Dewpoint pressure - Flue gas

[dianad]

Hi,

I'm calculating the flue gas dewpoint of a gas boiler. I'm finding some problems, because we know that the dewpoint temperature is the saturatrion temperature at water partial pressure. My problem is that the partial pressure is function of the total pressure, wich in the most cases, we don't know. I assume some values like: 1 bar, 2 bar, etc. and i found that the results were quite different. So, we can not assume any value that we want, but the one that is correct.
Another problem, is that no one talks about this total pressure. Is it absolute or gauge pressure? I saw many people using each one, wich leads me to no conclusion...

Can anyone answer this questions?

 

[davefitz]

My chemistry may be rusty, but the pressures of interest in determining dewpoint are absolute total pressure Pt and the condesing gas constituent partial pressure Px . Px is usually taken as the product of the gas total pressure times the gas constituent's mole fraction.

One then approaches the constituent's saturation curve to estimate whether one has dropped below the dewpoint, but I seem to recall that there are add'l interactions between unequal gas consituents that complicate the calculation of condensing fraction; the term fugacity comes to mind.

 

[dianad]

Hi, thanks.

OK, so the total pressure is absolute. Good.
The following calculations related to partial pressures, etc, i already know. My major problem still related to this total pressure, that i don't know in most cases. Doesn't it depends also of the chimney height? Dynamic pressure also?

Thanks! I hope you can reply to this last questions!

 

[rmw]

Total pressure is going to vary according to several factors, among them atmospheric pressure and boiler load as well as tuning factors such as excess air, etc. Do you have the capability to take a pressure reading at the point at which you are most concerned about acid dew point, or is it all the way to the top of the chimney that concerns you?

If you have the ability to take some pressure reading between the boiler and the chimney you should be able to calculate the duct losses and stack effect at various loads.

You should take your worst case condition and the worst case location for acid dew point corrosion and determine the total pressure at that point and design around your worst case scenario.

rmw

 

[dianad]

Hi rmw,

thanks for you reply!

So, you say that the total pressure might be the sum of:
P(static) + P(dynamic)-Head loss, where P(static)=P(air) + P(flue gas height column-chimney).
I still have some doubts:

1)I supose that the worst point is at the top of the chimney, where the static pressure is = P(air) and P(dynamic) is less than in the bottom of the chimney, because of the head loss.Is this correct?

Thank you very much!

 

[imok2]

CONDENSATION WITHIN BOILERS
(and dew point calculation

This will give you a complete run down on your question:

http://www.google.com/search?hl=en&ie=ISO-8859-1&q=dew+point+boiler+stack+temperature

 

[rmw]

dianad,

That isn't what I thought I said. I'll try again.

One point that I made was that there must be some point, a sample port or similar location in the flue gas ductwork that you can get a pitot tube type pressure reading device that will give you total pressure at that point. Unless you have an infinite number of sampling points to take the pressure readings along the flue gas path you will then have to calculate the pressure at points upstream and downstream of the point at which the pressure was taken.

Another point I tried to make is that since pressure losses throughout the system are flow (load) related, total pressure is load related and you will have to take differing loads into account.

I assume that the purpose for your exercise is to make sure that the proper temperature is maintained throughout the flue gas system so that the dew point is not reached anywhere it is not desired within the system.

The total pressure just gives you a starting point for determining the partial pressures of the constituents so that you can determine the dew point. It is the temperature of the flue gas that you are really worried about.

rmw

 

[dianad]

rm, thanks for your replay, again!

Yes, you are absolutely right. My problem is that i can't measure this with a pitot tube, because the installation is far away from where i am now. I'm working only with the given values by the client. But if i have:
1) the stack height -> Static pressure
2) fluegas flowrate -> Dynamic pressure
2) stach diameter -> Dynamic pressure (for calculate velocity)

i think i can work out a solution. Do you think so?
Can you answer my question related to the worst point of the stack, reffered above?

imok2,

Thank you very much for this. I'm reading now some of this documents and they are very good, but i still searching for a complete approach.

I know that this is not an easy question, if we try to make it complete. For example i know that we have to take in account the heat losses along the stack, also, but i would like to discuss this things step by step so that i can get concrete conclusions!

Thanks!

 

[athomas236]

Gentlemen,

Seems to me that we are getting things out of proportion here considering that we do not know the composition of the natural gas and the level of excess air but appear to be concentrating on the effects of dynamic and static heads.

The data we need to calculate the water dewpoint is the absolute pressure at the point of concern and the % water content of the flue gas, the water content depends on the composition of the natural gas and excess air.

Atmospheric pressure is 1.01325bar absolute which is equivalent to about 407 inches of water. If we make some bold assumptions like the boiler is balanced draft and the friction loss over the heating surfaces is 30 inches of water we get an absolute pressure at the point of concern of 377 inches of water.

Taking a typical UK natural gas (Bacton) with 92.6% methane working at 15% excess air, we get water content of 17.488% on wet volume basis.

Taking the pressure as 407 inches the partial pressure water at is 0.1772bar abs giving a dewpoint of 57.5C.

Taking the pressure as 377 inches the partial pressure water at is 0.1641bar abs giving a dewpoint of 55.8C which is a difference of 1.7C.

Conclusion the atmoshperic pressure dominates over the effects of dynamic and static heads so spend more time getting information on the gas composition and excess air.

Best regards,

athomas236

 

[dianad]

athomas236,

Thank you very much for your reply!
I saw it with attention and of all is the one that approaches more to my conclusions. But i still with a couple of questions that i would like to see them clear. Let me tell you that i've already did that work of the gas composition and it gave me a 18% H2O.
I made some simulations to see what is the relevance of the flue gas pressure for the same situations: excess air and water content. The result is attached.
As you can see, it's quite important to know tha absolute pressure. You mentioned 407 inches-the air pressure, but in most cases it isn't this pressure that is observed.

What is your comment to this?