Choosing the right nozzle for optimal productivity
Choosing the right nozzle is the key to achieving maximum productivity with whatever air compressor you have available to you. Choose a nozzle with too small of a bore and you’ll be leaving some blasting capacity on the table. Choose a nozzle with too big of a bore and you’ll lack the pressure to blast productively. There’s a balance to be struck, and it starts with your air compressor.
The more air volume you can compress per minute, the higher the pressure produced at the nozzle. This improves your production rate in two ways:
High pressure increases the velocity of the abrasive particles, delivering more kinetic energy upon impact, imparting a deeper anchor pattern. This allows you to use a finer grade of abrasive, which produces more impacts per volume.
More air capacity lets you use a bigger bore nozzle, which allows you to put more abrasive on target.
Assuming that the air compressor capacity available to you is not unlimited, you’ll need to select a nozzle with an orifice small enough to maintain a productive air pressure at whatever volume your compressor can supply.
For vapor abrasive blasting, the most commonly-used nozzle orifice sizes ranges from 3/8” inner diameter to 3/4”, increasing by increments of 1/16”. A 3/8” nozzle is sufficiently constricted to produce a effective blast pressure with a 185 CFM compressor. A ½” nozzle is sufficient to produce an effective blast pressure with a 375 CFM compressor.
It’s important to note that when you double the diameter of the orifice, you quadruple the size of the orifice and the volume of air and abrasive that can pass through the nozzle. To find your optimally productive nozzle, determine what nozzle pressure (PSI) you need to maintain for productive blasting, and what volume of air your available compressor can supply per minute (CFM), then consult the following chart to find the nozzle orifice size that meets those parameters.
||Required minimum liters
Bear in mind that your compressor will produce less pressure at the nozzle than what is indicated on the chart, depending on the condition and configuration of your equipment.
Nozzles wear out. Abrasive passing through the nozzle will wear away the lining, enlarging the orifice. A worn nozzle requires more volume per minute to maintain your target pressure. For example, If you bought a #8 (1/2″) nozzle to get 100 PSI with your 375 CFM compressor, once it has worn out 1/8″ diameter to the next nozzle size up (#9 – 5/8″), you’d be lucky to maintain 65 PSI using the same compressor (on the #9 line, the 375 CFM compressor falls between 350 and 400 CFM, so you can only maintain 65 PSI), because the volume of air passing through the enlarged orifice would be inadequate to maintain the pressure.
However, if you purchased a #7 (7/16″), still using a 375 compressor and targeting 100 PSI, once it had worn out to the next size up (#8 – 1/2″), you’d still be able to achieve 100 PSI, but you’d sacrifice some productivity earlier in the nozzle’s life (compared to a #8 (1/2″) nozzle) because the amount of air and abrasive that could pass through the orifice at 100 PSI would be less than what your compressor could supply.
Pressure can drop significantly between the compressor and the nozzle, up to one PSI per 50’ of hose (1” I.D.). In addition, pressure will drop with each bend in the hose or change of direction: the shortest, straightest hose configuration is best. On top of that, if there are ill-fitting couplings or leaks in the hose, the pressure drop could be total.
Not only does the size of the nozzle bore effect volume and pressure, but it effects the size of the blast pattern. For each nozzle size increase, expect up to a 10% increase in the size of your blast pattern. But it’s the shape of the nozzle, rather than orifice size, that has the most impact on the size of your blast pattern.