16 replies to this topic

[fatimah]

hi everybody

Do we have any standard for min shell side velocity? of course for max velocity, it is limited by Rho-V2 value 4000 as given by TEMA. too low velocity will promote fouling but i'm not sure the exact value. Does anybody have opinion about this? Thanks.

Edited by fatimah, 09 April 2010 - 01:48 AM.

[ankur2061]

hi everybody

Do we have any standard for min shell side velocity? of course for max velocity, it is limited by Rho-V2 value 4000 as given by TEMA. too low velocity will promote fouling but i'm not sure the exact value. Does anybody have opinion about this? Thanks.

Fatimah,

My understanding of velocities in S&T heat exchangers is that they have to be restricted below the point where tube vibrations start. Most of the S&T design & rating softwares give you warning for tube vibration. This could be a combintaion of several factors such as incorrect baffle cut or inadequate baffle spacing, but one of the reasons could also be high shell velocities.

So for a given S&T design I would also check the effect of shell side velocity on tube vibration & if possible adjust it to avoid tube vibration keeping the other variables such as shell diameter, baffle cut, baffle spacing etc. as constant.

If by adjusting the velocity, I can eliminate the tube vibration warning than that is my threshold or maximum velocity that I should be using.

Hope this helps.

Regards,
Ankur.

[fatimah]

Hi ankur2061

Thanks for your information. But, my confusion is regarding the min velocity for the shell side. From my 2 years experience using HTRI, they do not provide any warning message about min velocity for shell side. most books mention about threshold velocity, but for min one, i havent found so far.

[ankur2061]

Hi ankur2061

Thanks for your information. But, my confusion is regarding the min velocity for the shell side. From my 2 years experience using HTRI, they do not provide any warning message about min velocity for shell side. most books mention about threshold velocity, but for min one, i havent found so far.

Fatimah,

I am so sorry. I misunderstood your query for maximum velocity.

All right let's talk about minimum velocity. The minimum velocity needs to be determined based on the skin temperature. The skin temperature is the temperature outside the fouling layer or the metal surface temperature for a clean heat exchanger.

Skin temperature requirements are strictly based on service and minimum velocities need to be such that the skin temperature increase or decrease does not cause fouling. Fouling due to increase or decrease in skin temperture needs to be evaluated on a case-to-case basis. Two such cases would be:

1. Seawater side skin temperature shall be kept below 60°C to avoid scale deposition on the heat exchanger wall.

2. The hydrocarbon side skin temperature shall be kept above the hydrate formation temperature or wax appearance point or below any temperature that will cause other fouling (e.g. coking,).

Hope this helps.

Regards,
Ankur.

[kkala]

Do we have any standard for min shell side velocity? of course for max velocity, it is limited by Rho-V2 value 4000 as given by TEMA. too low velocity will promote fouling but i'm not sure the exact value. Does anybody have opinion about this?

N.P. Lieberman in his book "Process design for reliable operations" (Gulf Pub. Co., 1983) says:
"A general rule of thump is to keep velocities on both the shell and tube sides of heat exchanger above 3 fps and below 10 fps".
For shell side probably there is not a relevant official standard.

Edited by kkala, 10 April 2010 - 09:33 AM.

[fallah]

hi everybody

Do we have any standard for min shell side velocity? of course for max velocity, it is limited by Rho-V2 value 4000 as given by TEMA. too low velocity will promote fouling but i'm not sure the exact value. Does anybody have opinion about this? Thanks.

As far as i know,Rho V2 limitations in TEMA wouldn't be relate to shell side velocity and apply for inlet nozzle of shell sizing and also knowing whether the impingement plate is required.

What would limit the minimum shell side velocity are fouling and shell side heat transfer coefficient.

While allowable pressure drop of 0.5-0.7 kg/cm2 for shell side is permitted,in situations of very low pressure drop the allowable shell side pressure drop would have not been utilized properly.Because shell side heat transfer coefficient and hence the overall heat transfer coefficient are low and result in a large and expensive HX.On the other hand,if the shell side fluid is dirty,the low shell side velocity may result in shell side fouling which will result in high operating cost.Of course in these situations,two shells in series or F-Shell (as per TEMA) would be selected.

As per above i think there is no specified minimum value for the shell side velocity and designer shoul try to use allowable pressure drop as high as possible.

Edited by fallah, 09 April 2010 - 05:14 AM.

[breizh]

Good day ,
Hope this helps .
Breizh

[fatimah]

Hi there,

thanks all. anyway i need to comment about fallah's post.
Rho-V2 as in TEMA is not for nozzle shell or tube. i have this thought before, and after have discussion with HTRI, i found out that;

Tube Rho-V2 as per TEMA
referring to the entrance to the tubes from the channels

Shell Rho-V2
applied to shell/bundle entrance/exit areas

[breizh]

Hi Fatimah ,
An extract of rules of thumb for chemical engineers.

Hope this helps .
Breizh

[fallah]

Fatimah:

I have quoted part of the statement in your first post as below:

"Do we have any standard for min shell side velocity? of course for max velocity, it is limited by Rho-V2 value 4000 as given by TEMA."

And also see the attached from TEMA Ed. 2007 relates to our discussion.Now compare your statement with what mentioned in attachment.

You refer to shell side velocity,while TEMA in same case referred to shell entrance/exit velocity,that in my opinion basically differ.
Indeed,i think (as i mentioned in my first post) shell entrance/exit velocity directly depends on shell entrance/exit nozzle size.

Therefore,i think as mentioned in RCB-4.6 (in attached),Rho*V2 criteria apply for impingement baffles and errosion protection based on above at least for shell side directly relates to shell/inlet/exit nozzles sizes.It means if Rho*V2 doesn't meet the specified value,impingement plate should be applied for tube bundle protection.

And finally,Rho*V2 criteria in TEMA doesn't relate to the shell side velocity of the heat exchanger.

Attached Files

•  Pages from TEMA20071.pdf   60.28KB   342 downloads

Edited by fallah, 12 April 2010 - 01:16 AM.

[fatimah]

Hi there

thanks again all. i apologize i havent got time to see all comments.

The shell entrance/exit areas are not the nozzles, but the areas within the shell corresponding to their location. Change the shell ID means the shell entrance will change. thus produce more surface area, lower shell side velocity.

i will come back again once i find the correct calculation for shell side Rho-V2.

[fatimah]

Thanks breizh

i'v seen this page before. not sure whther you still remember that you suggest this book to me before. a good book but it does not satisfy my query. anyway, thanks!

[breizh]

Fatimah ,
Not the same book , this one is from Carl Branan .
Regards
Breizh

[fatimah]

Hi there

ankur2061;
most of the time designer is not equipped with limited skin temp then only option we have is to follow rule of thumbs as mentioned by kkala; velocity between 3 fps to 10 fps.

Majster
usually the shell side pressure drop is already determined by client, the fluid will flow to another process later with the required inlet pressure. if i'm not wrong, there is one normal practise engineering always do is to have pressure drop at 10% of the inlet pressure.

Fallah
seems i need to correct myself about shell velocity; there are 5 points of velocity exist.
- inlet/outlet nozzle
- shell entrance/exit
- shell velocity
anyway, for RHo-V2 4000 as stated by TEMA is for second item. the velocity is measured after the nozzle inlet/outlet before the impingment plate. Please take note that it's not the nozzle velocity.

Thanks!

[fallah]

Fallah
seems i need to correct myself about shell velocity; there are 5 points of velocity exist.
- inlet/outlet nozzle
- shell entrance/exit
- shell velocity
anyway, for RHo-V2 4000 as stated by TEMA is for second item. the velocity is measured after the nozzle inlet/outlet before the impingment plate. Please take note that it's not the nozzle velocity.

Thanks!

Fatimah:

I agree with you regarding above velocity classification in shell side of heat exchanger.

Actually,based on definition mentioned in TEMA (included in the page i have attached in my previous post) i think the value of shell entrance/exit velocity would be between inlet/outlet nozzle velocity and shell velocity.

[kkala]

ankur2061;
most of the time designer is not equipped with limited skin temp then only option we have is to follow rule of thumbs as mentioned by kkala; velocity between 3 fps to 10 fps.
Majster
usually the shell side pressure drop is already determined by client, the fluid will flow to another process later with the required inlet pressure. if i'm not wrong, there is one normal practise engineering always do is to have pressure drop at 10% of the inlet pressure.

Further notes on the matter are as follows.
1. While v>3 ft/s (practically 1 m/s) is widely recognized for tubes, it is less recognized for the shell. For instance, local colleagues say they do not find a low limit for shell velocity in their practices/ standards. But this is supported by N. Lieberman and seems quite reasonable.
By the way the thread "Liquid velocity in tubular heat exchangers" by kkala in this forum (most recent post 21 Mar 2010) covers some matters of the present thread.
2. The condition v>3 ft/s seems also to make the risk of low skin temperature remote, as explained in N. Lieberman's book for a fuel oil case.
3. Much depends on local conditions concerning scales. We have looked into several operating local exchangers, cooling desalter brine. Hot brine (more scaling) is always in the tubes, usually with v> 3 ft/s. Cold water is in the cell with v=0.3-0.8 m/s.
4. Usually allowable frictional ΔP in the exchanger is a fixed value according to practices here (e.g. 0.7 bar for liquids, both in tubes or shell; some higher ΔP can be specified for viscus liquids).

Edited by kkala, 18 April 2010 - 03:41 AM.