Heat transfer of hydrogen gas, Outlet temp

[jmrec100]

I'm brain dead and can't get the outlet temp for a simple problem. Please help.
Here's the problem. I have a hydrogen gas stream flowing at approx 5 l/min. It's temp inside the tube is at 800 deg C. It will be flowing thru a 1/4 OD , 3/16 ID ss tube. The temp outside the tube will be approx 60 deg C. I need to determine the temp of the gas in the tube at 12 ft distance away. I just can't remember how to find it.

 

[jmrec100]

I only need an approx.

 

[moltenmetal]

At what pressure? Matters quite a bit...It depends on how the outside of the tube is HELD at 60 C with that very hot, very thermally conductive, fairly high heat capacity gas flowing in it. You need to consider both the heat transfer coefficient INSIDE the tube and OUTSIDE the tube, unless you can be sure that one of these is greatly smaller than the other and hence is the controlling resistance to heat transfer.

Gut feel? If the tube OD is immersed in water at 60 C and the gas is flowing at atmospheric pressure, the answer is ~ 60 C.

If the tube is exposed to free convection and radiation conditions in air at 60 C, and the flowing gas is at 60 C, and you care whether the answer is 200 C or ~ 60 C, you need to do the calc.

Get yourself a heat transfer text and review forced convection heat transfer. The inside film coefficient can be obtained from Nu = 0.023 Re^0.8 Pr^1/3. And with the inlet gas being at 800 C, you can't neglect radiant heat transfer either.

 

[jmrec100]

Thanks. It's been a while but I am digging into my old text. The situation has been clarified a bit.
It's a 12 ft long 1/4 ss tube with incoming gas at 800C. The tube leads to the inlet of a pump with a flow rate approx 5 L/min. The tube sits in outside air around 30C ; no mechanically induced flow across the tube just in outside air. The concern is not whether the pump can take the heat but rather the sensor components down stream from it.
Been a while since I have even dealt with any heat transfer application. Any help would be appreciated.

 

[jmrec100]

The outlet pressure of the pump is guesstimated to be around 5psig. Inlet very low , near atm. Guesstimate 1 to 2 psig.

 

[IRstuff]

You still haven't indicated what the gas pressure is; that's critical to determining what mass flow corresponds with the 5 L/min volumetric flow. The gas flows at 4.7 m/s and is exposed to the pipe for 0.8 seconds.

Ostensibly, atmospheric pressure hydrogen at that temperature is roughly 22W; the flow rate implies some pressure though, but the first foot of pipe could convect and radiate about 200W at 800C.

Attached is a PDF of my calcs

TTFN

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[IRstuff]

Sorry, crossed posts. You can crank up the various parameters to be more consistent with 800°C gas temp and the pressure.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[jmrec100]

Thanks.
I'll look it over and jar my brains again. Inlet pressures were near atm or use 1 to 2 psig. And outlet guesstimate 5psig

 

[jmrec100]

Have a question. The Reynolds number is so small that assume laminar flow which should be at such low flow rate. I am assuming the 7 W/m2K is your h value? Where did that come from? I get a much smaller value from a different table at both 800C and 30C. Am I missing something? And dividing by 12?
7W/m2K x surf/12 ×770K = 32.7738W h x A x Tdifference = q
Is that the 12 ft length not converted to meters ?

So now with this energy heat loss. To convert to actual outlet temp? Sorry such remedial questions but it's coming back slowly.

 

[IRstuff]

Sorry, 7 W/m^2-K is just a swag for the external convection. I wasn't trying to completely solve the problem because there were too many parameters that I didn't know, hence, I completely neglected anything going on inside the pipe, except for the thermal mass flow.

The objective was solely to confirm whether moltenmetal's assertion was plausible, i.e., that the heat removal processes were sufficiently larger than the thermal mass being supplied.

Since the surface temperature decays as a function of distance from the inlet, I essentially modelled only the first foot of the pipe, 1/12th of the total length given. And, since the amount heat that's removable in that distance is so large, discretization of the pipe temperature as a function of distance is warranted.

The expectation is that you'd need to break up the pipe into a number of sections, each with its own heat transfer, and then propagate a cooler gas into the next section, etc.

TTFN

FAQ731-376
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[jmrec100]

Ok. Thank you. Now I understand it better.
Appreciate the assistance.

 

[jmrec100]

Ok. Now I am frustrated. How do you find the external convestion value? It is a function of temperature but could I use and average? But now, how to determine it?
I took an approach of wanting a final temperature of around 60C to 100C and find a length of tube needed to attain it. With this approach I ended up with a very long length which maybe makes sense? Radiation was not considered but it should be. Please enlighten me. I have been reviewing the heat transfer but a little slow at it.

 

[IRstuff]

The external convection coeffcient sounds like it's too high to be of much concern.

I can't seem to find anything direct on hydrogen heat transfer coefficient, but given that the ID is only 3/16", the htc should be quite high. The htc is proportional to k.hyd*Nu/L, where k.hyd is thermal conductivity, and L is the layer thickness.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize