Mechanical vapor recompression is a heat-recovery process in which vapor is compressed so it can be reused at a higher temperature.
Evaporation is widely used to concentrate milk, whey, fruit juice, sugar solutions, starch products, plant extracts and other liquid foods. In a conventional evaporator, vapor generated from the product may be condensed or reused in another evaporation effect. MVR instead captures the secondary vapor and mechanically increases its pressure.
Raising the vapor pressure also raises its saturation temperature. The recompressed vapor can therefore return to the evaporator’s heating side and transfer its latent heat back to the product. The system operates as an open-cycle heat pump, recovering energy that would otherwise be rejected in a condenser.
Where the vapor flow, compression ratio and required temperature lift are suitable, a specially designed roots-type vapor compressor may be used. This is not an ordinary ambient-air blower transferred unchanged into steam service. An MVR Roots compressor must be engineered for:
- Wet or near-saturated vapor
- High inlet volume flow
- Corrosion resistance
- Controlled temperature rise
- Appropriate rotor clearances
- Condensate management
- Reliable shaft sealing
- Liquid-droplet separation
- Process-compatible materials
- Stable operation over the specified pressure ratio
Entrained liquid droplets can cause erosion, thermal shock, rotor damage and severe mechanical loading. Efficient separation must therefore be provided upstream of the compressor. Drainage, start-up procedures and instrumentation must prevent accumulated condensate from entering the compression chamber.
The achievable temperature lift must be matched to the evaporator’s heat-transfer area and approach temperature. An excessively high pressure ratio increases power demand and discharge temperature, while an inadequate temperature lift reduces heat-transfer capability. MVR compressor selection must consequently be integrated with the complete evaporator design.
Large MVR installations may use high-speed centrifugal vapor compressors because of their very high flow capacity. roots-type vapor compressors are applied where their displacement characteristics, pressure ratio and capacity range suit the process. Neither technology should be selected solely on equipment price.
Information Required for Technical Review
Before a final model is selected, the supplier should receive the normal, minimum and maximum operating cases; the required flow reference condition; predicted pressure-loss breakdown; expected contamination; maintenance access; electrical supply; hazardous-area classification if applicable; and any redundancy requirement. Vacuum, vapor, hazardous-gas, gas-tight or multi-bar duties require separate written confirmation and may call for another machine class.
System Integration and Protection
A reliable package also depends on the equipment around the blower. The installation should include an appropriate inlet filter, flexible connectors, non-return and isolation valves, a correctly rated pressure-relief device, discharge silencing and instrumentation for pressure and temperature. Variable-speed control can match capacity to demand, but it cannot authorize operation below minimum speed or above motor, temperature and pressure limits. Start-up logic should establish an open flow path before loading the machine, while shutdown logic should prevent reverse rotation and uncontrolled process backflow.
Engineering Selection Framework
Equipment selection should begin with the complete duty rather than a nominal flow figure. The enquiry should state inlet pressure and temperature, required actual flow, maximum differential pressure, gas or air composition, operating schedule, site altitude, ambient range and control philosophy. Pipeline, diffuser, filter, valve and process losses must be evaluated together. A positive-displacement roots blower will attempt to deliver displacement as resistance changes, so an unanticipated restriction normally appears as greater absorbed power and discharge temperature rather than a harmless reduction in flow.
Equipment Scope and Selection Limits
MVR service normally needs a purpose-designed vapor compressor selected for vapor composition, condensate exposure, pressure ratio and temperature rise; ambient-air performance alone does not establish suitability.
Process Conditions to Confirm
A useful MVR specification states vapor composition, saturation condition, flow reference, pressure ratio, condensate load, required temperature lift and acceptable materials before compressor sizing begins.
Glossary
Secondary vapor: Vapor produced by evaporation and recovered for reuse as heating vapor.
Saturation temperature: Temperature at which vapor condenses at a stated absolute pressure.
Temperature lift: Increase in vapor temperature required to reuse it as process heat.
MVR compression ratio: Absolute discharge pressure divided by absolute compressor inlet pressure.
Droplet carryover: Liquid entering the vapor compressor with the process stream.
Heat-transfer approach: Temperature difference available to transfer heat across the evaporator surface.
Condensation pressure: Pressure at which compressed secondary vapor condenses and transfers heat.
Compressor discharge superheat: Outlet temperature above the saturation temperature at discharge pressure.