Dairy Engineering

Module 3. Separation equipment

Lesson 12
SELF-DESLUDGING CENTRIFUGE, BACTO -FUGE, CARE AND MAINTENANCE OF SEPARATORS AND CLARIFIERS

12.1 Introduction

Cream separator and also clarifier, have to be stopped, and dismantled for cleaning often, especially when the quality of the milk is bad. This involves interruption of HTST pasteurization operation also. Hence, a method of ejecting the sediment that is separated during clarification or cream separation was developed. This involved certain changes in the design, especially the lower part of the bowl which encloses the separator discs.

12.2 Self Desluding Centrifuge Operation

The milk sludge, produce during the separation in a separator collects in the sludge space of the bowl, which is designed as double taper. The bottom disk of the sludge space can be controlled hydraulically to be moved downwards, so that the sludge can leave the bowl through the double conical portion, where holes get uncovered. The sludge ejected from the bowl is collected in the upper transmission part consisting of stainless steel, and is fed through the sludge collector to the sterilizing vessel. The sludge collected in vessel is sterilized by adding caustic soda, under high temperature. After the sterilizing process has been terminated, the vessel must be emptied in order to make provision for having space for fresh sludge ejections. The hydraulically controlled bowl opening mechanism can be operated both manually and automatically.

12.2.1 Control system diagram

Fig. 12.2 Control system

12.2.2 Operation of control system

The removal of the sludge from the bowl is through the openings arranged at the widest circumference of the sludge space, and closed by an axially mobile bottom during the separation process. The opening and closing movements of the mobile bottom valve are controlled by a hydraulic control system. The control system associated with the bowl consists of the mobile bottom valve hydraulically movable in the bowl; the control water feed , the control water valve system with the solenoid and manually controlled valves for automatic and manual control, and the program control system. The bowl control system is operated in normal operation by a preselected desludging program. Any required full desludge operation can be controlled semi- automatically by the valve control cabinet. In the case of current failure or other possible defects, putting the control cabinet or the electrical control valves out of operation, the separation process can be brought to an end by operating the manually-operated valves.

The water used for hydraulic operation must be clean, soft fresh water. The control water pressure also must be sufficient, usually about 2.5 kgcm²

12.3 Bactofugation

Bactofugation is the process of separation of microorganisms from milk by using centrifugal force.

12.3.1 Advantages

· It is useful when UHT process is being done for the milk

· In the production of low heat powder, both aerobic and anaerobic spore formers can be considerably reduced. The low heat powder is useful in preparation of recombined milk.

· Cheese making for special cheeses using unpasteurized milk e.g Mozarella, Camemberti

· Most of the spores can be removed, and only that component which is around 2.5 to 3.0% of milk will be given the UHT treatment, thereby saving energy and nutritional quality.

· Because only part of the milk is heated, it is economical.

12.3.2 Densities of components

As the separation by centrifugal force depends on density differences, its is useful to know the densities of various components.

Table 12.1 Densities of various milk components

As we observe the above values, the bacteria is denser than most of the components, and hence, it can be separated by regular centrifugal process. Care must be taken to safely dispose of the sludge that is separated. A self-desludging type of separator is most suitable.

12.3.3 Effect of bactofugation

By Bactofugation at 50 to 55°C about 89 to 93% of the bacteria is removed. It be still more effective, if the milk is first clarified and then Bactofugation is performed, eliminating bacteria upto 99%.

Table 12.2 Percentage of components removed by clarifier and bactofugation

12.4 Bactotherm

Fig. 12.3 Bactotherm process

12.5 Maintenance of Separators ans Clarifiers
Centrifugal separators are high speed equipment, with clarifier bowl having discs held vertically by support of bearings. The passage of the fluids like milk, cream, skim milk are separated.

12.5.1 General observations in maintenance

1. Rubber gaskets, ‘O’ rings must be replaced before they wear out

2. Lubrication oil in the oil sump must be of required quality and should be upto appropriate level.
3. Bearings holding the spindle must be replaced regularly, to prevent bending of the shaft itself, in addition to rapid wear out of ‘o’ rings, drop in efficiency of separation and capacity.
4. Discs should be cleaned with softer material and not with wire mesh, and should be placed on rubber mat during cleaning.
5. The discs should be stored properly without damage.
6. Assembling of discs and other components must be properly done, without causing any vibrations, damage to parts and decrease in capacity.
7. Cream separator should not be run dry without any fluid.
8. No adjustments, assembling, disassemble to be done while the cream separator is revolving.
9. Worn out worm or worm wheel to be replaced before damage to other parts due to jerks, and loss of speed of rotation of bowl.
10. Centrifugal clutch shoe should be replaced to prevent any loss of power transmitted to worm wheel. They have to be roughened from time to time to give grip.
11. Bearings to the shaft of worm wheel should be replaced when worn out.
12. Under no circumstances, the direction of rotation of spindle be changed. This is likely to happen, when motor for the drive is repaired and reconnected with the three phase line, without checking for the direction of rotation.
13. Milk being fed to the separator must match the desired feed pressure recommended.
14. While assembling and disassembling, the nature of the threads whether right hand threads or left hand threads should be taken note of and accordingly done.
15. Under no circumstances, the weight of the connecting pipe lines should strain the separator.
16. Some of the cream separators like self desludging type, the lubricating oil is further cooled by cooling water. It is important that the cooling water should be of sufficient flow and pressure required, as well as soft water type. Under no circumstances, the cooling water by pass valve from the control water line should be closed or disturbed once the flow requirement is set.
17. The cream separation should not be done at too high temperatures than recommended, as the life of the rubber collars, packing etc will be reduced.