Heat Exchanger Selection 1

[Smith55]

Hello,

Looking for advice and experience on pros and cons of hairpin heat exchanger versus shell and tube for application involving highly cyclic thermal conditions, high temperature differences, temperature cross and high pressure on the tube side.

Any advice, experience or suggested resources for further guidance appreciated.

Many thanks.

 

[georgeverghese]

Pls state the terminal temps on both sides that result in temp cross. Is there non uniform vaporisation or condensing occuring on either side - pls describe.

 

[MJCronin]

Suggest that you post on the "Heat transfer and Thermodynamics" forum

MJCronin
Sr. Process Engineer

 

[EdStainless]

When there are large temp swings a u-bend HX is often preferred.
The tubes are free to expand and contract only having to slide in the support plates, no moving seals or high stresses on closures.
The classical example is high pressure feedwater heaters in power plants.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Christine74]

Either a hairpin exchanger or a "U" tube shell and tube exchanger can handle thermal cycling, large temperature differences, and high pressure on the tube side. But if you have a temperature cross you normally need to have pure countercurrent flow, which is easily accomplished with a hairpin exchanger but trickier with shell and tube exchangers (you either need multiple shells in series, or you need to use a longitudinal baffle in the shell).

If the required surface area is small (less than say 300 sq. ft) it's an easy decision, you want to use a hairpin. If it's significantly larger than that it might be better to use multiple shell and tube exchangers or use a TEMA "F" shell.


-Christine

 

[EdStainless]

I mentioned FWHs because we regularly supply tubes for these.
5/8" OD x 0.109"mw, 304LN, 45' long straight legs, 1600 tubes in each bundle.
Yes they are a little heavy.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Smith55]

Thanks all.

georgeverghese - Typical temperatures are Tubeside: T1 - 440, T2 - 240 deg.c, Shellside: t1 - 230, t2 - 430 deg.c.

MJCronin - Thank you I will post to heat transfer & thermodynamics forum.

EdStainless - Thanks for the explanation.

Christine74 - Thank you. Regarding the easy decision to select haripin if area is less than 300 sq.ft - presumably this would be that in larger sizes multiple shell and tube exchangers becomes more cost effective?

In terms of plot plan space, would you have any recommendations for best arrangement for space optimisation (vertical, horizontal, stacking, etc). Many of our heat exchangers are large (over 100 tonnes) and space is a constraint.

Also, we would like the heat exchanger to be self venting and self draining on the shell side - any recommendations appreciated.

 

[georgeverghese]

Okay, terminal temps are such that shell and tube units cannot be used here, no matter how many shells you line up in series.
See if spiral heat exchangers suit your application - temp cross not a problem since it is pure countercurrent flow. There is an account of these units in the 7th edn of Perry Chem Engg Handbook on page 11-54 to 55 - max design pressure stated to be 20barg. Thermal cycling to be discussed with manufacturer.

https://youtu.be/V7c-2VZFwrM

 

[Christine74]

Even though they offer pure countercurrent flow, spiral heat exchangers typically aren't specified unless you have a slurry or other particulate-containing process stream and they generally aren't suited for high-pressure applications such as this.

Other types of exchangers that offer pure countercurrent flow include welded-plate exchangers, hairpin heat exchangers, double pipe heat exchangers, and fixed tubesheet shell-and-tube heat exchangers with one pass on the tube side.


-Christine

 

[georgeverghese]

There is an account of the use of various expansion joints on shell and tube type HX which would be of use in one tube side pass, pure countercurrent flow in Perry 7th edn - see Fig 11-36b (TEMA type BEM) and narrative on page 11-38, which also mentions thermal cycling issues.

 

[Smith55]

Thank you all - much appreciated.

 

[georgeverghese]

Can you tell us what type HX and TEMA configuration you have for these large units , and design pressure / temp on both sides ? Would these be TEMA floating T type head ?