Our old standard was to put cooling that pushed air into the cabinet either by a fan as you describe, or from a duct from a common cooling fan unit.
I still think there is no better solution.
Reason being, having in our case, one fan unit supplying cold air to many cabinets and over pressure makes it more energy efficient and it's less of a maintenance cost, then to have several small fan units and filters.
The installation cost is of course higher, but that is a one time cost.
Today usually they put in heat exchange units on each cabinet when needed.

The reason for for pushing air into the cabinet instead of sucking it out, is, regardless of which filters you use or how you try to isolate the cabinets you will always suck dust into the cabinet.

We have electrical cabinets that are over 35 years old which are supplied with incoming air which makes the cabinet "over pressurized" and they are almost as clean as they where 35 years ago.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

Not conclusively, but in my general experience the desired approach tis to have the "high pressure" zone inside the cabinet. The basic reasoning is that I want one of two things to occur: either I keep the "dirty environment" out of my cabinet, or I "exhaust" my dirty secrets out to the rest of the world, thereby keeping my cabinet clean(er). Is the OPs "blow down from top" approach truly optimal? In my opinion, no - it always works better to use physics to your advantage (hot air DOES rise!). Is it perhaps better because the parts that need the most cooling (or at least largest temperature differential) are located at the top of the cabinet? Maybe.

The trick to "clean" air flow is to have filtration on the air inlet (to capture the incoming "crud"), and some form of anti-varmint protection on the exhaust - usually louvres or screen, not actual filter - to minimize pressure drop through the whole system. For a "pressurized" system, that filter would normally be BEFORE the fan intake where it's easy to see, check, and change. For a "depressurize" system, the filters are at the actual air intakes (again easy to see, check, and change)which are going to be somewhere other than where the fan is located.

Converting energy to motion for more than half a century

Dear Mr. PaulKraemer
#204A87]".......In the old control panel, there are two filtered ventilation openings on the lower part of the control panel (one on each side). (The filters look like a metal mesh). There is a fan mounted dead-center at the top of the old control panel that sucks air from above the panel and blows it down into the control panel. The intake side of this fan has the same type of filter as that used on the sides....."
I am of the opinion that:
1. assuming that there is NO over-temperature/heating problem with the old control panel and that new components do not generate more heat; the existing cooling air-flow volume is in order. But, see 2 below.
1.1 If you are considering to re-arrange the new components, try to locate those (heat generating parts) at the [bottom]. NOT at the top .
2. you mentioned that "....a fan mounted dead-center at the top of the old control panel that sucks air from above the panel and blows it down into the control panel. ....".
This is unusual to me. Usually, the top fan sucks in the (cool air) from/through the bottom two sides and blows/discharges the (hot air) through the top. Reason: hot air rises. No reason to blow (hot air) down to be discharged to outside, at the bottom!
3. Strongly recommend to reverse the top fan direction of rotation to blow/discharge instead of suction.
3.1. Refer the fan manual on how to reveres the rotation by electrical connection or reverse the fan installation, if possible?. Do NOT pull out the fan blade from the shaft and flit it over.
Che Kuan Yau (Singapore)

Something else to consider is that in some cases, the SCR controllers can be ordered with a "fins-out" flanged heat sink so that the majority of the heat is radiated out of the enclosure. It's worth looking into if that's the main source of the heat.

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

Yes, hot air rises, and there would be significant improvement to heat rejection if the hot air is exhausted from the top of the panel. You may also consider mounting the relays directly to the inside surface of the enclosure s the enclosure becomes the heat sink.

All cabinets I have seen have the fan at the top, or more correctly at the side at the top and the outlet at the bottom.
The only reason I can think of for this is that the air is much cleaner at the top then at floor level where all the dirt sooner or later ends up, and having the fan at the bottom can also create a under pressure socking in air from the cable entries if it is at the bottom, which is common, in old cabinets they are not always so tightly sealed either.

Most electrical components at least here can stand at least +55 C, so pushing hot air down and out at floor level is usually sufficient to keep the components cool it is after all the amount of air exchange and the temperature on the incoming air that makes the biggest difference not which way the air goes.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

@ Dear Ms RedSnake
I am of the opinion that cool air should be in from the bottom and hot air shall be exhaust/discharge out through the top. NOT reverse.
My opinion is based on my field observation:
1. (a) All VFD with enclosure of say > 0.5 to <2m in height are installed with cooling fan. The fan(s) are invariably located at the bottom, drawing cool air in and exhaust/discharge the hot air out [through the top]. NOT reverse.
(b) All Dry type power transformers of any kVA rating without enclosure; if equipped with cooling fan; the fan is always located at the bottom and blows upwards. NOT reverse.
2. The air temperature is always higher at the top (see IEC). NOT reserve.
3. Air filter if required is placed ahead of the fan, i.e. at the cooling air inlet. NOT reverse.
Che Kuan Yau (Singapore)

You are correct in what you say Che, I was merely pondering over why old cabinets where never built like that.

Still pondering.... even with filters on the fan, more dust will be pulled in when you open the door if the fan is at the bottom.
And for the function these old cabinets with the "wrong" airflow direction have, they still have done the job at least the ones we have for more then 35 years.

Back to the original question.
As someone said new components usually are more energy efficient and emits less heat, so if there where no overheating problem with the old cabinet the same functionality would work on a new one.

Today most cabinets whit great need of cooling is usually fitted with a heat exchanger, Rittal have several versions there are probably other brands too.
With these there is no air exchange from inside to outside of the cabinet, you put in your preferred temperature and they only run when needed and when you open the door it stops cooling.
Of course this is a more expensive solution at installation compared to installing a fan, but it saves on energy in the long run since it only runs when there is a need for it.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

@ Dear Ms RedSnake (Electrical)8 Jan 23 08:33
" #1. ..... till pondering.... even with filters on the fan, more dust will be pulled in when you open the door if the fan is at the bottom. And for the function ....... they still have done the job at ...... for more then 35 years."
Look at thousands/millions of VFDs on the market:
(a) those with plastic enclosure are NOT intended to be in (normal operation) with [the enclosure/cover removed]; even with the [fan is still in position/operation!].
(b) for those with higher kW rating with enclosure say 2m in height or with multiple panels, they are NOT intended to be [in normal operation with the doors open]. The doors if open for testing etc. are of very short duration (say within <1h). Any dust drawn in, during this short time is minimal.

" #2. ......Back to the original question. As someone said new components usually are more energy efficient and emits less heat, so if there where no overheating problem with the old cabinet the same functionality would work on a new one...."
If the design was with the cooling air entering from the bottom and hot air is discharge/exhaust through the top, (i.e. with better heat ex-changing); a smaller kW fan ,or no fan is required. Ultimately save fan running cost.

" #3. .....Today most cabinets whit great need of cooling is usually fitted with a heat exchanger, ...... . With these there is no air exchange from inside to outside of the cabinet, ....... only run when needed...... is a more expensive solution at installation compared to installing a fan, but it saves on energy in the long run since it only runs when there is a need for it."
(a) Thousands/millions of single plastic enclosed (small kW )VFD are NOT installed with [externally mounted/attach heat-exchange]. Reason: One or tow/three fans are adequate. See also below FYI.
FYI: (a) Externally/attach mounted heat exchangers are NOT useful on plastic covers. They are to be mounted on metallic (i.e. high heat conduction material) panels.
(b) Extremal heat exchanger are expensive. Very very low efficiency, as the cooling effect is "indirect" (i.e. going through cooling the metallic panel to cool down the heat generating parts attached on the panel)
(c) All heat exchangers work on same basic principle : cool air enters from bottom with hot air exhaust through the top. NOT reverse.
(d) when the heat generated is very low and the metallic enclosure is very large plus if the environmental temperature is low (i.e. temperature difference is high); NO fan is required.
Che Kuan Yau (Singapore)

UL doesn't like openings on the top that can let things drop into the enclosure. Typical is fan or vent in on a wall at the bottom and fan or vent out on a wall at the top.

FYI, Rittal makes some nice filter fans.

In the past, I've used these: https://www.vortec.com/en-us/vortex-ac, mostly in Hazardous location applications.

Mike

We always put the fans about knee high or so, blowing air into the enclosure.
There were two filters, coarse and fine ahead of the fan.
Our exhaust vents were either on the sides or the back near the top, never actually in the top surface.
The vents were commonly louvers with screen behind them to keep bugs and animals out.

When we had a very dirty location, we would install heat exchangers through the sides of the enclosure.
There were circulation fans inside and out and this kept the box watertight.

But out test equipment was all in sealed enclosures with AC installed.
Delicate electronics being what they are.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Thank you Red Snake, Gr8blu, Che, Jraef, CompositePro, Lional, mparentau, and Ed Stainless for your detailed responses.

My take is that while physics dictates that the most efficient cooling would occur if we put intake filters at a lower position and use the fan to suck air out of the panel at the top (making the inside of the panel negative relative to the room), the panel might stay cleaner if I stick with the current arrangement where we push cool air in from the top and allow it to exhaust at a lower location (making the inside of the panel positive relative to the room, which is the arrangement in my existing panel).

The ideas of a "fins out design" for my SCR's is interesting, but the location for this panel will be pushed right against a wall, so I think "fins out" might be difficult in my case. I think mounting the SCR's directly on the inside of the enclosure and using the enclosure itself as the heat might be less than ideal for me for this same reason. (Also, the enclosures I typically use come with a panel that is offset a small distance from the back enclosure wall, so I usually mount my components on this panel rather than directly on the enclosure wall).

I do believe (in fact, I am fairly certain) that the new components I use in this new panel will generate less heat than the old components in the old panel, and I have observed no sign that the old panel has been getting excessively hot. I suspect that I will have no over-heating issues whatever I choose.

Lionel's point that UL does not typically like openings on top is well taken also. This makes sense, as I have had panels in locations that I believed to be totally out of harms way, and water somehow found its way in as a result of an unexpected event.

Thanks to you all, I feel like I am in a much better position to make an informed decision on how I proceed. I still have some work to do on my actual layout. I'll make the final decision where I'll put my fans and intakes after I get a little farther with that.

I really appreciate all of your help.

Thanks again,
Paul

Dear Mr. PaulKraemer
There are numerous learned and good advice. I would like to make the following summary:
1. Suggest the Exhaust outlet to be located on the (highest position sides/back) instead of the [top] is a good proposal.
2. The fan may be located at the bottom or at the top; but the (cooling air flow) shall be in from the bottom, with hot air exhaust/discharge out at the highest level sides/back; where the panel is >200mm away from the wall. But NOT reverse flow.
BTW: This is also observed on switch-room ventilation design. The ventilation fan may be located at high level exhaust/discharge hot air out. But NOT reverse flow.
3. Prevention of dust entry is to use filter, placed at the [cooling air] inlet.
Che Kuan Yau (Singapore) .

Watch out with vortex coolers, the amount of air flow they require is much higher than you may realize. I once did a small one for some PLC panels that were above aluminum pot lines (where they melt the bauxite), to cool a single 24" x 24" x 10" cabinet, it required what amounted to 7-1/2HP of compressed air!

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

Some thoughts about optimal cooling:
Hot air rises.
Yes, absent any other influence, hot air rises. Hot air expands, and the specific gravity drops, hence, the hotter, light3er air rises above the cooler, denser air. The effect is called convection.
It doesn't take much of a fan to overcome convection, and in the case of an fan cooled electrical panel, I tend to ignore convection.
Where to direct the incoming air?
All else being equal, although that is seldom the case, I would direct the cooling air first owards the hottest components.
With the cooling air stopped, the various components will be at different temperatures depending on the internal heat generated by each component.
The greater the difference between the cooling air and a particular component, the greater the cooling, or temperature drop of the component.
With the air directed first to the hottest components, they will experience the greatest cooling or temperature drop. As the air continues over the cooler components, it will have been warmed by the hotter components and the temperature difference and the cooling effect will be less.
With the air directed first to the cooler components, the air will be heated somewhat before it reaches the hotter components. There will be less temperature difference and thus less cooling.
So, comparing air first to the hottest parts with air first to the coolestparts:
Air first to the hottest parts, the hotter parts will run cooler and the cooler parts will run hotter.
Air first to the cooler parts, the cooler parts will run cooler and the hotter parts will run hotter.
How much hotter or cooler?
Probably not enough to worry about.

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

The first time that I installed a punch press ( metal stamping ) press was a 400 ton Minster press that had been manufactured in the late 1950s. When the Akron-Selle Company purchased this press from Monarch Rubber in 1993 they had a millwright contractor rebuild the press and check out its mechanical systems.

The control panel ventilation had an air intake grill with metal mesh near the near the bottom on the door. At the center top was a centrifugal blower rotating on its vertical axis. Every control relay and motor controller except 2 ice cube relays had metal stamping oil in them and were otherwise gummed with gunk. I had to clean all of those. The president of The Akron-Selle Company had to get me some detergent so I could wash the terminals at the bottom of the panel after taking them out and dry cleaning fluid to clean the other electricals.

Part of how this happened was that the blower accidentally acted as a cyclonic separator and Monarch Rubber had used oil spray to lubricate their punch press dies. This put a lot of oil droplets into the air. What we used to lubricate to top side of the sheet steel going in was a rag and chain which is LESS messy. What we used to lubricate the lower portion of the die was one of those dishwashing plastic sticks that has a whole bunch of synthetic foam rubber on the end of it such that at about say 20 to 50 parts the operator would stop the press, dip the stick into a bucket of stamping oil, then lubricate the die.

The greater productivity of oil spray is a moot point if you get enough oil all over your plant that it catches fire. Oil spray is what one of my stepbrothers would call Milking A Duck which was his metaphors for a futile and/or asinine endeavor.

One of the other things I had to do was to replace a bunch but not all of the control wires in this press because vibration over about 35 years had broken a lot of wire strands so you need to check that too. Also, some of the electricians who wire these would cut off 1/3rd or 1/2 the wire strands because they would use the wrong notch on the wire stripper. some of the wire strands had also been nicked by using the wrong notch of wire strippers.

For direct air ventilation of a control panel you should prefer to use an intake fan or blower. The volute of a blower should have a drain hole drilled into it. The ventilation system should be dimantlable so it can be cleaned.

No matter how you design your control panel it needs to be easy to fix. Also, allow more wire bending space than what National Electrical Code requires. The Cram Method of building stuff causes assembly cost to rise dramatically which was how the Shoreham nuclear power plant had such a gigantic cost overrun.

There are also air conditioners that are built for cooling control panels. These will generally be more economical than vortex coolers IF the environment is clean enough for an air cooled air conditioner. The disadvantage of compressed or is that about 2/3rds of the work done by the compressor motor turns into waste heat during the compression process. Compressed air as an energy transmission means does have some advantages for SOME applications that outweigh the inefficiency of an air compressor. An air cooled air conditioner may be HARDER to keep clean in some environments and in those cases a vortex cooler or an air conditioner that runs on chilled water from a remote chiller or cooling tower would be better choices.

Also, some compressed air components have marginally overstated overstated ratings - you generally do not want to run air compressors, aftercoolers, refrigerated filter dryers, etc. at their maximum ratings. Instead, you need to run them a little less than their ratings.

Thank you mc5w for your thoughts. I won't have to worry about cleaning the parts inside my panel because I will be building an entirely new panel and discarding the old panel (with all its components). I will pay attention to the condition of the field wiring that I will have to disconnect from the old panel and re-connect to the new panel, replacing any wires that need to be replaced.

That is an interesting point that a fan blowing air into a panel can have the side effect of spreading lubricant or anything that might be suspended in the air all over the inside of the panel. Luckily, I don't think this will be the case in my installation location. There will be a little dust, but I don't expect much more than that.

I do have a question about your comment that "The volute of a blower should have a drain hole drilled into it." I was thinking I would use a fan similar to the one below...

https://hoffman.nvent.com/en-us/products/enchf0916...

... I don't think I will need a more expensive or more complicated fan because the there is no evidence of over-heating in the existing panel I will be replacing and I am certain the new components in my new panel will be more efficient than the old components in the old panel. I am just trying to come up with the best strategy for locating the fans relative to my components.

Would you expect that a small fan like the one in the link about would afford me the possibility of drilling a hole into the "volute", or of finding a similarly sized fan that would come with a hole pre-drilled.

To be honest, "volute" is a new term for me. I googled it to get an idea, but I am not even certain at this point if all fans have a volute. I apologize for my ignorance, and appreciate your help.

Thanks and best regards,
Paul

So what do you do when the fan stops / trips?

These look like simple straight through fans.
Blowers would have a volute. You drill a small hole in the bottom to allow drainage of any collected liquids.
I worked in a shop that was not heated much and we would have condensation collect in blowers.
In fact some of our control cabinets had heaters in them in order to keep them dry.
We never tried sensing if the fan was running, but we would put a high temp alarm on the most critical (or expensive) item in the panel. If it got hot then checking the filter and fan was step #1.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Change it.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

Something else to consider with the fan at the top blowing down:

In days gone by, passive /convection cooling of components was common, so as was said, it didn’t take a lot if fan power to overcome that. But a lot more devices have cooling fans in them now and I’d venture to say that 99% of them are blowing the fans UPWARD to take advantage of convection. So if you have things like drives and power supplies with cooling fans blowing Up, and your panel fan is trying to blow Down, you might inadvertently create dead zones in components where you get neither convection cooling nor fan cooling. I have seen this happen more than a few times in VFD cabinets.

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