Static Tubular Evaporators
Falling Film Evaporator
Operating discripton: The falling film principle requires an even distribution of the feed liquid. To achieve this distrubution one or more distribution trays are located in the top head of the evaporator over the complete tube sheet. Our proven distribution system makes sure that the inner surface of each individual tube is properly wetted.
The liquid film moves down the tubes by gravity force. Heat applied through the tube wall effects boiling-off the light fraction from the film. The picture shows a co-current operating evaporator with vapour and liquid leaving the bottom of the evaporator into the separator vessel.
Forced Circulation Evaporator
Operating discription: The feed liquid is fed into the circulating stream which is moved by a pump through a heat exchanger and a separator vessel. The heat applied through the tube walls superheats the liquid which flashes into the separator vessel where the vapour separates from the concentrated liquid. The high velocity of the liquid effects good heat transfer values but requires a large circulation pump. Further, the high velocity reduces the tendency for fouling and scaling.
Forced circulation evaporators can handle liquids with solids or salts precipitating them during the concentration step.
Natural Circulation Evaporator
Operating principle: Natural circulation of the liquid is induced only by density difference of liquid / liquid-vapour mixture. The heat applied through the tube walls superheats the liquid which starts boiling and generates a two-phase mixture. This forces the liquid to move from bottom to top of the evaporator tubes - the thermosiphon principle. Hence, no pump is required for liquid circulation.
However, a certain minimum temperature difference between heating steam and tube side liquid is vital to maintain evaporation and circulation.
Typically the tube bundle is of short tube length design.
Rising or Climbing Film Evaporator
Operating principle: The rising film evaporator operates with the density difference of liquid / liquid-vapour mixture inside the tubes. The liquid fed at the bottom section passes through a non-boiling zone before evaporation starts generating the two-phase mixture. From a certain point on, the vapour forces the fluid to rise as a film toward the upper end of the tube.
Long tubes are used for the bundle design but larger temperature differences are necessary for proper operation. The long tubes allow high concentration ratios in a once-through mode with an application for viscous and foaming liquids.
Whitlock evaporators are single stage air swept falling film evaporators with dried hot air blown counter-currently to the liquid flowing down inside the tubes. The dry air improves mass transfer for low moisture content in the product. Whitlock evaporators typically serve in the fertilizer industry for dehydration of:
- Ammonium Nitrate
The resulting melt can be processed directly in a prill tower to obtain dry granulates.
The advantages of the Whitlock Evaporator are:
- large capacities up to 2,000 metric tons per day
- moisture content down to 0.1 % water
- atmospheric operation
- high turn-down ratio
- easy to operate
- low maintenance cost