Chose your nozzles with care

Shot blasting efficiency depends on the right nozzle selection

Perhaps the most critical component for reaching production goals is chosing the nozzle. The nozzle’s bore shape determines its blast pattern as needed for the application.

  • Straight bore nozzle creates a tight blast pattern and is ideal for blasting small areas.

  • Venturi bore nozzle creates a wide blast pattern and increase particle improving productivity when blasting larger surfaces.

Normally it is the venturi type of shot blasting nozzle that is used. It is designed with a unique internal shape to achieve different objectives.

The venturi shaped nozzle gradually tapers outward from the orifice to the exit-end of the nozzle. This gradual expansion allows a mixing of air and abrasive within the nozzle causing them to expand uniformly before leaving the nozzle. A venturi nozzle provides excellent peening intensity and cleaning capability with a wide blasting pattern. The performance of the venturi nozzle depends on a precise ratio of length to orifice size, and to entry and exit tapers. This design creates a large blast pattern that produces maximum acceleration for cleaning.

As nozzles wear from continuous exposure to high-velocity media, more air and media are allowed to pass through the orifice. The resulting larger area within the nozzle consumes more air volume placing greater demand on the compressed air source. Unless air volume can keep up with the increased flow, pressure at the nozzle will drop. With reduced pressure, peening intensity and pro- ductivity falls and efficiency goes downs. A rule of thumb to follow for ensuring continuous high production is to replace the nozzle when the orifice wears to the next larger size.

Air consumption compared with nozzle wear

Nozzle Size Air flow Increase in air consumption
    4     1/4"     2,3 m³/min  
    5     5/16"     3,8 m³/min 96% more than no. 4
    6     3/8"     5,4 m³/min 43% more than no. 5
    7     7/16"     7,0 m³/min 29% more than no. 6
    8     1/2"     9,4 m³/min 33% more than no. 7

Information shown is based upon air consumption at 100 psi (7 bar/700kPa)
 

Nozzle pressure
Maintaining adequate nozzle pressure is essential to high-production blasting. The gauge on the compressor shows the air pressure at the compressor only. It does not indicate blasting pressure. Hoses, air filters, blast machines, and other components between the compressor and the nozzle all contribute to friction and pressure losses. To accurately determine nozzle pressure, use a hypoder- mic needle gauge. This simple tool consists of a needle mounted on a pressure gauge. Insert the needle into the blast hose at a 45° about 15 cm behind the nozzle holder, with the tip of the needle pointing toward the nozzle.

Nozzle pressure below 5,8 bar (85 psi) is indicating that something is not as it should be. Then you have to check the settings of the air compressor, and then check for restrictions at all fittings and hoses, moisture separators, and any system components. You also have to check the nozzle orifice for excessive wear.

Nozzle pressure compared with nozzle diameter

The scheeme above shows calculated consumption rates of air and abrasive for new nozzles. When you select the compressor then add 50% to the figures shown above to allow for normal nozzle wear and friction loss.

NOTE: The figures may vary depending upon working conditions. To maintain desired air pressure as nozzle orifice wears, air consumption increases. The effects of nozzle wear on air consumption must be considered when selecting nozzles and the compressors that support them.

Minimum air volume at 7 bars

Nozzle Size of orifice Min. airconsumtion
    4     1/4"     4,2 m³/min
    5     5/16"     6,6 m³/min
    6     3/8"     9,0 m³/min
    7     7/16"     11,6 m³/min
    8     1/2"     16,1 m³/min

 

Frederik Nielsen

"Need more information about shot blasting nozzles and how the right choise of a nozzle may improve your shot blasting process? Please call me! "

Frederik Nielsen CEO, Clemco Denmark

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