[TheForge] Plasmas

[Kenny O.]

Bruce Freeman wrote:

>Kenny,
>
>"Bar" is a pseudo-metric unit of pressure.  1 bar ~ 1
>atm ~ 15 psi.
>
>The spec you quoted says nothing about air consumption
>(which is what SCFM is all about).  You'll have to
>find that out separately.  I don't know much about
>plasma cutters, but I don't think they use all that
>much air.  (In fact, I thought they used nitrogen, not
>air.  But whaddo I know?)
>
>Bruce
>Nj
>
>
>  
>

S Ramakrishnan 
<http://www.iop.org/EJ/search_author?query2=S%20Ramakrishnan&searchfield2=authors&journaltype=all&datetype=all&highlight=on&sort=date_cover&submit=1> 
and M W Rogozinski 
<http://www.iop.org/EJ/search_author?query2=M%20W%20Rogozinski&searchfield2=authors&journaltype=all&datetype=all&highlight=on&sort=date_cover&submit=1>
CSIRO Division of Manufacturing Technology, Locked Bag No 9, Preston Vic 
3072, Australia
*
abstract.* /The properties of plasmas generated for the air plasma 
cutting process have been investigated in this study. The plasma arc 
cutting process employs a plasma torch with a very narrow bore to 
produce a transferred arc to the workpiece at an average current density 
of within the bore of the torch. The energy and momentum of the 
high-velocity plasma jet generated by the plasma torch melts, vaporizes 
and removes the metal from the region of impingement of the jet. 
Measurements have been made of the total arc voltage, nozzle voltage, 
air flow rate and nozzle pressure over a range of arc currents of 40 - 
160 A for a nozzle with a bore of 1.5 mm. Using high-resolution digital 
photography, the radius of the arc at the nozzle exit has been measured 
over the current range. Photographic observations indicate that an under 
expanded supersonic plasma jet emanates from the nozzle. An approximate 
two-zone arc model has been developed to estimate the arc radius, 
voltage and pressure of the arc at the nozzle exit as a function of 
current and the predicted results have been compared with experiments. 
The study reveals that the nozzle of the plasma torch is heavily clogged 
because of the presence of an electric arc with a very high current 
density in the nozzle. The nozzle clogging effect increases the pressure 
in the chamber upstream of the nozzle as the arc current is increased 
for a constant mass flow rate of air. The nozzle clogging phenomenon is 
crucial to generate a plasma jet with the high momentum required to 
remove material from the molten workpiece and to maintain plasma 
stability./