Hello everyone,

I would like to ask you a conceptual question about an indirect heater. It is not mi intention you to solve mi problem, instead of that I want you to clear me same doubt about the approach or method I used.

 

Problem to solve:

I have a present indirect heater (vapor bath) with the next variables:

·         Mass Flow: 24270 kg/hr (Gas – Oil – Water)

·         Q (heat flow) required to be exchange: 1426 KW

·         Area: 59.88 m2

·         Dt ml: 83.72 °C

·         Overall heat transfer coefficient 298 W/m2°C

·         Present Burner: 2.300.000 Kcal/h  (2673KW)

 

Now, my client is asking me to verify which the maximum capacity of this heater is. How much mass flow rate can be processed?

What should I do?

I take two different approaches. I ask you if I am making a big mistake, please let me now!!!!

Approach A:

I take the Equation:

Q: U x A x DT

Q heat flow: I Asume a 65% maximum efficiency based on the capacity of the present  burner so, 2.300.000 Kcal/hr * 0.65 : 1.495.000 Kcal/hr heat flow available to transfer the fluid. Converting units: 1737 KW. Then:

 

Q heat flow : 1737 KW

Area: 59.9 m2

Dtml: 83.64 °C

New Required U: 346 W/m2ºC

 

Then a raise the flowrate to raise velocity, because of that Reynolds Number increase, because of that Internal heat transfer coefficient hi increase and finally I GET my 346 W/m2 °C.

It is right?  My final result is: 41510 Kg/h

 

Aproches B:

I take this two Equation:

Q: (H_outlet-H _Inlet)*Q_mass

Q: U x A x DT

 

Then I make the next iterative calculation:

 

Raise Mass Flowrate by a 10% factor and the Calculate new Q_{10% Mass flowrate}

After that, take the equation:

Q: U x A x DT

 

Q: Q_10% calculated by Q: (H_outlet-H _Inlet)*Q_{10% Mass flowrate}

Dtml: 83.64 °C

U: Calculates from new Qmass flowrate

A: Check that the required area versus the present area of 59.9m2

                If I have enough area I still raising mass flowrate.

                If I don’t have enough area, a reduce mass flowrate.

 

At the end of this iterative calculation, I will find a new mass flowrate of: 26330 Kg/hr

               

 

Which of these approaches is the best? Some of them will give me an incorrect answer? I ask this because these approaches give me different results (very different) . If B is the correct one, what I am missing in approach A?

 

Thanks you!

Ariel

It's difficult to follow what you are doing. And we don't know what an indirect heater is. Maybe you can give a bit more concise description of what you are doing. And all the temperatures, too. A picture is worth a thousand words.

 

Bobby

Hello bobby. Y made a picture but I have a basic problem. I cant find the way to attach it to the post. It is a pdf File. Can you post a capture where I can attach a file? Or Send some instructions?

 

Thanks

Ariel

Choose Edit at the bottom of your post above. Then select full editor. You will have to scroll down to find it. With the full editor, you will see how to attach a file. You will have to scroll down to see it.

 

Bobby

Hello Booby,

 

Thanks for the answer. I mean an “Indirect Type Oilfield Heaters” used in the production of oil, gas, and their associated fluids as API 12 K define.

 

In my case, the coil is in vapor bath, not water bath.

 

I make a picture to describe the present situation. Since that point, I need to define the maximum mass flowrate that can be processed keeping temperature differential as now.

 

As I said, mi intention is to check if the way I am doing things are right. After that, it is a matter of checking numbers and excel formulas.

 

I hope this help you to help me.

 

Thanks

Ariel

So, is this simply a convection heater? Or is there liquid vaporized by a firetube and the vapors condensed to heat the fluid?

Tube Side (Coil): Fluid to be heat form 7°C to 60°C

Outside: Water Vapor at 120°C (saturated).

 

Yes, in the outside water condensate to heat the fluid. Usually External heat transfer coefficient h_o is taken from tables, 5800 W/m2 °C is a normal value.

It sounds like this is a newly purchased item and is not yet operating. How do you intend to control the fluid outlet temperature? If my client asked the same question of me, I would respond that the vendor guarantee is what should be expected. You can ask the vendor if there is any overdesign (extra surface) beyond the calculated requirement. If your client wants more capacity, then it should be the basis of design. And they should pay for it. Maybe you should send your client to your supervisor.

 

Bobby

Hello Bobby,

 

I answer on your mail:

 

It sounds like this is a newly purchased item and is not yet operating.

No, it started up two years ago. It always has working with flowrate below design flowrate.

 

How do you intend to control the fluid outlet temperature?

I think is beyond the post I wrote, but in this case vapor bath temperature, around 120°C is controlled by a TCV (gas to the burner control valve).

 

If my client asked the same question of me, I would respond that the vendor guarantee is what should be expected. You can ask the vendor if there is any overdesign (extra surface) beyond the calculated requirement. If your client wants more capacity, then it should be the basis of design. And they should pay for it. Maybe you should send your client to your supervisor.

 

It was already done. Vendor informed 8% overdesign (always talking about the area). Vendor said 55.43 m2 are necessary and the heater has 59.8m2.

 

In my opinion, I can “take off” more heat from this equipment. Sometimes, and this is a personal opinion, vendor technical answer is mix with commercial interest. In this case, if there aren’t enough capacity to heat the fluid with this equipment, vendor has the opportunity to sell me another one.

 

In some cases, like this (simply convection heater), I think company’s need to have the technical skill to check de performance of the equipment. That´s why I am wondering if the methodology I am using is correct because I get two different result. Again, I only want to check de methodology, and not equation by equation.

 

In other words, How anyone can calculate the maximum capacity of an already fabricated indirect heater?

 

Thanks a lot.

Ariel

Well, what you should be doing is gathering data for the heater operation. And compare the data to the design values. It's not unusual for heaters such as this to become fouled, thus reducing heat transfer capability. Or, maybe the vendor did not size it properly. Or there could be an accumulation of inerts on the water side. Also, it seems to me that your client should be dealing with the vendor, not you. As to your calculations, neither seems appropriate.

 

 

You need to give us all the information if you want help. It took a long while to get to this point, the beginning.

 

Bobby

Thanks bobby,

 

I Answer on your mail again.

 

Well, what you should be doing is gathering data for the heater operation. And compare the data to the design values.

What for? I don’t want to check present service. I want to check maximum capacity. Now is working well (as expected)

 

It's not unusual for heaters such as this to become fouled, thus reducing heat transfer capability.

Of course. That why usually we consider fouling factor in U Calculation. The “U” value Y gave you consider fouling resistance on both sides of the coil.

 

Or, maybe the vendor did not size it properly.

That something that could happened. Checking maximum capacity will be a kind of verification of initial design.

 

Or there could be an accumulation of inerts on the water side.

 

Also, it seems to me that your client should be dealing with the vendor, not you.

Not agreed. The client pay me to check vendor design and estimate maximum capacity. They paid me to give them a solution, not to say, “talk to the vendor”

 

As to your calculations, neither seems appropriate.

I think you are talking about two approaches presented. This is exactly where I have my doubts.

You said that the two approaches are correct and if the result are different it have to be a calculation mistake?. Just a matter of number?

 

To sum up, the two approaches need to give the same result?

 

 

You need to give us all the information if you want help. It took a long while to get to this point, the beginning.

Not agreed that we are at the beginning but your opinion. What information you think is missing?

 

 

Thanks

Ariel

Since you have a working exchanger, you can analyze the data collected for the operation. This is much more accurate than what you have proposed. The designer of such equipment probably has proprietary methods for rating, which you won't have access to. You will need to determine which surface is limiting the capacity. Either the fire tube or the exchanger bundle. And you will be limited by the shell design pressure and resulting bath temperature. Operating data will give you a means to achieve a very accurate estimate of the maximum capacity.

 

Good luck.

Bobby

Ariel:

 

I have written prior thread responses on this subject, but like Bobby I have been confused by your description and your sketch of just exactly what kind of heater you are describing.  We may just have a language problem.  Please refer to the attached document and tell us if it describes exactly what I think you have.  The basic problem I have is interpreting your statement of "Vapor Bath Heater".  I have never heard of such a heater.  In plain English this would be a steam heater.  For crude oil line heaters I have designed and used water bath heaters and have detailed design instructions if that is what you are seeking for confirmation on your calculations.  I always used the old Smith Industries method of design that I believed was used by the majority of designers of this equipment in the Texas-Oklahoma-Louisiana-New Mexico oil patch since the early days of crude oil production.

 

If you concur with the description of the equipment in the document, I can submit copies of the design instructions for your use.

Please confirm that you are not using a steam boiler (caldera) to heat the crude oil.  Water - or a water+glycol mixture is used in our oil patches because a steam boiler is more difficult and more expensive for oil patch use.  Hot water gives a favorable heat transfer coefficient and can be controlled more easily and safely.

 

I don't understand why new member Ijaz2852 is injecting his own specific query into this thread and am forced to delete his post because we can't have members sky jacking threads with their own topics.  This thread belongs to you and we will keep it that way.

 

 QB Johnson Water Bath Heater.docx   411.65KB   1320 downloads

 

 

 

Approach B is correct and Approach A is incorrect because the transferred heat duty through the heat exchanger must be equal to the heat duty received by the process fluid.

 

U (Overall Heat Transfer Coefficient) is a design character of a heat exchanger and it varies with multiple factors and conditions.  It is incorrect for you to assume that U value increases from 298 W/m2-C to 346 W/m2-C just because of flow rate increase in your Approach A calculation. 

Dear Art: I read your article. It is exactly what I wanted to describe but with a little comment: The process coil inside the heater could be immersed in: 1) Water. It is what you call “water bath heaters”. You can see the expansion tank and the equipment is open to the atmosphere. Bath Temperature: 75/80 °C 2) Vapor. It is what I wanted to describe as “vapor bath heaters” In spanish we call this equipment “calentadores indirectos en baño de vapor”. You can see a PSV installed and the equipment is not open to the atmosphere. It works, usually with 1.2 bar. Temperature 105°C (vapour pressure of water at 1.2bar) I would appreciate if you send the design instructions. I´ll see if it works with steam bath as well as water bath. My e-mail address is arielmartinez79@yahoo.com.ar Ok. Is not a steam boiler. Thanks for all Ariel