Sugar processing makes extensive use of evaporators for the concentration of raw juice. Evaporation is an energy-intensive operation that uses steam to heat evaporator vessels, called calandrias. There are a number of factors that can impact the efficiency of this process, one of which is non-condensable gas in the steam.
Problems Arising from Non-condensable Gases in Steam
Even though boiler feedwater is treated prior to steam generation, steam still contains a certain amount of non-condensable gases, such as oxygen and carbon dioxide, which can cause severe problems if allowed to build up in an evaporator. Some of these problems include:
• Corrosion – Carbon dioxide combines with condensate to form carbonic acid, which is highly corrosive. Oxidation of metal components of the evaporator also leads to pitting corrosion.
• System binding – If non-condensables are not vented from the system, they can accumulate to the extent where flow of steam and condensate is blocked, thereby hindering the efficiency of the evaporator.
• Heat transfer reduction – Non-condensable gases act as insulators, forming a film on the tubes of the evaporator and reducing the transfer of heat to the process stream. This forces equipment to work harder to meet heating demand, leading to reduced efficiency and increased power consumption.
Design Considerations for Problem-free Evaporator Operation
Venting of non-condensable gases will help to avoid these problems. Gases can be vented from both the top and bottom of the calandria. To ensure efficient operation, certain considerations should be taken into account when designing vents, including:
• Gases at the top and bottom of the vessel should vent independently through separate pipes and isolation valves. If these gases are vented through a common pipe, there may be preferential venting from one end with consequent under-performance and accumulation of non-condensable gases at the other end.
• At the bottom of the calandria, a certain head of condensate is necessary to maintain flow. Care should be taken to position the non-condensable gas extraction manifold to clear the level of condensate in the vessel.
Evaporators are vital components of sugar processing and their design should not be left to chance. Engaging with a highly experienced equipment manufacturer will help to ensure that they are installed and maintained in a manner that maximizes efficiency.