We know that the high pressure liquid refrigerant (comming from condenser) passes through the expansion valve and lowers the temperature (due to decrease in pressure).

My Question: From the formula pV = nRT: (p1V1) / T1 = (p2V2) / T2 [ n & R are constant] So, if the pressure (p2) decreases, either volume (V2) should increase or temperature (T2) decrease. Then, why only the temperature (T2) decreases. It can rather be isothermal process in which temperature remains constant (T1) and volume (V2) may increase. Moreover, this process of expansion of gas is very fast and we know that transfer of heat will not get sufficient time. So temperature should remain conatant and volume should increase. But actually, it does not happen. Why?


Refrigerant is NOT an ideal gas, so the ideal gas law is inappropriate in this application. The temperature of the refrigerant is related to the pressure that the refrigerant is experiencing through the Antoine equation. See https://en.wikipedia.org/wiki/Antoine_equation. As the refrigerant flows through the expansion valve, it experiences a significant drop in pressure. Due to this, the refrigerant is superheated on the low pressure side of the expansion valve, and it boils as a result. The heat required for boiling comes from the refrigerant itself, so the temperature of the refrigerant rapidly drops as boiling occurs, and this temperature drop does NOT require any heat transfer to or from the environment. Thus, on the low pressure side of the expansion valve, there is a mixture of low pressure vapor and low pressure liquid that goes to the evaporator.

  • $\begingroup$ the expansion valve also throttles the flow so as to hold pressure high on the condenser side and thus maintain the refrigerant there in a liquid state. $\endgroup$ – niels nielsen Aug 29 '18 at 17:50
  • $\begingroup$ When the refrigerant flows through expansion valve, it expands. So why it is super heated. It should rather be cooled. $\endgroup$ – Yash Mittal Aug 30 '18 at 2:11
  • $\begingroup$ Also, when it is super heated, it boils and takes heat from the pipe. So why does it become cooler than the room temp. It should maintain the temperature of room. (I'm new to this topic, please be patient) $\endgroup$ – Yash Mittal Aug 30 '18 at 2:13
  • $\begingroup$ @YashMittal, when the refrigerant boils, it doesn't take heat from the pipe to any substantial degree. The heat to boil the refrigerant comes from the refrigerant itself, and the temperature drops as a result. The final temperature of the refrigerant is directly dependent on the pressure that the refrigerant experiences ... lower pressure results in lower temperature. $\endgroup$ – David White Aug 30 '18 at 18:36
  • $\begingroup$ @YashMittal, the term "superheat" means that the refrigerant is above its boiling point. Please take the time to investigate refrigeration, vapor pressure, the Antoine equation, etc., on Google, as some research would give you more understanding of the issues. $\endgroup$ – David White Aug 30 '18 at 18:39

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