air temperature

[bigTomHanks]

Here is what I have:

several hundered feet of 3/8'' OD copper tubing with 0.032'' wall thickness that is placed in a thermal chamber

Thermal chamber capable of -40F with internal fan that blows air around at about 5-10 mph.

125 psig air inlet

350 CIM flow rate through the tubing

I would like the air leaving the tubing to be approximately the same temperature as the thermal chamber.

My question is how many feet of copper tubing do I need to achieve this outlet air temperature?

Does anyone have the equations I need to calculate this?

Thanks

 

[IRstuff]

Starting temperature?

TTFN

 

[Montemayor]

You say: "I would like the air leaving the tubing to be approximately the same temperature as the thermal chamber."

The basic heat transfer driving force requirement of the Fourier equation does not permit you to have what you like. If you need the outlet air to be -40 oF, you're going to have to furnish a lower temperature than that in your "thermal chamber". You'll get close, but you won't get there from here otherwise.

By the way, is the 350 CIM flow rate supposed to mean 350 cubic inches per minute? That's an odd engineering flow unit.

 

[MintJulep]

Art missed the obvious answer that you need an infinite length of tubing .

How do you plan to deal with the water that will condense out of the air and freeze long before it reaches the end of the tube?

 

[TD2K]

Art was being kind ;-)

 

[pennpoint]

Right! as MintJulep so aptly points out. You do need to be sure you are supplying low dew point air (say -60 degree F) through the tube because any moisture will freeze along the walls of the tubing and will build up, eventually cllosing off flow.
How does your test chamber get to -40? What kind of refrigeration system does it have?
Can you use dry nitrogen gas instead of air?

When you clarify (answer) "Montemayor" questions I am sure more information will come to you by him and others.

pennpoint

 

[bigTomHanks]

Starting temperature of ambient approximately 70F.

The air coming into the system has a dewpoint low enough for our application if not we have driers to deal with any moisture problem that we can put in line.

We are using too much volume to use nitrogen.

Our chanmber is capable of going lower than -40F in about 10-15 min from ambient.

Yes. CIM is cubic inches per minute. It is the units that our flow meter measures in and better than CFM for this application.

 

[TD2K]

In all seriousness, how close to the room temperature do you need/want? Within 10F, 5F or 1F is going to make a big difference because as the gas in the tube gets closer to room temperature, there is less and less driving force (temperature difference in this case) to further cool the gas.

At the inlet, you have 110 deg F temperature difference (70F - -40F) to cool the gas. If you cool the gas to -39F at the outlet, you are down to 1F driving force and 1% roughly of the inlet.

 

[sreid]

I think that there are too many unknowns in this situation to come up with an analytical solution that would give more than a wild approximation.

However, at your flow rate I calculate that the residence time for the air in 100 feet of tubing is about 40 seconds. That should be long enough to get substantial cooling. As pointerd out above, if the temperature drop is significant in the first 100 feet, an additional 100 feet would not add much.

I'd coil 100 ft. of tubing and do the experiment.

 

[bigTomHanks]

sreid

What more do you need? What are you basing this comment on?

 

[sreid]

The first big unknown is the convection coefficient between the 40 deg C air being blown aroung by the fan at the copper tubing. If the air flow was very high across the tubing (7 ft/sec is not very high), one could assume an isothermal OD for the copper tube.

With the air flow internal to the tube at about 30 inches/second. the flow is not likely to be turbulant (although it it would certainly become turbulant somewhere in the 100 feet of tubing). For non turbulant flow there is little mixing and the air and the air at the ID of the tube acts as a thermal insulation barrior to air at the center of the tube.

The basis for my statement is then that the heat transfer is likely to be dominated by thermal resistances at the OD and ID of the pipe (and the thermal impedance of the copper pipe can likely be assume to be zero). But the thermal resistance at the OD and ID of the tube are difficult to quantify but likely to have a large influance on the heat transfer.

Having said that, there must certainly be emperical data-formulas somewhere that addresses your case. What comes to mind is outdoor pipe in a breeze carrying hot or cold gas.

 

[sreid]

Here's a website that might be helpful.

http://www.wlv.com/

 

[bigTomHanks]

sreid
Thanks a lot for your help. I now have everything I need to solve the problem.
bigTomHanks

 

[25362]

bigTomHanks, even when assuming a reasonable [Δ]T, the formulas are not simple. If it is at all possible I'd say try 10 ft of tubing and see what happens.

 

[bigTomHanks]

25362
I did what any lazy engineer would do. I skipped the calculations and just did an experiment to determine the air temperature exiting the hose. I will then approximate length from that. I remember from heat transfer that you can do an analytical calculation for this case but a lot of assumptions must be made and I recall that the finding the solutions are time consuming. I just thought that because this seems like a pretty common problem that there might be some kind of shortcut equation used for this one special case.
Good day,
bigTomHanks