CONDENSED AND DRIED MILKS

Module 4. Manufacturing techniques

Lesson 5
BASICS OF EVAPORATORS

5.1 Introduction

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

b. Viscosity

c. Density

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:

5.2.1 Falling film evaporators

Over the years the falling film evaporator has practically replaced the forced recirculation evaporator. This type of evaporator is desirable from a product point of view, as it offers a short holding time. Further, the amount of product in the evaporator is reduced and the surface from which the evaporation takes place is increased. shows a diagram of a falling film evaporator.

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 . The steam will condense and flow downwards on the outer surface of the tube. A number of tubes are built together side by side. At each end the tubes are fixed to tube plates, and finally the tube bundle is enclosed by a jacket, see Fig. 5.3. The steam is introduced through the jacket. The space between the tubes is thus forming the heating section. The inner side of the tubes is called the boiling section. Together they form the so-called calandria. The concentrated liquid and the vapour leave the calandria at the bottom part, from where the main proportion of the concentrated liquid is discharged. The remaining part enters the subsequent separator tangentially together with the vapour. The separated concentrate is discharged (usually by means of the same pump as for the major part of the concentrate from the calandria), and the vapour leaves the separator from the top. The heating steam, which condenses on the outer surface of the tubes, is collected as condensate at the bottom part of the heating section, from where it is discharged by means of a pump.

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.

From Fig. 5.5 we can see that 1 kg of steam can evaporate 2 kg of water and by applying a third effect 3 kg of water is evaporated using only 1 kg of steam.

5.3 Separators

Separators with Tangential Vapour Inlet: As the vapours generated from the evaporation are used as heating media in the "next" calandria, any product must be separated, since it would otherwise contaminate the condensate and further represent a loss. The majority of the concentrate is discharged from the bottom of the calandria below the tube bundle. Due to the high vapour velocity some of the concentrate will be carried along with the vapour as small droplets. The separation is done in a separator with tangential vapour inlet, see Figure 5.6 connected to the calandria below the tube.

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.