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ROLLER DRYING / DRUM DRYING
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Basic principle
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Theproduct to be dried is applied in a thin layer (film) onto the surface of oneor two steam heated drums by using different application systems. The temperature on the drum surface is above100 ºC, and heat can be transferred rapidly to the product. This helps in transferring moisture that isevaporated to the ambient air and removing it by airflow.
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Aftera three-quarter turn of the drum, the drying process is finished, and the filmis removed from the drum by special knives (doctor blades) as a thin film,air-cooled, and milled into powder in a special mill. After intermediate storage in silos thepowder is filled and packed. (OR)
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Theprinciple of the roller drying process is that milk is applied in a thin filmupon the smooth surface of a continuously rotating steam heated metaldrum. The film of dried product iscontinuously scraped off by a stationary knife located opposite the point of applicationof the milk. The drier may consist of asingle drum or a pair of drums. The milkmay be pre-concentrated before it is being applied to the drum, and the degreeof concentration varies with the design of the dryer. The rule is that a single-drum dryer canhandle milk of higher concentration than that of a double dryer. The dried milk film is milled to break up thefilm. The particles of a roller drierpowder are solid, flat, irregular flakes containing no air. The absence of occluded air retardsoxidation, resulting in good keeping quality.
Design and Operation of drum driers
Drum driers may be of the following types,
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Monocylinder/single drum drier with applicator
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Double drum drier with spraying device/doubledrum drier
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Double drum drier with sump
Flow diagram of drumdrying system
Mono-cylinder with applicator
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Fromthe product trough, the concentrated milk is picked up by the concentrate drumand is transferred to the applicator drum. From the applicator drum the product is delivered to the surface of thedrying drum which is steam heated and has an inside temperature of 115-130ºC,and rotates at 10-30 RPM. Ambient airflows over the surface of the drum, mixes with the vapor, and is sucked throughthe vapor hood to the outside atmosphere. The process is controlled by the film thickness by adjusting theapplicator drum and drum speed.
Double drum drier with spraying device
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Herethe drying process is similar to the above explained one, but the productapplication is through small spraying discs, which rotate in the concentratetrough and transfer a thin concentrate film, which is transferred onto thedrying drum by the air flow. Theconcentrate droplets, which gain surface area by this process, undergo gentleand rapid drying.
Double drum drier with sump
Technical notes
Control of drying, ratios, and capacities.
Drumdrying is influenced by the following factors:
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Thickness of the film
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Uniformity of the film
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Drying temperature and time, controlled by steampressure, speed of the drum velocity
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Concentrate temperature and dry matter contentor concentration ratio
Inorder to achieve constant operation and therefore constant product quality allparameters must be kept constant, such as
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The drying time should be kept within 1.8-2.5seconds.
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The specific steam consumption is 1.3-1.6 Kg/Kgof evaporated water and 4.3-5.0 Kg/kg of powder
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The drying capacity of drum driers is in therange of 300-2000 L of concentrate per hour with a dry matter of 45 %.
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Factors which affect the capacity of the drumdriers include:
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Concentration ratio or dry matter content of theconcentrate
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Viscosity of the concentrate
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Acidity of the concentrate (higher aciditydecreases the capacity)
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Surface temperature the drum.
Advantages and Disadvantages (over Spray-drying System)
Advantages
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Relatively low capital and operating costs
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Plantis movable and occupies little floor space
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Plantis easy to handle
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Suitablefor operating small quantities of milk economically
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Producesmilk of better keeping quality
Disadvantages
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