Mechanicals-The Refrigeration Cycle

All of us have experienced an air conditioned space, whether it’s your home, a business you visit, or in your car, there’s nothing better than entering a cool space on a really hot day.  Yes, Northern Minnesota does experience heat.  We all also have a refrigerator, some of us more than one.  How do they work?

A year ago I wrote a blog discussing the second law of thermodynamics, which you can read here.  Basically, that law says heat moves from something hot to something cold.  Refrigeration works by the second law of thermodynamics, moving heat.  I’m going to use an air source heat pump (ASHP) as an example.  An ASHP has something called a reversing valve, which can move heat in two direction, into or out of a home.

Let’s start with a catalyst that moves the heat.  Most new ASHP’s use a refrigerant called R-410A, a fluid designed to move heat.  It has a boiling temperature of -48.5°F, that’s really cold, (just like a crisp January morning in northern Minnesota).  As you move a fluid through a bunch of copper metal piping with a surrounding air temperature greater than the fluid within the metal pipes, heat will move from the higher air temperature to the lower fluid temperature.  The heat is captured by the refrigerant and moved either inside or outside the home, depending on whether you are heating or cooling.

Outdoor condenser

OK, so we know how we capture the heat, now for the rest of the process.  Lets say we are using the ASHP to cool the home, starting from the inside air handling unit, a blower fan moves inside, warm air across a coil of copper tubing containing the very cold refrigerant.  We know that heat moves from hot to cold, this warm air moves heat from inside the home to the cold refrigerant.  (This process will also cause the warm air, which also contains some moisture, to condense on the very cold copper tubing, which helps to reduce humidity within the home.)  As the refrigerant is warmed, it is moved to the outdoor unit, called the compressor which, you guessed it, compresses the refrigerant fluid.  This compression substantially raises the temperature.  Now that the refrigerant is hotter than the outside temperature, this heat will be moved to the outside air by moving through the a bunch of copper piping surrounding the outside compressor.  (Because the fluid was compressed, the refrigerant temperature is now much greater than the outdoor temperature.  The heat that was removed from the home is now left outside.)  The refrigerant is still compressed at this point and is moved through a device called an expansion valve.  This valve allows the refrigerant to expand and become much cooler.  The fluid is then moved back to the indoor coil and the process begins again.  The beauty of the air source heat pump, this entire process can be reversed, moving heat from outside the home to inside.  (Some cold weather ASHP’s can still efficiently move heat with an outdoor temperature as low as -13°F, I’ve heard of some units still operating at -20°F.)  A standard air conditioner can only move heat from inside to outside.

An interesting fact about an ASHP, they are very efficient at moving heat.  Most have a coefficient of performance (COP) rating of around 3, meaning they are 300% efficient.  How can that be?  Because they move heat instead of create heat.  For every $1 of energy supplied to the ASHP, $3 dollars worth the heat is moved into or out of a home.  A ground source heat pump COP can be over 4!

Refrigerators, freezers and the air conditioning in your car all work in the same way.  Another common home appliance, a dehumidifier also work this way.  Warm, moist air moves across a very cold piece of tubing containing a refrigerant.  The moisture in the air condensing on the cold metal tubing, much like moisture forms on the outside of a glass containing a cold beverage on a hot summer day.  The water collects and drips in a collection pan which either needs to be periodically dumped or is attached to a hose where it is discharged to a drain.

The refrigeration cycle is much more complex than I described, I hit the basics of the process.  I believe the ASHP will be the heating and cooling system that is installed in nearly every home in the coming years.  As a matter of fact, I will be changing my main heating system to an ASHP with a natural gas back-up in the future.

Speaking about my home, as you may know, I purchased my 1952 Cape home late last year.  The snow is now gone and it’s time to start some outside improvements.  Stay tuned, next week’s blog will be about what I’ve started and some of the semi-expected surprises I’ve found.

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