When you have a fluid flowing past a fixed object you will observe a swirling into the space behind the object. A vacuum of sorts (low pressure) forms behind the fixed object and draws the fluid back into the space behind it. You can see this if you fill your tub with water and then move your hand through the water in one direction. You will see cavitation behind your hand and swirling currents in the water moving back towards your hand, the faster you move the more this will occur. This happens when you sit in the air flow going past you into your hood. Air will swirl around into the space in front of your body and can pull some of the torch exhaust back with it. Much of the heat you may be feeling is radiant heat from the flame and glass in it. A smoke test will show you if you are pulling torch exhaust into the area where you are breathing.
If this happening you may need to change where you sit relative to the torch (back a little will help), check that your air flow rate is such that it clears your torch plume but isn't strong enough to create this zone in front of your face that pulls in air from directly above the flame. I use makeup air coming in from under the table in front of the torch and it reduces the turbulence when I am in front of the torch by reducing the amount of air flow coming around my body.
Robert
Quote:
Originally Posted by philomena
I need to come back to this point.
How does this low pressure zone come? I always have the feeling when I'm sitting on my torch the fumes come back to my face. But I have to apologize, I have never heard of a low pressure zone. What is the theory behind it?
I really read a lot about how a ventilation set up should look like for beadmaking and in my class with Loren stump we discussed a lot about it. Everybody said my set up seems to be fine but I always complained about my feeling the fumes are coming back to me.
It would be a big help for me to know what I have to change. 
Cheers
Kathrin |