How much mass is released to decrease the pressure

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SUMMARY

The discussion focuses on calculating the mass of vapor released when lowering the pressure in a pressure vessel containing saturated water and vapor from 1 MPa to 0.9 MPa. The initial approach involved estimating water height and assuming it remains constant, which is incorrect. The correct method involves applying the First Law of Thermodynamics and considering enthalpy balance to accurately determine the change in vapor mass. Participants suggest clarifying conditions such as air presence in the headspace and whether the temperature remains constant or if the system is insulated.

PREREQUISITES
  • Understanding of the First Law of Thermodynamics
  • Knowledge of enthalpy balance in thermodynamic systems
  • Familiarity with saturated water and vapor properties at various pressures
  • Basic principles of pressure vessel operations
NEXT STEPS
  • Study the application of the First Law of Thermodynamics in closed systems
  • Research enthalpy balance calculations for phase changes in thermodynamics
  • Learn about saturated vapor and liquid properties using steam tables
  • Investigate the effects of pressure changes on vapor-liquid equilibrium
USEFUL FOR

Engineers, thermodynamics students, and professionals involved in pressure vessel design and operation, particularly those working with phase changes in fluids.

wasmaro
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Homework Statement
I have a pressure vessel with saturated water and vapor at 1 MPa. I'd like to determine how much vapor mass is released if I lower the pressure to .9 MPa.

Any ideas how I can do that?
Relevant Equations
First law of Thermodynamics
My best estimate was to find the water height and assume it remains constant when venting the air. I know the initial weights of vapor and liquid ( I know the total mass of substance inside the vessel), so I find the new density of water and vapor and calculate the "new" mass and take the difference from the initial mass. However, that's obviously wrong since the water height should not be constant.
Any suggestions?
 
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wasmaro said:
Problem Statement: I have a pressure vessel with saturated water and vapor at 1 MPa. I'd like to determine how much vapor mass is released if I lower the pressure to .9 MPa.

Any ideas how I can do that?
Relevant Equations: First law of Thermodynamics

My best estimate was to find the water height and assume it remains constant when venting the air. I know the initial weights of vapor and liquid ( I know the total mass of substance inside the vessel), so I find the new density of water and vapor and calculate the "new" mass and take the difference from the initial mass. However, that's obviously wrong since the water height should not be constant.
Any suggestions?
Are you saying that there is air in the head space? Is the temperature remaining constant, or is the system insulated? or something else?
 

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