The efficiency of a heat recovery system, based on natural circulation, is determined by assuring satisfactory boiling conditions across the section of each heat exchange modules There are four main aspects for optimization:

By definition, this is the mass water flow in circulation divided by the steam flow, which can be understood as the number of times the water flows around the evaporator section to be fully converted into steam, considering steady latent heat at constant pressure and temperature High circulation ratio corresponds to low steam fraction and vice versa Low volumes of steam fraction by volume prevent tubes from overheating

Another aspect of significant importance is to reach adequate wet wall flow to maintain nucleate boiling under any given condition. Water velocity optimization will enhance heat transfer by increasing turbulences while preventing from steam blanketing, overheating and deposition of entrained solids.

The reliability of any of the previous design aspects is directly linked to the criteria for selecting an appropriate chemical treatment program.
Chemistry related problems will affect the performance of the plant by altering the water velocity, the hydraulic stability and ultimately the heat transfer The chemistry conditioning depends on the quality of the makeup water, on the chemical treatment program selected for the cycle, on the performance of the drum internals and it is also linked to the blowdown system design The image represents similar conditions to those of water flow through a boiler tube at different heat inputs.

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