Heat exchanger sizing

Just a couple quick questions -

Is this an actual design, or a school project?

Are you trying to design an air preheater for a boiler/furnace?

Hello!

Well this will be an actual design, which I have already finished the three dimencional design. The objective is to cool the hot air gases temperature down to 150 dergess, in order to be able to dedust the air stream through a large self cleaning filter unit. The pollution of the gases is relatevly small, below 200mg per cubic meter of air.

The cooling fresh air that will be warm after the cooling of the gases, will be used for room heating during winter time.

The idea for preheating the furnace is an option too.

The problem is that I do not have experience for heat exchangers.

I have serched a lot during the last couple weeks and I have managed to understant the maths of the heat transfer. I have made a two way aproach of the solution, one described in the books and one analyticaly of my own... giving similar results. I am preparing for a third, but in engineering the experience is of prime importance.

We are interested for the "know how" co-operation with others that will support us with their experience.

This is a pilot project and we have included emengency safe arrangements in order to maintain the gases temperature below 200 degrees.

But other projects may be folow after that, and they would be much more demanding and precise with the use of many and complex heat exchangers, including more technologies that we are not familiar of.

If you can help us and prove that you are capable of such designs, this would be a open invitation for co-operation.

Best regards

Fair enough,

I am not going to get into the math of your application, because, quite frankly, it would be best left for a sophisticated computer program. But I do have a couple points/comments.

1. You don't explicitly state it, but it looks like you are also going with bare tubes on the cool air side. This is surprising because gases have a low heat transfer coefficient and the fins greatly increase the heat transfer per area. I think you would be better served by having fins, and I am sure you will find you can have much shorter tubes.
2. Depending on gas composition / scrubbing arrangements - you can run into trouble with gas side corrosion from sulfuric compounds (sulfuric acid) because the acid dew point of flue gas is typically around 116-163 deg. C. What material of construction are you using? How much sulfur is present in your fuel/exhaust?
3. Be cognizant/aware of cracking problems in these applications - especially due to thermal cracking.
   

Hello again,

First of all I would like to thank you for your interest and the advices.

Now, concerning the maths... I love them, so don't make the calculations, I would make them. But do you have detailed formulas to suggest?

I have made already an analytical approach, and I used 1.200 different points inside the exchanger (with the assistance of the computer of course), and I may even raice them to double or triple quantities with no big effort now... but this has to be tested and-or verified by cross examination with other calculations. However, my feeling, as an engineer, is that I am not far away from reality, but this is just a feeling. The diagrams and charts that have been produced are amazing... even for me. At least we may impress the client (Joke).

Concerning the sophisticated program that you suggest, do you have something in mind? Usually these programs are expensive, but most important need people that know well how to use them. As we would like to consider ourselves professionals, the cost should not be an obstacle, but to choose the right collaborator is a very difficult matter and needs time. We are searching about... but the time is pressing a lot for this project.

Concerning your comments:

1. The tubes are going to be bare... The pressure inside gases is close to atmospheric, and the gases are actually air that have been pre-heated through a special chamber and after have been heated with the combustion of natural gas. I believe that due to the low pressure the fins will not help drastically, but will increase the cost considerably.

2. The sulphuric compounds are not expected to be high... We may keep the temperature up to 170 - 180 degrees via the help of the electronics that will monitor the system. I shall take your advice seriously...

3. Thermal cracking is an important factor, but we have agreed along with the client that we shall use normal black steel, coated with special high temperature painting.

I would like to thank you in advance for your help

Best regards

If my memory serves me well, then in the book, Chemical Plant Design by Peter and Timmerhaus of the McGraw-Hill Chemical Engineering Series, it is stated that the optimum flow-velocity in the Tube-side is about 3ft/sec.

So, if you know the Tube dimensions, and flowrate, then you can calculate the mass-flowrate per tube; then divide the total mass flowrate (12.1 Kg/hr per Heat Exchanger) by the per-Tube mass-flowrate and that should give you the number of tubes you need in each heat exchanger. Then check to make sure that the exit temperature of the flue gass is in fact 150^oC. Otherwise increase the mass-flowrate of the outside air for the Shell-side until you get the flue-gas temperature at 150^oC.

This is the easiest way I can think of, for you to get this done. Computational design of heat exchangers is usually very intense.

I hope that this helps, and Good luck.

Hello!

First of all I would like to thank you for your comments and your time!

Now concerning the speed of the gases inside tubes: the velocity of 3ft/s ~ 0,9m/s, is a rather low value. I think that has to be applied of liquids. For this speed is required around 4,33 times quantity of tubes, that needs a lot more material (not for tubes that may be less in length but for the frames). Moreover, the speed inside the tube is dropping during the process due to the temperature reduction - density increase - and finally air volume reduction. Thus the gases speed would be extremely small at the end and the gases have pollutants inside that will accumulate on the internal surface of tubes rapidly, requiring high maintenance actions.

Never the less you input a good question: If are used more tubes but less lenght, keeping the same surface area the temperature drop will be higher or lower?

What is your opinion about?

Thanking you in advance for your help.

Best regards.

About software, yes you are correct they are very expensive. I am intimately familiar with one - an Enterprise Heat Exchanger design Software - and has a price tag of half a mill written in C++ with nested classes to capture the internal structure of specific heat exchanger type, and very customizable. I should not mind providing help to you guys regarding the software at some later date, irrespective of your location on the globe of earth. However, let us talk about that some other time.

You are pressed for time and we need to help you finish the project. So I suggest with respect to my earlier approach that you set the flow-velocity at the exit at 3f/s and temperature of 150oC; find the density of the flue gas at 150oC and recalculate the number of tubes.

Let us know what you have.

For the design of the HE adjust the inlet flow velocity by recalculating the flow velocity for the same mass flowrate at 450oC which should give you the inlet flow velocity.

Hello again,

Well I tried to do what you suggested…

With my approach (model) I have the following results:

TOTAL MASS FLOW OF GASES = 36.282 kg/h

TOTAL FRESH AIR QUANTITY = 200.000 m³/h

1. Heat exchangers 8 pieces, diamond arrangement 10x20 tubes = 200 tubes (your proposal). Tubes are in vertical position. Gases inlet is on top. Fresh air inlet & outlet side of tubes (mixed flow).

Each heat exchanger:

Gases inlet = 450 °C - 0,4252 kg/m³ (x = 0,2 Kg/Kg) - 10.666 m³/h - 4.535 kg/h - 1,62 m/s

Gases outlet = 150 °C - 0,7979 kg/m³ (x = 0,2 Kg/Kg) - 5.684 m³/h- 4.535 kg/h - 0,86 m/s

Fresh air inlet = 30 °C - 1,1135 kg/m³ - 25.000 m³/h - 27.838 kg/h - 1,89 m/s.

Fresh air outlet = 79 °C

Tubes = 200 pieces – outer diameter 114mm (no fins) – inner 108mm – 3,8 meters each. Total length = 760 m.

For all the 8 exchangers = 6.080 m!!!

2. Heat exchangers 3 pieces, diamond arrangement 10x20 tubes = 200 tubes (my approach). Tubes are in vertical position. Gases inlet is on top. Fresh air inlet & outlet side of tubes (mixed flow).

Each heat exchanger:

Gases inlet = 450 °C - 0,4252 kg/m³ (x = 0,2 Kg/Kg) - 28.443 m³/h - 12.094 kg/h - 4,31 m/s

Gases outlet = 147 °C - 0,7319 kg/m³ (x = 0,2 Kg/Kg) - 16.524 m³/h - 12.094 kg/h - 2,51 m/s

Fresh air inlet = 30 °C - 1,1135 kg/m³ - 66.667 m³/h - 74.233 kg/h – 3,20 m/s.

Fresh air outlet = 79 °C

Tubes = 200 pieces – outer diameter 114mm (no fins) – inner 108mm – 6 meters each. Total length =1.200 m.

For all the 3 exchangers = 3.600 m…

The final difference is great… Seams that obviously I have done a mistake, because for the same result I have a way far different area…

What is your opinion?

Thanking you in advance

Regards

The previous "Gest" comments was for me actually.

Regards