Shell and tube HX - Maximum tube passes for a given shell ID

[espnloser]

From a fabrication point of view what is the maximum number of tubes passes for a given shell ID. I have an 8 tube pass BEU HX and the shell ID is 21 inches. I wanted to know if this is possible and if anyone has any guidelines for this.

For example, no more than Y tube passes for a shell diameter less than X inches. (for a given OD tube)

Thanks for any replies.

 

[Montemayor]

espnloser:

An 8-tube pass, fixed tube sheet shell & tube heat exchanger with a 21.25” I.D. will have the following estimated number of tubes:

3/4" tubes on 15/16" Triang. pitch = 258
3/4" tubes on 1" Triang. Pitch = 224
3/4" tubes on 1" Square pitch = 174
1" tubes on 1-1/4" Triang. Pitch = 120
1" tubes on 1-1/4" Square pitch = 94

If you talking about a U-tube, removable bundle, then you can have:

3/4" tubes on 15/16" Triang. pitch = 254
3/4" tubes on 1" Triang. Pitch = 226
3/4" tubes on 1" Square pitch = 194
1" tubes on 1-1/4" Triang. Pitch = 118
1" tubes on 1-1/4" Square pitch = 98

I am citing Ernie Ludwig’s figure’s from his classic book, '"Applied Process Design for Chemical and Petrochemical Plants"; Vol. 3; p.24. If you need 100% accurate figures for fabrication, you should layout out the tube sheet on a drawing board and actually plot your tubes on it, taking into consideration your requirements for tube O.D., partition plates’ thicknesses, tie rods, Outer Tube Limit (OTL), and TEMA’s recommended spacing allowances. In my opinion, the latter method is mandatory if you are fabricating the unit yourself.

The same recommendation applies if you want to find out the maximum number of tube passes in a given shell ID with a given tube OD. Ludwig lists no more than 8 tube passes. I don’t know if that’s the maximum. I certainly have never considered that many and have never done more than 4 or five. I believe anything more than that is impractical and uneconomical – not to mention that it may be self defeating in some cases due to temperature crosses. You don’t furnish sufficient basic data nor do you explain your need or application, so I’m unable to comment further.

I hope this answers your query.

 

[Christine74]

A fairly conservative rule of thumb would be that the maximum number of tube-side passes can be estimated by taking the inside diameter (in inches) and dividing by 2.5. You could probably go as far ID/2.0, depending on the exact details of your exchanger, but you had better confirm that with the fabricator.

-Christine

 

[srfish]

As someone who has been a thermal designer for a fabricator,I believe That the rule of Christine74 is good. Although I would use a factor of 2.0 instead of 2.5. In large heat exchangers I have seen factors even lower than 2.0. For small exchangers (less than or equal to 10" ID), take the pass plate arrangement and see if there is room enough above the top pass plate to have enough tubes where there is good distribution of tubes between the tube passes.

 

[Casimo5]

Senior Fish,

What would be the rule of thumb for the shell side of a 4 pass exchanger? What I mean is, if you have 4 passes on the tube side, would you typically want the shell side to have 2 passes or is that based on the fluids you are exchanging heat between?

Thanks in advance,

Soapy

 

[Casimo5]

In other words, I've heard of a 1-2 and a 2-4, but have you seen a 1-4 exchanger? And if so, would the hot side typically be on the shell or tube side of a 1-4 exchanger?

 

[srfish]

Soapy73:

There are plenty of 1 shell pass & 4 tube pass heat exchangers in existance.

As far as which side to put the hot fluid on is mostly a matter of experience. There are numerous factors to consider. Factors such as fluid fouling, corrosiveness, viscosity and pressure. The flow volume ratio of the two fluids is also to be considered.

Some thermal design software has the option to run the hot fluid on one side and then make another pass with the fluid on the other side. Then you can estimate the cost of the two selections.

 

[espnloser]

thanks for the replies.