SS tank conductive heat loss 2

[vij36]

Dear All,

I would like to calculate heat loss of hot water stored in an SS 304 tank

Water temperature 90 C
Dia of tank 2100 mm
Height 3100
shell Thickness 5 mm
rockwool insulation 100 mm
K 0.036 W/m-k
SS 304 K value 16.2 W/m-k (this is at 100 C. I don't know K value at 90 C)
combined K 0.036 (1/(1/16.2 + 1/0.036))

I am referring this formula from
[link conductive heat transfer through a pot wall]

https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

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This results in 3800 watts. screenshot of calculation attached

So what would be the heat loss in degree centigrade if the tank is exposed to open air for a duration of 10 hrs.

Thanks,

 

[LittleInch]

You appear to have used the K figure for rockwool, but the thickness of the steel.

for heat transfer use the thickness of the rockwool - 0.1m. your heat out will reduce a bit. Heat insulation of the steel is negligible so forget it.

Temp drop is a transient thing as the heat loss drops off as the temperature difference reduces.

You can do a simple calc by looking at energy of the water at the start then reducing it by the heat outflow for say 30 minutes as a flat figure. Then recalculate the water temp. So long as its less than say 5C you are good, but if higher then reduce the time period.

Then recalculate based on your new temperature as the heat flow out reduces. Rinse and repeat.

Note that this is quite simple and ignores any impact of air flow over the outside. It will probably conduct heat away at ambient temperature, but not if it is e.g. inside an enclosure...

So heat loss in the open air with a breeze would be different to indoors inside a room.

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[EnergyProfessional]

There is heat transfer from the fluid to the tank, then from the tank to the air outside. So you are missing 2 resistances that depend on if the fluid moves or not. But in your case you could assume still water and air. Obviously a warm tank would introduce some convection.

 

[IRstuff]

There are number of errors in the graphic. The area should be closer to 24 m^2. The heat transfer coefficient is incorrectly inputted, since your equation in OP ignored the thicknesses of the materials. The deltaT is incorrect, although close, since you neglected air convection

Here's my calculation

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[pierreick]

Hi ,
To support your work document attached .
Pierre

 

[pierreick]

Hi ,
Another document in Spanish .
Pierre

 

[vij36]

Thank you every one for the help.

Will go through all the inputs and will come back with right amount of thickness of insulation and type

 

[LittleInch]

It's a pretty big Pressure vessel, but 100mm of rockwool is also pretty good insulation, so 0.5kW heat loss sounds about right.

but location and air movement outside of the vessel could change in a lot. And if it rains.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

With the tank being insulated, the wall thickness of 5mm will not create a significant temperature drop across the thickness of the tank even with all the factors mentioned by LI impact the vessel. Another point is temperature of water mentioned by IRs as 20K would exist as a solid and not liquid. Your biggest challenge is to determine the coefficients of convective heat transfer inside the tank wall and outside the insulation of the tank. Also in the calculations, I would use the bulk temperature of the water inside the tank which I would estimate to be an intermediate value between the inlet and outlet temperatures of the water. Be aware that the coefficients of thermal convective heat transfer on the outside surface of the insulation may not be the same between top, sides and bottom of the tank. Use the procedure presented by IRs but be careful about the temperatures in degree Kelvin in his presentation which is for steady state.

 

[IRstuff]

I was lazy and used K instead of °C, which requires more keystrokes in Mathcad; for heat transfer, it's all temperature differences, once you get past the heat transfer coefficient itself, which does use absolute temperature to get it right.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[SAK9]

Hi OP IRstuff has gone to the extent of working out a solution to your problem.It is a forum courtesy to thank him by starring his reply.

 

[vij36]

Dear Sir,
@IRstuff grateful for your reply.
Apologies for my oversightedness.

With your reply as the base now I am working on to find a an effective insulation also with less k value. There are some other calculation showing per sqft 4 w as the heat transfer. Then the total transfer is coming to be 3500 w. However there is no substantiation for arriving at 4 w per sqft. Attached the sheet.

 

[pierreick]

Hi ,
To support your work , Consider the document attached.
Pierre

 

[LittleInch]

Well that's the problem with using a spread sheet with some in built values you can't see

So it's not clear what k value the rock wool is, what ambient temperature is assumed or how they get twice the heat loss from manholes as the rest of the tank??

For something like a tank you either need something flexible like rock wool or spray it with PU foam to whatever thickness you need. Much more than 100mm and the effectiveness runs out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.