The aim of atomization of concentrate is to provide a very large surface, from which evaporation can take place. The smaller droplets, the bigger surface, the easier evaporation, and a better thermal efficiency of the spray dryer is obtained. The ideal from a spray drying point of view would be a spray of drops of same size, which would mean that the drying time for all particles would be the same for obtaining an equal moisture content. In practice, however, no atomizing device has yet been designed to produce a completely homogenous spray, although present designs have a high degree of homogeneity. From a powder bulk density point of view a homogenous spray is not wanted, as this would mean a powder with low bulk density, and that would mean an increase in packing material. It is, however, so that today's achievement of atomizing facilitates both the drying and the powder bulk density.
As mentioned previously the air distribution and atomization are the key factors to a successful utilization of the spray dryer. The atomization is directly responsible for many distinctive advantages offered by the spray drying. First, the very short drying time of the particles can be mentioned, secondly a very short particle retention time in the hot atmosphere and low particle temperature (wet bulb temperature) and finally the transformation of the liquid feed into a powder with long storage stability ready for packing and transport.
Summarized, the prime function of atomization is:
- a high surface to mass ratio resulting in high evaporation rates,
- production of particles of the desired shape, size and density.
To comply with these requirements many atomization techniques have been used in spray dryers. However, the most common ones can be summarized as follows:
- Pressure energy as in pressure nozzles
- Kinetic energy as in two-fluid nozzles
- Centrifugal energy as in rotating discs
The mechanism of atomization has been studied by many scientists, and though the first pioneers started more than 100 years ago, the subject is still highly controversial in spite of many published data.
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