Basic Principles of Evaporators|Dairy Engineering|Technical Superintendent milma|Technical Superintendent Milk Marketing Federation Limited
Evaporation and vapourisation are two processes in which simultaneous heat and mass transfer process occur resulting in the separation of vapour from a solution.
Evaporation and vapourization occur where molecules obtain enough energy to escape as vapour from a solution.
The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to the product.
VAPOURIZATION AND EVAPORATION
Evaporation and vaporization are quite different from each other. The differences are shown in Table
Vapourization | Evaporation |
Vapourization occurs when entire mass of liquid is raised to the boiling point. | It is only a surface phenomenon with only surface molecules escaping at a rate depending upon area of open surface. |
It is a much faster process for the production of vapours | It is a relatively slow process and depends mainly on temperature difference and on difference of vapour pressure between air and liquid. |
Boiling point is related to the pressure above the liquid surface and the amount of solute | Evaporation occurs at normal room temperature and application of heat is not necessary under normal evaporation process |
Vapourization of liquid is visual in the form of vapour bubbles rising and escaping at the surface. | Evaporation is not usually visual and hence not detected easily. |
Vapourization can be controlled by variation in pressure. | Liquid will evaporate until the pressure of its vapour is equal to the equilibrium value. It is the vapour pressure of the liquid for the given temperature of the liquid, for closed system. In case of open system, evaporation will continue till there is no more liquid in the vessel |
Evaporation and vapourization occur where molecules obtain enough energy to escape as vapour from a solution.
The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to product.
In a closed container with air space above the liquid, evaporation will continue until the air is saturated with water molecules.
Removal of water from a liquid product by evaporation is enhanced by adding heat and by removing the saturated air from above the liquid.
This is done by removal of vapour from the space above the liquid surface and thereby creating a vacuum.
The boiling point of solution due to dissolved solutes is higher than that of pure water and depends on the molecular weight of the solute.
The vacuum is utilized to remove water from liquid/solids at lower temperatures to reduce damage to heat sensitive products which might decompose at higher temperatures.
In the dairy industry evaporation means the concentration of liquid milk products containing dissolved, emulsified or suspended constituents.
During this process water is removed by boiling. This process is used in the dairy industry for manufacture of evaporated milk, condensed milk and traditional Indian Dairy products i.e. Kheer, Basundi, Khoa etc.
In milk condensing plant, milk is condensed by evaporating a part of its water content by using saturated steam.
The milk is boiled under vacuum.
As the milk boils, water vapour is formed. This vapour is utilized for heating the milk further in the next stage which is at a higher vacuum.
Modern dairy plants use evaporators to remove part of water from milk by boiling it under low pressure.
The process of evaporation takes place at a maximum temperature of about 70 0C corresponding to an absolute pressure of 230 mm (9.0 inch) of mercury (Hg).
Evaporation of milk under low pressure or vacuum is carried out in a specially designed plant.
The plant design depends much on the characteristics of liquid milk during boiling at low pressure than any other factor.
Some of the important properties of evaporating milk are as under.
I Concentration of solids (initial and final)
ii Foaming under vacuum
iii Heat sensitivity
iv Viscosity change
The engineering design of plant requires certain other factors which provide a suitable milk contact surface, cleaning without frequent dismantling, faster heat transfer and the economy of steam/power used for operating the plant.
Following factors are important for evaporation process.
(i) Concentration: The initial and final concentration of solute in the solution should be considered. As the concentration increases, the boiling point rises.
(ii) Foaming: Few products have tendency to foam, which reduce heat transfer and there is difficulty in controlling level of liquid which ultimately increases product (entrainment) losses.
(iii) Heat sensitivity: Milk, like many other food products, is sensitive to high temperatures. If time of exposure is more, there will be severe damage to milk proteins.
(iv) Scale formation / Fouling: It is a common phenomenon of deposition of solids on the heat exchanger surface.
However, the scale-forming tendency can be very much reduced by maintaining a reasonably low-temperature difference and relatively clean and smooth heat transfer surface.
The flow velocity of the product has also a significant effect. If scale formation starts, the rate of heat transfer decreases and cleaning becoming more difficult.
(v) Materials of construction:
Stainless steel is the most common metal for evaporators in the dairy and food industry. Other metals may be used in chemical evaporators.
The factors like strength, toughness, weldability, non-toxicity, surface finish, cost etc. are important in the selection of the material of construction.
(vi) Specific heat: It changes with the concentration of the solution. More heat is required to be supplied at high specific heat values.
(vii) Gas liberation: Few products liberate gases when heated under boiling pressures.
(viii) Toxicity: The gases liberated in few cases may be toxic and should be handle carefully.
(ix) Viscosity: There is an increase in viscosity of solution during evaporation which increases the time of contact and hence chances of burning or damage the product.
(x) Capacity: It is expressed as the amount of water evaporated per hour. It depends on the surface area of heat transfer, temperature difference and the overall heat transfer co-efficient.
(xi) Economy: It is based on the amount of water evaporated per kg of steam used. It increases with the number of effects