This is more of an update and report than anything else.
A few months ago I was faced with a problem of how to keep equipment cool inside a completely enclosed rack but also keep out dust and dust bunnies (mostly cat hair here), and also smoke from occasional incense or pipe tobacco, although that wasn't so much. The real problem here was the dust in this 70 year old house with many small hidden cracks and crevises. I have to constantly clean here.
It had been almost impossible to keep the equipment clean because it has fans that suck that stuff right in. Once inside it collects and forms dust bunnies that are obviously too big to have entered on their own. It's the silent killer of electronics, getting on tubes internasl heatsinks and clogging up fan screens and power supply enclosures and even vents intended for natural convection.
For big stuff, like broadcast transmitters with high airflow located in outbuildings, it apparently makes a cleaning nightmare. This article shows how one person added a HEPA filter and modified a Collins 831-G cooling system so that the inside of the equipment was kept much cleaner. The 831 has a so called reverse flow cooling system. It draws in air from the top and dumps it out the bottom of the cabinet.
https://www.thebdr.net/articles/ops/xmtr/AirFilters.pdfHe used a very expensive industrial filter because of a dirty room, but it may not be necessary for all applications. It is an amazing filter though. It's 20x20 inches and 12 inches deep, rated MERV 14. Not a household item but nether is a 10KW transmitter (for most of us).
People don't like the top-down cooling and I don't understand why, except that it seems 'wrong'. But if the cabinet is well-closed, it is better for one reason, that is it does not suck in air and dirt from the floor level.
Having a dust and cat hair issue, the last thing I want is to suck that stuff up from under the rack or at floor level and push it up into the thing. Vacuuming and sweeping is fine, but I don't want to move the racks and cabinets becauee of all the cables and other stuff, so I can't remove the dirt from underneath them.
Top-down is not new, and has also been used in the kinds of 1980s minicomputers intended for desk-side use in office environments for the same reason, that floors are dirty no matter what.
By one means or another I have converted most of my enclosed ham radio racks to draw air in from the top, or the sides right at the top. Things run just as cool. As the air is pushed down, it has a lot of turbulence so that any hot air convected from the equipments is entrained and moved out the bottom. It has nowhere else to go. Some may swirl around but in the end, the air in a 14 cubic FT cabinet is getting replaced by a 50-100CFM blower no matter what. A cheap infrared thermometer at inlet and outlet and on the equipment itself will prove it works.
In the old house, the problem is worse due to dust bunnies that evolve in just a week! I use a 4 FT rack to enclose two large and expensive custom-made desktop computers, a 100W/channel amp (PC speaker amp), and a rack mount power distribution unit. There is still room for a couple of compact PC type units in there. Before putting the equipment in the rack, I had to constanly open up the equipment and clean out dust. I would know when the time had come when the CPU temperature started rising or when I saw a dust bunny monster starting to grow on the fan grille. Vacuuming is fine, but the stuff is already getting sucked inside all week long.
A large printer/scanner sits on top of the 4 FT cabinet rack (and is wisely kept covered when not turned on), so the only place to put the cabinet intake fan was on one of 4 sides. The cabinet has doors, front and back. The front door was left alone. In the back door I cut a 10" circle and mounted a low speed DC fan on the inside of the door.
On the outside of the door, I mounted a residential type HVAC filter-holder type of return air grille. I used as large a size as possible. These grilles are deeper than the filters they house, so it was possible to space the filter away from the fan's intake. This lets air in through the whole filter, not just over the area of the fan. The effect is that the grille makes the dust bunnies form up on the outside, and only the smaller stuff gets through to the HEPA filter so it stays cleaner and lasts longer.
An alternative would have been to put a cheap $2 fiberglass filter on the outside of the HEPA filter to keep it even cleaner. A fiberglass job cab be washed. A $10 HEPA filter can be cleaned with compressed air or a brush, but is not very durable so 2-3 cleanings is about it.
The rack is now at positive pressure and filled with cleaned air. All of the air exits through the bottom at gaps between the sides and frame, and same at the bottoms of the doors. The rear door has a large gap behind it, a horizontal gap between the bottom of the door panel and the metal floor of the rack. This was where the power/signal/data cables used to come out of the rack. They still do, but to keep pressure up, I cut a piece of high density, non-rotting foam plastic about 4x4" section and wide enough to seal that space off. I then cut a 1.5" diameter hole in it for the 'umbilical' cable, which is a fancy way of saying -the bundle of data, audio, and power cables. I am looking for some 2" diameter split loom to finish this up, which will reach from there up to behind my displays on the desk, but that size is expensive unless you buy 10x more than you want. So to run the cables through the hole would be a pain, and I cut a slit in the foam and slkipped the cables in through that so they are all in the hole.
The filter holder is a 16x25" air return filter grille made by Everbilt. The size was chosen based on the size of the door, then the cheapest air return filter grille. Apparently, for unknown reasons, that size was 1/2 to 1/3 the cost of similar sizes.
The filter is a 16x25x1" unit rated "FPR 10", equivalent to the industry standard specification of MERV 13.
The 'MERV' rating is the industry-standard ASHRAE specification.
Home Depot's 'FPR' rating and 3M's 'MPR' rating are intentionally confusing techno-babble designed to funnel the confused home owner into buying those branded products. MERV 13 stops everything down to most bacteria and smoke particles.
Comparison charts from generic filter vendors:
https://www.airfiltersdelivered.com/c/merv-mpr-fpr-ratingshttps://filterbuy.com/resources/merv-mpr-fpr-ratings/The fan pulls about 100-150CFM into the cabinet. The fan is a centrifugal type, 400CFM, 24VDC unit salvaged from a big computer, running at a lower voltage and RPM via a rheostat, so it is quiet. That's the noise trick -oversized fan run slow and makes enough air. Similar fans are available online, but make sure they can run at reduced voltages. Brush types will, but almost all fans are now brushless and they have unpublished low-voltage limits below which the low voltage IC driver will not drive the inverter transistors into saturation and they can overheat.
All this seems like a lot of work compared to the genlteman's Collins 831-G cooling modification, but keep in mind that transmitter already had its own air flow system. The guy just did his engineering and put a better filter on it. I also suspect that his 12" deep filter was used at much less than its rated pressure drop @ CFM rating so that the original Collins fan would still do its job moving enough air to cool the transmitter. Be sure there will be a larger pressure drop across a HEPA filter than a standard metal mesh unit that may have come with the equipment.
Those with existing filters may want to measure the drop across the filter and have some idea of the CFM before deciding what filter size to use. In my case the 16x25" filter is made for home HVAC use and that means 800-1000CFM, so the measly 100-200CFM running here is no problem at all for a centrifugal fan, which unlike various similarly-quiet bladed fans, can easily handle some resistance in its air path without the air flow partially stalling at the blades. Most filter manufacturers have a chart for their filters showing CFM and pressure drop across the filter.
He shows a liquid type manometer. I believe it is a Dwyer Series Mark II Molded Plastic Manometer.
http://www.dwyer-inst.com/Product/Pressure/Manometers/Stationary/SeriesMarkIIThese are great and have a nonlinear scale so small pressures are easily read, but the correct fluid has to be used. It comes in red or blue.
I have a few Dwyer Magnehelic mechanical gauges.
http://www.dwyer-inst.com/Product/Pressure/DifferentialPressure/GagesI like them better, especially the 2-5000 series which is 65mm (2.6" diameter hole) size with black bezel like a panel meter, but they have linear scales so the appropriate one must be purchased for easy reading the pressure drop across the filter.
One can also make one's own manometer at home from clear flexible tubing at the appropriate angle (for a 0.5" scale) mounted to a piece of wood, using water as the fluid. I made a nice vertical one from varnished wood and an aluminum yardstick and clear tubing and use it to test blowers.
http://bunkerofdoom.com/3cx3000/100_2086a.MOVClear water may be best as food coloring can stain plastic tubing. Glass is high class! Water can unfortunately grow mold so there is part of the reason for the special fuid and the storebought item. I don't use a manometer or gauge on my 4 FT indoor PC rack at this time. It's easy to see that it's working properly. I believe I will use them for the (temporarily stalled) 3CX3000 project's air filter and tube blower - lots more heat and air to move there.
After about 6 months of operation, the project has met every expectation without obnoxious screaming fans. The filter has been cleaned twice, might replace it in the Spring.
Other interesting articles on transmitter environment and clean cooling here:
https://www.thebdr.net/articles/rf/site/XmtrSiteCooling.pdfhttps://www.thebdr.net/articles/rf/site/CleanAir.pdfhttps://www.radioworld.com/tech-and-gear/is-your-transmitter-starving