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Figure 2. Case studies of plate heat exchangers and tubular designs. All tubular designs were carried out with the aid of the HTRI program ST3.

is shown most graphically in Figure 3, where a Series R405 Paraflow is being installed next to tubular units that need twice the amount of space for the identical duty. It is further illustrated in the volumetric comparisons of Figure 4.

If the tightened plate pack of a Paraflow is regarded as a rectangular box, each cubic foot contains 50-100 ft2 of heat-transfer area according to the type of plate used. Allowing for metal thickness, the contained liquid is some 80% of this volume. Thus, the total of both heating and cooling media is about 5 gal. Expressed in another way, the liquid hold up per square foot for each stream varies according to plate type from about 0.06 gal down to 0.03 gal.

By comparison, one cubic foot of tubular exchanger of equilateral triangular pitch with a tube pitch-tube diameter ratio of 1.5 has a surface area of 10 ft2 for 2-in.-OD tubes or 40 ft2 for l/2-in.-OD tubes. The average contained liquid is proportionately 0.27 gal/ft2 down to 0.07 gal/ft2 of heat-exchanger area with no allowance for the headers. If

Figure 3. At the Glidden Durkee Baltimore plant, a Series R405 Paraflow is being readied to cool 1000 gpm of deionized water. This unit requires only half the space of a tubular exchanger for the same duty.

the heat-transfer coefficient ratio between plate and tubular is conservatively taken as 2, the plate exchanger volume to meet a given duty varies from 1/10 to 1/3 of that of the tubular. For a lower tube pitch-tube diameter ratio of 1.25, the comparison becomes 1/7 to 1/5. These facts demonstrate why the Paraflow plate heat exchanger is referred to as "compact."

Thermal Limitations

While it would appear offhand that the plate heat exchanger always provides a better performance at usually a lower price than the tubular exchanger, consideration must be given to the thermal as well as mechanical limitations of the plate-type machine. These usually are based on allowable pressure loss.

For single-phase liquid—liquid duties, the plate heat exchanger can be designed for moderately low pressure loss. However, if the pressure loss across any plate passage that has liquid flowing downward is lower than the available liquid static head, the plate will not run full and performance therefore will be reduced. This is termed low plate rate. Use of a plate below the minimum plate rate is inadvisable since it causes a wastage of surface area and results in unreliable operation. It is, however, possible to function below the minimum plate rate in a single-pass arrangement by making sure that the low plate rate is operated with a climbing liquid flow.

These problems are not quite so severe with a tubular exchanger and therefore, operation at a moderately lower available pressure loss is possible.

Conclusion

To summarize, the gasketed plate heat exchanger generally will be the most economical heat exchanger for liquid—liquid duties provided the material of construction is not mild steel and provided the operating temperatures and pressures are below 500°F and 300 psig respectively. For other types of duty such as gas cooling, condensation or boiling, the plate heat exchanger can be a very econom

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