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The evaporation process is designed to concentrate food products by boiling out unwanted water. The goal is for the food processor to recover the concentrated product with the desired level of solid content, and with the desired product quality.
In determining what type of evaporator you will need, several questions need to be answered.
- Is the evaporator best suited for the process that is required?
- Is the evaporator designed for the most efficient and economical use?
- What are the space limitations at the proposed process facility?
- What are the capital limitations of the operator?
- How many hours per day is the evaporator going to be used?
Single Effect Evaporator
Multi Effect Evaporator
In order to increase stability and also to improve efficiency in storage and transportation, it is desirable to concentrate Aloe. Final products that are currently marketed in the liquid form range from 5 times concentrated (5X) to 40X. However since Aloe is sensitive to heat, the concentration must be carried out at the lowest possible temperature. There are numerous designs of evaporators that work under high vacuum to facilitate water evaporation at low temperature. It is also important here to maximize heat transfer efficiency to lower the operational cost of the unit, therefore several evaporation chambers are placed in series and the hot evaporated gases from the first go to heat the second and so forth. This is called a multiple effect evaporator. The more the effects, the more efficient the operation. Typical operating temperature is about 140F (60C).
Rising Film Evaporator
The rising film evaporator was one of the first continuous operation type evaporators that was used in the food industry. This system uses a vertical tube bundle with steam condensing on its outside surface. Liquid on the inside of the tube is brought to a boil with the vapor generated forming a core in the center of the tube. As the fluid moves up the tube, more vapor is formed resulting in a higher central core velocity that forces the remaining liquid to the tube wall. Higher vapor velocities, in turn, result in thinner and more rapidly moving liquid film. This provides higher heat transfer coefficients and shorter product residence time.
The rising film evaporator has vastly improved product quality over previous types of evaporators. In addition, higher heat transfer coefficients have resulted in reduced heat transfer area requirements, thus resulting in a lower initial capital investment.
Falling Film Evaporator
Falling film product distribution generally is based around use of a perforated plate positioned above the top tube bundle of the calandria. The main problem in this design is figuring how to design for the even distribution of liquid to each of the tubes. Once designed, spreading of liquid to each tube is oftentimes further enhanced by generating flash vapor at this point. The falling film evaporator does have the advantage that the film is 'going with gravity' instead of against it. This results in a thinner, faster moving film and gives rise to an even shorter product contact time and a further improvement in the value of heat transfer coefficient.
One advantage that falling film evaporators have over rising film evaporators is in efficiency. The rising film evaporator requires a driving film force, and requires a temperature difference of at least 25 degree F across the heating surface. In contrast, the falling film evaporator does not have a driving force limitation-permitting a greater number of evaporator effects to be used within the same overall operating limits. However, using the falling film technique, it is feasible to have 6 or more effects.
Rising/Falling Film Evaporator
The rising/falling film evaporator is a combination of the rising and the falling film evaporator. It has ease of liquid distribution of the rising film unit and requires lower head room requirements of either the rising or falling film evaporator. This accomplished for two reasons. First, the tube bundle is approximately half the height of either a rising or falling film evaporator, and secondly the vapor separator is positioned at the bottom of the calandria.