Evaporator Design Calculator

Evaporation Rate comes entirely from the mass balance -- Feed Rate and the feed and product concentrations -- because that's what determines how much water physically has to leave to reach the target concentration, independent of how the heat is delivered. Steam Pressure, Boiling Point Elevation, and Overall Heat Transfer Coefficient only affect the second stage of the calculation: sizing the heat transfer equipment needed to actually deliver that fixed evaporation duty.

Required Area equals Heat Duty divided by the product of the Overall Heat Transfer Coefficient (U) and the temperature driving force (steam temperature minus the feed's elevated boiling point) -- the standard Q = U x A x deltaT heat transfer equation solved for area. The steam temperature itself is estimated from steam pressure with a simplified linear approximation valid only near atmospheric pressure, not full steam-table data.

A single-effect evaporator uses roughly 1 kg of steam to evaporate 1 kg of water, since the steam's latent heat is transferred once and then condensed away. Real industrial evaporators often use multiple effects in series, where the vapor produced by one effect is reused as the heating steam for the next effect at a lower pressure -- raising steam economy toward 2-5 kg evaporated per kg of fresh steam. This calculator only models a single effect, so it will not show that improvement.

Dissolved solids raise a solution's boiling point above pure water's -- Boiling Point Elevation is that increase, in degrees, which reduces the effective temperature difference driving heat transfer (since the heating steam's temperature stays fixed by its pressure, but the solution now boils hotter). A higher Boiling Point Elevation shrinks that driving force, which this calculator reflects as a larger Required Heat Transfer Area for the same duty.

The temperature driving force (steam temperature minus the solution's elevated boiling point) is floored at 1°C rather than allowed to go to zero or negative -- a combination of low Steam Pressure and high Boiling Point Elevation can otherwise put the approximated steam temperature below the solution's own boiling point, which is physically inconsistent (the "heating" steam would be colder than what it needs to boil). When that floor kicks in, Required Heat Transfer Area is computed off that artificial 1°C value rather than a real driving force, so a results page showing a very large Required Area alongside a low Steam Pressure is a sign the input combination has hit this floor and should be revisited -- raise Steam Pressure or lower Boiling Point Elevation until the estimated steam temperature is comfortably above the solution's boiling point.