Juice thickening in sugar plants relies heavily on the heat transfer ability of evaporators. Efficient heat transfer is also fundamental to juice heating and sugar boiling, as well as other sugar production-related processes.
In multiple-effect evaporators, cascade-type heating allows for the utilization of multiple evaporation phases. However, this heating process only succeeds if intensive heat transfer is achieved during each phase. It is therefore essential to ensure the proper functioning of each effect in the evaporator station.
In Robert-type evaporators, heating tubes are used. The optimal juice level ranges from 10 – 30% of tube height and heat transfer intensity may only change slightly with small changes to juice level. Nevertheless, marked deviations from these values may cause undesirable consequences on juice thickening.
In falling-film evaporators, tube wetting is a primary concern. Insufficient juice flow can cause tearing of the film, resulting in supersaturation and the formation of incrustation. Typically, liquid wetting can be assessed by measuring the volume of flow per unit length of tube circumference at the outlet.
Factors that Impact Heat Transfer
Over time, scale build-up on tube walls can reduce heat transfer efficiency to the point where juice thickening is negatively impacted. In such instances, maintaining processing performance will require lower concentrations of thick juice and lead to increased heat consumption. It is therefore necessary to observe heat transfer coefficients in the evaporator in order to decide when to remove scale.
Steam condensation irregularities on the tube’s outer surface can also have an influence on heat transfer. By ensuring that condensate levels do not rise above the outlet nozzles via draining and venting of non-condensables, this problem can be prevented. Caution is advised, however, as excessive venting and leaking steam traps can cause vapor losses.
When using a high-quality and well-maintained evaporator, checking differences in temperature between effects, and overall concentration, is enough to assess proper functioning.
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