Mechanical Vapor Recompression (MVR) Evaporator Sizing Calculator

Conventional multi-effect evaporators condense water vapor using external boiler steam, discarding latent heat in each effect. In contrast, an MVR evaporator acts as an open-loop heat pump: overhead vapor generated in the boiling chamber is compressed by an electrically driven centrifugal turbo-fan or Roots blower by 4 to 12 deg C in saturation temperature. Because the latent heat of water vapor is massive (~2,300 kJ/kg), compressing 1 kg of vapor requires only 60 to 120 kJ of electrical compressor shaft work. The recompressed hot vapor is returned directly to the shell side of the calandria, where it condenses completely, transferring 100% of its latent heat back into the boiling liquid. This produces extraordinary Coefficient of Performance (COP) values of 25 to 50, slashing energy consumption by up to 90% compared to traditional steam-driven evaporation.

Boiling Point Elevation (BPE) is the increase in the boiling temperature of a solution caused by dissolved non-volatile solutes (salts, sugars, or caustics) according to Raoult law. The effective temperature driving force across the evaporator calandria is $\Delta T_{eff} = \Delta T_{lift} - \text{BPE}$. If a concentrated salt brine exhibits a BPE of 7 deg C and the compressor delivers a saturation lift of 10 deg C, the net thermal driving force is reduced to only 3 deg C, requiring huge heat exchanger surface areas. When BPE exceeds 12 to 15 deg C, the pressure ratio and compression work required escalate to the point where multi-effect thermal evaporation or TVR becomes more economically viable than MVR.

During vapor compression, isentropic inefficiencies and compression work heat the vapor above its new saturation temperature, creating 20 to 50 deg C of superheat. Superheated vapor is an exceptionally poor heat transfer medium; it behaves as an insulating dry gas until cooled to its dew point. To prevent calandria heat transfer rates from collapsing, a precisely metered spray of demineralized condensate (de-superheating quench water) is injected into the compressor discharge duct. The quench water evaporates instantly, cooling the vapor to exact saturation while augmenting total mass flow returning to the calandria.

State-of-the-art MVR falling film evaporators achieve specific energy consumption (SEC) rates of 15 to 28 kWh per metric tonne of evaporated water (equivalent to 54 to 100 kJ/kg). In contrast, a single-effect steam evaporator consumes ~700 kWh equivalent of thermal heat per tonne, a 3-effect evaporator consumes ~240 kWh/t, and a 6-effect evaporator consumes ~125 kWh/t. MVR delivers the lowest operating cost of any industrial evaporation technology wherever electricity is reasonably priced.

Single-stage and multi-stage high-speed centrifugal turbo-fans (operating with titanium or duplex stainless impellers at 8,000 to 22,000 RPM via high-frequency variable speed drives or integrated gearboxes) handle large volumetric vapor flows (5,000 to 150,000 m3/h) with isentropic efficiencies of 78% to 84% and temperature lifts of 6 to 9 deg C per stage. Positive-displacement Roots blowers operate at lower rotational speeds (1,000 to 3,000 RPM), handle lower volumetric flows (<15,000 m3/h), achieve higher single-stage pressure ratios (up to 15 deg C lift), but operate with lower isentropic efficiencies (65% to 72%) and higher mechanical noise.