The rate of heat transfer and evaporation in any system is directly proportional to the surface area exposed to the heating element. In spray drying, this means that the greater the surface area of the liquid being sprayed, the faster the water will evaporate from the slurry.
The surface area of a given volume of liquid is inversely proportional to the average drop size that makes up that liquid. If the average drop size of a liquid is halved while keeping the total volume of liquid constant, the total surface area of all the drops will double. Thus, smaller droplet size leads to greater surface area and therefore more efficient drying in a spray drying system.
To achieve fine droplets, spray drying nozzles operate at high pressures and utilize the high internal energy of the slurry to break it up into a fine spray.
Consistency in droplet size is also crucial. While many nozzles may produce a wide range of droplet sizes, this is undesirable in spray drying applications, as larger drops may not have enough moisture removed and fail to dry, while medium-sized drops may have excess heat damage the resulting powder particles. To ensure consistency, spray drying nozzles are designed to provide a high level of droplet size uniformity, with a low relative range of particles. For more information on relative range and how it is measured and controlled, please visit our engineering resource section.