Roller driers

i. Classification of Roller Driers

Roller driers (also known as drum driers) may be classified in a number of ways viz:

i) Number of hollow drums and direction of turning of drums

(a) Single drum (b) Twin drum (Turn away at the top) (c) Double drum (Turn together at top towards centre)

ii) Pressure surrounding the product

(a) Atmospheric (b) Vacuum

iii) Method of placing the product on the drum surface

(a) Trough or reservoir above for top feed (b) Spray or splash feed (c) Trough below for pan feeding d) Sump below for dip feeding.

iv) Method of obtaining vacuum (For a vacuum drier)

(a) By steam ejector (b) By vacuum pump

v) Material of construction of drum

(a) Steel (b) Alloy steel (c) Stainless steel (d) Cast iron (e) Chrome or nickel

plated steel.

Note: i) Cast iron is usually used. The wear is excessive on stainless steel drum and the double drum atmospheric drier is most commonly used in the dairy industry.

ii) Vacuum drum driers are essentially the same as atmospheric units except that the drums are enclosed so that a vacuum can be maintained on the product during drying. The single drum with top feed is more commonly used for vacuum drying. A thicker film is obtained with top feed.

ii. Drum Description and Steam Flow

The drums are normally horizontal, hollow cylinders, 2 to 4 ft. in diameter and 3 to 12 ft in length. Drums are carefully machined both from inside and outside so that the thickness of the drum throughout its length is same. This assists uniform heat transfer and drying. The most used construction in the dairy industry is a double drum drier, which works under atmospheric pressure. Drums are mounted parallel to each other, about 0.5-0.75 mm (0.02-0.03”) apart. Care is taken to properly align the drums. Further, one drum is mounted on a stationary bearing, while the other on a flexible one, so that it can be moved to provide the desired clearance between the drums. The speed of drums is adjustable and usually kept between 14-19 rpm depending on concentration of milk and on the pre-set moisture content in the resulting milk powder. The product is removed after 3/4 to 7/8 of a revolution by a scraper knife. The blade of the knife (also called the Doctor blade) forms an angle of 15-30o with the roller surface. The metal used for the knife should be softer than the drum. The product is in contact with the drum for about 3 sec or less.

The drums are heated internally by feeding steam at about 60-70 psi (150°C) into the centre of the drum at one end of the shaft through the hub. The requirement of steam is 1.2-1.3 kg per kg water evaporated. The condensate, that moves to the bottom of the drum, is removed by pump or siphon continuously.

In atmospheric driers, drying takes place at atmospheric pressure. In vacuum driers,however, the drier is enclosed in a vacuum chamber, which is maintained at 27-29 inch Hg vacuum. A better quality product is obtained in vacuum driers, but besides being costly, the process becomes complicated.

iii. Flow of Product

The milk may be placed on the rollers in the natural form or pre-concentrated in an evaporator before it is fed to the drum drier. The degree of concentration varies with the design of the dryer. The milk is usually preheated and placed in a reservoir between the upper portions of the drums. Milk level between drums must be uniform for control of the moisture in the dried product. The general rule is that a single-drum dryer can handle milk of higher concentration than a double-drum dryer. The scraper blade scrapes the dried material from the drum. The film of dry milk forms a continuous sheet from knife to the auger trough, which is about level with the bottom of the drum. The broken film is subsequently milled, sized and packaged. Usually quality difficulties are encountered briefly while starting the drying operation. A common practice is to exclude the initial dried product from the lot until the product is satisfactory in appearance and the drum operation is normal. Water vapours above the drier are continuously removed by providing a hood above the drums.

iv. Factors Affecting Production

Following factors affect the rate of drying on drum dryer:

Pre-concentration of milk: Increased pre-concentration of milk up to 18% T.S. increases rate of drying.

Milk Feed Temperature: Increased milk feed temperature up to 160oF increases drying rate

Height of milk in the trough: Increased height of milk in the trough over the drum increases capacity.

Drum gap: If drums are farther apart, leakage may occur. The thickness of the film on the drum is directly related to the distance between the two drums.

Drum speed: Only slight increase in drying rate occurs as drum speed is increased.As speed increases the film becomes thinner so the amount of water evaporated and amount of powder produced remains approximately the same.

Steam Pressure: With increase in steam pressure, the drying rate increases. Too high a steam pressure results in scorching of the film. An increase in steam pressure from 55 to 65 psi results in approximately 10% increase in production.