Bibliography

Adapted from:

J. D. Usher and A. Cooper, "Paraflow Seminar, Part One," CPI Digest 2, 2-7 (1973). Copyrighted APV Crepaco, Inc. Used with permission.

APV Crepaco, Inc.

Lake Mills, Wisconsin

HEAT EXCHANGERS: PLATE VERSUS TUBULAR SELECTION

In forming a comparison between plate and tubular heat exchangers, there are a number of guidelines that will generally assist in the selection of the optimum exchanger for any application. In summary, these are

1. For liquid—liquid duties, the plate heat exchanger usually has a higher overall heat-transfer coefficient and often the required pressure loss will be no higher.

2. The effective mean temperature difference will usually be higher with the plate heat exchanger.

3. Although the tube is the best shape of flow conduit for withstanding pressure, it is entirely the wrong shape for optimum heat-transfer performance since it has the smallest surface area per unit of cross sectional flow area.

4. Because of the restrictions in the flow area of the ports on plate units, it is usually difficult, unless a moderate pressure loss is available, to produce economic designs when it is necessary to handle large quantities of low density fluids such as vapors and gases.

5. A plate heat exchanger will usually occupy considerably less floor space than a tubular for the same duty.

6. From a mechanical viewpoint, the plate passage is not the optimum and gasketed plate units are not made for operating pressures much in excess of 300 psig.

7. For most materials of construction, sheet metal for plates is less expensive per unit area than tube of the same thickness.

8. When materials other than mild steel are required, the plate will usually be more economical than the tube for the application.

9. When mild steel construction is acceptable and when a close temperature approach is not required, the tubular heat exchanger will often be the most economic solution since the plate heat exchanger is rarely made in mild steel.

10. Plate heat exchangers are limited by the necessity that the gasket be elastomeric. Even compressed asbestos fiber gaskets contain about 6% rubber. The maximum operating temperature therefore is usually limited to 500°F.

Heat-Transfer Coefficients

Higher overall heat-transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer because of the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well designed tubular unit are partially bypassed by liquid and low heat-transfer areas thus are created.

Bypassing in a plate-type exchanger is less of a problem, and more use is made of the flow separation that occurs over the plate troughs since the reattachment point on the plate gives rise to an area of very high heat transfer.

For most duties, the fluids have to make fewer passes across the plates than would be required through tubes or in passes across the shell. Since a plate unit can carry out the duty with one pass for both fluids in many cases, the reduction in the number of required passes means less pressure lost due to entrance and exit losses and consequently, more effective use of the pressure.

Mean Temperature Difference

A further advantage of the plate heat exchanger is that the effective mean temperature difference is usually higher than with the tubular unit. Since the tubular is always a mixture of cross and contraflow in multipass arrangements, substantial correction factors have to be applied to the log mean temperature difference. In the plate heat exchanger where both fluids take the same number of passes through the unit the LMTD correction factor is usually in excess of 0.95. As is illustrated in Figure 1, this factor is particularly important when a close or relatively close temperature approach is required.

In practice, it is probable that the sea water flow rate would have been increased to reduce the number of shells in series if a tubular had to be designed for this duty. While this would reduce the cost of the tubular unit, it would result in increased operating costs.

Design Case Studies

Figure 2 covering a number of case studies on plate versus tubular design demonstrates the remarkable heat-transfer performance that can be obtained from Paraflow units. Even for low to moderate available pressure loss, the plate heat exchanger usually will be smaller than a corresponding tubular.

Because of the high heat-transfer rates, the controlling resistance usually is fouling, so an allowance of 20-50% extra surface has been made based on APV experience.

One limitation of the plate heat exchanger is that it is rarely made in mild steel; the most inexpensive material of construction is stainless. Therefore, even when the plate surface requirement is much lower than the tubular on some heat-transfer duties, the tubular will be less expensive when mild steel construction is acceptable. If a close temperature approach is required, however, the plate unit always will cost less and where stainless steel or more exotic materials are required for process reasons, the plate unit usually will cost less.

Physical Size

One important advantage of the plate over a tubular unit is that for a particular duty, the plate heat exchanger will be physically smaller and require far less floor space. This

Tubular

Plate

Size

Overall coefficient MTD Pressure loss

3 shells in series: 20-ft tubes, 1 shell-side pass, 4 tube passes, 2238 tubes, 12,100-ft2 area 270 Btu/h • ft2 • °F

RIO Paraflow with 313 plates arranged for 3 process passes and 3 service passes, 3630 ft2 of area

Figure 1. Duty: demineralized water/seawater. To cool 864,000 lb/h of water from 107°F to 82.4°F using 773,000 lb/h of 72°F sea water.

Tubular design Plate design pressure loss pressure loss

Tubular design Plate design pressure loss pressure loss

(Ib/h)

(°F)

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