Air in the falling film evaporator product feed

Posted on 1st November 2013

in Falling Film Evaporators

by Jan de Geest

GLM Hydro Evaporator Problem Solving: – Throughout the execution of many falling film evaporation projects I often run into issues that are not directly evaporator related but have a high impact on the performance of the evaporator. One of them is the introduction of ambient air into the evaporator product feed. At one of our customer’s production facility, one of the falling film evaporators was not performing to the design and was very inconsistent in the performance. Obviously, the customer was not happy at that point.

The Product Entry

After studying the problem we found out that it was not a problem within the evaporator system or it’s design, but a product condition existing before the product feed hit the evaporator system. The customer had a large silo with a mid-level product entry. When the inlet-nozzle was submerged by the level, everything worked well. Once the level in the silo dropped below the inlet-nozzle, the product splashed into the tank level which caused a non-desirable high level of in-condensable gasses in the evaporator feed. These gasses release from the product in the first effect boiling, which caused the unpredictable behavior of the falling film evaporator.

Simple Solution

The solution in this particular falling film evaporator was a simple elbow on the tank inlet. This way the product deflects onto the silo wall avoiding the product to mix in air while dropping into the level. Another solution might have been keeping the level high when feeding product into the silo. We could even have considered relocating the inlet nozzle to a lower point of entry in the tank.

Avoiding air mixing into the product in an early stage of the production is a much easier and cheaper solution to the problem than to try to get the air out once the product enters the falling film- or forced circulation evaporation system. When your product has a lot of an in-condensable gas in your product feed that cannot be avoided, you could begin with a flash tank for degassing. This is often presented as the solution for degassing, although this means an extra investment, which is not always necessary.

Contact us for more information when your installation has these or other kind of issues.

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Tags: air, evaporator, falling film, product feed