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Figure 20. Steam heating in an R4 Paraflow (water flow rate 16,000 lb/h, inlet water temperature 216°F, inlet steam temperature 250°F, total number of plates 7).

Figure 20. Steam heating in an R4 Paraflow (water flow rate 16,000 lb/h, inlet water temperature 216°F, inlet steam temperature 250°F, total number of plates 7).

This example, however, well illustrates the application of a plate heat exchanger to condensing duties. If sufficient pressure loss is available, then the plate type unit is a good condenser. The overall coefficient of 770 Btu/h • ft2 • °F for 4-5-psi pressure loss is much higher than a coefficient of 450-500 Btu/h • ft2 • °F, which could be expected in a tubular exchanger for this type of duty. However, the tubular design would, for shell-side condensation, be less dependent on available pressure loss and for a 1-psi drop, a 450500 Btu overall coefficient still could be obtained. With the plate, the calculated coefficient at this pressure is 746 Btu but the effective coefficient based on total area is only 60% of that figure or 445 Btu/h • ft2 • °F.

Gas Cooling

Plate heat exchangers also are used for gas cooling with units in service for cooling moist air, hydrogen, and chlorine. The problems are similar to those of steam heating since the gas velocity changes along the length of the plate due either to condensation or pressure fluctuations. Designs usually are restricted by pressure drop, so machines with low pressure drop plates are recommended. A typical allowable pressure loss would be 0.5 psi with rather low gas velocities giving overall heat transfer coefficients in the region of 50 Btu/h • ft2 • °F.

Evaporation

The plate heat exchanger also can be used for evaporation of highly viscous fluids when as a Paravap the evaporation occurs in the plate or as a Paraflash the liquid flashes after leaving the plate. Applications generally have been restricted to the soap and food industries. The advantage of these units is their ability to concentrate viscous fluids of up to 5000 cP.

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