Cd

Process condensate

Process condensate

Centrifugal compressor

Condenser Excess vapor

Process condensate

Centrifugal compressor

Product

Product

rator is 17.5°F before losses are considered. Table 4 shows the total temperature differences for a number of conditions.

Centrifugal Compressors

The relatively high volume of vapor encountered in evaporators requires the use of centrifugal compressors in most cases. Initially, centrifugal compressors on steam were limited to a compression ratio of 1.4, which, in turn, limited the available temperature difference to between 13 and 17.5°F depending on boiling temperature. Only film evaporators could use MVR since there was not sufficient temperature difference to consider forced-circulation designs. Furthermore, products having significant boiling point elevation were excluded for this technique.

Two trends developed in recent years, however, have increased MVR capabilities and applications.

First improved design allows centrifugal compressors to run at higher speeds, increasing compression ratios approximately to 2.0 to 1. Consequently, a (AT of 27-36°F now can be obtained and thus, MVR can be used with forced-circulation systems and for the evaporation of products with boiling-point elevations.

Secondly, advances in evaporator design allow the operation of film evaporators with a lower (AT), thereby minimizing energy requirements. For some products, film evaporators can be calculated with a (AT (excluding losses) in the range of 6°F.

By increasing the AT available and decreasing the AT required, it also became possible to design multiple—effect MVR evaporators. Figure 29 illustrates a double-effect MVR system with a forced-circulation finisher operating across all effects. With this arrangement, the flow to the compressor is reduced and the finisher, operating at the higher concentration, takes advantage of the full AT available.

Fans

The next development in MVR design was an apparent reversal of prior advances.

Table 4. Saturated AT(°F) at Various Compression Ratios

Boiling temperatures

Table 4. Saturated AT(°F) at Various Compression Ratios

Boiling temperatures

Compression ratio

130°F

170°F

212°F

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