Stack effect for natural ventilation of electrical enclosure

[123MB]

Hello all mechanical people! This is my first post in the area. I am an Electrical guy, but I hope we can be friends.

We are designing an outdoor electrical enclosure containing a brake resistor which generates lots of heat. I have estimated, based on thermal rise (buoyant flow) calculations that the interior of the enclosure will be around 100 deg C.

The intention is to provide natural ventilation for the enclosure which is designed to take advantage of the temperature differential between inside and outside through the stack effect.

The enclosure is designed as follows:

1) 800Hx400Wx400D mild steel
2) Ventilation inlet opening at the bottom of the enclosure, mounted on one side, 400Hx200W covered with 2.1mm aperture mesh and protected by a rain hood.
3) Outlet opening in the roof of the enclosure, 400Wx400D covered with 2.1mm aperture mesh.

My question is: what is theoretical optimum size ratio between the inlet and outlet openings to maximize natural ventilation and cooling for the enclosure.

I have done lots of research online and read research papers, some say the inlet and the outlet should be the same size and others say different.

At the moment we have sized the outlet at 150% of the cross sectional area of the inlet as we have been advised to do so in order to maximize natural ventilation and cooling, but I do not understand why this is and I would like to understand this further.

I understand that a smaller inlet opening with a larger outlet opening should increase the velocity of air being sucked into the enclosure, but I don't see how this improves cooling as the volumetric flow rate is the same as what would be achieved with a larger inlet opening and lower air velocity.

I also understand that sucking the air in at a higher velocity creates a lower pressure area at the bottom of the enclosure. Does this effectively increase the differential pressure between inside and outside, resulting in more air being sucked in?

Essentially I am after some justification for sizing the inlet opening smaller than the outlet opening, otherwise I am concerned that from a common sense point of view it doesn't make sense and in that case we will probably just make them the same size.

Thanks.

 

[chicopee]

The volumetric flow rate may not be the same if there is a significant difference in temperature between the inlet air and exhausted air. The mass rate of air would be the same but not the flow rate. If the temperature difference between inlet and exhausted is not significant then you are correct for all intent and purposes.

 

[itsmoked]

1) How can you have the vents be the same size as the enclosure? You will be losing a lot of the enclosure's rigidity. Think of a cardboard box with NO ends at all.

2) You are over-thinking the opening sizes. There are so many things that would make a bigger difference here than small differences in the inlet and outlet sizes of your enclosure.

3) What you describe completely nulls the stack-effect aspects. Skip thinking about this as a stack-effect system and only think about convection cooling of that resistor standing alone in space.

4) If you are truly interested in using stack effect you actually NEED A STACK! Put the resistor in a sheetmetal pipe that's 3 meters long. Use standard sheetmetal pipe with a standard ventilation cap. Make the pipe only slightly bigger than the resistor as this will maximize the velocity past the resistor. Cut a piano hinge into the wall of the pipe where you need to service the resistor. Have the pipe end a short distance below the resistor so the heated air length is most of the stack. Paint the pipe black above the resistor to take advantage of the sun.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[123MB]

Thanks Keith

The opening dimensions aren't exactly as I specified. The opening at the top is more like 365x365.

Oh I'm definitely over thinking this.

My overall problem is that there's more realestate in the enclosure where vent openings (ie louvres) could be installed. Common sense tells me I should put in as many as I can, but this 'stack affect' has me restricted to this ratio between inlet and outlet size.

 

[itsmoked]

Oh I'm definitely over thinking this.

LOL

Leave the screening in small enough to exclude swallows. Cover it with a rain shield still leaving the box hole size cross section and it'll be fine.

Like I said, it's now just a resistor-in-a-room. Keep the top open about as much as the bottom. Stick a fork in it.


Keith Cress
kcress -

http://www.flaminsystems.com