In the art of evaporation or distillation, flash evaporators and boiling water evaporators both are well known. Each type of evaporator has its advantages and limitations.
Flash evaporators have to move large quantities of water per pound of vapor created and the heat exchanger design heat transfer rates are in the order of 450-550 Btu/hr/ft.sup.2. The vapor release rates obtainable are about two to two and one-half lbs/hr/in.sup.2 at 165.degree. F. Conventional boiling water evaporators are subjected to the same heat transfer rate range of 450-475 Btu/hr/ft.sup.2 and release rates of the order of one and one-half to two lbs/hr/in.sup.2 are obtainable at 165.degree. F. It is common knowledge that conventional boiling water evaporators have a smaller vapor release rate than flash evaporators. Also, boiling water evaporators are subject to foaming, and additives have to be used in a continuous manner to prevent this. Boiling water evaporators also are subject to scaling and fouling of the evaporating surfaces which results in lower efficiency of operation and increased maintenance and increased down time for the apparatus.
A great deal of research effort, time and money have been spent on boiling water evaporators to develop enhanced heat transfer surfaces which have grooves or the like machines or formed therein in an attempt to double the heat transfer rate obtainable. Thus, even though heat transfer rates of 1500 lbs/hr/in.sup.2 are obtainable, the cost of heat transfer surface is increased from two to three times, and when suitable fouling allowance is made in the process design, the advantage of higher heat transfer rates are offset completely by the increased cost of the heat transfer surface.
Ideally, the approach would be to find a simple, inexpensive means to increase the heat transfer rates obtainable in a boiling water evaporator and at the same time increase the vapor release rates obtainable. If this could be achieved, boiling water evaporators would be in a more competitive position with respect to flash evaporators.