Evaporators
Mechanical Vapor Recompression (MVR) Evaporators
Mechanical vapor recompression (MVR) evaporators use electricity or a gas turbine to drive a compressor that recycles the heat in the evaporator. This energy-efficient, cost-saving system has made MVR evaporators increasingly popular as energy costs continue to rise.
Benefits of Mechanical Vapor Recompression Evaporators
- Significant energy and cost savings
- Ideal for low boiling-point rise liquids
How Mechanical Vapor Recompression Evaporators Work
The principle behind mechanical vapor recompression (MVR) evaporators is simple. Vapor from an evaporator is compressed (with a fan or compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger.
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.
Design Considerations for Mechanical Vapor Recompression Evaporators
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.