Heat Transfer / thermal radiation

[MechG]

I have designed a machine and now I need to guard it with minimum space. The motor gets to about 100 degrees C so how far away does my steel mesh guard have to be to get to a safe working temperature????

The materials etc have already been specified so can not be changed. There is a 50% air flow.

Basically all i need is an equation/graph to show the distance 40c air would be from a motor that is 100c (air = thermal conductivity of 0.024W/mk).

Can anyone help?

 

[MiketheEngineer]

Depends on ambient air temp ,air flow, humidity, etc.. etc., etc.

I saw an equation one time that had like 20 variables in it....

Why not just fire it up - let it get to temp and measure the temp at various distances.

If you find a simple answer - let me know

 

[amorrison]

I would think of controlling the thermal radiation off the machine with a radiation shield which could be a as simple as a piece of sheet metal.

 

[IRstuff]

Depends on how good your air flow really is, i.e., how many CFM you're blowing, and whether you can get the air flowing all around. The minimum separation for full convection to develop is roughly 1/2 inch, but you'll need way more than that to get air flowing everywhere.

Your inside surface should be mirror finished to increase the reflectivity. A perfect blackbody radiates about 640 W/m^2 into a 300K ambient.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MechG]

Ok ill do a practical experiment with this as there doesn't seem to be a simple equation for this.

Ill use 1/2 inch gap with a mirror finish steel plate to test it!

Thanks for your help

 

[racookpe1978]

Put your shield as far from the motor as possible to get maximum air flow through as possible. Get as much air flow under the shield as poosible.

You're at 100 C (close to the boiling point of water!!! depending on elevation) - so your FIRST attempt should be cooling the motor, THEN shielding the not-very-often-present workers from the too hot motor!

 

[JKEngineer]

I'm missing something here. For one, I don't understand what a 50% air flow is.

However, your initial problem statement sounds like you are treating the problem like a conduction through air calculation. I don't think that's appropriate. You have air flowing around the system, so there is convection. In addition, you have radiation from the hot parts to the guard. The temperature of the guard is the point of interest, I think, not the air temperature at the guard. They are likely to be different. If this is true, then you need to be concerned about the convection to the guard and the radiation to it.

As the others have suggested, the radiation to the guard can be reduced by shielding or by making the machine reflective. The problem with that approach is it will actually raise the T of the machine by reducing its ability to reject heat.

An alternative is to make the inner faces of the guard reflective, reducing its ability to accept the radiated heat, and only mildy impacting the dumping of heat by the machine. The operator's side of the guard should be an emissive surface so it dumps heat to the surroundings as much as possible. You could also, if the situation warrants it and costs are not prohibitve, make the guard out of something other than a single layer of expanded metal. I am thinking here of a double layer of metal with an air gap or something tubular.

JK

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com