mean temperature of shell material
mean temperature of shell material
(OP)
We have a vessel that is heated with steam in an atmospheric jacket.
The steam is thus at atmospheric pressure. The bulk water inside the vessel is at approx 35C and static. How can I calculate the mean temperature of the material of the vessel shell (in this case stainless steel).
I recognise that any condensate will create a barrier on the steam side and that the water in contact on the inside would have a hot layer close to the vessel wall.
We are concerned that thermal expansion on the vessel could cause issues and I need a mean material temperature in order to undertake some stress calculations.
Any advice much appreciated!
The steam is thus at atmospheric pressure. The bulk water inside the vessel is at approx 35C and static. How can I calculate the mean temperature of the material of the vessel shell (in this case stainless steel).
I recognise that any condensate will create a barrier on the steam side and that the water in contact on the inside would have a hot layer close to the vessel wall.
We are concerned that thermal expansion on the vessel could cause issues and I need a mean material temperature in order to undertake some stress calculations.
Any advice much appreciated!





RE: mean temperature of shell material
Where is the tank situated and what is the ambient temp?
RE: mean temperature of shell material
-Mike
RE: mean temperature of shell material
The tank is indoors and the ambient is 25 - 30C.
Wall thickness 3mm
Taking the temperature of steam seemed a good starting point, but that is also true of the outer jacket skin too, so the jacket and vessel shell would expand together = no problem.
We have experienced cracking at the vessel / jacket weld though, and believe that the inner vessel is not expanding equally with the jacket.
I have taken some measurements of the jacket skin externally (80C max) and can assume that the surface of the jacket in steam contact is at almost 100C (after an allowance for an air film), But I am struggling to find a method to calculate the mean inner wall temperature to calculate the relative expansion of the vessel.
Graham
RE: mean temperature of shell material
RE: mean temperature of shell material
corus
RE: mean temperature of shell material
We know the worst case scenario is sufficient to cause stress points. For info the assmbly (horizontal cylinder) is 6 metres long.
I have been given an analysis by an unknown "expert" who has suggested that with steam at a nominal 105C will give a temperature on the jacket wall of over 100C (reasonable as it is then insulated). The expert has sugested that the vessel wall in contact with the water at 30C will have a mean temperature of 92C (this 8C difference is not enough to cause stress issues).
My feeling is that this calculation is incorrect but I have no way to demonstrate why.
My gut feeling is based on the assumption that the water is able to dissipate the heat quicker than the steam can add it, so the skin temperature should not be above the mid temperature between the two (i.e steam 105, water 30, mean temperature max <62C. This equates to a 40C difference between jacket and vessel. This is probably enough to cause our stress issues.
Is it a case of knowing the U value for steam to st/st transfer, similar for water to st/st, then ariving at a temperature where the delta T's and U values cause the two energy transfers balance? i.e a state of equilibrium?
This would be reasonably easy to calculate but I do not want to argue a point based on flawed logic.
Regards Graham
RE: mean temperature of shell material
But the expansion on the inner jacket is going to be sloped toward the side closest to the steam and drop off closest to the water.
How full is this tank going to be with water?
-Mike
RE: mean temperature of shell material
That condition makes a very good arrangement for heat transfer, I would expect a heat exchange coefficient well in excess of 1000 W/m²°C. Water side the exchange is much worse, as the water is not flowing: I would expect a heat exchange coefficient with an order of magnitude of 100 W/m²°C. These numbers are just gut feeling and could be calculated much more precisely, but, at least to me, that would require quite a long time.
If I'm correct, it seems that your expert is well to the point.
prex
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RE: mean temperature of shell material
corus
RE: mean temperature of shell material
For clarification the jacket is outside of the vessel. The 92C mean temp calculated is the wall between the steam and the water.
The measured 80C figure was on the outside of the jacket (which is the outer skin excluding the insulation and cladding.)
We all expected the measured 80C figures to be closer to 100, and this added to my concerns that the calculations were flawed.
Prex, thank you for the post, You seem to have hit the nail on the head suggesting that the steam condensing is much better at heat transfer than the water is at taking the heat "away", as this would agree with the figures that were previously calculated.
In the course of this discussion, I have realised that the calculations etc are based on the availabilty of sufficient steam, The fact the jacket only reaches 80C suggests to me that there is simply not enough steam to raise the equilibrium point to it's maximum.
This would make Prex's (and our "experts") calculations valid, but explain the findings from the field. I now just need to work out what is actually happening....
RE: mean temperature of shell material
Actual case will be more complicated but the simple explanation given just to emphasise the possibilities.
Regards,
athomas236