Specific Heat Capacity of Nitrogen 3

[heieup]

I'm working on a project in which I'm assessing the suitability of injecting nitrogen into a space in order to cool it down more rapidly than natural cooling (to assist in maintenance work and minimise downtime)

However, for this I need to know the specific heat capacity of Nitrogen (i.e. how much heat energy it will take from the space in order to achieve equilibrium) and have a figure for this of 1.042 j.g-1.K-1 at 300K.

How does this change in relation to the starting temperature ? Will the energy required to raise 1g of N2 by 1 deg K always be the same, or will it rise ?

Thanks

 

[IRstuff]

What temperature?
What heat load?
Dead load or continous?
Liquid nitrogen?
Can maintainer breathe nitrogen continuously?



TTFN

 

[CRG]

Try this web site:

http://webbook.nist.gov/chemistry/

CRG

 

[heieup]

Its in the range of 140-150 deg C and needs cooling to below 50 deg C before we can allow entry to work.

The idea would be to purge with nitrogen to cool the space and then flush out with air.

The heat source will be removed as plant will be off line the only source being from radiation from the steelwork.

Liquid Nitrogen ? I don't know yet, it depends on what temp the nitrogen needs to be to reduce the space temperature from 150 to 50 !

Thanks

 

[kmpillai]

Dear Heieup,
The energy required to raise 1g Nitrogen by 1K is not always same. Since Cp is increasing with temperature the energy required to raise the N2 by 1K is also more for at higher temperatures. Cp values of Nitrogen with respect temperature can be calculated using the polynomial coefficients published in some text books. I having the coefficients, using that I calculated for 300 to 500 K

Temp Cp Temp Cp
(K) (kJ/kg K) (K) (kJ/kg K)
300 1.0397 405 1.0444
305 1.0397 410 1.0449
310 1.0397 415 1.0453
315 1.0397 420 1.0458
320 1.0398 425 1.0464
325 1.0399 430 1.0469
330 1.0400 435 1.0475
335 1.0401 440 1.0480
340 1.0403 445 1.0486
345 1.0405 450 1.0493
350 1.0407 455 1.0499
355 1.0409 460 1.0506
360 1.0411 465 1.0512
365 1.0414 470 1.0519
370 1.0417 475 1.0526
375 1.0420 480 1.0533
380 1.0424 485 1.0541
385 1.0427 490 1.0548
390 1.0431 495 1.0556
395 1.0435 500 1.0564
400 1.0439 505 1.0572

I really don't know at what temperature ranges you want the data? If you need the polynomial coefficients for higher temperature range, please send a mail to me I will send the coefficients, which will be very useful for writing programs also.

Regards,
KMP
kmpillai@hotmail.com

 

[25362]

The Cp of nitrogen at amospheric pressure changes for about half a percent between 50 deg C (0.24895) and 150 deg C (0.25021 kcal/kg), hardly a change to be taken into account.

For more thermophysical nitrogen properties the website recommended by CRG is excellent.

 

[heieup]

Thanks to all,

The info has certainly pointed me in the right direction and I've got more of an idea how to progress, I'm sure there will be other questions though, as I'm a process engineer rather than mechanical engineer and seem to be obsessed with coming up with ideas which require me to do a whole heap of research before I can decide if there's any milege in them or not !

 

[IRstuff]

What was posted, while interesting, is only part of th answer, since there's been no mention of the thermal conductivity of the nitrogen. The issue will not simply be the displacement of the hot air in the compartment, but cooling the walls and equipment surfaces to less than 50°C just to keep the maintainer from burning his hands on the equipment.

Given that the compartment is large enough to hold at least 1 person and that there would therefore be the equivalent of something like 72 ft^2 of surface to cool down, you'll need lots of flow and flow rate.

Assuming a reasonable 3 ft/s flow rate and a brute force blower, you'd need something like 3*9*60=1600 cfm of nitrogen for a continuous cooling of a steady state heat load. Since the heat load is turned off, you can certainly take more time and therefore less nitrogen flow, but it's still a big problem.

Given that you're talking about starting maintenance while the temperature is still at 50°C, you'll need additional air conditioning to keep your maintainer from keeling over from the heat, anyway.

This suggests that the initial temperature of 150°C problem is essentially irrelevant, since you still need to get the temperature down enough to keep your maintainer from overheating.

Sounds like a portable A/C unit or swamp cooler with lots of air flow might be more realistic.

It will solve your initial cooldown problem more effectively, because you can easily run 300 cfm of air for the initial cooldown and then throttle back for the maintainer to do work. There's no practical way of running that much nitrogen to make any sort of meaningful cost-benefit.

TTFN

 

[heieup]

Thanks,

I had become aware of the requirement to cool the steelwork within the space. In essence as the space allows flow through it the only concern is steelwork cooling rather than air cooling.

Currently we push air through the system until the exit air is at a temperature of 50 deg C, however this can take hours. And even after this cooling period is over, the space increases in temperature due to the retained heat in the structure.

The ideal temperature for working would be under 30 degrees C, but entry for work can be permitted between 30 and 50 if work/rest regimes are in place (at 40 20 mins in, 20 mins outside in fresh air with plenty of drinking water)

It would appear after gathering the information, and the invaluable help from this posting, that the problem is in fact huge ! But is currently a serious issue and one we must address soon in order to ensure effective maintenance can take place within plant downtime allocation.

I will progress further !

 

[25362]

Viscosity, thermal conductivity, heat capacity at constant pressure and Pr number are not srongly dependent on pressure. IRstuff is right in that for h/t calculations you'll need more properties than only the Cp.

FWIW, values for nitrogen at atmospheric pressure, from J.P. Holman's Heat Transfer:
300K 400 K

density, kg/m³ 1.1421 0.8538
Cp, kJ/kg.K 1.0408 1.0459
abs. visc., kg/m.s X 10^-6 17.84 21.98
th. conductivity, W/m.K 0.02620 0.03335
Pr number 0.713 0.691

 

[IRstuff]

Are you pushing cold air through? and at what rate?

TTFN

 

[heieup]

Currently no the air is still hot coming partly from the process and making its way trhough a lot of hot steelwork before we get to the space. The idea is to substitute this cooling (which is not very effective) and use ambient compressed air or nitrogen (inert).

Incidentally the space is a precipitator.

 

[IRstuff]

Don't know what that is exactly, but sounds like something I don't want to spend time in.

It sounds like this maintenance is sufficiently regular that you're contemplating all this effort.

I would still think that either a portable cooler or possible a semi-permanently installed high-capacity blower would be more effective and overall, more useful, as you'll also be able to provide continual air flow and cooling to the maintainer. This could potentially alleviate some of the work/rest downtimes, because even if the walls are still hot, the cooling air will get and effect wind chill and keep the maintainer relatively cool.

TTFN

 

[imok2]

heieup
There are cooling vests made specifically for service men who have to work in hot environments they are pre cooled and last for about 2 hours. Many AC service people use them now. You might want to look into this.

 

[patrickraj]

K M PILLAI,

Is the equation you have used is from CARL YAWS.
Is there any accurate equation to calculate liquid waterviscosity.

Thanks in advnce

 

[derek3]

Hi heieup,

My suggestion may not be suitable for your application, but have you consider a fine water spray rather than nitrogen?

Heating water from 20C to 140C takes 4.8148 MJ/kg of energy.

Therefore each kg of water sprayed on metalwork at 100C or above will absorb almost 5 MJ of energy.

Hope this helps,

Derek3

 

[Fizzhead]

heieup:

Cooling a precip is difficult due to the mass of metal you are dealing with. I assume that following isolation from the boiler, the hot air you are dealing with is essentially due to stack effect. What I have done in the past was to open the ductwork upstream of the precip and install an extra expansion joint. When I wanted to work on the precip I then removed the expansion joint and inserted a metal plate. After that, the stack effect pulled in ambient air and effectively assisted in the cooling process. Of course this depends on your ductwork configuration and size, but it has worked for me. As for cooling with compressed or liquid nitrogen, this does not seem to be cost effective since you must first inject the nitrogen to cool the precip interior and then purge to remove the nitrogen. Since the purge must preserve the lower temperature environment, seems best to must use the air in the first place and save time and money.

 

[heieup]

Thanks Fizzhead (!)

I've abandonned the nitrogen idea after the potential nitrogen purging problems game to light. I've been looking at purging with compressed air instead.

Your suggestion sounds good but there are two issues with our set up, one is that the air drawn through is from a suction fan rather than chimney effect which is stopped for maintenance after a few hours. There are vanes to shut off the air flow as well so this removes much of the cooling effect although the flow is still quite considerable. The ducting is also huge and an extra expansion joint not suitable (it would actually be the only expansion joint !). However, I will progress compressed air purging which is at ambient temperature and should aid in cooling.

Heidi

 

[StoneCold]

I don't think you will ever get enough flow using compressed air. You should really latch on to the blower idea. You can suck cool air from out side and blow it into the space to cool it down. You can easily get 1500 cfm with a very small blower. You can also leave the blower going while your people are inside, provided that it does not blow so much dust around that they can not see, I assume your people have full face respirators. It also sounds like you could cool the space using the forced induction fan. That should cool the unit fairly rapidly. I would not try the water idea. It sounds good but if you put water in with the ash it will turn to concrete.

 

[rmw]

Not only as fizzhead has pointed out are you having to cool the mass of the structure, plates and wires of the precip, you will also have to cool down any collected material that has not fallen into the hoppers and been removed by the ash handling system.

How many doors does this precip have on it. Surely each hopper has an access door, as well as different zones of the box. Some models of precips have an inordinate amount of access doors due to the need to get in and service the internals.

Your quickest method is going to be to open all the doors, and put ventilation blowers, they come in electric and/or air driven versions, in some of the lower doors that are pushing cooling air in, and others in the upper doors removing the hot air from the inside. You want to attempt to put enough positive pressure on it (no more than fractions of an inch of water) so that hot air from the boiler does not continue to enter the work area. In other words, more fans putting air in than pulling air out.

Short of that, it is just a waiting game. And, going in with a full face mask with it still pretty warm is a miserable experience.

rmw