MARKET MILK

Module 6. Common dairy operations

Lesson 20
SEPARATION OF MILK - CREAM SEPARATORS

20.1 Introduction

Centrifugal separation is a type of sedimentation process, applied for solid-concentration, clarification and liquid-liquid separation with simultaneous separations of solids. There are around 450 designs of centrifuges made of 1200 varieties of materials. The bowls are available with discs from 10 to 250 in numbers. The disc spaces vary from 0.3 to 1 mm. Similarly, density difference which could be operated upon varies from 20 to 400 kg/m³. Concentration and washing of finely dispersed solids is often carried out for the processing of relatively high concentrated suspensions (about 25% by vol). Centrifuges have now capacities as high as 45,000 L/h. The concentration of suspension in the exit stream is as low as 0.04% by volume.

Centrifugal cream separators are similar to centrifugal clarifiers in that they consist of a stack of conical discs housed in a separator bowl and rotated at high speed by an electric motor driving either through friction coupling or a direct drive coupling. The distribution holes in each disc are vertically aligned and milk is fed to the base of the disc stack to pass up through the holes.

20.2 Separation by Centrifugal Force

Centrifugal separation is the most commonly used method in modern dairies. In this method, milk is fed into the rapidly spinning bowl, at the centre of the discs. Milk distributes itself in the space between the parallel discs rotating at high speed creating centrifugal force. The heavier of the two products, namely, the skim milk is thrown to the periphery of bowl, whereas the lighter one, i.e., fat gathers in the central portion. The incoming milk forces the separating layers from the bottom and out at the top of the bowl. Two passages are provided at the top of the bowl for skim milk and cream to pass out of the bowl (Fig.20.1).

Regulating the cream discharge opening (affecting the flow) by adjusting the cream screw, we can maintain the desired fat % in cream. The ideal temperature for separation is 35°- 45°C. Some amount of dirt present in the milk may also be separated during the process and get deposited in the space between the outer edge of the discs and the inner surface of the bowl shell. This slime is removed when the unit is shut down for cleaning, after milk is separated. This limited running time of separators leads to the development of a mechanism for automatically removing the solid material (dirt and cellular material, bacteria) without having to interrupt the operation of the machine. Such separators are called 'Self-desludging' or "self-cleaning" separators. Cold separation results in more fat loss than in warm milk separator.

Fig. 20.1 Flow of cream and Skim milk in the space between discs in a centrifugal separator

20.3 Important Components of a Separator

20.3.1 Bowl and disc assembly

The bowl is the key component. It consists of an outer shell within which a large number of cone-shaped discs are housed. The construction is such that a very small space (< 0.5 mm) is provided between each pair of discs (Fig. 20.2).

Fig. 20.2 Cross Section of centrifugal separator

It is very important that the bowl is accurately balanced and suspended over the spindle to ensure freedom from vibration. Milk is introduced to vertical aligned distribution holes in discs at a certain distance from edge of discs from start. Under influence of centrifugal force, the particles and droplets of milk will begin to settle either outward or inward in separation channels radially according to their densities related to continuous medium. There are up to 120 discs arranged one over other, angle of inclination being 45-60° to the horizontal. Outward diameters are 200-300 mm. Disc are generally made up of stainless steel and wall thickness will be 0.4 mm. The number of discs in a cream separator relates to its capacity. The space between discs allows the milk to flow through narrow gaps in laminar flow thereby ensuring the separation. Larger gaps between discs are necessary if there is a danger of clogging. Bowl discs used revolve at rpm of 5500-6000 and separate milk at mass flow rate of 20,000 liters per hour. The space between the discs must be increased if the cream of high fat content is to be obtained. In a normal cream separator, we can expect clogging if fat in cream is above 55-60%. The required high fat content can be obtained only after subjecting the cream again for fat separation.

20.3.2 Product inlet arrangement

Fully enclosed power driven units use a constant pressure pump with float balance arrangement at the pump suction, to maintain constant head. Milk enters the separator through a pipeline at the bottom of the machine and travels upwards through the hollow space in the spindle. Since the operation takes place in a sealed compartment, there is no possibility of foaming.

20.3.3 Product outlet arrangement

Separate outlets for skim milk and cream are provided on top of the bowl. The cream outlet is near to the centre of the rotating spindle and the serum outlet is at a lower position than cream outlet.

20.3.4 Cream regulating screw

For small capacity units, a screw arrangement is provided on the cream line, within the top disc. By manipulation of this screw, the passage of the cream could be varied to raise or decrease the fat content of cream. In the case of large capacity units, a regulating/back pressure valve is provided on the cream outlet line.

The cream screw can also be used for standardization of the fat content of the milk to any desired level below that of the fat content of the feed.

20.3.5 Driving motor

For rotating the bowl at the desired high speed, a gear arrangement is provided to increase the speed of bowl as compared to the speed of driving motor (about 1440 rpm). To lubricate the gears, a closed lubrication system is incorporated within the gear assembly.