Physics Help Forum heating/cooling cycle in closed, expandable system

 Feb 2nd 2017, 07:47 AM #1 Junior Member   Join Date: Feb 2017 Posts: 2 heating/cooling cycle in closed, expandable system Hello. Here is a real world problem. I have syringes filled with liquid water (air is completely removed) that are heated to 121C at 32 PSIG. Some of the water boils during this process, expanding the plunger of the syringe. As the system cools back down to room temp and is depressurized, I find that there is a gas bubble in it. Is it possible that the gas bubble is purely water? If so, why wouldn't all water vapor condense back down to a liquid when brought back to standard temperature and pressure?
 Feb 2nd 2017, 10:27 AM #2 Senior Member   Join Date: Jun 2010 Location: NC Posts: 419 It must be air... Hi Rupert1776, You give properties of the water of your test as: T = 121C and p = 32 psig. so it would be 121C and 0.322MPa (or T = 149.8F and 46.7 psi) I checked both units, approximately... at 121C, liquid water boils for pressures at or below 0.200Mpa. But p = 0.322MPa is greater than 0.200MPa. So there is no boiling. and at 149.8F, liquid water boils at 3.7 psi or lower. The pressure is 46.7psi. But our p = 46.7psi is greater than 3.7psi. So there is no boiling. So my thought is Boiling did not happen. I hope I did this right... Good Luck, JP 32 psig = 46.7psi(101.3kPa/14.7psi) = 321kPa 121C = (121C+273)(1.8) -460 = 149.8F
 Feb 2nd 2017, 12:52 PM #3 Junior Member   Join Date: Feb 2017 Posts: 2 I looked back at the data, and the pressure is 30 psi absolute, which is right at the boiling point for 121 C. Some water does boil, but also as we cool it, the pressure, I believe, drops more rapidly than the temperature does, causing further boiling. The important portion here is the second half. If there is a closed, expandable system, and a small portion of the water is boiled (independent of the conditions) and then the system is brought back to standard temperature and pressure, will the water vapor condense, or remain a vapor?
 Feb 3rd 2017, 05:13 AM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,071 Just seeking a bit of further clarification You indicate the system at the start is at 30psi or about twice standard atmospheric pressure. You later indicate that the system finishes at "standard temperature and pressure", By this do you mean 14.7psi at 15C... Note that a liquid will produce vapour from it's surface without boiling. Boiling is the point at which it starts to produce vapour within the bulk of the fluid.
 Feb 3rd 2017, 08:48 AM #5 Senior Member   Join Date: Jun 2010 Location: NC Posts: 419 Other details? Keep in mind. When the water is heated to 121C, some liquid forms vapor AND the specific volume of the vapor phase is about 800 times the liquid. More detail is needed to say anything meaningful...

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