thermal conductivity question/confusion
thermal conductivity question/confusion
(OP)
I have a machine that blows hot air (350F) into a tube that is 7 inches ID by 18.5 inches long. The tube is currently made of 1020 CRS, 18 gauge (.048" thick). The idea is that I want minimal heat loss in this tube, ideally having 350F air at the exit end. Here are thermal conductivity ratings of three different materials:
1020 CRS: 51.9 W/m*k
304SS: 16.2 W/m*k
glass: 1.1 W/m*k
So one would think that if I made the tube out of 304SS I would get a higher air outlet temperature (less heat loss), and if I made it out of glass the outlet temp would be higher still. However, this is not the case, in fact there was almost no measurable difference when I tried it. What am I missing in my thinking? Is there another property I need to be looking at? Is the tube so thin that it doesn't matter what it is made of? Thanks for any help thermal guys and gals!
1020 CRS: 51.9 W/m*k
304SS: 16.2 W/m*k
glass: 1.1 W/m*k
So one would think that if I made the tube out of 304SS I would get a higher air outlet temperature (less heat loss), and if I made it out of glass the outlet temp would be higher still. However, this is not the case, in fact there was almost no measurable difference when I tried it. What am I missing in my thinking? Is there another property I need to be looking at? Is the tube so thin that it doesn't matter what it is made of? Thanks for any help thermal guys and gals!





RE: thermal conductivity question/confusion
As you mention temperature measurements, did you measure the inlet temperature together with the oulet one? And how much is the flow rate?
prex
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RE: thermal conductivity question/confusion
It's not an instrument error, I have used several diffferent RTD's and thermocouples and they all read the same.
RE: thermal conductivity question/confusion
RE: thermal conductivity question/confusion
You should have two heat loss mechanisms, convection and radiation, assuming your tube is conductively isolated, otherwise, you have conduction as well. The largest, because of temperature, is radiation.
You didn't describe the external environment. Is it at room temp? How much air flow is there across the tube, either from air conditioning or just convection from the tube?
A simple solution might be to place a highlt reflective tube with an ID 1 inch larger than the OD of your tube. This will minimize radiated loss as well as convection.
TTFN
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RE: thermal conductivity question/confusion
Temperature around the tube is ambient (say 70F). No airflow across the tube, it's stagnant inside the cabinet of a machine. The temperature measurements are actually done in 2" OD tubes at the inlet and outlet of the big tube (it necks down on either end with flat plates, no taper/transition). Any heat that's being lost is through the walls of the tube, I'm sure of that. My question is really about thermal conductivity, and whether or not I understand it properly. Why would a glass tube with such a low thermal conductivity now show and difference from a carbon steel tube?
RE: thermal conductivity question/confusion
If my numbers are correct, you're losing about 1 kW, mostly from radiation. Convection might only account for about 200 W, tops.
However, I would disagree that your cabinet temperature is only 70ºF if there's no air flow, since you're clearly dumping a fairly substantial heat load into the cabinet. It would need to have forced air cooling fans to remove the heat from the cabinet, therefore, it would have to have air flow. Something with external walls at nearly 350ºF is NOT going to have 70ºF air around it.
TTFN
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RE: thermal conductivity question/confusion
You're right, the inside cabinet temp is higher than 70F while the unit is running, like you said due to the heat loss that we're talking about in the first place. It might be closer to 120F inside that cabinet.
So you're saying that what's really killing me is radiation? Is there a readily available reflective foil I can put around that thing to keep the radiation down to a mimimun?
RE: thermal conductivity question/confusion
The sum of the resistances of inside convective film, inside fouling, tube metal conductivity, outside fouling, and outside convectivew film are calculated in a conventional manner. The effect of tube metal is so small, you can neglect it in this case, just as it is nearly negligible in tubular condensers.
RE: thermal conductivity question/confusion
RE: thermal conductivity question/confusion
Oops, had a slight glitch, I was trying to do two different things. The radiated component is only 1/2 of convected component, so they're both contributing to your heat loss.
You need to accept that you might not practically have zero heat loss. How much can you tolerate? How much are you willing to spend to achieve whatever you can achieve?
A simple, 1-in thick foam insulation with aluminum reflective foil might reduce the total heat loss by a factor of 5.
TTFN
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RE: thermal conductivity question/confusion
As also recalled above, the heat loss through the tube wall is the result of various thermal resistances in series, and, as for electrical resistances, the largest one dominates. With your conditions (thin wall, sustained flow at inside) the controlling resistance is on the outside.
To evaluate the losses through the tube wall you can account for a heat transfer coefficient of 10 W/sq.m/degC on the outer surface: this includes convection and radiation losses, each contributing for some 50% to the total.
To crunch some numbers, 50 SCFM of air means some 0.03 kg/s of air, or some 30 W per degC of heat content. In other words a loss of 30 W would cause a temperature drop of 1 degC.
The heat loss through the tube wall can be estimated, with the coefficient above, to some 400 W, say 500 W to include some losses by conduction to the tube supports.
As a consequence we would expect a temperature drop of 17 degC or 30 degF (!).
By suitably insulating the tube wall (and a portion of the connected pipes and equipment), you can definitely reduce that loss by a large amount, say a factor of 10.
prex
http://www.xcalcs.com : Online tools for structural design
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads
RE: thermal conductivity question/confusion
RE: thermal conductivity question/confusion
RE: thermal conductivity question/confusion
RE: thermal conductivity question/confusion
TTFN
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RE: thermal conductivity question/confusion
RE: thermal conductivity question/confusion
Are they aluminum or steel? If aluminum, they could be contributing to the heat loss.
TTFN
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RE: thermal conductivity question/confusion
Cheers
RE: thermal conductivity question/confusion
I think davefitz and Serrand are on the right track. very often in gas-to-gas heat transfer, almost all the resistance comes from the boundry layers (inside convective film and outside convective film) compared to conductance of the tube