6 replies to this topic

[PSB]

While Designing a STHE, BEM type, with the following process data for normal operating case :

Shell Side Fluid: Cooling Water
Shell Side Flow : 29000 kg/hr
Shell Side Inlet : 33 deg C
Shell Side Inlet : 45 deg C
Fouling Factor : 0.0004 m2.hr.C/kcal

Tube Side Fluid: Nitrogen
Tube Side Flow : 7969 kg/hr
Tube Side Inlet : 210 deg C
Tube Side Inlet : 38 deg C
Fouling Factor : 0.0002 m2.hr.C/kcal

HTRI calculates the Mean Metal Temperature (MMT) in Final Results for the normal operating case, however the licensor has asked for MMT at the event of coooling water failure (no fluid on shell side).

How can I determind the MMT for such cases, required for mechanical design of tubesheet?

[mvp]

PSB:
1. For CW failure case, change the flow of CW in HTRI to some low number, say, 0.000001 kg/h and run.
Do not worry about other Heat Transfer results; just use the MMT.
2. The fouling factors on water: is the water that bad? or is it too much over design in specifying?
If water is bad, see to that you have good velocity on the shell side?
3. Nitrogen may be considered clean; fouling factor is high
4. What is the metallurgy for the SS & TS??
5. You are having a temperature cross. watch out. do not have multiple passes or increase your N2 outlet temperature if allowable
Good luck
MVP

[mvp]

PSB:
Is there a possibility of switching the sides of the fluids
In that case, the expansion issue would be of lesser magnitude when CW fails.
The other advantage >> it is easy to maintain a higher CW velocity in tubes
MVP

[kkala]

"instead, this is looking at the metal temperature, such as might be used for calculating thermal expansion/contraction of the exchanger components, or for unacceptably hot/cold metal temperature during upset conditions (e.g., the cold fluid runs dry, melting the exchanger". (Guess by Tornado, May 1st 2010,
http://cr4.globalspe...-Heat-Exchanger).
Although other interpretations of MMT can be seen in Web, above is probably what the licensor means. In case of no cooling water flow, tube side (and shell side a bit later) can approach 210 oC. This can be considered as MMT, but you had better consider tube design temperature being a bit higher, (say) 230 oC.
Instead of said MMT, we use design temperature for excahngers.

Edited by kkala, 15 November 2010 - 07:09 AM.

[PSB]

PSB:
Is there a possibility of switching the sides of the fluids
In that case, the expansion issue would be of lesser magnitude when CW fails.
The other advantage >> it is easy to maintain a higher CW velocity in tubes
MVP

MVP:


MVP:

Thanks for the suggestion. When I try to run the case with low flow ,the run fails.
Incidentally another idea struck me, I ran the case in sumulation mode of HTRI keeping the Shell side and Tube side inlet temperature as in the normal case and keeping the outlet tube side and shell side temperature undefined, allowing HTRI to assume the duty. Then I gradually decreased the cooling water (Shell side) flow till it approached the tube side temperature i.e 210 deg C. The run failed just before the Shell Side flow of 450 kg/hr when it approached 207.6 deg c. (which is actually the scenario).

If you lose cooling water it usually causes a shutdown. You won't have cooling water flow but you will have water in the shell to act as a heat sink until it drains out. The wall temp might go up a little but not much.If for some reason there is no shutdown then the water will still provide a heat sink for a while as it evaporates and there still won't be a significant change unless this goes on for a very long time and enough water evaporates to expose tubes.

MVP, I am afraid, can't question the parameters like fouling factor that are provided by the licensor. Metallurgy on both the sides is CS which is also provided by the licensor. The temperature cross is avoided as the exchangers chosen is BEM counter-current, single pass. Swapping the fluids does not help much.

Would appreciate if u can shed more light.

Regards,
PSB

[mvp]

PSB:
1. CW on shell-side; if you have MMT for CW failure case, send to Mechanical design department or vendor to find out if there are expansion issues. If none, go ahead.
2. If there are, then try the CW on tube side case; it will have a slightly smaller exchanger; and the difference between the shell and tube temperatures (in case of failure) would be low.
3. If possible post an excel output of your case
4. An air cooled exchanger would do the job; if the client is willing to accept a higher N2 Outlet Temperature; The CW will have to exhaust the heat to air anyway.
Best Regards
MVP

[kkala]

If you lose cooling water it usually causes a shutdown. You won't have cooling water flow but you will have water in the shell to act as a heat sink until it drains out. The wall temp might go up a little but not much.If for some reason there is no shutdown then the water will still provide a heat sink for a while as it evaporates and there still won't be a significant change unless this goes on for a very long time and enough water evaporates to expose tubes.

Above can be right when cooling water pump stops, while relevant valves upstream and downstream of exchanger remain open. But consider the case (e.g. at startup) when both cooling water valves are inadvertedly closed and both nitrogen valves have been opened. Then blocked water (being heated) will be expanding out through the thermal expansion PRV until its temperature reaches 210 0C. At that moment both tube and shell sides are at 210 oC (see Post of 15 Nov 10).
We have recently applied this practice for water / brine exchangers using design temperature (177 oC) instead of max operating (in practice Process here does not specify MMT for exchangers).

Edited by kkala, 16 November 2010 - 06:55 AM.