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Economist
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Reduce Natural Convection

10/15/2013 6:51 PM

How to reduce natural convection from a hot exposed pipe. Can blowing air in the reverse direction (but at a speed matching the air blowing up) to cancel it out help?

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SolarEagle
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#1

Re: Reduce Natural convection

10/15/2013 6:53 PM

Why not just insulate the pipe???

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lyn
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#2

Re: Reduce Natural convection

10/15/2013 7:00 PM

Forget blowing air. You can't stop air molecules from bumping into the pipe and carrying away heat.

A vacuum would be best. Evacuated tube around the hot pipe, ala Thermos® jug.

Or, cheap, relatively.

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lyn
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#3

Re: Reduce Natural convection

10/15/2013 7:05 PM

A brief narrative about what you want to accomplish would be VERY helpful, in both (all) cases.
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#4

Re: Reduce Natural Convection

10/16/2013 3:21 AM

No

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#5

Re: Reduce Natural Convection

10/16/2013 3:23 AM

No.

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#6

Re: Reduce Natural Convection

10/16/2013 6:43 AM

The effect of pipe insulation on total costs of energy and insulation.

Even a small amount of insulation (0.02 m = 2 cm thickness) yields a dramatic reduction in costs due to reduced convection.

From ScienceDirect, via Google.

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JE in Chicago
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#7

Re: Reduce Natural Convection

10/17/2013 9:19 AM

jacket the pipe with water.. internally or externally . Supply the water at the top and drain away the heat at the bottom.. Not ideal? What about a sleeve ar ound the pipe drawing in cool air near the floor and exhausting it

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#8

Re: Reduce Natural Convection

10/17/2013 10:19 AM

As far as just reversing the air flow from one direction to another to increase heat transfer rates:

1. It would depend on the equipment orientation in respect to the ambient surroundings.

2. If the air available at the bottom of the equipment is cooler than at the top of the equipment and the cooling air is blowing downwards it would definitely improve heat transfer to reverse the direction of cooling air flow and/or vice versa.

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#9
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Re: Reduce Natural Convection

10/17/2013 8:33 PM

According to this, for low Pr flow, which is what I have, the wall velocity seems to scale as the the square root of the Prantl number times the Raliegh number. So, reducing this product can reduce natural convection. This can be done by using a rough pipe for example, so boundary layer is formation is suppressed. Adding insulated fins in the horizontal direction can also reduce natural convection..

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#10
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Re: Reduce Natural Convection

10/19/2013 3:37 PM

A rough surface leads to a boundary layer increase and can have a positive effect. With the fins it much more complicated since at same time you increase the convection area so that even if the boundary layer is thicker the increased area has a negative effect the sum could lead to nothing. Insulated fins are more expensive than a simple pipe insulation. Keep it KISS.

What is the reason to reduce convection ? And of course how much ?
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