Dairy Engineering

Module 3. Separation equipment

Lesson 11
DIFFERENT TYPES OF CENTRIFUGES. APPLICATION IN DAIRY INDUSTRY, CLARIFIES, TRI-PROCESSORS, CREAM SEPARATOR

11.1 Introduction

Mechanical separation by centrifugal force can be classified in broad terms as below, depending on physical state of the components being separated.

1. Liquid from liquid e.g. cream from milk by cream separator

2. Solid from liquid e.g. Ghee residue from ghee by Clarifier

3. Solids from gas e.g. powder from air by Cyclone separator

4. Gas from liquid e.g. vapours from Concentrated milk in Vapour separator

11.2 The Different Types of Centrifuges can be Classified as

Fig. 11.1 Types of centrifuges

11.2.1 Clarifier

One of the important applications of centrifugal force in dairy industry is the Clarification of milk. It is also based on the density difference between milk and the extraneous matter like dust, hair, leucocytes, and other contaminants. The clarifier is also used in separating Ghee from Ghee residue. The separation process is fast and almost complete. The Clarifier has one entry for the milk or the fluid to be clarified and one exit for the clarified material. The clarifier sludge gets collected in the inner wall of the bowl carrying the discs. The discs are conical in shape and are mounted on a spindle that rotates at high speed and have holes to allow passage of milk to the gaps between the clarifier discs. The location of the holes, called the neutral zone is located nearer to the periphery, such that heavier particles that are to be separated is moved more easily. (Fig. 11.2: Milk clarifying bowl)

11.2.2 Open type of cream separators

These types of cream separators are fed by milk open to atmospheric pressure, and flow down in separator discs by gravity from a vessel also open to atmosphere. These separators are of smaller capacity, maximum of 1000 lph. They are suitable for sour milk separation and are driven by belt and pulley, powered by small motor. The feed from the vessel flows by gravity through a spout. A float positioned on the top of the inlet prevents entry of air, and allows only milk to enter into the bowl.

The separator disks in the bowl are about 16 to 25 in number, depending on the capacity, and the size . The cream outlet is controlled by a small screw, by which the fat percentage of cream can be varied. As the open type cream separators are not expected to be connected to pasteurizers, and are of small capacity, they are designed to operate with milk not being warmed.

The drive mechanism for the bowl spindle is by a flat belt connected to the driver shaft. The driver shaft is driven by single speed motor. In some smaller capacity designs, the bowl spindle forms the worm and the driver shaft is the worm wheel, which may be driven by hand or by motor connected through a V belt and pulley.

Fig. 11.3 Small capacity cream separator

11.2.3 Hermetic cream separator

The disadvantage of open type cream separator is that it is suited for only small capacities and the discharge is to atmospheric pressure. It cannot be linked to pasteurizer so that the separation can be done at the optimum temperature of 40 to 45°C. The other disadvantage is the product being open to the atmospheric contamination and ingress of air, making the product foamy. The Hermatic cream separator is of larger capacity, and both inlet and outlet are connected to pipes, making it possible to operate under completely air tight condition.

Tri-purpose or Tri-process separators are designed for three purposes, to do separation, standardization and clarification in a single unit. Milk contains dust, leukocyte cells etc, which are removed before processing. This function of Triprocess separator is called “Clarification”. Depending upon the requirement, the fat may be removed partially to a fixed percentage known as standardization or completely skimmed termed as cream separation. The control of cream outflow can lead to these variations in operations.

11.2.3.1 Description

1. The separator has horizontal and vertical drive systems for the separator bowl.
2. The horizontal drive system consists of an electric motor, centrifugal clutch and drive shaft with worm wheel.
3. The vertical drive system consists of a shaft , called the bowl spindle on the top of which the separator bowl is mounted. The shaft has teeth to match worm wheel.
4. Power from the motor is transmitted from the worm wheel on the horizontal drive shaft to a worm gear of the bowl spindle. So the bowl spindle rotates at higher speed (say 4 to 5 times higher) than the horizontal drive shaft speed. (Fig. 11.5: Worm wheel)
5. Precision ball bearings support the drive components and thus eliminate vibration.
6. Also mounted on frame is a hand brake acting on the clutch disc, a tachometer and revolution counter button for speed indication, an oil filter plug and a drain plug, an oil level glass, etc.
7. The bowl consists of two main sections: the body and the hood, which are held together by a threaded lock ring.
8. The disc stack is clamped between the hood and the bell shaped distributor at the axis of the bowl.
9. The gap between discs is about 0.5 mm. The disc wall thickness is about 0.4 mm and angle of inclination to the horizontal is 45 to 60 degrees.
10. The bowl speed is usually 5500 to 6000 rpm.
11. The bearings in the frame are splash lubricated by the worm wheel, which runs partially, immersed in an oil bath in the base of the frame.
12. Separators separate about 98.5 to 99% of fat in milk (0.02 to 0.05% fat in skim milk).
13. Cream outflow from the separator normally represents about 10% of the total milk feed.
14. The feed to the separator is by a pump, in case the supply tank is far away. Otherwise, a centripetal pump with impeller leads the milk suction line into the spindle, from where the milk enters the bottom of the distributor.
15. For the requirement of standardization, the outlet of cream is kept at a higher discharge pressure to that of skim milk outlet. The flow rate of cream is measured by a rotometer.

The internal parts of the bowl and the drive mechanism is shown in figures 11.6 & 11.7

Fig. 11.6 Hermetic cream separator

(Adapted from Operators manual of Alfalaval Pvt Ltd)

Fig. 11.7 Cream separator (Adapted from operators manual of alfalaval Pvt Ltd.)