Module 2. Production of cream

Lesson 5

5.1 Introduction

Cream separation is the mechanical separation of milk into cream and skim milk by means of centrifugal force. Cream separator is a device used to separate cream from milk. There are different types of cream separators available in dairy industry. Performance of cream separators will largely be influenced by its design and operating conditions.

5.2 Cream Separators in Dairy Industry

There are two basics types of separators depending on their mode of operation

1. Hand operated and

2. Power driven

The power driven cream separators are further classified as:

1. Open-bowl separator

2. Semi-closed or semi-hermatic separator

3. Closed bowl or Hermatic separator

4. Self-desludging separator

5.2.1 Open bowl separators

These are characterized by an open top milk inlet and pressure less, skim milk and cream outlets open to atmosphere. The main parts of a cream separator are: Supply can/milk basin, milk faucet, regulating chamber with float, cream screw and skim milk screw, cream spout and skim milk spout, Separator bowl (consisting of bowl shell, milk distributor, discs and rubber ring), spindle, gears, crank handle and bowl nut. The base consists of horizontal, vertical drive prime consisting of electrical motor, centrifugal pump, drive shaft with a worm wheel. The vertical drive prime consists of a shaft known as bowl spindle. At the top of the bowl spindle a separator bowl is mounted. Power is transmitted from worm to a worm gear connecting to the spindle. Discs, shaped as cones, are stacked in into disc assembly and placed in the bowl shell.

Milk is introduced on to vertically aligned distribution holds 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 are arranged one over other and, angle of inclination is 45-60° to the horizontal. Outward diameters of discs are between 200-300 mm. Disc are generally made up of stainless steel and wall thickness will be around 0.4 mm. The space between discs varies the flow through narrow gaps in laminar flow ensuring the separation. Larger gaps between discs are necessary if there is a danger of clogging. Disc bowls used have an rpm of 5500-6000 and at mass rate flow 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, clogging can expected if fat in cream is above 55%-60%. The required high fat content can be obtained only after subjecting the cream again to fat separation. Suppose milk is to separate below 25°C then the distance between the discs must be increased due to higher viscosity. The number of discs in cream separator relates to its capacity.

In gravity fed type cream separator, a supply tank is mounted on bowl unit. It consists of two separate outlets one for cream and another for skim milk. To handle small capacity units, a screw arrangement is provided on cream line just at the outlet point. This cream screw helps manipulate the flow rate of cream.

Precautions for Efficient Working of open bowl separator

1. Proper assembling and fitting of different parts

2. Proper lubrication in machine

3. Quality of milk to be separated

4. Temperature of milk (30-40°C)

5. Speed of the bowl

6. Uniformity in speed of bowl

7. Uniform supply of milk

8. Sanitary condition of machine

The disadvantage with this separator is that lot of excess foaming in skim milk.

5.2.2 Semi-closed separators

These are the separators with milk feed at atmospheric pressure and paring discs located in the cream and skim milk outlets which discharge into closed pipes at elevated pressure. The milk inlet is so designed as to prevent any air getting into the milk stream. This helps to prevent foaming in cream and skim milk.

This paring disc converts rotational energy of milk and cream to a linear kinetic energy. They are constructed like the impeller of centrifugal pump and dipped in rotating ring of liquid. The liquid is made to flow with a high peripheral speed into channel openings of the paring discs. These discs are fixed to the feed pipe. The liquid enters horizontally along spiral channels, which are divided, and flows vertically through the discharge channel in the shaft of the paring disc. Here the kinetic energy is converted into peripheral energy and 2.5-5 bar pressures can be generated depending on the rotational speed and diameter of paring disc. The generated pressure can be used to push the exiting cream and skim milk through variable flow. Restrictive devices and outlets can be used to generate back pressures which can control flow so that a fairly accurate control of fat content is feasible.


Fig. 5.1 Semi closed

5.2.3 Closed bowl or hermetic separators

In these separators, milk inlet and the cream and skim milk outlets are all connected to closed pipe lines. The milk is transferred in a closed pipeline under pressure, which is mostly flow controlled. This design is normally used for self-cleaning (self-desludging) separator; i.e., the dirt is discharged periodically without interruption of the process.
With closed separators incorporation of air is nearly totally excluded, which results in the following advantages:

(a) Gentle treatment of the milk in the bowl, as its entire volume is occupied by the liquid
(b) High skimming efficiency ( up to 0.005% of fat in skim milk)

Closed bowl separators are usually provided with self desludging mechanisms


Fig. 5.2 Hermatic closed Separator

The separator shown in Figure 5.1 is also equipped with a self-cleaning bowl. The dirt (which is separated from the milk) is collected in a bowl (which is conical to the outside) and is ejected periodically through ports. This permits the separator to be included in a CIP circuit without requiring time for disassembly. The operating time is independent of the sludge container capacity which permits a multi shift continuous operation.

5.3 Construction and Working Principle of Cream Separator Bowl

Whole milk is fed via the central pipe to the distributor at the bottom of disc stack and is subjected to rotation. The distributor serves as the base for the assembly of 110-130 discs. The coarse dirt particles settle down in the conical bottom part, i.e the sludge space. The milk flows upwards through the openings in the discs which are inclined towards the rotational axis. Thus, in the disc assembly, milk rises in channels, which are parallel to the rotational axis. The channels are formed by the rising holes of the disks and the milk is distributed in thin layers in the space between the disks, (distance of 0.5-1.0 mm). It is here that separation of milk into cream and skim milk takes place and at the same time the dirt is deposited as slime in the sludge space.

The heavier part of the liquid (skim milk) moves toward the outside, and the cream (which is lighter) moves in the opposite direction toward the center. In a slit between the bowl base and the disc inner rim, the cream rises to the cream retention chamber and goes into the cream outlet. After passing the disk space, the skim milk goes into the bottom part of the bowl and flows as a liquid ring into the slit between the bowl cover and the separation disks into the skim milk chamber. The separator discs serve to keep the skim milk separate from the cream. The skim milk is then directed via the paring disc of the skim milk chamber into the skim milk outlet. As explained above, the fixed separator discs have the effect of a rotating centrifugal pump, so that in each chamber the required pressure (for transporting the liquid) is generated.

5.4 Self Desludging Separator


Fig. 5.3 Self Desludging Separator Bowl

Working principle of the self desludging separator bowl: The particulate impurities such as dust and cellular material from blood are dense solid material and hence collects on the outside of the rotating bowl. If left like that, it fill up the sludge space and inhibit the flux of skim milk there by hindering the separation process. Self desludging separator does not permit the accumulation of sludge in the bowl. Self Desludging ia a mechanism for automatically removing solid material without having to interrupt the operation of the separator. Slots are cut in the outside of the bowl and are normally kept close by a sliding bowl bottom which is elevated by hydraulic pressure of running water in a reservoir underneath. Intermittently hydraulically operated valve opens up and the reservoir drains to allow the bowl bottom to fall and this causes opening of these slots. The outward pressure on the sediment forces it into the outer bowl. The valve then closes to allow the reservoir to refill and the slots to close. The action is very rapid and it discharges at regular intervals, which depends upon the volume of sediment, space in the separation bowl and condition of the milk.

Last modified: Thursday, 25 October 2012, 5:42 AM