Natural circulation and forced circulation evaporators
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In natural circulation and forced circulation evaporators, is there any relationship that must exist between the evaporation rate and the circulation rate? Some data indicate that the circulation rate in a thermal siphon reboiler equals the evaporation rate divided by the gas fraction at the outlet. So what is this gas fraction? What value is appropriate for the gas fraction in forced circulation, one that facilitates enhanced heat transfer without causing scaling in the main pipes?
There is no gas inside the forced circulation pipe! So there is no such thing as a main pipe!
Given a constant evaporation rate, the volume of circulating water is related to the temperature rise after the water is heated. The extent of this temperature rise depends on the choice of pump; for a smaller temperature rise, a pump with high flow rate and low head is selected, while for a larger temperature rise, a pump with high head and slightly lower flow rate is chosen. The head of the pump is needed to overcome the resistance in the pipes as well as the pressure losses caused by the temperature rise, thereby preventing scaling and blockages in the pipes due to boiling of the circulating water.