Exit Velocity from Chimney of Thermal Power Plant

Sure. With the information provided, I'd say around and 8-25 m/s would be ideal.

Thank you for your prompt response.

Can you assist me in finding a reference/standard for this recommended velocity range? If you need more information; I can provide here.

Aside from being by the sea, where are you?

Where have you looked for the codes/standards so far?

Why are you asking?

Why does your engineer not know this already?

If you know the throughput of your boiler, I'd think this would be a matter of simple calculations.

Dear Lyn,

Calculation is simple; i just need recommendations for minimum velocity range acceptable and the criteria/factors to keep in mind (including exit velocity) for air dispersion. If the velocity at the exit of the stack is too low; then flue gas can flow downwards from the chimney effect the buildings/humans nearby.

You have not provided any useful information.

You should not be asking strangers to do your job.

Good luck.

Lyn,

this is my job to find out the recommendations from Mechanical Engineers working in the same field around the World. That is why i joined this community for learning purposes. I am not asking anyone to do my job or do calculations for me on my behalf. I suppose; this portal is for Knowledge sharing and discussions on various aspects of Mechanical Engineering design. If you cant help in constructive way or If you dont exactly understand what I am asking about than please dont answer. It's way better to not answer than being very rude in replying. Hope you have a nice day.

Funny, I thought lyn had already provided an answer. Don't bite the hand that is checking that you understand that answer. Check out the phrase "socratic learning" and youu'll understand what hes up to!

Welcome to CR4. I hope you both learn and share a lot.

Dear Rose:)

What is funny in this?

You are right it is actually sad.

Asking for advice and not taking any seems foolish.

Here is a link to look into the socratic teaching method.

So my question is (starting with the easy one):

In what country are you and further

What standards do regulate power plants where you are? One of them might be the one you are looking for!

From that response, I'm guessing you're a US citizen. In the UK "funny" can mean funny ha-ha or funny-peculiar (similar to ironic). I was using the latter meaning; perhaps you are unfamiliar with this usage.

I appreciate that you are new here - have you read the CR$ Rules of Conduct? They give some good advice about what to include in a question to get positive, helpful and meaningful answers.

English Rose;

Thank you for your guidance. Well, the general rule is to include as much information you feel is appropriate and enough. The person to answer the question shall ask specific questions for positive, helpful answers leading to constructive discussions. Rather than asking questions which LYN asked in #3 (where are you? Why are you asking? Why does your Engineer not know this already?) are not appropriately asked. What shall i answer to them? Where are you? I am in my home?? why are you asking? sorry, shall i not ask? why does your engineer not know this? Because we all are stupid?

Anyway, cheers

Asking for a standard would have required the information of your location or the location of the power plant in the first place.

This is important information.

Run a bernoullis analysis, be sure inside pressure of chimney wont allow condensation to happen in your chimney. Water and flue gas will sooner or later form acidic or corrosive agents to your stack.

However pls, note that equation is for incompressible fluid, you may substitute ideal gas (isentropic) in the term "rho" to have a good approximate.

legolaz

Thanks for your response. I know how to calculate pressure, velocity etc. I am not asking for calculations. I am just asking if anyone of you have experience in chimney design so he would know that there is some recommended minimum exit velocity criteria at exit of the chimney by experince/codes/his company standard etc. Hope you guys understand:)

You could go to EPA website. There is a standard plume to consider and dispersion modelling on the local where your plant is located.

I bet this conditions were already given on your EIA/EIS "Environmental Impact Statement before your plant was designed and could operate. You may review the docs.

Some designs are hot air bouyancy supported.

Spades

Thank you for your detailed response. It is to the point and insightful. The civil aviation, height calculations etc are civil engineering problem. For mechanical engineering side; velcoity, pressure, flue gas flow rate calculations are important. For estimation of velcoity 35 m/s; may I ask how you estimated it? I know its calculation is simple. You know the flow rate and area of the stack; you can easily calculate it. However, for a fixed diameter and max. flow rate; when you get a specific velocity at the top exit of chimney; that very velocity may have some range/minimum limit (specified in some code/standard) to be satisfied in order to minimize the stack-tip downwash. I was reading somewhere that the rule of thumb for avoiding downwash is to make the exit velocity 1.5 times the wind speed at the top of the stack. Also, one of the suppliers of the chimney recommended to have a velocity in the range of 18 m/s to 30 m/s to maximize the dispersion of pollutants. I just wanted to have other opinions. Thanks.

My estimation was based only on previous experience with a similar height stack to the one you mentioned, I can offer no further info on it. You need to input far more parameters than you have provided with regard to the variables that I mentioned previously before you can do any calculations, that's why I said to do the math.

Your local EPA office should be able to offer some assistance in this regard.

Below are a few links that may be worth investigating to get a better idea of stack requirements as per codes, but this is not something that you want to get wrong as the remedial costs can be substantial, employ a suitable environmental consulting engineer to take the responsibility of getting it right first time..

Arrow energy

EPA

Dispersion modelling

Spade

Great. These are very resource-ful contents. I will review them.
Lastly, could you please tell me that for your experience with same height stack; the estimated 35 m/s minimum velocity at the top of the stack should satisfy all boiler loads (100%, 75%, 50%, 25%)? or at just full load operation? Since, at lower loads the flow rate of flue gas will reduce and with the same area the velocity at the top will also reduce. So i guess the chimney exhast minimum velocity criteria shall be satisfied at the worst case (whcih is 25 % load). Any views on this?

Exit velocity 1.5 times the wind speed at the top of the stack - what wind velocity would you choose? Average, 50-year maximum or what? In UK for structural design I use 50m/s which covers most likely eventualities, but 1.5 x that seems excessive. Wind like that would blow the emissions away, and the locals would likely have worse things to worry about.

Also you haven't answered Lyn's questions in #3, specially the 1st one. The design might vary depending on the country.

Codemaster;

Let me clear it to you; the fluegas velocity at the exit of stack shall be 1.5 times that of wind speed at the top of the stack (that is what i read). So, if for example, the max wind speed at the top of the chimney is 50 m/s then 50 x 1.5 = 75 m/s shall be the exit flue gas velocity. To achieve this velocty, diameter (size) of the chimney may be adjusted (reduce dia).

Anyway, the best response was given by Spades. Thank you.

Let me clear something to you.

You came here giving no usable information, such as locality, local topographical information or prevailing winds, local regulation knowledge on your part and no clue what you are doing.

Your statement about civil engineers being responsible for things you don't know, but should, indicate your lack of acceptance of responsibility for the project in general.

Finally, your dismissal of a member who offered you help in good faith is a breach of etiquette and an insult.

Good luck, you're going to need it.

Dear Lyn,

I can see that you are GURU here. Howeve did you see your response to me? You are telling me that i have no clue on what I am doing? Then you are judging me, without knowing anything on what I am up to, by saying I lack in taking responsibility for the project in general. Then, in the previous questions, you are telling me others can't do you job. Then, you are saying why does your Engineer doesn't know the answer? If my engineer knew why I was here? Can you analyze how rude and on personal level you are offending me here.

Hope you can help others. Good day to you.

Clarity is always useful! But you haven't answered the question. If max wind speed is 50m/s, flue gas velocity obviously = 75 m/s, but how will you select the design wind speed? Will you select what you think is appropriate, or get it from published data, or what? I don't suppose you would design for US-style twister wind speeds, if they happen in the locality, as that would probably destroy the chimney.

You still haven't told us what part of the world the plant is. Recommendations/standards are likely to vary from place to place.

I've had a look at the links spades posted in #12, and the only reference I can find to velocity is in Arrow energy, which mentions 4.3 and 10.6 m/s, but these don't appear to be stack exit velocities, unless I'm missing something. And in any case they're much lower than the velocities banded about in this thread. The other links are about determining stack height, no reference to velocity that I can see. If you or spades can point me to where something is said about exit velocity I'll hold my hands up.

Codemaster:

The power plant is being built in Phillipines. It is in the Engineering design phase of EPC (Engineering, Procurement, Construction). I (as an EPC mechanical designer, responsbile for designing the power plant flue gas flow path) have calculated the exit velocity (with fixed size of stack and fixed flue gas flow rate at 100% load) which came out to be about 22m/s with the 200 m height of the stack and plant is located in the seashore with high wind speeds.

This 22 m/s calculated velocity has been forwarded to our client (owners of the plant in Phillipines) who are currently reviewing this value in regard to the factors raised by Spades and others here such as locaility, local codes, EPA regulations, civil aviation etc. Also, this 22 m/s velcoity was calculated based on max load of the plant. If the plant is running at, for instance, 25% load this velocity reduces to 10 m/s. I was just wondering if the exit velocity range of 10~22 m/s is considered low for ths same capacity, height, location of power plants (according to someone's experince). Thanks.

OK, so the maximum flue gas flow is known (naturally, from power plant design), and somebody has selected a flue outlet diameter. You have calculated 22m/s (quite straightforward). You then submit that figure to your client and leave it to him to decide if that is an appropriate velocity. Fine if you can get away with it! Unless I've missed it, this is the first time that has come out, we thought you were selecting velocity (hence diameter).

Your comment I was just wondering if the exit velocity range of 10~22 m/s is considered low for the same capacity, height, location of power plants (according to someone's experience) is a reasonable one, but I'm not sure you've heard much to help you. There have been suggestions but without much in the way of justification, apart from previous experience, which might have been in a different topographical situation. The one about 1.5 x wind velocity doesn't tell us much without somebody selecting a design wind speed.

I don't know whether you've read the links in #12 in any detail (I've flicked through, but there's a lot of it), but you didn't comment on the last paragraph in my #33.

But the best of British luck!

Codemaster:

According to the ID fan design and at the plant's max load of (100% TMCR); including air ingress and leakages; the max flow rate expected was determined. Then, for the chimney design a specific velocity criteria was given to the chimney vendor which was minimmum exit velocity = 22 m/s at the exit and max velocity at the entrance of chimney = 30 m/s). Based on this velocity criteria for the design of the chimney, given by me from another referenced project, the chimney vendor calculated the size of the chimney and pressure drops and the chimney draft. So, the size and flow rate became fixed based on the velocity criteria (min 22 m/s at the exit and max 30 m/s at the enterance) given to the chimney vendor.

Now there are 2 questions:

First question: When the load changes from 100% to 25 %. The flow rate of flue gas reduces. With the flow rate reduction and the fixed size of the chimney (which was already calculated with 100% load); the velocity at the exit changes from 22m/s to 10/ms. Here, the question comes; would this 10 m/s be too low?? or we can ignore this low velocity since, the plant will not operate at 25%~50% load for long times? That's why i was asking for range of accepted velocities from some standard.

Second question: the velcoity criteria (min 22 m/s at the exit at 100% load and max 30 m/s at the enterance at 100% load) which i gave to them from some reference project of same conditions. There is no valid reference available for that range of velocity critera. That velcoity criteria for that reference project was sent to us by owners of that project whose information is unknown and whose criteria may be based on their site conditions. So, currently, I am waiting for the current project's owner response. If min 22m/s is acceptable to them at 100% load; then for the other loads of 25%-50% will also be acceptable to them based on their local standards, regulations etc.

Regarding the 1.5 times wind speed rule and the recommendations by ANSI (3000 fpm = 15.24 m/s and ASHRAE (2000 fpm ~ 3000 fpm = 10.16 mm/s ~ 15.24 m/s); please see page 3 and page 6 of the following link (from EPA and DOE) which states the following:

http://www.i2sl.org/documents/toolkit/bp_modeling_508.pdf
L A B S F O R T H E 2 1 S T C E N T U R Y 3
7. Maintain an adequate exit velocity to avoid stack-tip downwash.
The American National Standards Institute (ANSI)/ American Industrial Hygiene
Association (AIHA) standard for laboratory ventilation, Z9.5-2011,(9) suggests that
the minimum exit velocity from an exhaust stack should Photo from CPP, Inc., NREL/PIX 13818 be at least 3,000 fpm. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)(6) recommends a minimum exit velocity of 2,000 to 3,000 fpm.

A rule of thumb for avoiding stack-tip down-wash is to make the exit velocity at least 1.5 times the wind speed at the top of the stack

OK not sure what you want me to do, I was just commenting on some aspects that interested me, I've no experience of stack design.

Still not clear who selected diameter and exit velocity, it seems to have gone back to you in your latest post, but don't worry about it!

In your link it says - This stack top wind speed is commonly taken to be 1% wind speed, which can be obtained from ASHRAE for various worldwide metropolitan areas²¹. Note that ASHRAE-provided wind speed must be adjusted from the anemometer location to the stack top²¹. I don't understand the 1% wind speed - top 1-percentile? - but hopefully it's made clear in the ASHRAE data.

Only other thing is I'm surprised velocity in the stack body is higher than at exit. I'd expect lower in the body, to keep friction losses low, with a taper at the top to give required exit velocity. I've seen stacks like that.

That's what a good question looks like!

Finally.

As ER stated, the information highway is a two way road.

Complete information at the beginning of the journey, will make the trip much more pleasant.

Thank you.

Depends on a number of factors including flue gas temp.

(minimum plant operational power /max plant power) × 16 m/s + 6 m/s

as per WAGs mrwags.osar.14u

assumes linear relationship between plant power and flue gas flow rate.

Hi,

According to this formula (minimum plant operational power /max plant power) × 16 m/s + 6 m/s; this comes about to be 10 m/s. Could you please elaborate on "WAGs mrwags.osar.14u". Couldn't find it on internet.

WAGs. - wild ass guesses

mrwags - 'em are WAGs.

osar.14u - oh yes they are one for you.

haha thank you for your guess:)

I do appreciate someone who can take a joke well.

I wish I had something that would help you resolve your problem.

Is there not an organization that provides oversite/regulates exactly these types of things where you are? If you are wary of contacting them out of a reasonable fear of exensive scrutiny and possible costly fines, it would still be a place to search.

Checking with reps for trade publications or contractors/suppliers who may support similar operations may be able to get you in touch with someone who can point you in the correct direction.

A GA from me but I can't avoid the feeling that your comments are wasted.

I was a coop student in a utility that didn't have a formal coop program, so generally I was handed all sorts of what could have been considered "make work" projects. The good part was that I was given complete access to the inner workings of the bowels of the company. I asked some really dumb questions but sooner or later some old timer would take me aside and explain why things worked the way they did. I then wrote a report with my findings. It always vexxed my managers when I came up with real answers to their imaginary questions.

RamConsult:

I am currently going through the dispersion modeling/EPA papers and going through the data givenby Spade and others here.

For your info, I work in a Mechanical Engineering design department of an EPC company who is responsible for building power plants around the globe. As a Mechanical Engineer, I am currently responsible for Engineering design on equipment that comes in the flue gas path for a thermal power plant including AH, ESP, IDF, FGD and Stack. The ID fans are surely designed to create certain flue gas flow rates by adjusting the dampers or vane angles but the exit velocity from chimney depends upon area of stack as well not only flow rate from ID fan.

I belive that there is no such thing as stupid question; but there might be stupid answers. I hope everyone here also believe on it. Thank you for your responses.

Just want to add one more point here: Designers can't forget to use ID fan or can't eliminate it due to cost contraints:) it's power plant not an icecream to forget to add flavors in them (no offense). You can't operate power plant unless you design the exhaust system well. You need to calculate the pressure drops accross the flue gas path to see if ID fan is required. Otherwise you use FD fan to create sufficient pressure to push the flue gas out of the stack. In some cases, very low power output, the natural draft (created by chimney effect) can create sufficent draft to pull the flue gas out.
Hope it adds to your information.

Option 1: clone the chimney design from a similarly-sized plant elsewhere.

Option 2: many years ago the Shell petroleum company produced a guidance booklet on chimney design. Though the particular one in mind went on a well-known internet auction site several years ago, it may be possible to obtain a copy using an internet search on chimney design codes.

Option 3: contact the agency responsible for environmental protection in the applicable jurisdiction, and ask for guidance; do this by telephone.

Good luck.