The development of jet nozzles has to be considered both as regards the material as well as the shape of the nozzle, in the past only short nozzles made of grey cast iron were used, later they used to make it of cast steel and malleable cast iron. Only by the introduction of the hard metal technology it has been possible to produce more wear-resistant nozzles.
Whereas they used to speak about “day nozzles” expressing the durability, nowadays they mainly speak about Venturi or Laval nozzles.
Cast nozzles showed a very high degree of wear and tear; this brought the disadvantage that the outlet openings became bigger every hour and the supply performance of the connected compressor was not big enough resulting in a rapid pressure decrease before the nozzle and also in considerable decreases of the abrasive blasting performance. During a short interim period of time porcelain nozzles were used which had a durability of 20 to 25 hours.
It is a further disadvantage of nozzles with short service life that the outlet openings are not equally round, but mostly ovally worn off influencing the blasting jet considerably in its laminar flow. The application of hard metals such as tungsten carbide and boron carbide has reduced decisively wear and tear of the nozzle borehole of the inlet and outlet channel. Service times of 500 to 1,000 hours are quite common nowadays, provided that the nozzle can be protected against vibrations caused by knocking. The extremely brittle hard metal gets finest hair cracks which are washed out immediately by the sharp jet.
The hard metal body is produced on the basis of the sintering method. By introducing the sintering technology the nozzle could be shaped in such a way that a respective inlet cone (inlet angel) as well as a respective outlet cone can be made out of one piece. In this regard they remembered the theory of Laval who had proven that only a very long nozzle was able to convert pressure completely into velocity because a conversion of pressure into velocity by 100% is not abruptly possible.
When selecting the optimal jet nozzle three criteria are of special importance:
1. the size of the inlet opening and the length of the nozzle
2. the shape of the nozzle channel
3. the wear resistance of the nozzle
The highest outlet velocity would result by a nozzle channel being very long and enabling expansion of the pressure up to the absolute outlet pressure of 0 b.
Of course, this would not be possible in practice.
A long nozzle always shows a better blasting performance than a comparably short nozzle. Because a short nozzle can be handled more flexibly this one is often used for complicated and flexible abrasive blasting processes; and in this case the lower blasting performance is accepted.
Gustav de Laval developed the Laval-shape, i.e. the theoretically optimal shape of a nozzle allowing higher outlet speeds than the critical speed for abrasives because further expansion takes place in the outlet cone. This principle was integrated in the “Venturi-Nozzle-Technology”.
This operating principle is quite simple.
Air flows through an air channel. The air channel narrows up to the centre of the reduction of the cross-sectional area and expands again to a bigger diameter.
There will be resistance against the air that flows through this Venturi channel. The speed of the air flowing through increases by growing reduction of the cross-sectional area and reaches its highest value at the narrowest point.
If there will be a so-called critical pressure at the narrowest point of the nozzle, the cross-section of the nozzle has to increase from this point on again to further increase the speed reached up to now.
Air-Outlet speeds at jet nozzles
with a straight outlet amount to about 120 – 150 m/s
with a laval-shaped outlet amount to more than 330 m/s
at an operating pressure of 7 b at the nozzle opening.
The added abrasive cannot, however, accept this speed of the air stream because of its shape and mass so that the real outlet speed of the abrasive is lower.
Outlet speeds of the abrasive at jet nozzles
with a straight outlet amount to about 80 – 90 m/s
with a laval-shaped outlet amount to about 160 – 240 m/s.
In order to reach the optimal outlet speed of the abrasive it has to be ensured that the jet nozzles will be provided with sufficient amounts of compressed air according to their diameter. The nozzle table shows the required minimum amount of compressed air (volume flow) in dependence on the operating pressure and on the nozzle diameter.
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Example of different jet nozzles
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