15 replies to this topic

[Dean Abd]

Hi. I'm designing a gas-gas heat exchanger with temperature range 20C to 600C, 30 bar and heat duty of 37 MW. I've been calculating using Kern's method for shell and tube Hex and i'm getting ridiculous number of tubes - about 5000 tubes per pass (2 passes). Any input? Thanks in advance.

[Saml]

5000 or 10000 tubes is big, but not unseen.

What I don't see right is going from 20 to 600 °C in one exchanger.

[Dean Abd]

It is apparent that the temperature range is pretty huge. I was planning to shift some of the duty to the fired heater that comes after, but i was wondering is there a limit to a temperature range for shell and tube Hex? Because the aim is to reduce the amount of duty of fired heater for lowering the operating cost. 

 

Thanks.

[srfish]

What are the other two temperatures?

[Dean Abd]

Hot stream : 600C - 135C

Cold stream : 20.7C - 580C

 

Thanks.

[srfish]

Because of the large temperature cross, the heat exchanger needs to be of the countercurrent flow type, not 2 tube pass.

 

37 megawatts is a very large heat duty. The heat exchanger surface will probably need over 5000 tubes.

[Dean Abd]

Is it practical to split the duty into 2 or 4 HEX in series to cater for the high flow rates with reduced duty say 9MW each? With Tcold : 20.7C to 400C and Thot 600C to 135C.

Edited by Dean Abd, 12 September 2016 - 01:13 PM.

[Saml]

This size of exchanger is something that is big, but not unheard of. As en example, a big hot-exchanger in a modern ammonia plant synthesis loop may well be above 50 MW in a gas-gas application.

 

However, these are specially designed pieces of equipment. It is the technology provider who makes the design. It is not an item that is designed  by the EPC contractor based on a generic spec. 

 

Here I will ask other members of the forum about their experience, but I have not seen exchangers with this range. There are some with a big range, like the preheater in styrene plants. Those represent a challenging mechanical design of the exchanger itself and all the piping around it in term of supports, stress calculations, etc... Also you have Waste Heat Boilers, TLE/TLX that go from 900+ to about 400. But again, these are custom pieces of equipment designed by specific vendors with quite a bit of specific know.

 

I have not seen  a design where you have 600 °C at one exhanger tubesheet end and room temperature at the other. Perhaps it is doable, but for sure, not common.

[Bobby Strain]

What's the process application?

 

Bobby

[Dean Abd]

Dear all,

 

so I've split the duty to get a smaller temperature range.

These are my thermal design details (conditions : vapour-vapour 1:2 STHE - no phase change)

Process fluid content: H2, CO, CO2, H2O 

Potential TEMA type: AES

Potential material: carbon steel or 304 steel

Duty: 9.69 MW

Thot : 370C - 220C (tube - safety reasons)

Tcold : 27.4-178.2C

Num of tubes : 469 per pass

Shell diameter : 2.19 m

fluid velocity: tube - 13m/s, shell - 25m/s

baffle spacing : 0.55m

U : 192 W/m2/C

Pressure drop : tube - 1.8 kPa, shell - 67 kPa

 

One of my many concerns are flow induced vibrations and in regards to the design, is this practical reality? Is there any particular concerns that i should address?

 

Thanks.

[srfish]

Definitely check for vibration. Depending on the answer, you may have to change the tube arrangement.

 

The overall heat transfer rate looks reasonable.

[Mathi]

Dean Abd

 

The tube side and the shell side velocity are very high. There will be erosion and vibration definitely. Try to reduce the fluid velocity on the shell and tube side.

 

Also, you composition includes CO2 which is corrosive. Therefore, it is better if you go SS 304 rather than Carbon Steel.

 

If you want to utilize carbon steel, then you might have to do a corrosion study and based on the study you will have decide the shell and tube thickness.

[Dipankarc84]

Mathi, it is a gas gas heat exchanger. The velocities are not high. The densities of the gases are much lower than liquids and hence the rho-v2 would not be huge at 25 m/s. 

[Dean Abd]

Srfish: Can you please elaborate on the tube arrangement? For now I'm using triangular pitch (the fluids are rather clean) with 1.83 m of length. 

 

Mathi: I found the velocity are within the range based on literature review, just as Dipankarc84 mentioned. Is there a way to calculate the critical velocity (vibration) without using HTRI software?

 

Dipankarc84: Thanks for the clarification. 

 

My other concerns :

1. Is the diameter too big? Because as far I am aware TEMA standards only cater for up 1.5 m. Or this is just a matter of transportation - crane?

2. Is there a way to further reduce the shell side velocity besides reducing baffle spacing?

Edited by Dean Abd, 19 September 2016 - 07:08 PM.

[Mathi]

Dean Abd

 

You are right. I left the information that it is a gas/gas exchanger into consideration

 

Use the link below to find information on calculating the critical velocity

 

https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiZyPeUnJ3PAhWILY8KHYA3DWAQFggoMAE&url=http%3A%2F%2Fwww.theijes.com%2Fpapers%2Fv3-i4%2FVersion-1%2FJ03401059064.pdf&usg=AFQjCNHCsyM7LxwoPRmKIhFjMy4zt8NR5w&sig2=CHYjoNXPJe6cTPkGk-3YRg

 

Also, I would suggest you to calculate the tube collision damage number and baffle damage number.

 

With regard to transportation, I have sized vessels of 3 m dia and 15 m length, there was no problem in transportation suggested by the logistic team.

Edited by Mathi, 20 September 2016 - 01:26 AM.

[srfish]

The best tube pattern to prevent vibration is triangular. Refer to page 28 in TEMA as to which triangular pitch is best. It is the 30 degree one. To find out more about vibration you can read section 6 in TEMA.