Why does it take more energy to heat nitrogen?

[SPIAction]

Hey guys,

If this is in the wrong section let me know and I'll move it.

So here's the simple question.

I work in an industry in which we pressurize fluid based dampers with nitrogen. Those units pressurized with nitrogen take more energy to heat up than those pressurized with ambient air. Therefore the nitrogen based units run cooler.

So what is it about the nitrogen that causes the unit to resist heat or run cooler?

Thanks!

Dave

 

[Mapes]

Dave, could the starting temperatures possibly be different? Nitrogen that comes from a high-pressure tank will immediately be cooler because it does work in displacing air. Conversely, air compressed by a compressor will immediately be hotter because work is done to compress it.

If the starting temperatures are equal, I'm not sure what's happening. Possibly the molar heat capacity of nitrogen is larger than oxygen at high pressure (this is something you could check - I don't know). Or perhaps the pressures are actually different in the two tanks, the pressure gauge being rated for nitrogen or air and giving the wrong reading for the other gas.

 

[SPIAction]

Dave, could the starting temperatures possibly be different? Nitrogen that comes from a high-pressure tank will immediately be cooler because it does work in displacing air. Conversely, air compressed by a compressor will immediately be hotter because work is done to compress it.

If the starting temperatures are equal, I'm not sure what's happening. Possibly the molar heat capacity of nitrogen is larger than oxygen at high pressure (this is something you could check - I don't know). Or perhaps the pressures are actually different in the two tanks, the pressure gauge being rated for nitrogen or air and giving the wrong reading for the other gas.

Ah...thanks.

So all factors are equal, and whatever it is, it makes a big difference as it takes a significant amount more heat to bring the nitrogen filled unit up to a given temp than the unit filled with regular "air".

In other words, I'm not talking about small amounts...it's a very noticable difference.

 

[SPIAction]

I guess this might have something to do with it.

Specific heat capacity
Specific heat capacity is the amount of heat required to change temperature of one kilogram of a substance by one degree. Specific heat may be easured in kJ/kg K or Btu/lboF. For comparing units, check the unit converter for more information!

Specific heat capacities for different materials can be found in the Material Properties section.

http://www.engineeringtoolbox.com/spesific-heat-capacity-gases-d_159.html

Since enthalpy of a fluid is a function of its temperature and pressure, the temperature dependence of the enthalpy can be estimated by measuring the rise in temperature caused by the flow of heat at constant pressure. The constant-pressure heat capacity - cp - is a measure of the change in enthalpy at a particular temperature.

Similarly, the internal energy is a function of temperature and specific volume. The constant volume heat capacity - cv - is a measure of the change in internal energy at a particular temperature and constant volume.

Unless the pressure is extremely high the work done by applied pressure on solids and liquids can be neglected, and enthalpy can be represented by the internal energy component alone. Constant-volume and constant-pressure heat capacities can be said to be equal.

For solids and liquids
cp == cv
The specific heat capacity represents the amount of energy required to raise 1 kg by 1oC, and can be thought of as the ability of a substance to absorb heat. Therefore the SI units of specific heat capacity are kJ/kg K (kJ/kg oC). Water has a very large specific heat capacity (4.19 kJ/kg oC) compared with many fluids.

 

[uart]

Hi SPIAction. How is the heat added to the unit. Are you directly applying heat or are you simply observing that during operation the nitrogen filled units heat up more slowly? If it's the latter then I'd suggest that it has nothing to do with the heat capacity of the nitrogen but rather that it's some indirect action, possibly related to the interaction of the air with the oil in the unit.

BTW. As far as I can tell the heat capacity of nitrogen is the same as air. Since air is over 70% nitrogen I find that "big differnce" hard to believe, unless as I say it's an indirect effect.

 

[Andy Resnick]

The heat capacity of pure nitrogen is different than air:

http://www.engineeringtoolbox.com/nitrogen-d_977.html
http://www.engineeringtoolbox.com/air-properties-d_156.html

But the differences don't appear to be significant at high temps.

 

[uart]

The heat capacity of pure nitrogen is different than air:

http://www.engineeringtoolbox.com/nitrogen-d_977.html
http://www.engineeringtoolbox.com/air-properties-d_156.html

But the differences don't appear to be significant at high temps.

Yes I should have said that they were the same to within a few percent, thanks for the nitpicking. Certainly not enough to account for "I'm not talking about small amounts...it's a very noticable difference".

 

[SPIAction]

So we tried two methods, one was via an enclosed oven, the other was via a heat gun.

The units have about 450ml of fluid/oil in them and about 200ml of a gas chamber (that remains isolated from the fluid) which is charged at about 150PSI.

As the main bodies of the damper are aluminum, the heat is dispersed very well, and with the heat gun, we could easily get the "air" unit up to 300F. With the N2, we could only get the unit up to about 220F.

I consider that a big difference.

Does this seem to add up?

 

[negitron]

Have you tried swapping which gas was in which unit, assuming this is possible, to rule out differences between units? If you put air in the unit which now contains nitrogen and you can heat it to 300, you know there's something about the physical unit rather than the type of gas.

 

[uart]

So we tried two methods, one was via an enclosed oven, the other was via a heat gun.

The units have about 450ml of fluid/oil in them and about 200ml of a gas chamber (that remains isolated from the fluid) which is charged at about 150PSI.

As the main bodies of the damper are aluminum, the heat is dispersed very well, and with the heat gun, we could easily get the "air" unit up to 300F. With the N2, we could only get the unit up to about 220F.

I consider that a big difference.

Does this seem to add up?

OK so the gas can't directly interact with the oil so that rules out the alternate mechanisms that I was thinking of. Yes this is really quite puzzling.

Edit. Oh yeah, re negitron's post. Are you sure that the units are otherwise identical (apart from the type of gas).

Also I would think that "difference in temperature which you could get it up to with a hot air gun" would speak more of the ability of the unit to disperse/dissipate heat rather than of it's heat capacity. So maybe we should be looking more at differences in heat conductivity between the two units.

 

[SPIAction]

It was the same unit.

We just let it sit for a few hours between tests.

 

[Andy Resnick]

CO2 has a significant difference in heat capacity than either N2 or air at the temperatures you are talking about. What about charging a unit with CO2 and seeing what's what?

 

[SPIAction]

CO2 has a significant difference in heat capacity than either N2 or air at the temperatures you are talking about. What about charging a unit with CO2 and seeing what's what?

Well...that's a good question.

Nitrogen is typically used in these units, which I assumed was because it ran cooler than air, and for the fact that it was free of moisture.

The units typically have a rubber bladder that isolates the gas from the fluid. This bladder is charged to 130 to 160PSI. Heat brings the pressure up, as does a rod that enters the unit that displaces the fluid and therefore the bladder. Pressures may rise to as high as 250 to 280PSI.

I have been told that nitrogen is less likely to permeate the rubber of the bladder, given the size of the nitrogen molecule, and therefore the charge will last longer. But I don’t know how true this is and I don’t know anything about the size and behavior of the carbon dioxide molecule in such an environment.

Thoughts?

 

[negitron]

Carbon dioxide is such a small component of ambient air (.03%) that I highly doubt it's a factor here.

 

[Lok]

What about the IR radiation absorbed is nitrogen any different from oxigen, maybe oxigen absorbs more so it can heat up far easier. That would explain the fact that if oxigen present in the mixture than the whole mixture gets heat up as the oxigen collides with the nitrogen molecules etc...

 

[russ_watters]

I may have missed it, but I didn't see if the OP had addressed this answer:

Dave, could the starting temperatures possibly be different? Nitrogen that comes from a high-pressure tank will immediately be cooler because it does work in displacing air. Conversely, air compressed by a compressor will immediately be hotter because work is done to compress it.

This difference would be significant.

 

[negitron]

I may have missed it, but I didn't see if the OP had addressed this answer: This difference would be significant.

He did, in post #3. He states that all starting factors are, in fact, equal.

 

[russ_watters]

He did, in post #3. He states that all starting factors are, in fact, equal.

Given the apparent simplicity of the issue and the lack of detailed descriptions from the OP, I'm not inclined to take that at face value...

To have all starting conditions be equal, they'd either have to be getting the air from long-term storage tanks kept at room temperature (not just a compressor's attached tank) or let their devices return to equilibrium after filling some from a nitrogen tank and others from a compressor. That would take a lot of effort.

A clearer description of exactly how these systems operate is needed to really know what the differences are because it isn't really possible for all starting factors to be equal here. The OP is here because he is unable to identify which starting factor is causing this issue. The answer is amost certainly a factor that was thought to be insignificant (and thus he didn't think to tell us), but wasn't...

 

[russ_watters]

A few questions:

How are the dampers charged with air/nitrogen? Ie, where does it come from? N2 tank? What pressure? Air tank or compressor? How long does it take to charge them?
How long is heat applied to get it up to operating temperature? Are the dampers at room temp to start with?

 

[Andy Resnick]

Carbon dioxide is such a small component of ambient air (.03%) that I highly doubt it's a factor here.

I think you missed my point. We are all supposing the effect is due to differences in the heat capacity; I proposed a test. Pure CO2 is commonly available.

 

[Andy Resnick]

Well...that's a good question.

Nitrogen is typically used in these units, which I assumed was because it ran cooler than air, and for the fact that it was free of moisture.

The units typically have a rubber bladder that isolates the gas from the fluid. This bladder is charged to 130 to 160PSI. Heat brings the pressure up, as does a rod that enters the unit that displaces the fluid and therefore the bladder. Pressures may rise to as high as 250 to 280PSI.

I have been told that nitrogen is less likely to permeate the rubber of the bladder, given the size of the nitrogen molecule, and therefore the charge will last longer. But I don’t know how true this is and I don’t know anything about the size and behavior of the carbon dioxide molecule in such an environment.

Thoughts?

The issue is the permeability of the rubber- what kind is it? Different materials have different permeabilities.

I'm having a hard time keeping track of all the different parts: is there a website or something with an image of the device?

 

[Lok]

The issue is absorbance ( the wavelength of infrared that gets absorbed ) of oxigen and nitrogen.

It is not the temperature or pressure of the source as working conditions of any mechanism are fixed.

Even heat capacity is useless so to say considering the small amount of gas usually present.

 

[Lambduh]

The issue is absorbance ( the wavelength of infrared that gets absorbed ) of oxigen and nitrogen.

It is not the temperature or pressure of the source as working conditions of any mechanism are fixed.

Even heat capacity is useless so to say considering the small amount of gas usually present.

If i understand this correctly the aluminum chamber is what's being heated which in turn heats the gas. The pressure in the chamber was stated to be 150-250 psi, so there is quite a bit of gas in the the chamber. That's 10-17 atmospheres of pressure. So while most of the aluminum will be giving off infrared wavelengths (i think ~5 um peak at 300ish degrees F) the gas is pretty dense. I'm trying to find absorption spectra for Oxygen and Nitrogen at longer IR wavelengths. Even if it isn't absorbed by the nitrogen the rubber bladder will... Then the heat capacity of the rubber is included. This is an interesting problem!

One question i haven't seen asked is how is the temperature of the inside being measured? I'm guessing it's a regular thermocouple? If so i wouldn't think it would be gas dependent in any way.

I just want to make sure i understand... Very crude drawing incoming
rubber_________↓___↓
aluminum___↓____________↓
Atmosphere |gas| fluid | gas | <--Heat Source

Let me know if I visualized this wrongly.