Milk Separators: Types and Functions in Dairy Processing
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Separators are among the principal machines used in milk processing, designed to split the product into fractions of differing density inside a rotating separating component — the drum.
Separators used in the dairy industry are classified by production purpose, design features, the degree of contact between the milk and air, the method of removing foreign impurities and sediment from the milk, and the type of drive.
What types of separators are used in the dairy industry?
By production purpose, separators are divided into several categories, each tailored to a specific dairy operation. The list below covers the main functional types:
- for separating milk (concentrators) and obtaining cream with a fat content of 10–45%, and, on repeated concentration, cream with a fat content of up to 85%;
- for cleaning milk of foreign impurities (clarifixators);
- for standardising milk by fat content (milk can be standardised on a cream-separating separator using a special standardising device, on a dedicated separator for milk standardisation and cleaning);
- for homogenising milk (clarifixators);
- for removing microflora (bactofuges);
- for separating, standardising or cleaning milk (universal units);
- for producing curd from the curd clot (soured milk);
- for clarifying whey and separating protein dust.
By design features and degree of milk-to-air contact, separators fall into three groups:
- open — with open milk feed and open discharge of cream and skim milk (the cream and skim milk are in direct contact with air);
- semi-closed — in which milk may be fed openly or under a closed but unpressurised supply, while the product outlet is closed and operates under the pressure created by the separator; during separation the product inside the drum is not isolated from contact with air;
- closed (hermetic) — in which the milk feed, the product outlet and the processing of the milk inside the drum are isolated from air; milk is fed into the separator under pump pressure, and the product leaves under pressure created by the separator or pump through closed pipelines.
By the method of removing foreign impurities and sediment from the drum, separators are available as follows:
- with manual sediment unloading after the separator is stopped and dismantled;
- with centrifugal periodic sediment unloading during continuous operation of the separator;
- with centrifugal continuous discharge of the heavy fraction through nozzles (in the curd separator).
By drive type, separators are manufactured with a manual drive, an electric-motor drive, or a combined drive (manual and electric motor). Only open separators with a throughput of up to 600 l/h are produced with a manual drive, and these are not used at dairy-industry enterprises.
The choice of separator depends on its purpose, the volume of product to be processed, the technological process scheme, and the technical and operating data of the separators.
How is a milk separator built?
The main assemblies of a separator are the separating device, the intake-and-discharge device that feeds the product into the separating device and removes the separation products, and the drive mechanism with an individual electric motor mounted on the separator frame. Together these units carry the milk through the centrifugal process from feed to finished fractions.
The separating device of the separator (Fig. 1) is closed by a lid. The body and lid are joined by a nut. Inside the body sits a disc holder carrying a stack of discs. A dividing disc is placed at the top of the stack and separates, inside the separating device, the flows of the light and heavy liquid fractions. The intake-and-discharge device has a central tube for introducing the raw product into the separating device.
Pressure discs are fixed to the tube to discharge the separation products under pressure. These discs are positioned stationary inside the separating device within the pressure chambers.
The horizontal shaft of the drive mechanism rotates in bearings and carries the driving wheel of the multiplier. Drive from the electric motor is transmitted to the horizontal shaft through a centrifugal clutch, bringing the drum up to its rated rotational speed in 5–10 minutes. The driving wheel on the horizontal shaft engages the multiplier wheel located on the vertical shaft (spindle), which sets the spindle in motion and, with it, the separating device mounted on the spindle's end. 
The discharge of solid sediment may be periodic — with full disassembly of the separating device — pulsed when sediment is removed during operation, or continuous through nozzles in the wall of the device. Separators for removing the fat fraction include cream separators, standardising separators, separators for high-fat cream, and dispersing separators.
How does a cream separator work?
The cream separator is designed to split milk into cream and skim milk. Its performance is judged by the mass fraction of fat remaining in the skim milk (0.03–0.01%), while the mass fraction of fat in the cream is 10–40%. A cream separator with pulsed sediment unloading runs to a program embedded in its control system, with separation taking place while the unloading channels are closed.
When solid sediment accumulates at the periphery of the separating device, the channels open and the sediment is released into the receiving hopper. The unloading openings then close and the process repeats after a time interval set in the program. Milk is fed into the separating device through a pipeline and the central tube of the intake-and-discharge device.
Through an opening in the disc holder and vertical channels in the disc stack, the milk is distributed into the inter-disc space, where separation takes place. The cream, forced toward the axis of rotation, first passes into the upper part of the separating device and then into the pressure chamber with its stationary pressure disc, from which it is discharged from the separator under pressure.
The skim milk is directed toward the periphery of the separating device into the contaminated space, from where it moves to the upper part of the separating device, to the skim-milk pressure chamber, and is forced out of the separator under pressure. Inside the separating device the cream and skim-milk flows do not mix, because they are kept apart by a solid partition formed by the upper disc and the dividing disc of the disc stack.
To regulate the fat content of the cream, a control valve and a cream flow meter (rotameter) are fitted on the pipeline. With a constant volume and fat mass fraction in the incoming milk, increasing the cream output reduces the fat mass fraction in the cream; conversely, reducing the cream output raises the fat mass fraction in the cream.
Separators for high-fat cream are used mainly in butter-production flow lines. These separators are designed to obtain cream with a high fat mass fraction of 80–85%.
The design of a high-fat cream separator differs somewhat from that of a conventional open-type cream separator. Because of the high viscosity of the product being processed (cream), the inter-disc gaps in the high-fat cream separator are enlarged, the throughput is reduced, and the fat mass fraction in the high-fat cream is regulated by a controller on the buttermilk line.
After 1.5–2 hours of operation, a batch-type separator must be stopped for sediment unloading, washing and cleaning. Separators for removing the heavy fraction include milk-cleaning separators, whey-cleaning separators, whey-protein separators, and bacteria-removing separators.
What does a milk-cleaning separator remove?
The milk-cleaning separator is designed to remove from milk both mechanical and natural impurities — microorganisms, particles of dirt and blood. On average, the mass fraction of impurities in milk is about 0.04%.
A milk-cleaning separator with pulsed sediment unloading consists of a separating device, an intake-and-discharge device and a frame with a drive. The separator is supplied with a control panel that has a program unit governing the operation of the sediment-unloading system.
Milk enters the separating device (Fig. 2) of the milk-cleaning separator through the central tube and is directed into the contaminated space. The cleaned milk moves toward the axis of rotation of the separating device along the gaps between the discs, while the sediment accumulates on the wall of the body.
The cleaned milk is discharged from the separator under pressure through the pressure system, which includes the pressure chamber and the pressure disc. The sediment that has collected in the separating device is unloaded through an opening in the wall of the body. While sediment is being collected, these openings are closed by a piston that forms a movable internal bottom inside the separating device. Working fluid entering the cavity beneath the piston from the hydraulic system creates an upward pressure that presses the piston against the gasket. 
Within the separating device, two valves of the working cavity are placed radially; they are connected to the cavity beneath the piston, to the hydraulic system, and to the openings through which the working fluid exits. When working fluid is supplied to the valves, the valve pistons open the openings, so that the fluid is forced out of the space beneath the piston through the openings in the drum.
As the piston in the separating device descends, it opens the opening for sediment discharge; the working fluid then refills the space beneath the piston. The piston rises to its upper position and the sediment-accumulation process repeats.
How is curd separated with a curd separator?
The curd separator of the open type, with continuous discharge of the heavy fraction (curd) through nozzles, is designed to defat curd and forms part of the equipment set for the line producing curd by the separate method. In the wall of the body of the curd separator (Fig. 3) there are 12 openings holding nozzle holders. By changing the size of the nozzles and their number, the moisture mass fraction in the curd can be regulated. 
The curd clot is directed along the central tube beneath the disc holder into the peripheral space of the separating device. The curd is continuously discharged through the nozzles into the curd receiver, while the whey is removed from the separating device through a guide ring in a free flow. Only a clot obtained from skim milk can be processed in this separator, because processing a clot made from whole milk inevitably causes losses of milk fat, which is carried away together with the whey.
