Scenario presets
Switches and actions
Alarms
Numerical checks
The science
What an MSF plant does
Seawater is heated to the top brine temperature (TBT) in the brine heater and then flows through a cascade of chambers kept at falling pressure. In each stage a little of the hot brine flashes to vapour; the vapour condenses on a tube bundle that carries the cooler recycle brine and drips into a distillate tray. The condensing heat pre-heats the recycle, so the same heat is used again in every stage. The vapour fraction produced per stage is x = cp ΔTb / λ.
Stage model
For stage j the condensing vapour at Tv heats a tube stream F with effectiveness ε = 1 - exp(-UA/(F cp)). Balancing brine enthalpy release against tube gain gives Tv = [B cp(Tb,in - δ) + ε F cp Tc,in] / [B cp + ε F cp], heat Q = ε F cp (Tv - Tc,in) and distillate D = Q / λ(Tv). The brine leaves at Tb = Tv + δ where δ is the boiling-point elevation (BPE, grows with salinity and temperature) plus a non-equilibrium allowance of 0.6 K. Brine and tube temperatures relax with a 45 s lag per stage, so changes ripple down the cascade.
Mass, salt and energy
Makeup M = Bd + D; salt balance M Sf = Bd SN so the concentration factor is CF = 1 + D/Bd. Heater duty is Q = R cp (TBT - Tc,out,1) and the gained output ratio is GOR = D / msteam. The ideal many-stage limit of the distillate fraction is 1 - exp(-cp ΔT / λ), which the numerical checks compare against.
Scaling, fouling and gas
Alkaline scale (Mg(OH)2, CaCO3) and CaSO4 deposit faster above about 112 C and at high concentration factor; antiscalant slows it. The model lets tube cleanliness decay as exp((TBT - 100)/6) / (1 + 0.9 dose) and lowers UA accordingly. Non-condensable gas (air, CO2) blankets the condensers and reduces UA by up to 35 %; the vent ejector removes it at a steam and power cost.
Honest simplifications
Equal heat capacity for brine and tube streams, one lumped effectiveness per stage, no flashing-orifice hydraulics, no distillate flash-box recovery, constant overall coefficient (2.6 kW/m2/K clean), perfect demisting and instant salinity mixing in the pool. Pumping power is a simple head x flow estimate. Numbers are textbook-scale, not a design for a real plant.