Condensed and Dried Milks

Module 13. Technology of dried milks

Lesson 27
DRUM OR ROLLER DRYING

27.1 Introduction

Milk and skim milk are dried commercially by the spray, drum or foam spray method. Today most milk and skim milk are dried by the spray method. In the past, the drum dried dairy products constituted a high percentage of the total amount produced. But since World War II decreased economic advantage of processing by drum compared to the spray process and its characteristic scorched flavour and poor solubility due to the severe heat treatment have drastically reduced the volume of drum dried milks. Now only a relatively small amount is manufactured by the drum process.

27.2 Drum or Roller Drying

Milk or milk products can be dried in a thin film (~ 0.1 mm) on an internally steam heated rotating drum. Often, two drums are set up side by side at a very small distance apart. The water evaporates within a few seconds, which is possible due to the high drying temperature (>100°C). On a drum drier, the product enters as a liquid and leaves as a solid. The product is removed from the exterior of the drum by a "doctor" blade. The dried film is scraped off from the drum by means of a steel knife, collected, and ground.

The process is usually called drum drying, or in the dairy industry, more often called “Roller Drying." The process is also known as "Film Drying". Drum drying requires less space and is more economical than the spray drier for the small volume.

The major disadvantage of drum drying is that the dry product may have a scorched flavor and solubility is much lower because of protein denaturation. Considerable product damage due to heating occurs, mainly because scraping off is always imperfect and, accordingly, a part of the milk is repeatedly wetted and dried. The quality of the powder can be improved by using a vacuum roller drier, in which the milk is dried at a lower temperature, but this method is expensive. Nowadays the roller drying process is little used.

27.3 Classification of Drum Dryers

Drum dryers may be classified as shown in Table 27.1.

Table 27.1 Classification of drum dryers

Generally cast iron is usually used. The wear is excessive on stainless steel drums. The metal used for the knife should be softer than the drum. The double drum atmospheric drier is most commonly used in the dairy industry. Vacuum drum driers are essentially the same as, atmospheric units except that the drums are enclosed so that a vacuum can be pulled on the product during drying. The single drum with top feed is more commonly used for vacuum. A thicker film is obtained with the top feed.

27.4 Flow of Product

The product may be placed in its natural form or condensed in a vacuum pan or evaporator before it is fed to the drum drier. Milk is usually pre-condensed for single drum units. The product is usually preheated and placed in a reservoir between the upper portions of the drums. Other devices may be used to provide thin film over the turning drums. The doctor blade, a sharp hard flexible knife, scrapes the dried material from the drum. The blade sits at an angle of 15 to 30° with the surface. The film of dry milk forms a continuous sheet from the knife to the auger trough which is about level with the bottom of the drum. The auger for each drum discharges the product into elevators, then to the hammer mill which pulverizes the product, after which it may be sized. After sizing, the dried product is packaged and stored, or moved to market. These preceding steps are carried out in one continuous operation. In some vacuum units, the product is accumulated in a tub or tray and removed periodically first from one side of an enclosed chamber (separate from the drying chamber) to the other side, while maintaining a vacuum on the drying unit.

Water vapor above the drier has a lower density than the air surrounding the unit, and will rise. A hood must be placed over the drums with a stack for vapour to move out of the area.

27.5 Drums

The atmospheric double drum drier is most commonly used for milk drying.

* Vacuum: The vacuum drum drier is used for foods and milk. The inside of the drum is heated with steam with a low pressure. For vacuum drum drying, the vacuum between the drum and the housing surrounding the drum is maintained at 68.5 to 73.6 cm Hg vacuum. The product temperature approaches the temperature of the steam heating medium.

* Size & Speed: The drums used for a drier are 61 to 122 cm in diameter, and up to 3.6 m in length. Drums must be carefully machined, inside and outside, otherwise a difference in thickness will alter, heat transfer and drying will not be uniform. The speed of the drums is adjustable usually being from 6 to 24 rpm; however, the range of speed may be from 1 to 36 rpm. Speed of the drums is important as it affects (a) the thickness of film, and (b) the time the product is on the roll. The speed of the drums may be varied according to the concentration of the product and the dryness desired.

* Product Contact: The product is removed after 3/4th to 7/8th of a revolution of the drum has taken place. The product is in contact with the drum for about 3 s or less.

* Mounting: One drum of a double drum drier is mounted on a stationary bearing; the other drum is mounted on a bearing which can be moved to provide the desired clearance between drums.

* Spacing: The spacing between drums of a double cylinder drier is about 0.05 to 0.1 cm when the drums are cold. The level of milk between the drums affects the capacity of the unit and is designated by the distance above the centerline of the drums. The product is contained over the drums with end plates.

* Uniform Film Feeding: A single drum drier may be equipped with upto five spreader rolls to increase the uniformity of thickness of the film on the drier. The concentrated product is fed onto the drum next to the spreader rolls. As the drum turns, a thin film is formed, and an additional film then forms on the next spreader roll.

27.6 Moisture Removal

Moisture is removed by circulation of air which carries away the moisture above the unit. A hood above the unit directs the water vapors from the drier. A product entering the reservoir above a double drum with 9% solids would leave at about 12 to 16 % total solids. A hood should be placed over, the sump or reservoir which is between and above the drums. Adequate airflow must move over the surface of the drums to carry away moisture. An excessive amount of airflow should be avoided since this causes low efficiency due to rapid cooling of the drum surfaces and of the product. In a vacuum drum drier, moisture must be removed from the chamber in much the same procedure as done in the evaporators.

27.7 Steam Flow

Steam is fed into the center of the drum at one end of the shaft through the hub. Steam pressures up to 6.3 kg/cm² are utilized with dry saturated steam up to 149°C. Superheated steam should be avoided because as heat is removed from the steam, the vapor condenses to a liquid. The liquid condensate moves to the bottom of the drum and must be removed by a pump or siphon. Flooding of the inside of the drum with the condensate reduces the rate of heat transfer.

27.8 Factors Affecting Production from a Drum Drier

(1) Milk Feed Temperature : As the milk feed temperature is increased, the rate of drying increased proportionally at about 2.2 % for each 5.5°C increase in feed temperature with little increase above 71°C.

(2) Height of Milk : Increasing the height of milk over the drum from 25 to 30 cm gives an increase in drying rate of 10%.

(3) Drum Gap : The roller gap should be adjusted between 0.05 and 1.09 cm. If the drums are farther apart, leakage will occur. The thickness of the film on the drum is directly related to the distance between the two drums.

(4) Drum Speed : Only a slight increase in drying rate occurs when the drum speed is increased. As the speed is increased, the film becomes thinner so that the production, both in kilograms of water evaporated and kilograms of dry product produced remain approximately the same with only slight increase in drying rate as the speed is increased.

(5) Steam Pressure : With an increase in steam pressure, the temperature is increased and thereby the rate of drying increased. Too high a steam pressure or super heated steam must be avoided because scorching of the film will result. Increasing the steam pressure from 3.87 to 4.57 kg/cm² (150 to 155°C) increases the production of dry product by approximately 10%.

The product may be damaged and scorched if there is uneven milk supply, incomplete removal of film, imperfect roller alignment, rough roller, too high a temperature of the product caused by too high a steam temperature or too slow a drum speed.

27.9 Operation and Maintenance

Several items must be checked periodically to assure maintenance of quality of the product at sufficient, rated capacities or output. Following factors must be considered.

(1) Drums must be properly aligned, particularly for double drum or twin drum units and have identical characteristics of speed, heat transfer, wear, etc.

(2) Knives must be reground regularly (~ every 100 hr) and be of uniform sharpness.

(3) Knives must be flexible, machined on both edges, have uniform thickness and be easily adjusted. Uniform knife pressure against the drum must be maintained. Excessive pressure of blade on the drum increases the energy to operate the drum and danger of metallic shavings in the product. .

(4) Drum surface must be kept smooth. It may be necessary to resurface the drums after 1000 to 3000 hr of operation. The drum should receive a film of oil or paraffin wax when not used regularly to prevent rust.

(5) Condensate must be promptly removed from inside the drum.

(6) Drum must be vented of air to assure that all of the interior heat transfer surface of the drum can be utilized for steam.

(7) Processing operations before drum drying consist primarily of removal of moisture, normally 2:1 concentration to 16 to 18% solids, in an evaporator or vacuum pan. Clarification helps to some extent. Homogenization before drying on a drum does not affect the rate of drying, but will help quality for whole milk.

(8) Starting the drier is accomplished by

(a) Lifting the knives,

(b) Starting the drums

(c) Turning on the steam, and

(d) Placing the product on the drums.

The very first powder is not of as high a quality as the remainder because of the tendency to be either over or under dried. The drums should be set in motion when the heat is applied by gradually turning on the steam to prevent warping.

(9) Preheating of product is desirable, especially if intended for bakery use.

(10) End plates or end drums, which are spring loaded and contain the product above double drums, must be properly adjusted to (a) prevent leakage and (b) accumulation of solids.

(11) The level of milk in the reservoir must be uniform and holes kept open in the distribution tubing. Change in the milk level affects the film thickness on the drum and thus the steam requirements.

(12) High moisture in the product is due to low temperature, thick film, high total solids, and fast speed (rpm.)