Nozzles used for spray drying are usually operated at high pressures. Feedstock is abrasive and travel through the nozzles at high velocities, removing material from the internal components. As the nozzle components wear, the performance degrades. The most common symptoms of worn nozzles are an increase in flow rate, degradation of spray pattern uniformity and an increase in droplet size.
There are three areas of the swirl that will show the first visible signs of wear: the narrow “extended part” that forms one side of the inlet, the corner between the bottom and wall of the chamber and the bottom of the chamber itself. Of these three areas, the one that will have the most effect on the spray pattern will be the extended part. Once the extended part shows significant signs of wear, the swirl should be replaced.
In most cases, the orifice will wear uniformly and the hole diameter will grow in size, causing the flow rate to increase. RACA recommends to change the orifice discs when the flow rate increases by 5%, which is equivalent to a hole diameter increase of 2.5%.
Body and Adapter
The velocity of feedstock through the internal passages of the adapter and body are much lower than the velocity through the swirl and orifice and, as a result, wear rates are much lower. The adapter is attached to the feed pipe either by welding or by tapered pipe threads.
The body is connected to the adapter with non-sealing, parallel threads. Several types of bodies are available, for example the standard and the cone face version.
Leaking nozzles in a spray dryer can cause serious problems, including ruined product, fires and explosions. For this reason, it is critical to ensure that the nozzles will not leak during operation. Most nozzles use O-rings as seals. Because of the high operating pressures and high temperatures of the spray dry chamber, RACA recommends that the O-rings be replaced each time that the nozzle is disassembled. RACA also recommends the use of a FDA approved lubricant on all O-rings.
Disassembly and cleaning
Dryer operators usually use a dilute acid solution for nozzle cleaning. When drying dairy products such as milk, cheese, and whey, the nozzles must be cleaned immediately following shut down in order to prevent the parts from getting stuck together. Some clean the complete nozzle in place (CIP), while some remove the wear parts and soak them in the diluted acid solution.
Use the right tools when dismantling the spray nozzles
Clean all parts thoroughly and replace any worn parts with new
Replace the seals every time the nozzle is opened regardless of their appearance
Reassembling and tightening the nozzle according to the instruction manual