825 Constructing Daily Waste Heat Source and Load Diagrams

The normal daily operating schedule for the furnace analyzed in Section 8.2.4 is plotted in Figure 8.11. It is shown that the present schedule shows furnace operation over two shifts daily, 6 days/week. It is necessary to identify one or more potential loads for each of the two sources.

If high-temperature exhaust stack streams from direct-fired furnaces are recuperated to heat the combustion air stream, then the load and source diagrams are identical. This kind of perfect fit enhances the economics of waste-heat recovery. In order to use the cooling-water stream it will be necessary to find a potential load. Because of the temperature and the quantity of enthalpy flux available, the best fit may be the domestic hot-water load. The hot water is used for wash facilities for the labor force at break times and at the end of each shift. The daily load diagram for the domestic hot-water system is shown in Figure 8.12. Time coincidence for the load and source is for two 10-minute periods prior to lunch and for two 30-minute wash-up periods at the end of the shifts. Because the time coincidence between source and load is for only 1-1/2 hours in each 16 hours, waste-heat recovery will require heat storage.

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