9 replies to this topic

[vne]

Dear all,

I am currently designing a BKU reboiler, in order to utilize the allowable tube side pressure drop, 4 tube passes have been chosen. By choosing 4 instead of 2 tube passes, size of the exchanger can be reduced significantly (tube bundle can be reduced from 1000 mm to 650 mm, tube length can be reduced from 9.7m to 6.7m). However, by increasing the number of tube passes, the F stream (pass-partition lane bypass stream) also increases from 0 to more than 30% which is not recommended. The following message is from HTRI:

"The F stream flow fraction is greater than 30%. This is outside the range of HTRI experimental data and the confidence in the results decreases as this flow fraction increases above 30%. You should check the exchanger geometry, as such a large F stream will negatively impact the performance of this exchanger."

To reduce the F stream, I added several seal rods in the field "Passlane Seal Device" of HTRI. However, I don't know if the number I putted is reasonable (for this case, 18 seal rods have been added and F stream reduced to 19%).

The question is that: what is the practice/guideline to determine the number of seal rods to be added for minimizing F stream?

Thank you

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[DB Shah]

In BKU where tube are submerged in boiling pool, should "F" stream matter?

[vne]

_{Yes, when seal rods are installed, % overdesign increases.}

[DB Shah]

HTFS does not takes number of baffles as input for BKU (I have old TASC version). Hence I understand that normally baffles are installed for support in BKU rather to enhance heat transfer. In case F stream is affecting so much, try double segmental, this will reduce F stream.

[vne]

It seems you misunderstand my question. Of course in BKU support plates are used to support the tubes. I am asking about the seal rods which are solid tubes installed in the passlane to prevent the flow in this area (F stream).

[Jiten_process]

@vne

For BKU type reboiler where there is mechanism of pool boiling, leakages does not matter much. The problem u faced with your case is because HTRI goes into theoratical model and do all calculations. To overcome this problem try and change the configuration of tube passes. The are four different arrangement for four tube passes. Go to tube pass arrangement tab in HTRI. you may see reduction in F stream.

If above doesnt help then seal rode is good option. But in my experience i have seen after providing the few seal rode, its effect reduces; that means with further increase in seal rode no. you wont see much decrease in F stream. The second constraint is mechanical. I would strongly suggest that you ask this to mechanical people who do the mech design of H.E. Allowable quantity of seal rode will depend the tube bundle diameter also.

I guess since it is pool boiling case, you can also block F stream while running the exchanger on HTRI. I dont see any problem in that.

Hope this might give you some pathforward.

[DB Shah]

I do appreciate your concerns, but as I indicated earlier and as Jiten also expressed, I do not envisage problem if you block F stream in HTRI.

[vne]

DB Shah and Jiten,

Thank you for your input and sorry for not response to you earlier.

I totally agree with you that F stream has little or no meaning for pool boiling heat exchanger. I refered to similar BKU of past projects and what they did to over come HTRI message was to increase the number of seal rods untill F stream fraction is less than 30. The number of seal rods entered is just to cheat HTRI and of course are not written in the heat exchanger data sheet.

[Pilesar]

In HTRI version 6, you do not have to enter imaginary seal rods to totally block the F stream. Instead, go to Input Summary -> Geometry -> Clearances -> Block Bypass Streams -> F Stream

[Robert Montoya]

The F-stream seal rod diameter is not internally limited to the size that fits in the pass partition lane. Previously, the diameter could be larger than the lane, and the resulting F-stream flow fraction would decrease as the seal rod diameter increased.