Lesson 5. BASICS OF EVAPORATORS
BASICS OF EVAPORATORS
The vacuum pan or vacuum evaporator is the heart of milk condensery. It is the retort in which the condensing proper is accomplished. It is used in the manufacture of every type of concentrated milks.
5.2 Developments in Evaporators
While unchanged in principle from that of originally used by Gail Borden, the vacuum pan unit has undergone marked improvement. The vacuum pan of today has appreciably increased evaporative capacity due to type and arrangement of heating surface and of condenser; it is capable of greater fuel economy, more efficient use of condenser water, better control of entrainment losses and more effective protection of the milk against heat damage. Systems are also designed to achieve the maximum plant efficiency, with the minimum downtime, and the desired balance between steam and electricity use.
The simplest evaporator is an ordinary open pan heated with steam or direct gas. The evaporation takes place from the surface while the liquid to be evaporated is heated up to the boiling point corresponding to the ambient pressure, which at sea level will be 100°C.
As the evaporation has to take place from the surface, which is limited in relation to the content of the pan, the evaporation will naturally take long time. The milk will be exposed to the high temperature with a deterioration of the proteins, chemical reactions such as the Maillard reaction, or even coagulation as a result.
As the development went on, the concentration is carried out in forced recirculation evaporators. The heating surface is provided either by a steam jacket or a series of steam coils or by milk tubes enclosed in a steam chest or calandria. Evaporating capacity of the plant depends upon:
(1) Type and arrangement of heating surface in condenser.
(2) More efficient use of water for condensing plant.
(3) More effective control of milk against heat damage.
The area, design, and arrangement of the heating surface play an important part in determining the evaporating capacity of the vacuum pan. The latter is directly proportional to
(1) Total area of heating surface,
(2) Temperature difference between steam and milk, and
(3) Overall heat transfer co-efficient which is influenced by
a. Velocity of milk and steam flow
d. Sp. Gravity
e. Sp. heat
f. Thermal conductivity of milk, metal, milk film, steam condensate film and incrustation.
Some systems used to regenerate heat are:
The liquid to be evaporated is evenly distributed on the inner surface of a tube. The liquid will flow downwards forming a thin film, from which the boiling/evaporation will take place because of the heat applied by the steam. See
Since milk, due to the protein content, is a heat-sensitive product, evaporation (i.e. boiling) at 100ºC will result in denaturation of these proteins to such an extent that the final product is considered unfit for consumption. The boiling section is therefore operated under vacuum, which means that the boiling/evaporation takes place at a lower temperature than that corresponding to the normal atmospheric pressure. The vacuum is created by a vacuum pump prior to start-up of the evaporator and is maintained by condensing the vapour by means of cooling water. A vacuum pump or similar is used to evacuate incondensable gases from the milk.
Fig. 5.4 One stage evaporator, definition of various specific quantities and the corresponding condensate
As vapour, see Figure 5.4, from the evaporated milk contains almost all the applied energy, it is obvious to utilize this to evaporate more water by condensing the vapour. This is done by adding another calandria to the evaporator. This new calandria - the second effect - where the boiling temperature is lower, now works as condenser for the vapours from the first effect, and the energy in the vapour is thus utilized as it condenses.
Multiple effects : In the first evaporation unit, steam is introduced and part of the water is evaporated from the milk. The steam condenses, and vapour is separated from the concentrated milk. The vapor is used to evaporate water from the concentrated milk in the second effect, etc. The flows of milk and vapour are cocurrent. The number of effects varies from 3 to 7. The vapour coming from the last effect is condensed in a special condenser; the temperature of the water in the condenser determines the boiling temperature in the last effect. The boiling temperatures in the other effects are determined by the pressure drop of the vapour when being transported to the next effect. The temperature difference ΔT between the condensing vapour, and the boiling liquid will be smaller for a larger number of effects N. A higher N implies greater saving of steam but needs a larger heating surface; hence, a bigger and more expensive plant, more loss of heat by radiation, and higher cleaning costs. Especially in the last effect, the heat-transfer rate may become very small: here, the temperature is lowest, and the concentrate viscosity is very high.
Fig. 5.6 Separator with tangential inlet
5.3.1 Wrap-around separator
To reduce space requirements a new development has taken place with the design of the Wrap-Around Separator. It is integrated into the base of the calandria. It has the same high efficiency as the classical separator with a low pressure drop. It is typically used on big calandrias with MVR compressors connected to the wrap-around separator with a very short vapour duct minimizing the pressure drop.