![]() The refrigerant cools in the condenser, restoring some degree of order as the gas molecules tighten back into a liquid. The compression phase is all about pressure and temperature increase (with almost no change in entropy), so the refrigerant enters the condenser as a hot, high-pressure gas. It is the quickening and separation of the molecules as they adopt a gaseous form. Entropy is the work performed during the phase change. The molecules begin moving so quickly that they break free from the liquid and vaporize. It transforms from a liquid to a gas, meaning that its molecular structure becomes disorganized. The refrigerant evaporates in the evaporator. Which situation sounds more chaotic? Driving at a slow to moderate pace on a high-traffic freeway or participating in a street race on the interstate? The latter is much more disorderly, and a similar principle applies to the higher entropy of gases than liquids. Liquid molecules slide past each other, but gas molecules zoom past each other at high speeds. That’s true, but they also move a lot faster than liquid molecules. I briefly mentioned that gas molecules are a lot more sparse than liquid ones. That’s because the molecules move very differently in gases and liquids. Remember when I said that changes in entropy are most noticeable between phase changes? ![]() Why does entropy vary across those stages? T represents temperature, and S represents entropy. Entropy slightly decreases and increases during the expansion phase, and it stays constant in the compressor.Ī T-S diagram like the one shown below shows how entropy changes in the system along with the temperature. Entropy rises while the refrigerant is in the evaporator, and it falls while the refrigerant is in the condenser. Phase changes occur in the evaporator and the condenser. (I’ll bet you can guess what happens in the evaporator.)Įntropy varies with each process, mainly where phase changes occur. Expansion occurs after the expansion valve or other metering device, preceding the evaporator in the refrigeration circuit. For example, compression occurs in the compressor, while condensation occurs in the condenser. It’s pretty easy to remember those parts because they are named after the processes. There are four main phases in the refrigeration cycle: compression, condensation, expansion, and evaporation.Įach stage of the cycle has a corresponding part within an HVAC unit. ![]() How does entropy fluctuate throughout the cycle? Temperature, pressure, or phase changes wouldn’t occur within a unit if there weren’t enthalpy and entropy. On a fundamental level, entropy indicates that the HVAC system has the capacity to perform work. That is one example of entropy at work during refrigeration.īut what does entropy actually do for us? How does the change in the molecules’ organization affect the way we make our HVAC systems work?īefore we answer any more complicated questions about entropy, we should establish what entropy means for system performance. Refrigerants exist in gaseous and liquid forms at different points of the cycle, and the molecules of gases are much more sparse and disorganized than liquid molecules. One way the molecules react is by undergoing a phase change. The concentration of refrigerant molecules responds to those changes in temperature and pressure. Refrigeration occurs in a cycle with temperature and pressure changes throughout. They don’t precisely describe entropy’s role in refrigeration. While these are correct, they are rather broad definitions of the term. Some people would simply describe entropy as a state of disorder, and my favorite is that entropy is a mathematical relationship between heat and temperature. Many people understand entropy as the condition in which molecules become more disorganized and spread out. Sometimes, you will bump into the word entropy, and I wanted to take a stab at making it more understandable. In the trade, we talk a lot about changes in enthalpy, especially when we are looking at total heat exchange over an evaporator.
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