Convective drying method
The convective drying method is most common and applicable to the fruit and vegetable drying process. Fruit and berry or vegetable puree can be dried with this method using spray dryers. The heat is supplied in the form of heated air, which mixes with the steam and thus dries the surface of the product.
Advantages of convection drying
The product during convective processing does not overheat if the temperature of the product itself is one hundred Celsius degrees, and the air temperature is 150-200. These conditions are obligatory for the preservation of all nutrients, which are best preserved if the product is immediately cooled after drying.
However, losses cannot be avoided with this drying method. Most of the losses occur at the preparation or final stage. The preparation stage consists in preparing the product for the drying process, the final stage is associated with storing the finished powder in the drying chamber.
Convective drying is carried out using spray drying equipment, which can be easily adjusted to any volume of work and performs them in an automatic mode, allowing you to save on maintenance staff costs.
Disadvantages of convection drying
However, there are also disadvantages of the convective drying process. For example, when it comes into contact with air, the product is oxidized, and aromatic substances lose their strength very quickly. Another disadvantage of spray dryers in their big sizes, i.e. it is non-compact equipment that takes up a lot of space and is rather difficult to relocate. The heat entering the dryer with the air is quickly dissipated and does not have full strength as a result of the air circulating at a high speed. Steam is also consumed in a large volume, as evidenced by the indicators: 1 kg of product absorbs 3.2 kg of steam. In spite of these disadvantages, spraying equipment is still used on different production sites.
The structure of spray dryer
Drying chamber, air filter, spraying mechanism, heat generator, ventilation system, cooling system and cleaning system are included in the spray dryer. The product can be sprayed centrifugally or pneumatically, thus it is possible to distinguish two types of the dryers: the pneumatic dryer and the centrifugal spray drying chamber. The pneumatic spray dryer is represented by a cone-shaped section. The drying process ensures the formation of water droplets, which immediately flow to the bottom of the drying chamber. The dry product enters and is collected in the tank with the conical bottom. Spraying of the centrifugal type is associated with the activity of a spraying disc, which is installed at the bottom of the tank or fixed in the upper point. The centrifugal spray dryer is a productive equipment with high power. The power required to spray vegetable or fruit puree is from seven to eight thousand rotations, and a centrifugal dryer gives much more — eighty thousand. This dryer is represented by a centrifugal chamber, where air is circulated so that the torch spraying the product goes up or down. The possibility of the appearance of a residual product is excluded in this chamber.
Disc and nozzle distributors
Two operations are the most important in convection drying: the method of spraying the mass and the removal of the powder from the drying process. The uniform distribution of droplets and the crushing of the product into particles of the same size depend on taking into account the method of atomization / spraying of the mass. Disc sprayers grind the product more intensively and into finer crumbs than spray nozzles. Larger droplets are generated at the nozzle distributors, while the size of the drop itself depends on the type of product.