TABLE OF CONTENTS The capital cost of chemical processes tends to be dominated by the number of items on the flowsheet. This is certainly true of heat exchanger networks and there is a strong incentive to reduce the number of matches between hot and cold streams.
Referring back to the flowsheet in Figure 1.4, three exchangers, two heaters and one cooler are used in the design, making six units in all. Is this the minimum number, or could the designer have managed with fewer units? If so, can we find a rapid way of identifying this?
As previously described, the bare flow diagram in Figure 1.5 shows that there are four separate process streams to consider. The target energy performance for this system as calculated by the Problem Table method shows that only heating is required, and no cooling. Straight away then, we know that the cooler is surplus to requirement! Figure 3.17 shows the heat loads on the one hot stream and three cold streams written within circles representing the streams. The predicted hot utility load is shown similarly. Note that the total system is in enthalpy balance (i.e. the total hot plus utility is equal to the total cold). If we assume that temperature constraints will allow any match to be made, then we can match the whole of cold streams 2 and 3 (total 1,405 units) with hot stream 1, leaving a residual heat load of...
