Repsol YPF

Drying barns

Basically, the drying process consists in removing the water for the product's surface by evaporation, transferring it to the air around it. The speed of this process depends on the air (on the speed with which it flows around the product, its degree of dryness, etc.), and on the product's characteristics (its composition, its moisture content, the size of particles, etc.).

The amount of water vapour that is present in the air is known as humidity: an absolutely dry air, that has no water vapour inside it, has a relative humidity of 0%, whereas one that is saturated with water has a relative humidity of 100%. As the air gets warmer, its
relative humidity falls and, thus, it is able to absorb further humidity. As the air heats up around the product, the latter dehydrates more quickly.

In practice, the maximum level of water that can theoretically be absorbed by the air is never reached, due to the efficiency with which the air mixes with the product, the nature and characteristics of the product, and other reasons. In normal conditions, the air can remove between 30 and 50% of the theoretical amount of water that is present.

Once the surface humidity has been removed by evaporation, the dryness level depends on the speed with which its internal humidity moves to the surface, which is something that varies between different products. Therefore, although one may may initially think otherwise, a better drying process is not necessarily a quicker one, because if the surface is completely dried out, a crust forms that prevents internal drying.

The real energy requirements per kilogramme of product to dry out are variable, depending on the year's climatological conditions, which determine:

• The product's initial degree of humidity.
• The air temperature at the entrance of the drying barn.
• The air's humidity at the entrance of the drying barn.   

The warm air that is necessary may be obtained:

• Through the burning of liquids, solids or gases.
• By using a heat pump.
• By using electrical resistances.
• By taking advantage of the residual heat from other processes.   

Below, a table is featured that may serve as guidance with regards to the temperature and humidity levels that we are referring to: