815Matching Loads to Source

It is necessary that the heating load which will absorb the waste heat be available at the same time as the waste heat. Otherwise, the waste heat may be useless, regardless of its quantity and quality. Some examples of synchrony and non-synchrony of waste-heat sources and loads are illustrated in Figure 8.1. Each of the graphs in that figure shows the size and time availability of a waste-heat source and a potential load. In Figure 8.1a the size of the source, indicated by the solid line, is an exhaust stream from an oven operating at 425°F during the second production shift only. One possible load is a water heater for supplying a washing and rinsing line at 135°F. As can be seen by the dashed line, this load is available only during the first shift. The respective quantities and qualities seem to fit satisfactorily, but the time availability of the source could not be worse. If the valuable source is to be used, it will be necessary to (1) reschedule either of the operations to bring them into time correspondence, (2) generate the hot water during the second shift and store it until needed at the beginning of the first shift the next day, or (3) find another heat load which has an overall better fit than the one shown.

In Figure 8.1b we see a waste-heat source (solid line) consisting of the condenser cooling water of an air-conditioning plant which is poorly matched with its load (dashed line)—the ventilating air preheater for the building. The discrepancy in availability is not diurnal as before, but seasonal.

In Figure 8.1c we see an almost perfect fit for source and load, but the total availability over a 24 hour period is small. The good fit occurs because the source, the hot exhaust gases from a heat-treat furnace, is used to preheat combustion air for the furnace burner. However, the total time of availability over a 24-hour period is so small as to cast doubt on the ability to pay off the capital costs of this project.

0 0

Post a comment