Heat exchangers with vertical shells and tubes and a vapor-liquid separator on top are known as rising film evaporators (RFE).
The feed enters the heat exchanger at the bottom and climbs via the tubes. The heating medium provides heat on the shell side. When the feed is heated, the vapor produced pushes the liquid against the wall and raises it upwards as a film. In a vapor-liquid separator, liquid and vapor mixtures are separated at the top.
The high velocity created by vapor lift is responsible for good thermal performance. This upward movement against gravity causes a lot of turbulence in the liquid. This is one of its most significant benefits for viscous compounds that tend to dirty heating surfaces.
Available Designs of Rising-film Evaporator
The essential concepts of the rising film evaporator, thermo-siphon, have been used to build a variety of other evaporator designs. Evaporator designs are often tailored by private enterprises and industries based on the needed use in manufacturing the desired goods. This is necessary to guarantee that the best product is produced while also maximizing the design's efficiency and cost-effectiveness.
Artisan Rising Film Evaporator
Artisan Industries specializes in creating specialized thermal separation equipment. The Artisan Rising Film Evaporator uses the same fundamental principles as a long-tube vertical evaporator, but it's been modified to handle more volatile and viscous compounds that a traditional design would not be able to handle due to significant fouling.
In this regard, the Artisan Rising Film Evaporators are utilized as a flash evaporator or pre-heater to remove the bulk of the volatile components before stripping. This design allows the operator to modify the feed rate or steam rate to remove residues, maximize steam efficiency and respond to product behavior. This evaporator is designed for high-temperature applications and materials with a high viscosity that tend to foul transfer surfaces.
Rising Thin Film Vacuum Evaporator
The Rising Thin Film Vacuum Evaporator is a variation of the original rising film evaporator, with the key distinction being that it allows liquid to evaporate at a lower temperature. This is made feasible by the fact that it acts under vacuum, which prevents unwanted formation in the liquid. This design was intended to allow evaporation to re-concentrate a dilute solution to its desired concentration while also enabling evaporated water to condense and be collected for recirculation or other applications. There are a variety of Rising Thin Film Vacuum Evaporator models available each with its own capacity, condenser design, and concentration control. This design is very small, allows for easy solution concentration management, and may be used with extremely corrosive and effervescent liquids.
Semi Kestner
In the sugar industry, the Semi-Kestner, also known as the Semi-Rising Film Evaporator, is commonly employed. This equipment includes a Polly-baffle catcher to prevent juice entrapment, as well as a juice coil and flushing system for more efficient juice delivery. Because less steam is used, this design has a high risk of syrup and high vapor pressure. The juice flows back without being discharged after just passing over the heating surface once, thanks to the short liquor holding time and efficient heat transmission design features.
Guidelines for Designs being Considered
Choosing an appropriate temperature difference
The temperature difference between the heating media and the boiling liquid, which is the log mean temperature difference, must be large enough to provide sufficient ascending force of the steam vapor in the tube side to cause the liquid film to flow upwards. In general, the bigger the temperature differential, the stronger the steam's pushing power. Furthermore, a large temperature differential increases the flow rate of liquid and vapor within the tube. As the flow rate rises, turbulence rises, increasing the heat transfer coefficient. However, the entire temperature differential must be within the boiling point ranges of the two components, otherwise, the quality and purity of the products may be compromised.
Sizing
The size of a rising film evaporator is often a delicate undertaking since it necessitates a thorough grasp of the process requirements as well as the behavior of the materials involved. Long and thin tubes are often considered to be more cost-effective than thicker shell sizes for shell and tube heat exchangers. Nonetheless, the size may always be altered and tailored according to the needed use in creating the desired items, while comparing the cost of building against the demand. The length usually runs between 4 and 8 meters, with a diameter of 25 to 50 millimeters.
Thermal Economy
The thermal economy is an important consideration when developing a rising tube evaporator. To achieve this, design characteristics with a significant impact on the thermal economy must be addressed. The entire heat transmission area is one of the most important factors. It is normally appropriate to maximize the heat transfer area in order to maximize the thermal economy, as a greater area will result in a higher heat transfer rate. Despite this, increasing the area of heat transfer may cause issues in terms of expanding the varied dimensions of the evaporator, which in turn raises construction costs while also subjecting the evaporator to additional limits such as space and design constraints.
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