Note: I\'m not looking for a solution for the problem. I need explanation for a

Question

Note: I'm not looking for a solution for the problem. I need explanation for a specific part of Following is a problem along with the solution 3-34 One kilogram of water fills a 0.1546-m3 weighted piston-cylinder device at a temperature of 350°C. The piston- cylinder device is now cooled until its temperature is 100°C. Determine the final pressure of water, in MPa, and the vol- ume, in m3. Answers: 1.8 MPa, O.001043 m3 Solution 3-34 A piston-cylinder device that is filled with water is cooled. The final pressure and volume of the water are to be determined. Analysis The initial specific volume is 1. 0.1546 m, =0.1546 m'/kg H2o 350°C 1 kg 0.1546 m This is a constant-pressure process. The initial state is determined to be superheated vapor and thus the pressure is determined to be T1 = 350°C P1.8 MPa (Table A -6) The saturation temperature at 1.8 MPa is 207.11°C. Since the final temperature is less than this temperature, the final state is compressed liquid. Using the incompressible liquid approximation, u2-00 0.001043 m3 /kg (Table A-4) The final volume is then u, = mv. (1 kg)(0.00 1043 m3/kg) = 0.001043 m3 What I want to know is how to identify that the problem involves a constant-pressure process. I don't see such thing mentioned in problem statement so there has to be a way where one can identify that the process is a constant-pressure proves. Please provide detailed and thorough explanation. Thanks

Explanation / Answer

Since there are no constraints for piston movements (up and down) such as stoppers, blocks, springs, etc.. The piston is free to move up and down. When we are heating the liquid or gas its temperature and pressure increase, if the piston is constrained. But whenever there is no such constraint (as in this case) only temperature will increase and as the piston moves upward the pressure remains constant. And its volume increases.

For a cooling process(in this problem) only temperature and volume are decreased and pressure remains constant as the piston will move downwards. If there are stoppers there will be changes in pressure. It may imcrease or decrease depending on where the stoppers are placed.

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