8 replies to this topic

[3bo0od]

Hello..

 

I have a design problem for designing a shell and tube heat exchanger for heating ethylene gas with superheated steam:

 

Shell side: ethylene gas at 32 bar to be heated from 22 Celsius to 65 Celsius, flow rate 340724.7 kg/h.

Tube side: steam at 5 bar it will be cooled and condensed ( saturation temperature of water at 5 bar is 151) from 237 Celsius to 151 Celsius, flow rate 14037 kg/h

 

I did all the calculation it was easy but the problem I don't know how to calculate the heat transfer coefficient for the condensing steam in the tubes because all the books and topics in the web are discussing the condensation of saturated fluids not superheated. This heat exchanger is existing in a petrochemical plant and it is my study case to be submitted for my chemical engineering project class. All what I have to do is to design a heat exchanger similar to the existing one starting from scratch. 

 

more information:

It is a BJS TEMA type

with 1 shell pass and 2 tube passes

heat transfer area is 200 m2

no of tubes 584 

tube length 6 m

[srfish]

In this situation where you have  desuperheating and condensing, the simplest procedure is to use a gas heat transfer rate for the desuperheating zone and a condensing rate for the condensing zone.There is another method to use for condensing at low operating pressures. You calculate the tubewall temperature where the tube wall becomes wet with condensate. Then use this temperature for the outlet temperature of the desuperheat zone. This will reduce the size of the desuperheat zone with its low heat transfer.

[3bo0od]

Thank you sir for your answer..

Actually that what I was thinking about, but how to identify these zones?

What I did, I have added the heat transfer coefficient for cooling gases (desuperheating) in tubes to the heat transfer coefficient for condensing in tubes. 

 

using the attached formulas:

 

https://www.dropbox....13_23-00-56.jpg

https://www.dropbox....13_23-03-30.jpg

[xavio]

3bo0od,

 

Your job is "to rate" an existing HE, not "to design", you aim to see if it suits the new application.

Your job can be conveniently done by a software like HTRI, but as student you might not have the resource.

 

Heat transfer mode in the desuperheating zone can be evaluated as dry-wall desuperheating or wet-wall desuperheating.

It depends of degree of superheating, temperature difference, and fluid properties.

 

From heat balance you should be able to identify two heat duties: one for desuperheating (sensible), another for condensing (latent).

By evaluating the U values for both duties, you can determine the required length of tube zone for each duty.

 

Examine this link to help you understand:

http://www.wlv.com/p...abook/ch3_2.pdf

 

A quote from the above reference, for relatively small desuperheating duty:

"...it is both simpler and more conservative (in the sense of calculating a larger condenser area) to assume that condensation will occur directly from the superheated vapor, using the saturation temperature and a condensing heat transfer coefficient in the rate equation, and of course, including the sensible heat in the heat load."

 

I am afraid that your desuperheating duty is so large that it requires excessively large area.

If this is your case, a separate steam desuperheater may be an economical option.

 

Good luck.

 

xavio

[PingPong]

I find it strange that this exchanger was designed with low pressure superheated steam condensing on tubeside, and high pressure ethylene gas on the shellside, but so be it.

 

The overall heat transfer coefficient U will be mainly determined by the h of the ethylene gas on the shellside, so do not get carried away too far by the desuperheating part of the exchanger.

 

Moreover. I think there will also be condensate subcooling taking place in view of the low temperature of the ethylene compared to the steam condensing temperature.

 

Also consider how the duty of this exchanger is controlled: By a control valve in the steam supply line, lowering the condensing pressure? Or by a control valve in the condensate outlet, backflooding the exchanger?

[xavio]

PingPong,

 

I agree with you, the fluid allocation is weird, even the use of TEMA BJS itself is awkward.

But, I think the OP doesn't have the authority to make the decision, he is just asked to rate the HE.

So, we have to assume that the plant engineer has considered all things before deciding this way.

 

I also agree that shell side h will be the limiting factor, so poor U should have been anticipated.

 

In addition to the subcooling concern, I am also concerned with mechancial integrity.

Large difference in shell/tube temperature can be detrimental to fixed tubesheet HE if not mitigated.

Let's hope the plant engineer has considered this, too.

 

Good day!

 

xavio

[latexman]

xavio,

 

The "S" head is a floating head, so both tubesheets are not fixed.  It can expand and contract somewhat freely on the "S" end.

[xavio]

latexman,

 

Oops, sorry my mistake. I was thinking of an "L" head rather than "S".

You're right, thank you for the correction.

 

Have a nice day!

 

xavio

[3bo0od]

Thank you guys for your answers..

It was really helpful