Spray drying nozzles

Engineering consideration 2 - Droplet size

The heat transfer, and hence evaporative power, of any system will be a direct function of the surface area presented to the heating element.  In the case of spray drying this means that the greater the surface area of the fluid being sprayed the quicker the water will be evaporated from the slurry.

The surface area of a given volume of fluid is inversely proportional to average droplet size that makes up that fluid.  If  one halves the average droplet size of a fluid, and keeps the overall volume of fluid the same, the summed surface area of all the droplets will double.  In other words, the smaller the droplet size the greater the surface area and hence the greater the drying power of the spray drying system.  (For a detailed explanation of surface area to volume ratio please see our engineering resources section.)

For this reason spray drying nozzles are designed to form very fine droplets.  They do this by operating at very high pressures and using the high internal energy of the slurry under such pressures to break it apart into a fine spray.

Uniformity of droplets

It is also important to ensure that the droplets formed are fairly uniform in size.  Many nozzles will produce a broad spectrum of droplet sizes but this is undesirable in spray drying applications, as larger droplets may not have liquid removed and thus fail to dry.  Smaller droplets will have their moisture removed but the remaining powder particle may be damaged by excessive heat.  For this reason spray drying nozzles are designed to give a high degree of regularity of droplet size.  They have a low relative span of particles - for more details on relative span and how it is measured and controlled please visit our engineering resource section.