SPRAY DRYING

In spray drying, achieving the right balance between atomization and flow rate is crucial. A low flow rate of liquid is needed to enable the necessary evaporation, while well-atomized liquid maximizes the surface area exposed to the hot air, enhancing evaporation rates. However, these two factors are at odds with each other. The ability of a direct pressure nozzle to atomize liquid depends on the pressure at which the liquid is sprayed, but higher pressure also means higher flow rate.

To address this issue, specialized spray drying nozzles have been developed to provide high atomization at low flow rates. These nozzles use a swirl chamber to atomize the liquid before it exits the nozzle, which maximizes atomization at the expense of flow rate. This design allows more of the energy from the liquid to be used for atomization, rather than simply increasing flow rate as in a typical axial spray nozzle.

These specialized nozzles operate at very high pressures to provide the energy needed for effective atomization, but this high pressure also presents challenges for nozzle wear. Overall, optimizing atomization and flow rate is a critical consideration in spray drying processes.

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.

DESIGNING CONSIDERATION 3 – CONTROL OF SPRAY PROPERTIES Achieving the optimal balance of bead size, flow rate, and spray pattern is essential for efficient spray drying of each unique liquid. This balance is not only crucial for energy efficiency but also to avoid overheating of the spray, especially in the case of food products.

The ideal balance may vary even among batches of the same liquid, requiring intimate knowledge of the process and product. Designs are measured and can be adjusted to modify flow properties, with numerous combinations providing a high degree of control over the spray drying process.

Spraying at high pressures causes wear and tear on the nozzle, which is compounded by the fact that the liquid being sprayed is typically viscous or contains suspended particles. As a result, the suspended particles are forcefully pushed into the nozzles at high pressure, leading to rapid wear and tear.

To address the issue of rapid wear and tear, most spray dry nozzles are equipped with tungsten carbide compound whirl chambers, but even this only serves to slow down the problem. The modular design of the nozzles helps to reduce the cost of replacements, as only the worn-out part needs to be replaced rather than the entire assembly.