General VFD Enclosure Cooling Question

[MitchusMaximus]

Hi all, I have a question regarding cooling an enclosure housing one or two VFDs. Maybe this belongs in the Heat Transfer and Thermodynamics forums, but I will start asking it here. Let's say I use a tool such as the Hoffman NVent Cooling Selection Tool to determine the size of a VFD enclosure I would need to keep the internal temperature below, say, 50C, and this tool gives me the minimum recommended CFM of the cooling fan to stay below that temperature. If the built-in cooling fans of the VFD exceed that recommendation, would that reduce the fan size needed to exhaust the hot air in the enclosure? Is there a point where it would only require using an inlet and outlet vent to provide enough ventilation and cooling?

 

[LittleInch]

You usually find that VFDs of any decent size kick ou more heat than you can deal with inside an enclosure or room without needing A/C.

50C is an impossible situation to actually be present in and do any work.

And don't double post.

But some details here would help such as enclosure size or heat loss etc

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Heaviside1925]

If the built-in cooling fans of the VFD exceed that recommendation, would that reduce the fan size needed to exhaust the hot air in the enclosure?

No, consider these two separate systems. VFD fan is only exchanging heat with the air inside of the enclosure, the additional enclosure fans provide the mechanism to exchange the required volume of air to remove the heat from the enclosure.

Is there a point where it would only require using an inlet and outlet vent to provide enough ventilation and cooling?

Possibly, at low enough ambient conditions simple convection could provide enough cooling. I'm not sure if it's Hoffman but there are enclosure manufactures that provide tools for vent size and location based on simple convection.
There are even cases, depending on ambient conditions, where the enclosure itself or a purge system provide enough heat removal that vents wouldn't even be required but you might need a heater.

 

[MitchusMaximus]

Thanks for the replies! In this case I would be looking at an assumed worst-case of 30C outside the enclosure, my drive is rated to run up to 40C before needing to be derated, so I'm going to aim for that internal temperature. There would be two drives going in this box, each with a heat load of 475W, and each of the drives' fan units moves 201CFM. If I went with a 60x36x12 box, assuming all sides can be used to dissipate heat (this is an outdoor enclosure, and I believe it will be set on a stand instead of wall-mounted), the tool recommends a minimum fan speed of 198CFM.
On this same topic, what impact on cooling do two fans mounted next to each other have on the enclosure? I don't imagine it's as simple as adding the CFM of each respective fan together.

 

[waross]

I don't imagine it's as simple as adding the CFM of each respective fan together.

Consider the increased Pressure drop across each fan due to the restriction of the inlet filter.
Or, in a contaminated ambient, (lots of airborne contaminants), you may consider a closed enclosure with external heat sinks and lots of fans on the heat sinks.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

I would tend to discount the VFD fans and as long as the VFDs manufactures separation distances were used, then your heat load is simply 2 x 475W. Now to the heat removal... Your delta T is 10deg C, so if you know the heat capacity of air, you can determine how much heat (joules) the air (volume of your enclosure) can adsorb from 30-40C. Now your drive heat is a rate, W=J/s. So, you need to exchange the volume of air (CFM / rate) in your enclosure to match or exceed the rate of your drive heat rate.
Hoffman should not only be providing fan volumetric flow rate (198CFM) but also vent size and location. What goes in must come out, so, the fan needs to be able to supply the required flow rate at a sufficient velocity/pressure to overcome the resistance of the vents.

 

[LittleInch]

"all sides can be used to dissipate heat"?? By simple convection?

What about the solar gain?

Stick a 1kW fan heater into a box 15 ft3 and watch the temperature rise.

A 12" single fan / or exit is about 5ft/sec to get to ~200 cfm.

Prob ok.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FacEngrPE]

Recently has the situation with just sun loading on a rather large enclosure. No VFD inside, bit did have control transformer, PLC contractors, and some other equipment, but no vent fans as this equipment enclosure was NEMA 4X for salt spray. Sun thermal loading was enough to fail some solid state components with a service limit of 55C.

There is at least 1 VFD OEM that has a model that puts the heat sink on the outside of the enclosure. Direct sun may still heat the enclosure too much for no ventilation.

 

[7anoter4]

If we neglect the heat evacuation through the enclosure wall-I hope it is a metallic box and the dimensions are in inches-we need-calculated from 40oC inner air to 30oC intake air-155 CFM. A 60 W may be evacuated through box wall then 132.5 cfm remains to be extracted by ventilation.

 

[Heaviside1925]

OP,
FacEngrPE makes an excellent point about location. Will this be indoors or outdoors? conditioned space? If outdoors, will it have any environmental protections?
In addition to sun loading, if unprotected, outdoors, there are exposure factors that come into play. Consider there could be greater heating requirements in the winter as well.

 

[LittleInch]

"this is an outdoor enclosure, and I believe it will be set on a stand instead of wall-mounted),"

From post six above....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Heaviside1925]

OP,
FacEngrPE makes an excellent point about location. Where will this be installed? indoors or outdoors? conditioned space? If outdoors, will it have any environmental protections?
In addition to sun loading, if unprotected, outdoors, there are exposure factors that come into play. Consider there could be greater heating requirements in the winter as well.

Edited per latter comment/clarification.

This is an outdoor enclosure

Does this mean, the enclosure ingress protection rating has an outdoor rating, is the enclosure to be mounted outdoors or both?

 

[7anoter4]

I think the heat capacity of the air cp=0.31 kcal/m^3/oC it is very high. It depends on pressure , temperature and humidity. So, for a dry air [30-40%],760 mmHg and 40oC it has to be 0.27 and then minimum CFM= 152[it is not 132.5 as I said above!]

 

[jraef]

I never assume that the VFD built in fans will move heat OUT of the box. Their job is to move air across the heat sink fins. By the time that velocity gets reduced in that task, I don’t assume there is enough flow to get it past the filters.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

I have the following opinion for your consideration.
1. The VFD is provided with built-in fans which provide sufficient cooling for their product placed in an open air room of ambient temperature < 40 [sup]o[/sup] C .
2. When it is enclosed, the heat generated would have to be dissipated by the enclosure or extracted. In your case, the total heat to be dissipated is 2 x 475= 950W.
3. In general, the heat dissipated by the enclosure metallic surface with 10K temperature difference is NOT very much. Therefore force air fan cooling is widely employed. The enclosure should be as big as practical with say 300mm clearance on top and bottom, 250 mm on all sides, for free air flow.
4. As an estimation, the fan air volume V=P/1.2 x dt(K) = 950W/1.2 x 10K = 79.16 L/s or 170 CFM. Depending on the environmental dust condition, add say a 15% more volume for the blockage etc... Fabricate the inlet at the bottom as large as possible taking into consideration of the IP . Place the outlets at the top sides with opening 1.2 times the net opening of the inlet.
5. Single fan would be cheaper but two fans would allow the VFD to run at half load when one is down.
Che Kuan Yau (Singapore)

 

[LittleInch]

There's only about 10ft3 of air in this box when you allow for the VFDs themselves.

So 170cfm is a change of air volume every 3 to 4 seconds.....

That's unreal.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MitchusMaximus]

Thanks for all the tips regarding this topic. The enclosure does need to be outdoor rated, so I'm looking at NEMA 3R. A think a 12" fan would do the job for keeping the drive cool enough even if this box is out in the sun.

 

[edison123]

If it is outdoor, wouldn't a split ac set up be better at cooling and preventing dust entry?

Muthu

http://www.edison.co.in

 

[LittleInch]

And rain / water.

This is a pretty small box for that amount of air movement

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Heaviside1925]

Even though I have spec'd them many times, cabinet AC units are my last resort. They are notoriously unreliable and not from a purely functional aspect because they are very well designed. The issue comes to proper maintenance of these units. They are usually very far down on the priority list and that's considering if preventative maintenance is performed at all. Typically, you don't know one has failed until you have an over temp or drive fault alarm, normally on a hot day and its a week lead-time to get a new unit, while in the meantime, the cabinet and VFDs are either taken out of service or some sketchy solution like a box fan and leaving the cabinet door cracked open is used.
I do agree with the other comments voicing concerns over dust/rain moisture ingress. My preference, if possible, for a remote, unconditioned, outdoor location is to use either direct purging or vortex cooling if a reliable clean compressed air source is available. These types of systems are much more robust, and work-rounds are much easier in the event of component failure.