Too Hot to Handle?

waltl

Hi. Often, when I am a tour guide at the Museum of Glass, Tacoma,WA. glassblowing shop. I get a question for which I don't have a definitive answer. I thought someone might know.
The question is "why don't the glassblowing pipes(steel tubes, 4.5 feet long, .75" wide) get too hot to hold"
Some of the tour guides say that it is the stainless steel which has a relatively low thermal conductivity(which it does), but that can't be the whole story since stainless pipes are relatively new and that doesn't account for at least 1900 years of glassblowing.
I tell them it is a combination of two things. One the shape of the pipe which has a high surface area to mass ratio so it radiates heat efficiently, the other is that once a pipe head is covered in glass it is to some extent insulated from the heat of the gloryhole.
Which is a bigger factor in the cooling of the handle end of the pipe? Conduction or convection from the surface?

What's your take??

PS. Do you know of anyone who has addressed this question experimentally?

Thanks for your help.

AlephZero

Glass is a very poor conductor of heat. The thermal conductivity of glass is about 10% of stainless steel, and only about 1% to 2% of iron and most steels.

So if you have a layer of molten glass at say 1500C around the end of the pipe, about 90% of the temperature gradient will be inside the glass. In other words the hot end of the pipe will be at about 150C, and you then have the length of the pipe to dissipate that heat by convection and radiation to the air.

It is counter-intuitive, but if you used an iron or non-stainless steel pipe which has an even higher thermal conductivity, the pipe would be even cooler, because the rate of heat flow is limited by what can conduct through the glass not what can conduct along the pipe, and the temperature gradient along the pipe would be smaller.

waltl

Thanks for your help, AlephZero.
I am not a scientist, so I am still a little unclear about the last paragraph.
Do you mean that the pipe with higher thermal conductivity is better able dissipate the heat before it reaches the handle?
Or perhaps something else?

NobodySpecial

No it means the higher conductivity metal is colder at the glass end - because it is connected to a cold thing = you.
The biggest temperature difference will then be between the glass end of the pipe and the molten glass - since this temperature difference is accross low conductivity glass there is less overall heat flow.

A good way of thinking about it is the traffic jams that would be caused if an 8 lane highway went down to 1 lane - compared to it being 4 lanes all the way