Entropy, oh Entropy, What Art Thou?

Ok, so Shakespeare did not wax lyrical about entropy. I was talking with Bryan Orr the other day and the subject of entropy came up. Bryan does HVACR podcasts and organizes educational resources for HVACR. His website is hvacrschool.com. The quickest way to discover that you don’t really know everything you should about a subject is to try and explain it to someone else. In talking with Bryan, I realized my grasp on entropy was a bit tenuous.

Anyone looking carefully at a refrigerant pressure-enthalpy diagram might have noticed the steep, diagonal lines on the right side of the saturation curve labeled entropy. Entropy is a measure of the level of disorganization in something. Entropy is a natural process: everything tends to become less and less organized over time. The inside of most service technician’s trucks towards the end of the week is an example of increasing entropy. Parts and tools scattered about, service tickets and coffee cups in the dash, and a copy of Fundamentals of HVACR, 3rd edition open in the front seat. So how does this concept of increased randomness have anything to do with air conditioning?

In researching for an answer to that very question I came upon many explanations that were honestly a bit beyond me. It seems I am not the only person who struggles with exactly what entropy is. The definition that I found which came the closest to something which made sense from an HVACR point of view was “Entropy, the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. Because work is obtained from ordered molecular motion, the amount of entropy is also a measure of the molecular disorder, or randomness, of a system.” Another idea that helped was “Thermodynamic entropy is part of the science of heat energy. It is a measure of how organized or disorganized energy is in a system of atoms or molecules.” From these descriptions I can visualize that entropy is a measure of the amount of energy required to keep something at its present condition and state. That energy is unavailable for useful work because that amount of energy is required just for the substance to remain as it is.

If you think about the different physical states and the way molecules are arranged, you can see that solids have a low entropy – the molecules have very few possible arrangements, liquids have a higher entropy – the molecules move freely around, and gasses have the highest entropy – the molecules are whizzing around nearly independently. Extending the mental concept a bit further, increasing the temperature of a gas increases the entropy because now the molecules are moving more, so there are more possible arrangements. On the other hand, increasing the pressure of a gas decreases its entropy because the molecules are packed in more tightly, decreasing their ability to move around. Increasing the volume of a gas increases the entropy because the molecules have more room to play, and thus, more arrangements.

OK, so we still have not really nailed down how this has anything practical to do with a refrigeration system. Entropy is measured in BTU per pound per degree. Basically, that is the definition of specific heat. Notice that if you follow any line of constant entropy from left to right, the gas increases in temperature and pressure. That is exactly what happens in a compressor. These two changes counteract each other in terms of the effect they have on entropy, leaving entropy unchanged. When gas is compressed its entropy remains the same. Mechanical energy is converted into heat energy, but the amount of heat per degree for each pound of refrigerant stays the same.

Next time you are on a job and want to plot the system operation on an enthalpy diagram, remember that you can use the lines of constant entropy to make it a bit easier. The compressor line starts where the evaporator pressure line intersects the suction temperature line. Compression will follow the lines of constant entropy up until you intersect with the condenser pressure line.