3 replies to this topic

I am 4th year mechanical engineering and i am working on the graduation project which is a design of waste heat recovery boiler , and i have some questions which i will be very grateful if you replied to me
I am working to make a design program so these data could be in a range

first the process gases is at 160 bar, 500 c , clean .
water 125 bar , 125 c , m.st = 50 t/hr
the design will be divided in to 2 parts
1.Economizer (raise the feed water temp. before the evaporator from 125 c to 313 c )
2.Evaporator.

I am not asking for the calculations but
My questions are general

1. the % of vaporization in the evaporator .
2. I can make the design of both parts as a shell & tube H,EXs
But how to make them as one part in one box
3.for the same data if the gases pressure is 1 bar I will require a large heat
transfer area what apparatus should I use ?

[Art Montemayor]

Ahmed:

To answer your specific questions:

1. The % of vaporization in the evaporator should be 100% --- this follows your basic definition of the problem: you are trying to make steam out of water, and therefore you have to vaporize all of the water that you are feeding the evaporator (a.k.a., the waste heat boiler).

2. As a 4th year mechanical engineer you should know better than most people that at 160 and 125 bars of pressure it is practically impossible to contemplate a “box” as a pressure vessel. You are dealing with a formidable pressure vessel at 160 bars and it would certainly be a cylindrical (or even perhaps a spherical) vessel. You know, as a mechanical engineering student, that these two geometric shapes are the ones that inherently can contain such high pressures. Plane surfaces – such as the walls of a box – cannot contain pressures on a practical level. You need to research the application of high pressure heat exchangers and the different type that can be applied.

3. Common sense will tell you that when you are dealing with a low pressure gas, you are now dealing with a very high volumetric flow rate, a very low heat transfer coefficient, and a very low allowable pressure drop. These outstanding features of a a low pressure gas stream should steer you in the direction of a a very large shell configuration --- something approaching a box-like structure. The high pressure water and steam generation belongs, of course, in the tube side and any shell that you select.

You didn’t supply any sketch or gas composition. Your basic data is scarce. Therefore, that is all I can comment on.

Thank you very much Mr. Art, but I have more questions.

Generally, in the evaporator (water tube boiler) I want 100 % steam, but i have read before that the steam will form like an insulating layer over the water since its heat transfer coefficient is less than water so the overall heat transfer coefficient will decrease and in case of high temp gases maybe cause overheating to the tubes.

Also, in the case of low pressure steam the specific volume difference between the steam and the water is large -- so I might evaporate about 20% of the water entering the evaporator and then this mixture enters the steam drum separating the steam from the water & recirculating the water back. Am I right or not?

In my previous case the steam is at a very high pressure, so its properties are near that of water -so as long as the specific volume & the temp. are not too high, there is no overheating. But what about the overall heat transfere coefficient?

I am little confused about this point, and I would be grateful if you cleared up this point for me.

[Art Montemayor]

Ahmed:

You fail to give us a specific case and seem to prefer to dwell in generalities. You also have not addressed my request nor answered my post. I have to presume that you want to discuss generalities and not specifics. If that is the case, then this thread is going to take a long time and the responses will be numerous and purely theoretical in nature. Additionally, your comments/questions do not target any specific area of design – either process or mechanical. You have not organized your basic data and, consequently, your comments/questions are confusing. For example, you basically write 3 points in your last post. (Note that I have edited your post so that it reads grammatically correct and can be better understood; I have also arranged it 3 paragraphs – which define the 3 points or thoughts that you want to convey. Please, please organize your thoughts and write them out accordingly so that they can be better understood.) I comment on these 3 points below:

“Generally, in the evaporator (water tube boiler) I want 100 % steam, but i have read before that the steam will form like an insulating layer over the water since its heat transfer coefficient is less than water so the overall heat transfer coefficient will decrease and in case of high temp gases maybe cause overheating to the tubes.” Do you concede that what I stated is correct? In other words, if you pump in 100 kg/min of feed water into your steam generator, you will generate 100 kg/min of steam – 100% vaporization. You are getting confused with the fact that when you have a rather large temperature difference between the two fluids you will have a tendency to form a vapor-bound surface on the side of the vaporized fluid. This vapor is being generated so fast that it forms an insulating barrier against heat transfer. This is remedied by incorporating a heat flux (energy transferred/transfer area) that allow for slower, steady heat transfer. In layman’s terms this means the heat transfer area must be larger than usual. I usually employ 8,000 to 10,000 Btu/hr-ft2 as the heat flux – depending on the magnitude of the temperature difference and the application. You should research this point out and understand it.

“Also, in the case of low pressure steam the specific volume difference between the steam and the water is large -- so I might evaporate about 20% of the water entering the evaporator and then this mixture enters the steam drum separating the steam from the water & recirculating the water back. Am I right or not?” I don’t understand your statement or what you are trying to depict. If you are boiling water in a vertical tube that is attached to a steam drum, you normally are circulating the water much as in a thermosyphon effect. That’s one of the functions of the steam drum – as used in high pressure boilers. You may evaporate 20% of the water recirculating around the steam drum, but you will evaporate 100% of the water entering the boiler. Do you no see how confusing this can all get if you don’t furnish at least a sketch?


“In my previous case the steam is at a very high pressure, so its properties are near that of water -so as long as the specific volume & the temp. are not too high, there is no overheating. But what about the overall heat transfere coefficient?” I still don’t understand your concern for “overheating” and what it is based on. I also fail to understand your question about the overall heat transfer coefficient. What about it are your specifically asking?