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Centrifugal compressor

Centrifugal compressor

Figure 29. Multiple-effect MVR, centrifugal compressor with finisher.

Figure 29. Multiple-effect MVR, centrifugal compressor with finisher.

As the required AT across a film evaporator decreased, it became possible to provide that AT by using a fan. This produces a compression ratio on the order of 1.2 to 1, providing approximately 7 or 8°F AT (before losses) for evaporation. The MVR system previously discussed and shown in Figure 30 is typical of fan use. It should be noted that the fan MVR technique can be used only on single-effect film evaporators handling product without significant boiling point elevation. Where the heat-transfer coefficient of the product is low, it sometimes is better to make use of the higher AT available from a centrifugal compressor.

Where fans can be used, however, the horsepower requirement usually is less than in centrifugal compressor designs. Furthermore, fans do not require that the inlet vapor be superheated. Instead, this is done either by a separate heat exchanger using steam or by recycling some of the compressor discharge vapor to the compressor suction.

It is, incidentally, a common practice to use a fan evaporator to concentrate a product up to a point where a large AT is required and then to switch to a small steam or MVR finishing evaporator for final concentration.

Other Designs

Rotary blowers are low-capacity positive compressors that occasionally are used with small MVR evaporators of up to approximately 15,000-lb/h capacity. This generally is a finishing-type evaporator.

In some cases, two compressors will be arranged in series as is shown in Figure 30. Here, the bulk of the evaporation is done in a single effect MVR system having a centrifugal compressor. Some of the compressor discharge vapor then is further compressed in a blower in order to operate a finishing evaporator.

Estimating Compressor Power Requirements

To calculate estimated power requirements for an MVR compressor power and AT values for both centrifugal compressors and fans have been plotted in Figures 31 and 32, respectively. These values compare reasonably well with installed MVR systems.

Table 5, meanwhile, compares the power requirements for different MVR designs. Note that it is possible to calculate an equivalent steam economy by converting horsepower to Btu/h and multiplying by 2545. This value then is divided by the latent heat to arrive at the equivalent steam used.

Thermodynamics

To comprehend the potential of MVR evaporation fully, it is important that the thermodynamics of this technique be

Rotary compressor

Rotary compressor

Figure 30. Single-effect MVR evaporator with centrifugal compressor and rotary-blower-driven finisher.
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