Natural convection from hot pipe in water

In summary, the conversation discusses a problem with heat transfer in a defrost water tray model. The individual is trying to find the heat transfer rate between the compressor coil and the water, but is getting a convection coefficient that is too high. They are using natural convection correlations for a copper tube in water, but the assumptions they made may have affected the accuracy of their results. After further discussion, it is discovered that the mistake was in assuming the coil temperature to be 60 degrees Celsius, when in reality it is only 42 degrees Celsius. This leads to more realistic heat transfer rates of 20-30 watts.
  • #1
prageet
2
0
Hey,

I was having this very silly problem in heat transfer. Please could you see what's going wrong !

I am working on defrost water tray modelling. In that, the water collected during defrost is heated by the compressor o/p coil so as to aid evaporation. I was trying to find out the heat transfer rate, in watts between the coil and water . It is given that the coil is at 60 deg Celsius.


When I try to calculate the h value and the Q value using natural convection correlations, I am getting a convection coeff (h) as large as 1500 W/m^2.K. Whereas it should be in the range of 20-100 for natural convection in water.

Its basically a copper tube in water. Its OD is 5mm. Length of coil is 75cm. For natural convection calculations I have taken charateristic length as its OD ie 5mm. Is that correct. ? The correlations that I apply are for horizontal pipe in fluid.

The value of Nusselt no. I land up with is 15. This give me h value of 1500 W/m^2 K (Q~600 Watts), which is very huge.. ! (using h=(nu*k)/d). k(thermal conduct. ~.5 W/m.K)

Which of these assumptions do you think could have made this go so off.. ?

Could anyone point out where I am going wrong with this !

Thanks !
Urmil
 
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  • #2
First, a Nusselt no of 15 for natural circulation is not unreasonable. So for a 5mm tube, it is what it is. On the other hand, the smaller surface area should affect your heat transfer rate.

This was for the outside surface of the tube correct? What are the conditions inside the tube? How many tubes? How long are the tubes? What is the temperature difference driving heat flow?
 
  • #3
I agree with edgepflow. Copper tubes in water will transfer heat at pretty good rates, so 1500 W/m^2*K is reasonable.
 
  • #4
Hey,

Thank you for your response..

Yes, I too verified, that a copper coil 5mm od and 75cm length at 60 deg cels. when placed in water at 40 deg cels. can provide heat rates as high as 1500 watts.

In my case, my mistake was in assuming the coil to be at 60 deg cels. This coil is actually the condenser in the refrigerator. The ref. fluid that flows through it is at 60 deg cels. and I wrongly assumed that to be the temperature of the coil.

On accounting for the 2 convection resistances (1/hA) at the ref. and water surface, I found that the coil (whose resistance is negligible) is at a temperature of 42deg, only 2deg greater than the temperature of water.

This gives me heat rates of 20-30 watts which is realistic for the case of the condenser coil in the defrost water tray.

Thanks,
Urmil
 

1. How does natural convection occur from a hot pipe in water?

Natural convection from a hot pipe in water occurs when the water near the pipe is heated and becomes less dense, causing it to rise. This creates a continuous cycle of heated water rising and cooler water taking its place, resulting in a convection current.

2. What factors affect the rate of natural convection from a hot pipe in water?

The rate of natural convection from a hot pipe in water is affected by several factors, including the temperature difference between the pipe and the water, the size and shape of the pipe, and the properties of the water such as density and viscosity.

3. How is natural convection from a hot pipe in water different from forced convection?

Natural convection from a hot pipe in water occurs due to the natural movement of fluids caused by density differences, while forced convection involves the use of external forces such as pumps or fans to move the fluid. Additionally, natural convection is typically slower and less efficient than forced convection.

4. Can natural convection from a hot pipe in water be used to transfer heat in industrial processes?

Yes, natural convection from a hot pipe in water can be used for heat transfer in industrial processes. It is commonly used in heat exchangers and cooling systems, as well as in heating and ventilation systems in buildings.

5. Are there any safety concerns related to natural convection from a hot pipe in water?

There are some safety concerns related to natural convection from a hot pipe in water, as the heated water can cause burns if it comes into contact with skin. It is important to take proper precautions and follow safety protocols when working with systems involving natural convection from a hot pipe in water.

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