M

2fG2L gpDe

The pressure loss can be predicted from equations 2 and 3.

Obviously, equation 1 cannot represent accurately the performance of the many different types of plate heat exchangers that are manufactured. However, plates that have higher or lower heat-transfer performance than given in equation 1 usually will give correspondingly higher or lower friction factors in equation 2. Experience indicates that the relationship for pressure loss and heat transfer is reasonably consistent for well-designed plates. In the Appendix, equations A1-A3 are further developed to show that it is possible for a given duty and allowable pressure loss to predict the required surface area. This technique has been used for a number of years to provide an approximate starting point for design purposes and has given answers to ± 20%. For accurate designs, however, it is necessary to consult the manufacturer.

Heat-Recovery Duties

In any heat-recovery application, it always is necessary to consider the savings in the cost of heat against the cost of the heat exchanger and the pumping of fluids. Each case must be treated individually since costs for heat, electricity, pumps, etc will vary from location to location.

One point is obvious. Any increase in heat recovery, and thus heat load, results in a decrease in LMTD and considering a constant heat-transfer coefficient, subsequently in the cost of the exchanger. This effect is tabulated in Figure 5. Because the cost of an exchanger increases considerably for relatively small gains in recovered heat above the 90% level, such applications, even with the plate heat exchanger, must be studied closely to verify economic gain.

The economic break-even point is far lower for a tubular exchanger. Situations where it is advantageous to go above 90% recovery usually involve duties where higher heat recovery reduces subsequent heating or cooling of the process stream. High steam or refrigeration costs therefore justify these higher heat recoveries.

As shown in Figure 5, the cost of increasing heat recovery from 85 to 90% at a constant pressure loss of 12 lb/in.2 is $2600. From a practical standpoint, going from 90 to 95% requires a significantly higher pressure loss and nearly doubles the exchanger cost. However, even with this 95% heat recovery and assuming steam costs at $6.00/ 1000#, payback on the plate heat exchanger would take 530 hs.

The plate heat exchanger thus provides a most eco-Xnomic solution for recovering heat (Fig. 6). This degree of heat recovery cannot be achieved economically in a tubular exchanger since the presence of cross flow and multipass on the tube side causes the LMTD correction factors to become very small or, alternately, requires more than one shell in series. This is shown in the Figure 7 comparison.

As detailed, this example illustrates that the plate heat exchanger has considerable thermal and therefore price advantage over the tubular exchanger for a heat recovery of 70%. Since the overall heat-transfer coefficient and the effective mean temperature difference both are much higher for the plate unit, reduced surface area is needed. Furthermore, because of cross-flow temperature difference problems in the tubular, three shells in series were needed to handle the duty within the surface area quoted. Using only two shells would have resulted in a further 40% increase in surface area.

The small size of the plate heat exchanger also results in a saving of space and a lower liquid holdup. For this type of heat recovery duty, a stainless-steel plate heat exchanger almost always will be less expensive than a mild steel tubular unit. Although the tubular exchanger physically will be capable of withstanding higher temperatures and pressures, there is a considerable and for the most part

12 lb/in.2 Pressure loss 25 lb/in.2 Pressure loss

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