Nozzle clogging is a frustrating problem, and it is easy to understand why.
A clogged nozzle cannot do its job because:
Coverage is impaired. A blocked orifice does not allow full spray angle formation.
Distribution is interrupted. The spray tends to get "lumpy" with heavy areas and voids.
Flow rates are reduced. With passages clogged, less liquid gets through the nozzle.
Droplet spectrum shifts. A clogged or partially clogged nozzle cannot create the desired droplet size.
Nozzles do not clog by themselves. The debris has to come from somewhere else in the process.
To deal with the problem, you can either make the debris go away or find a nozzle that can tolerate the debris.
A permanent solution usually involves a combination of both.
Removing the debris
The best approach to eliminating nozzle clogging is to remove the debris. If you can identify the material, you might be able to figure out its source.
My company once dealt with a customer who was using a group of air-atomizing nozzles. The air passages kept clogging. Through a simple investigation, the customer identified the material as rust flakes. That reminded him that he had no air dryer and that the piping was carbon steel. Water in the lines was rusting the pipe from the inside, finding its way to the nozzle. A dryer helped, but eventually he had to replace the piping. The problem then went away.
If you cannot eliminate the source of the debris, you might be able to capture it before it reaches the nozzles. This approach involves strainers, filters or some other separation method. Remember: Make sure you choose the right size mesh or capture medium.
The rule of thumb for selecting the degree of filtration is to keep in mind the size of the expected debris and the smallest-size nozzle on the system. The measurement you need for the nozzle is the "free passage" dimension. The filter should be able to capture anything larger than one-third the free passage. If the smallest free passage measurement is 0.090 inches (in.), the filter should be able to capture anything larger than 0.030 in.
If you do resort to filtration, make sure the filtration device is positioned appropriately. In one case I remember, a customer thought he was using appropriately sized and selected strainers. An analysis based on nozzle and debris size suggested everything should work. However, the nozzles clogged occasionally, especially after someone disturbed the piping. It turned out that small particles that would normally pass through the strainer and nozzle accumulated in the pipe. Over time, they agglomerated into a coating on the pipe walls.
Eventually, a chunk of coating could peel off the wall and clog a nozzle. The situation improved when the strainer was moved closer to the nozzles.
Before adding filtration, however, try to find "more tolerant" nozzles to mitigate the problem before you go overboard.
These two axial flat-fan nozzles have the same flow rate; however, the one on the left has a 15-degree spray angle, and the one on the right a 110-degree angle. Both could be listed in literature with the same "equivalent orifice," although their free passage measurements are very different.
Choosing a better design
The other main approach to solving clogging problems is to find a more suitable nozzle. First, evaluate the clog resistance of the current nozzle array. "Free passage" ," or "narrowest cross section" ," indicates the largest particle that can pass through the nozzle at its smallest point.
The normal assumption is that the particle is spherical. A nozzle must have a point at which it meters the flow, so a maximum theoretical open section exists for any design in that capacity. Much of the time ," but not always ," this section is the final orifice. Sometimes, the smallest point is at the inlet or somewhere inside. The debris might not be visible when you look into the nozzle's final orifice.
This information is not mentioned on every page of a nozzle catalog. If you do not see "free passage" listed specifically, you might see a measurement for the "orifice diameter." Often, this dimension also is the free passage, but not always. Do not be confused by the term "equivalent orifice." This measurement can be used to compare the flow rate of different nozzle configurations, but it can be very misleading if you are trying to evaluate clog resistance.
The free passage dimension is the clearest. You should know exactly what will fit through the nozzle when you have that figure.
Orifice diameter could help you if you know something about nozzle design. Frequently, it is as good a number as free passage if you know the orifice is, in fact, the smallest point. A catalog should provide this information in most cases.
Usually equivalent orifice diameter turns up only during discussions of axial flat-fan nozzles. Most of these nozzles have an elliptical orifice. If you look at a catalog chart, you will find manufacturers have the same flow rate available in various spray angles. Some customers make their own flow calculations based on orifice diameter, so the equivalent orifice is an attempt to make an approximation of the orifice if it were round. In virtually every case, free passage is smaller than this measurement, sometimes drastically smaller. Consequently, it is much safer to find the true free passage measurement.
Nozzle families have their own unique characteristics. If you understand these characteristics, you will be better able to find the most clog-resistant designs.
Standard axial designs.These designs are called "mill and drill" designs. The orifice is formed through a milling process from the outside through to where a hemispherical hole has been drilled from the inside, forming an elliptical orifice. The size and severity of the ellipse determine the flow and spray angle.
Narrow-angle nozzles are nearly round. Wide-angle nozzles are more of a slit. A 15-degree nozzle of a given flow rate will have a larger free passage than a 90-degree nozzle of the same flow rate. The minor axis of the ellipse is the real free-passage measurement for a nozzle such as this.