from high-temperature exhaust gas streams. In the application pictured in Figure 8.13 the hot exhaust gases will be cooled by the incoming combustion air. Because of the temperature of the gases leaving the furnace, the heat exchanger to be selected is a radiation recuperator. This is a concentric tube heat exchanger which replaces the present stack. The incoming combustion air is needed to cool the base of the recuperator and thus parallel flow occurs. Figure 8.13 includes a sketch of the temperature profiles for the two streams. It is seen that in the parallel flow ex changer, heat recovery ceases when the two streams approach a common exit temperature. For a well-insulated recuperator the conservation of energy is expressed by the equation

Qs stack gas (h stack gas, in h stack gas, out-

Both the right- and left-hand terms represent the heat-recovery rate as well as the decrease in fuel energy required. If the burners or associated equipment have maximum temperature limitations, those temperatures

Figure 8.11 Furnace operating schedule.
Figure 8.12 Daily domestic water load.
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