9. Jet nozzles
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”.
Venturi-principle:
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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