Thoughts From the Front Porch

Here’s an interesting experiment:  fill a large plastic bag with air from your living room and twist it shut. Then take it outdoors, stick a tube in it, and hook the other end of the tube to the input of an air compressor with a small, removable tank.  Run the compressor until the bag goes flat.  You will notice that the compressor tank has become a bit heated up by the air becoming compressed.

Let it cool off to normal temperature, then take the compressor tank indoors.   Release the valve and let the compressed air escape into the room, noting that it gets very cool as it escapes and also makes water condense around the nozzle.  Now carry the tank back outdoors and wipe off the condensation.

Congratulations! You’ve just performed one complete cycle of room air conditioning.  Repeat this process enough times, and the room air will become cooler and dryer.

Of course this ridiculous example has no practical value, but it does demonstrate exactly what happens when your air conditioner kicks on.  The AC repeats this cycle continuously until your room air has been chilled to the temperature you selected, with a very comfortable drop in humidity as well.

Any fluid, be it air or water or Freon, loses or gains a tremendous amount of heat as it changes state from liquid to vapor and back again. The trick is to make it gain heat indoors and lose it outdoors.

Your air conditioner’s closed plumbing system is a continuous loop, filled with a fluid that changes state easily within its temperature range of operation.  In most cases the fluid is Freon, but other fluids such as ammonia or even ordinary air will work depending on how the mechanism is designed.  Now let’s follow this fluid as it makes it way through the cycle we’ve just demonstrated.

We’ll start with the compression part; Freon vapor has made its way through sealed tubing from inside your home to that big, noisy monolith sitting in your back yard.   Upon entering a very powerful pump, this vapor is squeezed into much smaller volume, getting real hot in the process.  This heat must be removed.

From the pump, the hot vapor passes into a condenser, which is the coils with black metal fins visible behind the grill of the outside unit.  A large fan draws outdoor air through these coils, cooling the vapor inside and causing it to change to a liquid state, giving up a tremendous amount of heat in the process.  That’s why the air blowing out the top is so hot.

This condensed vapor, now a liquid, is pumped back into your home through another tube into a device called an evaporator. It’s the shiny aluminum & copper A-frame you see when you change the air filter on some units.    As the fluid enters the evaporator area under high pressure, it passes through a tiny hole called an orifice.   As it passes through this final restriction, it enters an area of much lower pressure on the other side of the orifice, and that’s where the magic happens.

Whether a fluid is in liquid or gaseous state depends on two things; temperature and pressure.  Upon passing through the orifice into lower pressure, the Freon instantly evaporates.  Remember how our experimental compressed air got colder when we let it expand?  Well, that’s exactly what’s going on here.  The Freon has once again changed state and, in doing so, it must take on heat from somewhere.

It gets this heat of evaporation from those shiny metal coils and makes them very cold, even icy, as a result of heat loss. The low-pressure Freon vapor now makes its way through tubing back outside to the compressor, and our cycle is complete.

A fan blows air from inside your home over the evaporator coil, picking up the coil’s coolness and adding it to your room air (actually, it loses heat, but it’s easier to imagine it picking up cool).  As this room air is circulated over those cold coils its humidity condenses on them, thereby drying the air as a lucky coincidence of operation.

The condensate water drips into a collection area and drains through a pipe to the outdoors, which is why window air conditioners always drip.  (I pipe this free water to my birdbath, keeping it full all summer, but that’s another story)

A few points need to be made.  For one, the huge volume of heat dissipated by that big noisy fan comes mostly from the compression process, not heat picked up the vapor as it passes through the evaporator.  The compression/expansion cycle really doesn’t care where it gets its heat as long as it happens at the right pressure and in the right part of the cycle.

Also, an air conditioner doesn’t even need electricity to operate, other than for the fan that moves your room air through the evaporator and for control operations.   It only has one moving part, the compressor, which can be turned by any mechanical input, be it a steam engine, gasoline motor, or even a windmill.   On early train cars, the compressor was operated by train wheels as they rolled over the rails.

In fact, compression can even be created by the heat of a small gas flame, as in those old Servel gas refrigerators whose only moving part was the door hinge. It’s all just a matter of exchanging one kind of energy for another.

A third point is that the factor of dehumidification is a freebie, an accident of process that makes conditioned air much more comforting.

And finally,  never worry that your AC is struggling in the summer heat. Regardless of temperature, they work pretty much the same all the time.  It’s actually just a side benefit that your home is made comfortable as a result.  April or August, it doesn’t really care where it gets its heat or where it loses it.  Your power bill only goes up because the AC has to run longer to satisfy your thermostat setting.

Pretty cool, huh?

Views From Benny Hill is a series by Jerry Smith

Views From Benny Hill