In AC 101 we learned that an air conditioner is a heat sink that overcomes the second law of thermodynamics. It can move heat from a hot space (inside a business IT closet) to an even hotter space (outside on a summer day). It takes energy, a compressor, and refrigerant to do it.

Air conditioners work because of a closed loop with a compressor setting up a high pressure side and a low pressure side in the loop. The low pressure side absorbs heat and the high pressure side rejects heat. Setting up the pressures in the system is what uses energy and what makes that “cold” air our customers enjoy.

Here we need another lesson in physics. Using Boyles Law we know that the lower the pressure a liquid is under, the lower the temperature at which it will boil and change state into a gas. And boiling absorbs heat – large amounts at the transition between liquid and gas.

“The lower the pressure a liquid is under, the lower the temperature at which it will boil.”

Did you do your homework? A watched pot will boil, and you should have noticed that it took a lot of heat energy to make it happen. Boiling absorbs heat. It is called a “state change” from a liquid (water) to a gas (steam). In every-day life we achieve this state change by introducing a lot of heat energy through a high temperature source like a fire or a stove burner.

But what if that liquid would boil outside on a 40 degree winter day? What if that liquid would boil at negative 41.5 degrees? If that was happening, the liquid would be absorbing heat energy from places at relatively low temperatures. And the liquid certainly wouldn’t be water. That would turn into ice. Check out AC 103 about refrigerants and evaporator coils.

To Remember:

  • Boyles Law
  • State Change
Author: corecontrol