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Figure 15. Guide to materials of construction. FFPR—fiber-filled phenolic resin: RLCS—rubber-lined carbon steel; NiLCS—nickel-lined carbon steel.

Figure 15. Guide to materials of construction. FFPR—fiber-filled phenolic resin: RLCS—rubber-lined carbon steel; NiLCS—nickel-lined carbon steel.

tion takes place has to be a forced-circulation evaporator and, in most cases, a tubular unit. However, when the problem is not one of crystallization but a general scaling of the heat-transfer surface instead, the Paraflash is much more efficient. As shown in Fig. 16, the circulation rate through the plate is considerably less than through a tube and results in much lower horsepower requirements.

If crystals are formed and are too large for a plate evaporator to handle, the designer has to determine whether the tubular forced circulation unit should be a single- or multiple-pass configuration. Normally, forced-circulation tubulars are designed with tube velocities of 6-8 fit/s or higher to minimize fouling. In the Figure 17 comparison between single- and three-pass configurations, three-pass units are shown to have lower circulation rates directly resulting in higher temperature rises, lower log mean temperature differences, and, therefore, greater surface area. Even though the circulation rate is less, the pressure drop on the three-pass versus one-pass is more because of greater overall tube length. This results in larger absorbed horsepower. It generally is concluded, therefore, that a single-pass arrangement is less expensive from both the capital and operating cost standpoints.

HTCs, Forced Circulation, and Film

Estimation of HTCs from physical properties is easier and more reliable for forced-circulation units than for film evaporators. As a general rule, HTCs used for the latter are determined by experience or test work.

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