Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapor from an evaporator is compressed (with a positive-displacement, centrifugal or axial-flow compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger. Various combinations are possible, including single-effect recompression, multiple-effect recompression, multiple-stage recompression, and single-effect recompression combined with a multiple-effect evaporator.
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 °F evaporator condensate requires 999 BTU. The energy savings possible through the use of a recompression evaporator are significant. 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:
- 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 (change in temperature) 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 superheat. 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.