Heat Transfer Through a Tube

[tntecheng]

I am currently using nitrogen gas as a coolant. The tube length from dewar to coolant nozzle is appox 20ft. It's coming out of the coolant nozzle at 72°F. I need to cool the gas to ~40-50°F. I was thinking of running the tube through a tub of ice to cool the Nitrogen as a simple solution. I can disconnect the tube from the dewar and add extra length as needed to coil it in the tub. I'm trying to calculate how much of the tube needs to be in the ice.

I've been trying to use Q=2*pi*L*k(Ti-To)/(ln(ro/ri))
where:
Tube - Polyurethane - k=0.02 (W/m*K)
L - unknown
Ti=72°F
To=32°F
ro=0.1875in
ri=0.126in

I've had no luck and seem to be going in circles. What am I missing? Any help is appreciated.
Thanks,
DJ

 

[EnergyMix]

You're trying to create an elementary heat exchanger. If you haven't taken heat transfer, this would be very hard to solve mathematically (especially as it appears you're wanting to use a spiral heat exchanger model.) Read through some of the other posts on here to get formulas, or get a heat exchanger book (hint, your T[sub]Nitrogen[/sub] is going from 72° to 40° and your t[sub]ice[/sub] is going from 32° to 35° (as a guess.) However, if you are willing to experiment, then nothing stops you from getting a tub of ice, a couple of thermometers, and some tubing.

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[SNORGY]

I am inclined to treat this the same way as I would for an indirect bath heater and approach it using NTU-effectiveness method for an essentially isothermal bath at about 33 F. If you base your coil length on that, then you will know it will work if you have "ice" at some temperature below "almost ice". The U-value for water-immersed polyurethane tubes will be your biggest challenge to find.

 

[racookpe1978]

Is this a intermittent use, always attended situation - like a lab experiment or batch-run process that is under reliable attention? If so, just run it through a water-ice bath. At 0 F, an un-monitored plain mix of ice in water will get you too cold very quickly, but a standing-room-temperature water bath will fairly quickly return to room temperature 70+ degrees F. Which you indicate is too hot.

So you're looking for a refrigerator-type regulated cooler whose control is based NOT on the water bath temperature but on the outlet of the N2 coming out of the cooler (if a irregular N2 flow) or on the water bath (if a constant N2 flow).

Just cooling the N2 is fairly simple - but we need to know the size of your N2 lione and the mass flow of the N2 - you didn't mention either! Cooling a small N2 line is easy. Over-cooling is even easier.

A big N2 line? Cool the N2 tank with that regulated cooler with lines wrapped around the N2 tank. Then just insulate the N2 line against heat flow.

 

[NoHtPp]

Throw it at Ernie Cherry Aquasystemsinc.com MI
he has a slew of observed points WITH REFRIGERANT gasses in his cool-ing HX coils

I just bought 3 , 1 for w:w radiant; and two for DX r22 system, shell around coils.

Mention "JP" and he may have a moment to square up your calcs- est's... (not to buy, just he is friendly)

he has surface sq ft and all from small to quite large.

2)

Any hope of non-refrigerant cooling of your gas with an oversized Earth-Coupled Loop (ECL) like we use in GeoThermal- Heat Pumps ground loops?

Ditch or pond or horizontal borehole Earth-Coupled Heat Exchange for the process in ? do you have 45f soil?

 

[tntecheng]

Thanks for all of the input. The polyurethane line is 1/4"OD and 3/8"ID. The N2 is flowing at 75SCFH @ approx 15 psi. It is a constantly monitored process. I think I'm going to by a SS tub, get some ice, and do some experimenting this week. Thanks again.