The size of drops in a spray coating system can have significant effects on the final coating quality. Finely atomized sprays have a higher surface area to volume ratio, which means they have more overall contact with the target and will adhere more quickly. This is particularly important in drum and liquid bed coating systems, where contact is randomized.
On the other hand, larger drops tend to bounce or break on impact instead of sticking to a surface. When they do form a layer, it tends to be uneven and thicker, meaning any required evaporation may take longer and result in an uneven coating. The prolonged presence of wet product increases the likelihood of product sticking together in liquid bed/pan coating systems or partially wet product on transport systems.
NOZZLES FOR SPRAYING VISCOUS LIQUIDS
Spraying viscous liquids presents several challenges, particularly in the food industry where many viscous liquids must be sprayed precisely.
Consistency is a measure of a liquid's resistance to shear stress and an approximate measure of its "thickness." It is measured in Pascal seconds, with water at 20 degrees Celsius having a consistency of 1.002 mPa.s. This is commonly reclassified as the unit centipoises (cps), with 1 cps equal to 1 millipascal second, such that water has a consistency of around 1 cps at room temperature.
AIR ATOMIZING NOZZLES
Many air atomizing nozzles mix the liquid and air inside the nozzle before discharge from the opening. This is sufficient for low viscosity liquids like water, but for liquids much above 100 cps, issues arise. The larger drop sizes and air pockets that form in the chamber severely disrupt the spray pattern, leading to blockages and degraded coating quality. To overcome this challenge, external mix air atomizers should be deployed. These mix the air with the liquid stream after exit from the nozzle, avoiding the problems encountered by internal mix atomizers.