Heat transfer calculation in open tank equalization

Thanks for your replies guys, sorry for unclear and undefined question. Let me try to reparaphrase my question.


Updated info:
This is the way i try to solve this problem.


I calculate the pipe length required to cool fluid inside open tank. With the data i had shown yesterday. I assume that no radiation, tank wall is insulated because the outer surface doesn't contact with ambient air, heat from ambient air is negligible (no longer using natural convection).


You can see my calculation and the pipe sketch inside the tank in a file I’ve attached below
With those data i did my calculation based on internal forced convection of fluid inside pipe. I also assume the pipe diameter with 2 simulations scenario using 10 inch and 1 inch pipe. I notice that the result doesn’t make sense. The calculation using 1 inch pipe has smaller pipe length compare with 10 inch pipe. The way i calculate is from the heat required from the tank then I assign it as heat for force calculation inside pipe (because Qin = Qout). I assume that outlet temperature of pipe is equal with desired tank (30 C). My reference is based on textbook Heat and Mass transfer fundamental and application by Yunus.A Cengel, Afshin J. Ghajar, in chapter 8.


Based on the Nusselt number (Nu) of each simulation, the smaller pipe has the greater value than the larger pipe. That’s because the flowrate in smaller pipe is higher, so that Re (Reynold Number) will be increased as well. With the higher Re can cause hl (convective heat transfer coefficient) higher too which will result to lower heat transfer surface contact. This is also applied on larger pipe with opposite result.


The focus here is to cool the tank. According to my understanding, to cool the tank with those data, the smaller pipe should has longer pipe length than the larger pipe. Based on my calculation, the pipe length for 1 inch is around 966 m, while for 10 inch is around 6810 m, so from this it’s likely the pipe diameter is proportional with the pipe length. I think that’s impossible, imagine if using pipe diameter closer to 0 it's likely that the pipe length is also very short. With short pipe length and smaller diameter, it’s not enough to cool the tank.


But that’s what happened to my calculation. My guess is because the way I calculate is (maybe unconsciously) focusing on heating the pipe from 28 to 30. But I don’t know, I don’t have any idea why this can be wrong.  

[enter link description here][1]
So what do you guys think?
\*I try to simulate the calculation without natural convection, when the force convection one is already clear I will try to consider this too.
\*Let me know if there is something still unclear for you, I will try my best to make it more understandable


Click the link below to see my calculation
[1]: <https://drive.google.com/folderview?id=1TlV9CxFvpx_A6q1GA0PLx6JI4_u5TXQC>


