# Airflow calculation

Gardiner
Hi,
I am new to the use of forums and have no experience in airflow but I was looking for some help with the calculation of the gas flow through the main stack of a cement plant, the measurements I have at the moment are as follows
Velocity= 12.5m/s
Temperature= 120°C
Differential pressure= 0.9mbar
Diameter= 3m
I am looking for what equation/equations I need to calculate the normalised flow and if I need more measurements
Any help would be greatly appreciated thanks
Mark

Gold Member
Is this a forced draught or a natural draught stack ?

Gardiner
Hi Nidum,
It would be forced draught as it's the emissions from a cement plant,the other measurement I have is the moisture content which was 18.4%
Mark

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Gold Member
I've just looked at some pictures of cement plant stacks and I see that they are generally very tall .

So what we have is probably mainly forced draught plus some additional component of natural draught . We may not actually have to use this information directly but always best to understand the complete problem from the outset .

Where and how are the pressures measured ?

Where and how is the flow velocity measured ?

What is the composition of the flue gasses ? Mainly just air or is there significant admixture of other gasses / water vapour / particulates ?

Mentor
Why isn't this just velocity divided by area?

Gardiner
I've just looked at some pictures of cement plant stacks and I see that they are generally very tall .

So what we have is probably mainly forced draught plus some additional component of natural draught . We may not actually have to use this information directly but always best to understand the complete problem from the outset .

Where and how are the pressures measured ?

Where and how is the flow velocity measured ?

What is the composition of the flue gasses ? Mainly just air or is there significant admixture of other gasses / water vapour / particulates ?

Yeah they are the stack in question is 400ft tall measurements would be taken at 200ft
The measurements in my first post were taken by handheld meters
The moisture content would be taken by an ABB analyer
The pressures would be taken by Foxboro transmitters
And temperature would be taken by pt100
The gas itself would be a mixture of gases such as S02,NO,CO,CO2 and O2, there would also be a dust content in the flue gas

To just clear up what I am trying to do here is prove that the plant instruments are giving me the correct readings by checking with handheld readings and being able to calculate the normalised flow from these

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Gardiner
Why isn't this just velocity divided by area?

Russ
I am new to the whole area of flow but I do know temperature is important as it affects the density and the other readings taken are used in some formula to calculate the normalised flow, what I need to be able to do is prove that the reading from the flow instrument we have is correct by use of the handheld readings and working them out through an equation

Gold Member
I understand the question . I'm just making sure that the readings are being taken in ways which give good enough accuracy .

So as @russ_watters says volume flow is just velocity times area . That gives us one answer .

What you want to do though is try to get another answer based the pressure differential and other data and see how good the match is .

When you say pressure differential what exactly do you mean by that ?

Do you have any actual figures for the density of the gasses ?

Gold Member

We need to know the pressure at two different locations in the gas flow path in order to do the flow rate calculation . Ideally widely spaced locations and where the cross sectional area is the same for both locations .

Gardiner
I understand the question . I'm just making sure that the readings are being taken in ways which give good enough accuracy .

So as @russ_watters says volume flow is just velocity times area . That gives us one answer .

What you want to do though is try to get another answer based the pressure differential and other data and see how good the match is .

When you say pressure differential what exactly do you mean by that ?

Do you have any actual figures for the density of the gasses ?

The D/P taken was with a handheld meter we also have a straight pressure reading from the stack also which is -2mbar
I don't have the actual density of the stack gas

Gardiner

We need to know the pressure at two different locations in the gas flow path in order to do the flow rate calculation . Ideally widely spaced locations and where the cross sectional area is the same for both locations .

Are you looking for the pressures at two different heights on the stack?
I think this is why we on site use the handheld

Gold Member
Ideally pressures and temperatures at several levels in the stack and including the entry and exit levels .

We also need information about the gas density .

Ideally measure it directly at the same levels as your pressure and temperature readings .

Stack emissions in most countries are subject to frequent analysis to ensure that they do not contain pollutants above allowable limits . A quantitative analysis of the gas components might allow us to work out at least the mean density even if it was not directly available .

This is quite a complicated problem to solve when all factors are taken into account .

Mentor
Russ
I am new to the whole area of flow but I do know temperature is important as it affects the density and the other readings taken are used in some formula to calculate the normalised flow, what I need to be able to do is prove that the reading from the flow instrument we have is correct by use of the handheld readings and working them out through an equation
Are you asking for mass flow? When you say "airflow" that says to me volumetric flow, which is 88 m3/sec. Or, maybe I'm not understanding what you mean by "normalized flow". Do you mean standardized to a specific set of conditions? What conditions?

It doesn't look to me like there is a difficult question here, but I'm not understanding what you are asking for. Could you give a little more detail please?