Swenson - Mechanical Vapor Recompression Evaporators

Based upon a 75% isentropic (adiabatic, reversible) compressor efficiency and a combined electric drive motor and gear reducer efficiency of 92%, the energy required to compress a single pound of vapor from 14.1 to 22.8 psia is only 49.3 BTU. To produce the equivalent steam from one pound of 234 oF evaporator condensate requires 999 BTU. The energy savings possible through the use of a recompression evaporator are signficant. In some cases multiple-effect evaporators have been converted to MVR to take advantage of changes in the relative cost of electric power and steam.

The compression ratio required is comprised of three components which are:

• The boiling-point rise, i.e., the temperature of the boiling liquor minus the temperature of boiling water at the same pressure.
• The Delta-T required for heat transfer.
• The pressure drop in the vapor pipe to and from the compressor.
• Mechanical recompression works best in a system with low boiling point elevation and where larger heat transfer surfaces can be used to lower the differential temperatures in the heat exchanger.

Vapor bodies use the Swenson vertical-inlet baffle design, which has proven to be effective in minimizing short circuiting. Vapor from the body is compressed with either a single-stage centrifugal compressor, a multiple stage fan or a turbine and condensed in the heat exchanger. Condensate is sprayed into the vapor discharged from the compressor to control super-heat. Depending on the application a small amount of make-up steam may be required.

Most submerged-inlet evaporators short circuit. That is, some of the heated liquor which enters the vapor body short circuits to the outlet instead of rising to the boiling surface. The boiling temperature of the liquor is increased above the equilibrium value (denoted as degrees of short circuiting), which decreases the, Delta-T available for heat transfer. It is particularly important to minimize short circuiting in recompression evaporators because this reduces the available temperature difference to drive the system. Overcoming this requires a higher compression ratio which increases power consumption.

Mechanical recompression is not limited to single-effect evaporation. It is sometimes economical to compress vapor from the last effect of a double- or triple-effect evaporator so that the vapor can be condensed in the first-effect heat exchanger.