Paramount Limited
 
 
 

 
Detail

 

In falling film evaporators the liquid product (A) usually enters the evaporator at the head (1) of the evaporator. In the head, the product is evenly distributed into the heating tubes. A thin film enters the heating tube are it flows downwards at boiling temperature and is partially evaporated. In most cases steam (D) is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product (C) form its vapor (B) is undergoing in the lower part of the heat exchanger (3) and the separator (5).

http://www.niroinc.com/images/spacer.gif

A: Product
B: Vapor
C: Concentrate
D: Heating Steam
E: Condensate
1: Head
2: Calandria
3: Calandria, Lower part
4: Mixing Channel
5: Vapor Separator

 

Falling film evaporators can be operated with very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These characteristics make the falling film evaporator particularly suitable for heat-sensitive products, and it is today the most frequently used type of evaporator.

Falling film evaporators are used extensively in chemical process industry, food and paper industry. Due to the absence of static head effect caused by liquid column as in other types of evaporators, evaporation can take place at very small effective mean temperature differences. The temperature difference is typically between 3 - 8C. This is significantly less than in other devices used for evaporation, e.g. forced reboilers or kettle evaporators, here the effective mean temperature difference is between 15 a 30C. The film heat transfer coefficients are in general high and characterised by surface boiling.

The absence of hydrostatic head allows this type of evaporator to operate at very low absolute pressures. 

Product residence time can be very short, especially in one through the operation. This characteristic of short retention time low operation pressure and small required effective mean temperature differences makes this type of evaporator particularly suitable for  concentration of heat sensitive liquids. The absence of nucleate boiling under normal operation conditions and low temperature differences reduces also possible fouling tendencies.

The flow pattern can be characterised as a liquid annulus with vapour core.

Conclusion:

In general very high heat transfer coefficients, which can be considerably lower in systems with viscous mixtures.
Very short liquid residence time in tubes, therefore, low liquid hold-up and inventory
Small effective mean temperature differences, where evaporation takes place at the liquid surface. In general, it is recommended to avoid nucleate boiling (fouling)
Pressure drop in tubes is often negligible, therefore, only in vacuum applications the calculated pressure drop has to be taken into account
Falling Film Evaporators can be operated at very low absolute pressures, they can be used at absolute pressures approaching the hydrostatic head of film thickness

 

Typical applications :

Distillery stillage
Yeast effluent
Grain wet milling
Brine concentration
Abattoir effluents
Gelatinous wastes
Black liquor
Textile industry
Wool washing effluents
Oil emulsions treatment
Pickling bath recovery
Effluent from chemical reactors and storage tank cleaning
Ammonium nitrate polluted condensates from reactors
White waters (waters containing paraffin)

 

Our Offices Downloads Site Map Contact Us
www.paramountlimited.com © 2015