“Keep it Cool” Transmitter Building Cooling Systems   by Mark Persons W0MH

We all know that inadequate cooling will induce transmitter failures.  As Ben Franklin once said, “An ounce of prevention is worth a pound of cure.”  There are many ways to design cooling systems for transmitter buildings.  This task is not to be taken lightly.  Just like adequate grounding, it should be high on the list of getting a facility built right to minimize problems later. 

Today let’s talk about non-air conditioned buildings, which is the norm in northern Minnesota where I am from.  Roof vents of any sort, including the turbine type, are a bad idea.  There will be weather conditions that force rain or snow down through them and onto a transmitter below.  I’ve witnessed this with disastrous results.

 The best system I have seen has an 8 or 9 ft flat ceiling inside a transmitter building with a gabled roof.  Then 2 ft by 2 ft holes were cut in the ceiling directly over transmitter exhaust ports.  Hot air goes up and into attic area.  In essence, the attic is a huge plenum where hot air collects.  Fans on the gabled ends of the roof then draw hot air to the outside as necessary to keep the transmitter room at the right temperature.

 When using outside air to cool, you will find that inside air is 5 to 10 degrees warmer than outside air.  That is about as good as you can do.  The system isn’t right if the inside temperature is more than 10 degrees warmer than incoming air.  Yes, that means that a 90 degree summer day could have a transmitter running in 100 degree room air.  Fortunately that doesn’t happen often in northern states.

 Plan for two or more fans in the highest end parts of a gabled roof.  (fan louver photo) Never use just one fan.  A breakdown could overheat the transmitter building in short order.  Each fan should be on its own thermostat and circuit breaker.  Failure of one will not trip a breaker to take the other fan or fans down at the same time. 

 For economy sake, I like barn fans with screens to keep mosquitoes and other critters out.  These fans permanently mount in a building wall.  Their gravity louvers usually work well.  Motor driven louvers often suffer mechanical failures as they age….sometimes in just a few years.  (finger-guards photo) Inexpensive squirrel cage fans are for sites with small transmitters.  Best to put ¼” hardware cloth over openings to prevent accidental injury.  

Don’t forget the incoming air.  (hoods photo) Plan for about twice the square foot area that the fans have.  Part of the reasoning is that air filters clog with dirt and therefore the effective area of the air opening is decreased.  Also they offer resistance to airflow even when there is a new air filter in place.  One remedy is to use pleated air filters.  They cost a bit more, but increase the area which air goes through and they last longer.  I like air filters that are 2 inches deep rather than only 1 inch deep when air volume is high.  It all depends on the situation and the amount of air necessary to get the job done.  By the way, you can never overdo it.

 (Top left photo) Even commercially installed filters may leak dirty air into the building like the one in the photo.  Shoddy work on this one!  Rarely have I seen one that adequately keeps dirt from getting around filter sides.  The best solution to the problem, in my opinion, is to make your own air filter frame. (block size photo)  I use ¾” plywood as an outer frame where an air filter might fit against a wall and then another piece of plywood with smaller opening to hold the filter in and prevent air leakage. (front and back photo) 

 Save the plywood that you cut from the air filter frame and use it in place of an air filter during winter months.  Do that on the sides of the building where the wind comes from.  In my area it is on the north and west.  Leave at least one air filter in place to provide some incoming air.  

 There are interesting fasteners, available in well-stocked hardware stores.  Look for a screw that has thread for wood on one end and a machine screw thread on the other.  (hardware) Use an acorn nut on the machine thread side to screw fasteners into a wood frame. Then remove the nut and use wing nuts to hold the second piece of plywood in place.  Replacement of the air filter is easily done by removing wing nuts.  Four should work in most instances.  No special tools are required. 

 The first example is for a filter that fits over a single concrete building block hole.  For larger openings, say 2 foot by 2 foot, just scale up.   Add burglar protection by having a welder create a frame with steel rods.  (2’x2” open photo) In that case, the frame could be made large enough to accommodate the filter.  (2”x2” complete photo) All you add is the plywood cover with a hole large enough to hold the filter in, but not restrict air flow.  With a filter of that size, I recommend 2” deep pleated filters.  They will handle a lot of air.    

 You should have some kind of primary air filtering such as ¼” hardware cloth, also available in a well stocked hardware store.  Think of it as galvanized window screen that has one-quarter inch square holes.  This will keep mice out.  If you have a rodent problem on an existing system, you could use ¼” hardware cloth over the outside of the hood. (screening photo)  Put it on with screws so removal for cleaning is possible. 

 For a transmitter building with a 20 KW FM transmitter, I like to have two 20 inch exhaust fans and four 2 ft by 2 ft pleated air filters about 3 ft above the floor distributed equally around the building perimeter. 

 I remember a poorly designed system that had an air duct pipe connected directly to the top of the transmitter.  The transmitter would quit running and then would start running again when someone opened the building door to investigate.  The transmitter’s air pressure switch saved the transmitter from burning up in that instance.  There were two problems.  The first was no incoming air vents on the building and the second is that there should never be an air duct attached directly to the top of a transmitter.  Manufacturers of transmitters specifically recommend against that practice telling their customers to leave 10 inches or more of space above the transmitter so the transmitter can do its own airflow work unimpeded by outside influences.

 Put electric baseboard heat in transmitter buildings to keep the temperature near normal when there is a transmitter failure during the winter months.  I like to keep temperature in transmitter buildings above 50 degrees just to make sure the equipment doesn’t quit working.  Maybe you have seen that too.