Evaporation under vacuum||why we need vaccum evaporation||Vacuum operation in evaporators||*TechnicalSkillsInDairySector*

Vacuum operation in evaporators:

            With a number of heat sensitive liquids it is necessary to work at low temperatures, and this is effected by boiling under a vacuum, as indeed is the case in the last unit of a multiple effect system. Operation under a vacuum increases the temperature difference between the steam and boiling liquid and therefore tends to increase the heat flux. 

At the same time, the reduced boiling point usually results in a more viscous material and a lower film heat transfer coefficient.

Examples:For a standard evaporator using steam at 13.5 kN/m2 and 380 K with a total heat content of 2685 kJ/kg, evaporating a liquor such as water, the capacity under vacuum is (101.3/13.5) = 7.5 times great than that at atmospheric pressure. 

 

Under vacuum conditions:

Boiling point reduce and achieved at lower temperature.
Driving force temp.difference is higher.
Heat loss in condensate is lower.
Heat sensitive items are not damage.

why need vacuum evaporation:

1)Boiling point of solution rises

– Due to increase in solid content

ΔT between steam and product decreases(driving force)

– Due to steam temperature remaining constant and

product temperature (boiling temperature) increasing

• Rate of heat transfer decreases

– Due to ΔT between steam and product decreasing.

2) Type of liquor :

Those which,at given pressure,boils at about the same temperature as water and those which has higher boiling point.

3) EVAPORATION OF HEAT SENSITIVE PRODUCTS:

lIn evaporators ,heat sensitive products may deteriorate and lead to general lowering of product quality.

l Foods, which are mostly volatile flavor ingredients, retain more of these ingredients if you prefer short contact times with hot surfaces when evaporated under vacuum.