Engineer's Corner
10:17 am
Thu May 22, 2014

TheEC: Air Conditioning at 88.1FM

New portable A/C. Note the pink foam insulation along the walls.
Credit Aaron Read RIPR

Regular readers of the ENGINEER'S CORNER might recall my story about air conditioning at our 102.7FM transmitter in Narragansett Pier.   Well now I'm pleased to report that we have air conditioning at 88.1FM WELH in Seekonk as well!

In many ways, this is an even bigger deal; 102.7 had a hefty vent fan system that could move a lot of air.  Sure, if the air outside was hot, it means the air inside was hot, too.  Usually you can't cool a room using outside air below about +10F degrees above outside air temps.   So if it's 90F outside, it's 100 to 105F inside...ugh!  

And at 88.1, we didn't really have even that.  The transmitter site is an 8x10ft shed with a single 12 inch desk fan blowing air out one of the wall vents, and no insulation whatsoever on the walls or ceiling.  Temps routinely broke 120F inside, even when it was only 70 to 80F outside.

At this junction, credit must be given to Elenos, the manufacturer of our ETG3500 transmitter.  Even in such harsh conditions, that little bugger just chugged right along.  Faithfully pumping out 3000 watts  (our effective radiated power is 4000 watts thanks to gain in the antennas) every day with nary a gripe. 

But now it's got much less reason to gripe!  We've lined the walls, ceiling, door and even the window with 3/4in insulated foam, and installed an Idylis 416710 portable air conditioner, with 12000 BTU of capacity.   Plus a spare fan is in there to help keep air circulating.   I also sealed up one of the wall vents and installed a simple 1in thick air filter across the other to help keep the incoming air from dirtying up the space.

The A/C's hot air vent on the back of the shelter, along with the condensation drain.
Credit Aaron Read RIPR

Remember: a portable A/C has to vent the hot air outside the structure.  That air being blown from inside to outside must be replaced by air flowing outside to inside SOMEhow (nature abhors a vacuum).  Best to channel it through a single entry point that you can control and filter.  Obviously it's impossible to hermetically seal the rest of the shed, but certainly that filter ought to help keep things a lot cleaner.

Believe it or not, we're not entirely sure this'll be enough!  The insulation only has an R-value of 4, and that Elenos still puts out a lot of heat.   What's R-value?  It's how insulating the insulation is, and higher is better.  A house with good attic insulation might an R-value of 40 to 50.   But that's thick, fluffy, fiberglass insulation.   In our case, getting much more than 4 or 5 just isn't possible.  But certainly every little bit helps!  Here's why:

To calculate an R-value, take the difference between the outside temp (let's say 80) and the desired inside temp (let's say 70).  Multiply that number by the square footage of the wall or ceiling space that's being insulated (let's say 6x10ft, or 60 sq.ft).   Then divide by the R-value (here it's 4).   So you've got ((80-70=10)*60)/4 = 150BTU.   So we need an air conditioner producing 150BTU's of cooling continuously.   Doesn't seem like much, but that's just one wall: multiply by four walls, plus the ceiling (it's about the same size), and not every square inch of the walls and ceiling is perfectly insulated, and you have a major source of heat inside the room with the transmitter and other assorted electronics.   That 12,000BTU capacity gets eaten up quickly!

But we had this A/C available to us and it's a good place to start.  If it turns out we need more, we can always cut a larger hole in the wall to put in a "window unit" A/C which tend to be larger capacity and more effective (since they allow a true "closed loop" air handling system, instead of one that's always drawing in outside air).

Even the doors and window get insulation!
Credit Aaron Read RIPR

  You might wonder WHY air conditioning is so important?  It's because of the electronics inside all the equipment.   Some gear, like the Elenos transmitter, can be designed to last longer in extreme heat.  But most stuff isn't.  The parts inside fail much faster in high heat: electrolytic capacitors literally dry out, cooling fan bearings wear out, integrated circuits overheat and melt (yes, melt).   When you have proper cooling, you extend the MTBF substantially.

Now your intrepid engineer just needs a little cooling of his own, in the form of aloe!  I was working outside much of the day to cut the foam and prepare it for installation.   Very pleasant, but very sunny - I got a slight sunburn.  Ouch!