Downdraft Gas Burners
Titan Industrial Heating Systems has pioneered the down draft gas burner system for heating:
The tube heating efficiencies decrease as build-up occurs, slowing the heating process and ends consuming more in energy.
This system allows you to have the heating source independent of the tank. When the tube builds up, as it does in phosphates and waste water applications: simply disconnect the power and shut off the gas, remove the vent tube and the burner and pull the fire tube out.
You replace it with spare tube you had made at the same time: while the crusted one is being cleaned.
For waste water evaporation more than 1 tube can be in a tank. The typical rate is 15 G.P.H per burner.
Your rate may be less due to humidity and other factors.