Check and make sure that the air jet and nozzle bore sizes are optimised.   It is a fact that when checked many venturie blast systems have air jets and nozzles that are entirely miss-matched.

The design of the venturie is offered in various ways, but essentially, by passing air over the end of the pipe, a vacuum is formed to suck abrasive from the hopper.  If the nozzle bore is too large, a lessor vacuum is generated and the volume of air is allowed to expand prematurely, losing speed and without the blast media quantity desired.

If the nozzle bore has worn and has increased from its original size by 20%, then your process parameters, productivity, air pressure increased compensation and ongoing costs have totally changed.
In the smaller type blast gun, an air Jet of 3.2mm (1/8”) bore will pass 25cfm FAD (Free Air Delivered*) continuously at a pressure set at  5.5 Bar (80psi). As a rule of thumb, a suitably sized compressor will require 5.5Kw of energy to maintain this level of flow.  (*FAD is used for the measurement of air placed through an air jet open to atmosphere.  Do not confuse with compressor displacement calculation, which fools most people not used to dealing with the dynamics of compressed air when blasting)

A compatible abrasive nozzle for a 3.2mm air jet, the nozzle will be between 6.4mm (¼”) to 8mm (5/16”) in bore. If the bore is worn to a size 20% larger, the bore becomes 9.6mm and allows the air to expand and will slow down the forward projection speed by upto 50%. A similar principle happens to larger bore sizes with significantly larger discrepancies.

Click to read Tip No. 9 Quality and delivery of your compressed air

For more information on Shot Blasting please visit www.rosler.com

Post written by
Haydn Kitchen
Shot Blasting Technical Manager