Effect of Pressure on Heating (not boiling) water

  1. I understand that if I apply pressure to water in a closed system, the boiling point increases. So at atmospheric pressure, it boils at 100 degrees C and at 100 PSI, this number increases to 164 degrees C. In my experiment, I'm using the sun to heat the water as it runs through a solar water heater on the roof. What I don't understand is why if I have the system at atmospheric pressure, it gets hotter than is I pressurize the circuit at 100 PSI. For example, at atmospheric pressure, I can get 65 degrees from my collector, but if I prime the circuit to mains pressure of 100 PSI, I struggle to get 47 degrees C. Why ? Can someone please explain this and provide a reference for me to research this more.
  2. jcsd
  3. SpectraCat

    SpectraCat 1,395
    Science Advisor

    Are you sure it's not just a question of flow rates? Unless you are regulating the flow somehow, the rate will be much faster at 100 psi, and so there will be less time for the water to absorb heat as it runs through the solar heater. There could also be a small increase in the heat capacity due to changes in the density at the higher pressure, but since water isn't very compressible, I would think that it is basically negligible.
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