Forced Circulation Evaporator notes

 Forced Circulation Evaporator

Forced circulation evaporator is used to avoid either crystallisation or boiling of the product on the heating surface due to fouling characteristics of the product. In this evaporator, the flow velocity in the tubes should be high for which high capacity pumps are required.

Principle

In forced circulation evaporator, the liquid requirs additional mechanical agitation for its movement, i.e., liquid is circulated through the tubes using high pressure pumps. As a result, liquid does not boil because boiling point is high sudden fall in pressure, observed when the liquid leaves the tubes and enters the vapour head, leads to flashing of super-heated liquor. This affects the evaporation.

Construction

Forced circulation evaporator consist of a pump for increasing pressure and circulation. The evaporator also has steam jacketed tubes held between two tube sheets. The internal diameter of the tube is 0.1m and length is 2.5m. The parts of the tubes project in to the vapour head, consisting of a deflector and connected to a return pipe which runes downward and enters the pump inlet.

working

Steam is introduced through the inlet into the calandria (heating tubes). Feed is introduced through the provided inlet and is forced by the pumps to the tubes with a positive velocity. While moving upwards through the tubes the liquid heats up and boils. the vapour-liquid mixture rushes out of the tubes at a high velocity and strikes the deflector which throws the liquid downward. This steps is also necessary for an effective separation of liquid and vapour. The separated vapour enters the cyclone separator and escapes through the outlet. The liquid moves down the return pipe to the pump for further evaporation. This continues till the desired concentrated products is obtained which is collected.

Use

Forced circulation evaporator has the following uses:                                                                1) It is used for concentrating heat sensitive materials.                                                              2) It is used for insulin and liver extracts.                                                                                  3) It is suitable for corrosive solutions.                                                                                      4) It is suitable for crystallising operations where crystals are to be suspended at all the times.

Merits

Forced circulation evaporator has the following merits:

1) It has a high heat transfer coefficient due to rapid liquid movement.                                    2) Salting, scaling, and fouling are not possible due to forced liquid circulation.                    3) It is suitable for heat sensitive materials because of rapid evapoartion.                                4) It is also used for liquids with high viscosity, as pumping mechanism is used for circulating the liquid.

Demerits

Forced circulation evaporator has the following demerits:

1) It has a high residence time for the liquids.                                                                            2) It is expensive because of the pump required for liquid circulation.

Economy of multiple effect evaporators

The quantities of vapour produced per unit steam introduced is the economy of a multiple effect evaporator. Feed is introduced into the evaporator at a temperature equal to its boiling point. Thus no additional heating source is required for increasing the temperature of the feed. The steam supplied condense and gives heat of condensation which gets transferred to the liquid. This heat now serves as the latent heat of vaporisation, i.e., liquid after receiving heat. Negligible loss of heat occurs here.

The economy of multiple effect evaporator depends on numerous factors:

1) Feed temperature,                                                                                                                    2) Temperature range in the evaporator,                                                                                    3) Ratio of weight of feed to the product, and                                                                          4) Pressure difference.