Convection over horizontal pipe in air

In summary: If the values are significantly different, it may be necessary to adjust the formula or consider other factors that could affect the convection component of heat transfer. If you are unsure of the accuracy of the natural convection formula, it may be helpful to consult a reputable source or conduct additional research on the topic.In summary, the conversation discusses a problem with heat transfer from a horizontal pipe in air and the use of formulas to calculate the convection heat transfer coefficient. The two formulas used are for natural and forced convection, and it is suggested to evaluate the values from both equations and consult additional sources for accuracy.
  • #1
hemi_fan
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I am working on a problem involving heat transfer from a horizontal pipe in air. I am having trouble with it and not getting the solution I expect, and I think it is because of the convection component of the heat transfer. The general formula I am following for convection from a horizontal pipe is:

Q = h*A*(Tsurface-Tambient)

where:

h = convection heat transfer coefficient
A = pipe surface area
Tsurface = surface temperature of pipe
Tambient = air temperature around pipe

I am using the following formulas for calculating the heat transfer coefficient:

Natural Convection:

h = 1.32*((Tsurface-Tambient)/D)^0.25

Forced Convection:

h = C*Re^n*Pr^(1/3)*k/D

where:

Re = Reynolds number
Pr = Prandtl number
k = thermal conductivity of air
D = outside pipe diameter
C,n = constants dependent on Re (can't remember their name off-hand, see chart)

Reynolds Number C n
0.4-4 0.989 0.330
4-40 0.911 0.385
40-4,000 0.683 0.466
4,000-40,000 0.193 0.618
40,000-400,000 0.0266 0.805

Are these formulas appropriate (natural convection in particular as this was given to me by a colleague and I have no clue where he got it)? The forced convection is from a textbook (which I don't have at the moment), however the book did not specify the orientation of the pipe. If anyone could chime in, or maybe provide a link to a 'how-to' on calculating convection around pipes that would be great, thanks!
 
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  • #2
I would evaluate the heat transfer coefficients from both equations and see how the values compare.
 

1. How does convection occur over a horizontal pipe in air?

Convection over a horizontal pipe in air occurs when the temperature of the pipe is higher than the surrounding air. This creates a temperature gradient, causing the air to become less dense and rise, while cooler air sinks. This movement of air transfers heat from the pipe to the surrounding air through convection.

2. What factors affect the rate of convection over a horizontal pipe in air?

The rate of convection over a horizontal pipe in air is affected by several factors, including the temperature difference between the pipe and the surrounding air, the velocity of the air, the surface area of the pipe, and the thermal conductivity of the pipe material.

3. How does the shape of the pipe affect convection over a horizontal pipe in air?

The shape of the pipe can affect convection over a horizontal pipe in air by altering the surface area and the flow of air. A larger surface area will allow for more heat transfer, while a more streamlined shape may reduce the amount of air flow and therefore reduce the rate of convection.

4. How does convection over a horizontal pipe in air differ from convection over a vertical pipe?

Convection over a horizontal pipe in air differs from convection over a vertical pipe in terms of the direction of air flow. In a vertical pipe, the air will rise due to its lower density, while in a horizontal pipe, the air will move horizontally over the surface of the pipe. Additionally, the surface area and thermal conductivity may also differ between the two orientations, affecting the rate of convection.

5. What are some real-world applications of convection over a horizontal pipe in air?

Convection over a horizontal pipe in air has many real-world applications, including cooling systems for electronic devices, heat transfer in HVAC systems, and natural ventilation in buildings. It is also important to consider in industries such as food processing, where maintaining a specific temperature is crucial for food safety and quality.

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