Typical overall coefficient of heat transfer, NITROGEN 1

[rjs286]

I am trying to find a starting value for an overall coefficient of heat transfer of Nitrogen. Does anyone have any ranges? Thanks.

 

[StoneCold]

We need more information. Are you just calling Air, Nitrogen so this is the tube outside heat transfer coefficient on a fin fan unit? Maybe you are heating nitrogen in a shell and tube exchanger and you want the tube side heat transfer coefficient?

Give us the situation and you will get a lot further.

Regards
StoneCold

 

[pennpoint]

Liquid or gas?

Regards
pennpoint

 

[25362]

While answering the previous questions, please also confirm whether you need the individual HTC coefficient for the nitrogen stream only, and not an overall HTC. The latter would oblige knowing against what is nitrogen changing heat, and, if gaseous, under what pressure is the nitrogen flowing.

For example, for S&T heat exchangers, nitrogen gas at high pressures exchanging heat with another gas might show an OHTC of 0.1 kW/(m^2oC), and up to twice if condensing steam were the other fluid.

On the other hand, low pressure nitrogen gas would show an OHTC of less than 0.05 kW/(m^2oC) with whatever is the other fluid, since its individual HTC is quite low.

 

[rjs286]

Thank you for your response. This application will be an air cooled heat exchanger. The Nitrogen is the tube side. There are four different exchangers.
The first one will have the following criterea:
Design PSI: 350 psi Design Temperature F: -20/350 F

Second:
Design PSI: 850 psi Design Temperature F: -20/350 F

Third:
Design PSI: 2200 psi Design Temperature F: -20/350 F

Fourth:
Design PSI: 4500 psi Design Temperature F: -20/350 F

This is a Gaseous state. If need be, the Individual HTC could be used to calculate the OHTC. This would be more preferrable I suppose. I have a spreadsheet set up to do some # crunching.

Thanks.

 

[speco]

jrs286,

As a ballpark, I would suggest using about 3.0 at the lower pressures and 4.5 at the higher pressures, based on the following asssumptions:

1. The operating pressure is reasonably close to the design pressure in each case.

2. There is a reasonably pressure drop allowance, say in the 5-10 psi range.

3. The finned tubes being used are of the more-or-less industry standard for air-coolers: 1" OD tubes, 2.25" fin OD, and about 10 fins/inch (yielding about 5.65 ft^2 of effective surface per linear foot of tubing).

4. The cooling air is in a "normal" range for mass velocity through the bundle.

Regards,

speco

http://www.stoneprocess.com

 

[25362]

In the absence of sufficient data I made some assumptions, as follows:

-Equal mass flow rates for all four units
-All flows are turbulent
-To assess the properties of nitrogen I assumed for all
cases the same average temperature of 185 deg F
-Please note that for the last three cases the gas is
supercritical and has relatively high densities.

The individual HTC for the lowest pressure case (#1) is taken from the literature. The others are pro-rated from the tabulated properties for nitrogen under the assumed conditions, using a conventional formula in which Nu is a function of the Re and Pr numbers for flow inside tubes.

The results, in W/(m².K) are:

245, 250, 280, and 300, respectively.

If the mass flow rates differ, these values should be reviewed.

To estimate the OHTC one needs to know the prevailing conditions on the chilled air side.

 

[rjs286]

Thanks for the data. I have been working on this spreadsheet for a couple of weeks off and on. I just started this project (Air cooled heat exchangers). Its been a while since I have done heat transfer problems so I am a little bit rusty.

I am not sure if I have my spreadsheet set up right. I have used the GSPM manual along with basic arithematic for Nu, Re Pr, etc.

I have a guide in a seperate sheet that explains all calculations and variables. I assume an OHTC to calc. area. Then work my way back through using Re and Nu to recieve h. After this, I calculate OHTC again with the basic formula for U. However, my values do not look right that I am getting. This is my first time trying to come up with this tool for design of heat exchangers. (besides school 5 years ago!)


Thanks guys,

Ron