Swenson Technology,
Inc.
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| THERMAL RECOMPRESSION |
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reduce energy consumption, water vapor from an evaporator
is entrained and compressed with high pressure steam in a
thermocompressor so it can be condensed in the evaporator
heat exchanger. The resultant pressure is
intermediate to that of the motive steam and the water
vapor. A thermocompressor is similar to a steam-jet
air ejector used to maintain vacuum in an evaporator. Only a portion of the vapor from an evaporator can be compressed in a thermocompressor with the remainder condensed in the next-effect heat exchanger or a condenser. A thermocompressor is normally used on a single-effect evaporator or on the first effect of a double- or triple-effect evaporator to reduce energy consumption. As with mechanical recompression, thermal recompression is more applicable to low boiling-point rise liquids and low to moderate Delta-T's in the heat exchanger to minimize the compression ratio. |
To
illustrate the energy effectiveness of a
thermocompressor, compare the steam usage for a
double-effect evaporator with that of the double-effect
evaporator with a thermocompressor (as shown in Fig. 15). Motive steam at 85
psig is utilized to compress first-effect vapor from
19" Hg to 32" Hg absolute. The
forward-feed, double-effect evaporator is used to
concentrate a weak sodium sulfate solution. A
rising-film evaporator is used for the first effect and a
forced-circulation evaporator is required for the second
effect where sodium sulfate crystals precipitate.
The double-effect evaporator with thermal recompression requires 33% less steam than the conventional double effect. In essence, the steam usage for the double effect with thermal recompression is comparable to that of a triple-effect evaporator. The main advantage of thermal recompression is improved steam economy for a moderate capital expenditure - less than that for an additional effect. |
Fig. 15. Swenson Double-Effect Thermal Recompression Evaporator
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Copyright 2009 Swenson Technology, Inc. |
