Types of Evaporation

Condensing is a process of removal of water from milk in the manufacture of concentrated milks. Evaporators are the equipment used for carrying out the process of evaporation. Evaporator is a major equipment used in the dairy factories for condensed milk. There are two types of evaporators:

• Vacuum pan/evaporator: Batch operating type evaporator is preferred when relatively small quantities of milk are to be handled in a batch operation for product like sweetened condensed milk.
•  Continuous flow evaporator: Continuous flow evaporator is preferred for large scale operation working in conjunction with milk drier.

i) Vacuum Pan: This type of evaporator is employed in the majority of condenseries for the manufacture of condensed milk (Fig 9.1). The principal parts of vacuum pan are: i) Evaporator body, ii) Heating surface,iii) Condenser and trap and iv) Vacuum pump. The tubular calandria pans, usually made of stainless steel, are provided with either internal or external heating systems and work under reduced pressure. The pans are internally fitted with steam coils and the lower portion is steam jacketed.The steam pressure is usually 1.4 to 1.5 kg/cm. The operation is usually carried out at 54-63°C, the milk boils at this temperature owing to reduced pressure (63.5 cm of mercury) which is maintained in the pan. A vacuum pump is used to maintain the vacuum in the pan. Condenser is one of the important parts of the system. When the water vapour, which rises from boiling milk inside the pan, comes in contact with cold metal surface or cold spray of water in the condenser, it is condensed and carried off as water by means of pumps or other means.

Vacuum Pan Evaporator

ii) Horizontal Tube Evaporator: In this type of evaporator, the material to be evaporated boils outside the horizontal tube and steam condenses inside the tubes. These types of evaporators are not considered efficient as the heat transfer is lower especially if the liquid is viscous.

iii) Vertical Tube Evaporator: These are distinct improvement over horizontal types. In this type the liquid boils inside vertical tubes with heating medium, which is usually condensing steam held in a chest through which the tubes pass. In this type of evaporator there is a large diameter hole in the centre through which the liquid to be evaporated flows down. Heating and boiling of liquid cause it to rise and flow upwards and un-evaporated liquid flows down through the central large diameter tube. Thus natural circulation is promoted in this type of evaporator.

iv) Forced Circulation Evaporator: This type of evaporator is a modified form of vertical tube evaporator. In this type a pumps is used to force the evaporating liquid through the tubes. This helps to further improve the evaporating capacity.

v) Long Tube Vertical Evaporator: This type of evaporator consists of one pass vertical shell and tube heat exchanger discharging the product to be evaporated into a relatively small vapour head. Normally no liquid level is maintained in the vapour head and the residence time of liquid is only few seconds. The tubes are usually 2 to 5 cm in diameter. The length normally varies from 6-12 meters. In this type the feed enters at the bottom of the tube, starts boiling as it moves up the tube and the mixture of liquid and vapour leaving at the top at high velocity strikes against a deflector placed above the tube. This deflector is effective both as a vapour separator and as a foam breaker. When the ratio of feed to evaporation is low, recirculation of liquid is provided. The liquid is recirculated by a re-circulating pump.

vi) Climbing Film Evaporator: In this type the liquid to be evaporated is fed at the lower end of the vertical tubes, heated by the condensing steam on the outside of the tube. The liquid starts boiling at the bottom, the vapour along with the liquid tends to rise further up the tube in the form of a thin film. As the film rises further evaporation occurs until they reach the top of the tubes. The mixture of vapour and milk then passes into a separate chamber from which the vapour passes to a condenser. The concentrated milk is removed by a pump.The main disadvantage of this type of evaporator is that a comparatively large volume of liquid is present in the evaporator at one time and most of it located at the bottom. Because of this, severe scale formation occurs more rapidly in this region.

vii) Falling Film Evaporator: This type of evaporator also consists of vertical tube bundles heated by condensing steam on the outside (Fig ). The liquid to be evaporated is fed to the top of the tube and allowed to flow downwards on the inside of the tubes. Arrangements are provided at the top to distribute the liquid evenly to all tubes in the bundle. As the liquid flows in a thin film rapid evaporation takes place. The resulting vapour and concentrated liquid flow into a separator at the foot of the evaporator.The vapour is drawn off from the top of the separator and liquid from the bottom by a pump.

Falling Film Evaporator

The advantages claimed for the falling film evaporator are short residence time of liquid, good performance with small temperature difference across the heating surface. Also more effects can be used with falling film evaporator because it will operate with a lower temperature difference than climbing film evaporator.

viii) Plate Evaporator: The tubular heaters described above are longer and occupy more space. Development of plate type heat exchangers indicates clearly the advantages of better heat transfer that could be obtained between metal surfaces and liquids when arranged in layers so that the liquid flows in highly turbulent manner in thin layer between the metal plates. Among the advantages claimed for this type of evaporator are compact nature of equipment, requirement of low head- room and the comparative ease for addition of plates or their removal from the assembly for increasing or decreasing the capacity.

ix) Centri-therm Evaporator: This type may be described as mechanical evaporator, the operation of which is based on number of conical surfaces rotating about a vertical axis, the gap between the cones providing the steam condensing and evaporating surfaces. Feed is introduced centrally from the top and steam centrally from the bottom. Boiling takes place on the conical surface and centrifugal force throws the concentrated product radially outwards to the rim of the cones from where it is expelled. Condensate likewise is thrown out radially and discharged in a similar manner. The concentric conical construction gives a compact form of heating surface and the high radial velocity obtained from the rotating action results in a thin high speed film moving across cones.

x) Mechanical Recompression Evaporator: In this system of evaporation process, a part or all of the evaporated vapour from an evaporating unit is compressed and this compressed vapour making up a large percentage of the heat-required for further evaporation. A mechanical recompression evaporator is generally limited to single effect, compressing the vapour by means of a positive displacement pump or centrifugal compressor which can be run by electric motor, steam turbine or diesel engine. All the vapours from single effect are compressed and returned to calendria with no vapour going to the condenser. This eliminates the cooling water requirement normally associated with conventional evaporators.In thermo-compression evaporator the compression of vapour is affected by the use of high steam pressure and jet type steam ejector. The vapour evolved from boiling milk are partially entrained by a jet of high- pressure steam and the vapours arising from this mixture of steam and vapour is injected into the calendria and forms the heating medium for incoming milk. This system enables a considerable saving in quantity of steam and water required for evaporator operation.

xi) Multiple Effect Evaporator: Considerable saving in steam and water can be effected by using multiple effect evaporator. The vapours from one effect have considerable latent heat and may thus be used for heating second effect and vapours from second effect to heat a third effect and so on. Thus great economies of operation may be made in large installations with multiple- effect evaporator. For example in a double effect unit, the steam and water consumption are just about one-half that required in a single effect unit. With double effect evaporator, the first effect operates at a lower vacuum than the second effect, which makes the boiling temperature higher in the first effect than the second effect. Since the vapour given off from the first effect are hotter than the boiling point of the milk in the second effect, they are used to heat the milk in the second effect. This cuts the steam and condensing water requirement to about half of what is required for a single effect evaporator. Units having as many as four effects are in use for evaporation milk with corresponding saving in steam and water. A four-effect unit will require only about one-fourth of steam and water per kilo-gram of water evaporated as compared to ordinary single effect. For a single effect evaporator steam required to evaporate one kilogram of water from milk is approximately 1.2 kilogram.