Falling Film Evaporator
Concentration of highly –sensitive material require a minimum time of exposure to the heated surface. This can be done in falling film evaporators. The tubes are vertical and the product forms a thin film on the inside of the tubes, which are surrounded by steam. The product is preheated to a temperature slightly above the boiling temp. in the evaporator. From the preheater, the product flows to the upper section of the evaporator.
By means of specially shaped nozzle, product is distributed over a spreader plate. The product is slightly superheated and therefore expands as soon as it leaves the nozzle. Part of the water is vaporized immediately.
The steam formed forces the product outwards against the inside of the tubes and it run down the walls of the tubes as thin film. The water content of the film evaporates rapidly as the product passes through the tubes. A steam separator is fitted under the evaporator and separates the vapor from the concentrated product.
The resident time in the falling film evaporator is very short, as only a small amount of the product is treated at the same time.
There is no hydrostatic increase in temperature in a falling film evaporator which means that the value of the saturated vapor pressure corresponds to the vacuum in the tubes. The vapor reaches its maximum velocity at the bottom of the tubes shortly before the discharge end.
Characteristics of multiple effects Falling Film Evaporator
- It can be operated at much lower Δ T (Difference between jacket and boiling temperature).
- The low Δ T, is favourable to vapor recompression and consequently, the energy consumption is low.
- Short holding time.
- Low power consumption.
- Can be operated at with or without TVR, with TVR efficiency is improved by 25%.
- Can be operated at high steam pressure at 9 bar and low steam pressure at 1.5- 3.5 bar on turbine steam exhaust.
- Suitable to concentrate the product up to 30-40% TS.
- CIP is required after every 8-10 days of continuous running of plant.
- After every 8-10 days of running start up of vacuum and steam is required.