High pressure humid air and thermodynamic tables

In summary, the conversation discusses the problem of determining the expansion pressure and extracted work of a container of humid air at a given pressure. The ideal gas law is not accurate in this scenario due to a high compressibility factor, and the use of thermodynamic tables and diagrams is recommended. Additionally, the effect of condensing water on the adiabatic expansion and the need to consider partial pressure of water vapor are mentioned. The use of a psychrometric chart is suggested as a solution for calculating the work extracted from the expansion.
  • #1
kandelabr
113
0

Homework Statement


This is my problem: i have a container with humid (say psi = 60%) air at a pressure of 450 bar. i need to know to what preesure can this air be expanded (adiabatically) so that it reaches temperature just above freezing.
i also need to know how much work is extracted from this expansion.

Homework Equations


this would not be hard to calculate using ideal gas law and laws for gas mixtures, but at this pressure the compressibility factor is 1.4, that means a 40% error done with ideal gas law. this should be done using thermodynamic tables and diagrams.

The Attempt at a Solution


as air expands adiabatically, it cools down, but condensing water adds some heat so expansion of "the dry part" is actually polytropic. if i neglect partial pressure of water vapor, i can calculate polytropic exponent n, since i know how much heat the water gives out during condensation.
but i don't know how partial pressure of water vapor changes with pressure.
if dew point changes with pressure, the complication is even bigger.
if i knew enthalpies in start and end state, calculating work would be simple (w = |h2 - h1|).
 
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  • #2
You need to use a psychrometric chart (http://en.wikipedia.org/wiki/Psychrometrics). Adiabatic cooling is a transition along a constant enthalpy line.

See also http://www.eitexam.com/Search2/ZMech/PsychroEq.asp?SB=1#Adiabatic%20Mixing

The answer should pop out.
 
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Related to High pressure humid air and thermodynamic tables

1. What is high pressure humid air?

High pressure humid air refers to air that contains a significant amount of water vapor and is under high pressure, typically above 1 atmosphere. This can occur in various natural and industrial processes, such as in weather systems and in steam power plants.

2. How does high pressure affect humidity in air?

High pressure can affect humidity in air by compressing the air molecules, which can increase their ability to hold water vapor. This results in higher humidity levels as the air becomes more saturated with water vapor.

3. What are thermodynamic tables?

Thermodynamic tables are charts or graphs that display the properties of substances, such as air, at different temperatures and pressures. They are commonly used in thermodynamics and fluid mechanics to calculate properties such as density, specific heat, and enthalpy.

4. How are thermodynamic tables used in relation to high pressure humid air?

Thermodynamic tables can be used to determine the properties of high pressure humid air, such as its temperature, density, and specific humidity. This is important for various engineering and scientific applications, such as designing HVAC systems and understanding weather patterns.

5. Are there any limitations to using thermodynamic tables for high pressure humid air?

Yes, there are limitations to using thermodynamic tables for high pressure humid air. These tables are based on ideal gas assumptions and may not accurately represent the properties of real-world air at high pressures and humidities. Additionally, they may not account for factors such as non-condensable gases and non-ideal behavior. Therefore, it is important to use thermodynamic tables with caution and consider other factors when analyzing high pressure humid air.

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