Evaporators

If we consider the heat balance of a single-effect evaporator we find that the heat content (enthalpy) of the evaporated vapour is approximately equal to the heat input on the heating side. In the common case of water evaporation, about 1 kg/hr of vapour will be produced by 1 kg/hr of live steam, as the specific evaporation heat values on the heating and product sides are about the same. If the amount of vapour produced by primary energy is used as heating steam in a second effect, the energy consumption of the overall system is reduced by about 50 %. This principle can be continued over further effects to save even more energy. The maximum allowable heating temperature of the first effect and the lowest boiling temperature of the final effect form an overall temperature difference which can be divided among the individual effects.Consequently, the temperature difference per effect decreases with an increasing number of effects. For this reason, the heating surfaces of the individual effects must be dimensioned accordingly larger to achieve the required evaporation rate, but with a lower temperature difference. A first approximation shows that the total heating surface of all effects increases proportionally to the number of effects. Consequently, the investment costs rise considerably whereas the amount of energy saved becomes increasingly lower.

In Single Effect Type Evaporator while the product is being forces through the tubes of the evaporator, heat is added to remove a specific amount of moisture. After this is completed, both the product vapour and the concentrated product are forced into the separating chamber where the vapour is drawn off and may be used elsewhere. The concentrate is then pumped off to another part of the process.

The Falling Film Type Evaporator consists of shell and tube heat exchanger called as calandria that is mounted in vertical position. The liquid product enters the evaporator at the head and is evenly distributed to the heating tubes. A thin film enters the heating tube and flows downwards at boiling temperature through the steam in the jacket and is partially evaporated. The vapour is separated from the liquid in vapour separator and the concentrated liquid is collected at the bottom or transferred to the next stage. The falling film evaporators can be single effect or multiple effects depending on the capacity. Falling film evaporators can be operated with small temperature differences between the heating media and the boiling liquid and they also have short product contact time. These characteristics make the falling film evaporator particularly suitable for heat sensitive products and it is the most frequently used evaporator.

Advantages :

• Ideal for clear, heat sensitive foaming and corrosive solutions.
• Continuous, single pass operation with minimized retention time.
• Single or multiple units with high heat economy.
• High heat transfer co-efficient.
• Low power requirement.
• Simple construction.
• Low floor space requirement.

Applications :

• Food
• Herbal extracts
• Pharmaceuticals
• Dairy
• Dyestuff
• Ammonium Nitrate.
• Sugar Syrups.

Rising Film Vacuum Evaporator operate on a "thermo-siphon" principle. Feed enters at the bottom of the heating tube and as it heats, steam begins to form. The ascending force of this steam produces during the boiling causes liquid and vapours to flow upwards in parallel flow. At the same time the production of vapour increases and the product is pressed as thin film on the walls of the tubes and rises upwards. This co-current upward movement against gravity has the beneficial effect creating a high degree of turbulence in the liquid which is advantageous during evaporation of highly viscous products and for the products that have tendency to foul during heating surfaces. These types of evaporators are often used with product recirculation, where some of the formed concentrate is reintroduced back to the feed inlet in order to produce sufficient liquid loading inside the heating tubes.

Advantages :

• Ideal for clear, heat sensitive foaming and corrosive solutions.
• Continuous, single pass operation with minimized retention time.
• Single or multiple units with high heat economy.
• High heat transfer co-efficient.
• Low power requirement.
• Simple construction.
• Low floor space requirement.

Applications :

• Food
• Herbal extracts
• Pharmaceuticals
• Dairy
• Dyestuff
• Ammonium Nitrate.
• Sugar Syrups.

Forced Circulation Evaporator is recommended for the concentration of high viscosities solutions/slurries which have a tendency to crystallize or produce scales. The circulating product is heated when it flows through the heat exchanger and then partially evaporated when the pressure is reduced in separator. The liquid product is typically heated only a few degrees for each pass through the heat exchanger. This type of evaporator is also used in crystallization applications because no evaporation and therefore no concentration increase, takes place in the heat transfer surface. Evaporation occurs as the liquid is flashed evaporated in flash vessel/separator.

Advantage :

• Ideal for high crystallizing and high viscosity solutions & Slurries.
• Single or multiple effect evaporation
• High operational flexibilities, heat transfer co-efficient, vapours liquid separation.
• Low temperature rise operation.
• Minimized fouling due to high velocities & static head.
• High turn down ration.