Tubular Falling Film Evaporator

In tubular falling film evaporators require uniform and adequate wetting of the tubes and so need the complication of juice recirculation and a distribution system feeding equal quantities of juice to every tube.
An emergency water feed to the evaporator is also required, in case the juice supply fails, the absence of adequate feed even for a short time can lead to severe fouling of the tubes. The main problem, with a descending-film evaporator, is the distribution of juice on the upper tube plate. It is necessary that all tubes from this plate, or from one section of the tube plate, should receive the required volume of juice so that the juice is distributed only in a thin layer around the tube, and that no section of the tube remains dry at any moment.
where
Q is the weight of juice in kg/hr,
N is number of tubes, and
D is the diameter of the tube in meters.
The falling film tubular evaporator has to have a minimum flow rate through each tube, and needs a pump-around system to recirculate liquid. This requires a level of liquid held in the sump of the vessel and a control system to ensure that the required liquid flow to the distributor is achieved. The tubular falling film evaporators need a similar system, although the wetting rate required is lower.

Critical operational and product characteristics of the solution to be evaporated have a major effect on the selection of the tubular falling film evaporator type most suited for the application.

● Heat sensitivity
Many foods, pharmaceuticals, chemicals and resins are heat- or temperature-sensitive and require either low heating temperatures or a short residence time exposed to the heat, or both. This can be accomplished by a combination of minimizing the volume of product in the tubular falling film evaporator at anyone time, minimizing the time in the tubular falling film evaporator, and reducing the product's bulk boiling temperature by operating the tubular falling film evaporator at reduced pressures. Reducing the internal operating pressure may also allow operation at lower heating temperatures while still maintaining a reasonable heat-transfer driving force.

● Fouling
Fouling of the heat transfer surfaces is usually caused by solids in the feed, precipitating solids in the concentrate, or degradation of the product. A slow buildup of a film on the heat transfer surfaces will cause a gradual reduction in the overall heat-transfer coefficient. Eventually this will require shutdown of the process and cleaning of the heat transfer surfaces, which results in production downtime and additional maintenance labor.

● Foaming
Product foaming during vaporization is common. It may range from a small amount of unstable foam that breaks easily to a very stable foam that is difficult to break and tends to fill the entire void of the tubular falling film evaporator system. Foaming can often be minimized by special designs for the feed inlet (separation of feed from the vapor stream) and the vapor/liquid separation area(special disengaging designs). Also, reducing the boiling intensity of the liquid on the heat transfer surface (by operating at a lower temperature or at a higher pressure) and reducing the vapor velocity in the tubes may significantly reduce foaming.Where the product purity specifications allow, introduction of an antifoam may solve or greatly reduce the problem.

● Solids
The properties of the concentrate may change as the solids concentration increases. Solids may plug tubes, causing loss of heat transfer surface, in turn resulting in reduced heat-transfer rates and requiring downtime for cleaning. Solids increase the tendency to foul the heating surface, which reduces the heat-transfer coefficient and boil-up rate. An increase in solids may also increase the concentrate viscosity, which affects the overall heat-transfer coefficient, reducing capacity.

● Viscosity
Any increase in the viscosity of the concentrate will reduce the overall heat-transfer coefficient.

● Distillate-to-concentrate ratio
There must be enough liquid passing through the tubular falling film evaporator to keep the heat-ed walls wetted. Lack of wall wetting and fluid velocity may cause serious fouling and salting of solids on the heat transfer surfaces, causing reduced heat transfer and possible product quality degradation as a result of hot spots on the heating surface. In processes where high distillate-to-concentrate ratios are required, recycling of some amount of the concentrate may be required.

● Distillate vapor velocity (pressure drop and entrainment)
Consideration must be given to the vapor velocity in the tubular falling film evaporator tubes and heating jackets. Adequate velocities are needed to produce sufficient heat-transfer coefficients with-out exceeding limitations on pressure drop, erosion and entrainment. Careful attention must be paid to the requirements of the vapor/liquid separator for separation efficiency and pressure drop.

● Heat transfer medium
The heat transfer medium (hot oil or steam) may impact the selection of the type of tubular falling film evaporator. Liquid-heated tubular falling film evaporators typically have lower overall heat-transfer coefficients and require more heat transfer area. If the product is temperature-stable, then hot oil heating may allow higher temperatures and overcome the lower heat-transfer coefficient. This, in some cases, could allow the use of a smaller tubular falling film evaporator.

● Required materials of construction (reactivity)
A major consideration in tubular falling film evaporator selection may be the required materials of construction. The heat-transfer surface material is extremely important, because it not only affects the overall material cost, but also determines the thermal conductivity of the material, which impacts the overall heat-transfer coefficient and the required surface area.

Zhangjiagang Changshou Industrial Equipment Manufacturing Co., Ltd
The company has a registered capital of RMB 80 million and a production base area of ​​35,000 square meters., and a high-tech and well-trained team of more than 260 employees, including 80 engineering and technical personnel (5 senior engineers and 50 professionals with junior and intermediate titles) and more than 100 certified welders. These employees have the extensive experience in the manufacturing and installation of pressure vessels and the on-site manufacturing of large equipment. In addition to the high-quality production supporting equipment, the company has the advanced and perfect inspection and testing equipment, pressure leakage test equipment, physical and chemical equipment, welding laboratory and so on.