Thermodynamics - Find initial pressure (given Temperatures)

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Discussion Overview

The discussion revolves around determining the initial pressure of R-410a vapor contained in a sealed glass tube at 20°C, given that cooling the tube to -20°C results in the observation of small droplets of liquid. The focus is on applying thermodynamic principles and tables to solve the problem.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant outlines the states of the system, noting the temperatures and the phase of the refrigerant at each state.
  • Another participant questions what occurs at -20°C, suggesting that condensation happens at this temperature.
  • There is a suggestion to use thermodynamic tables to find the vapor pressure at -20°C.
  • One participant proposes that the vapor pressure could be read directly from the table, speculating it might be 399.6 kPa.
  • Another participant expresses uncertainty about accessing the tables but agrees that the value should be read directly from them.
  • A participant inquires about using the ideal gas law to relate the pressures and temperatures of the two states.

Areas of Agreement / Disagreement

Participants express uncertainty about the application of thermodynamic tables and the ideal gas law. There is no consensus on how to proceed with the calculations or the exact values involved.

Contextual Notes

Participants mention the need for thermodynamic tables and express varying levels of familiarity with them. The discussion does not resolve how to definitively calculate the initial pressure based solely on the given temperatures.

Who May Find This Useful

Students or individuals interested in thermodynamics, particularly those dealing with phase changes and pressure calculations in sealed systems.

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1. Homework Statement

For a certain experiment, R-410a vapor is contained in a sealed glass tube at 20°C. It is
desired to know the pressure at this condition, but there is no means of measuring it, since
the tube is sealed. However, if the tube is cooled to −20°C small droplets of liquid are
observed on the glass walls. What is the initial pressure?

2. Homework Equations

Thermodynamic tables
ideal gas equation?

3. The Attempt at a Solution

I set up both states:
State 2: T = -20 (saturated)

State 1: T = 20, v1=v2=0.0648 P = ? (superheated)

I just don't understand how we can figure it out from just the temps. I just can't wrap my mind around the thermodynamic tables.
 
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What happens at -20?
 
At -20, the condensation occurs?
 
Can you use it to find vapor pressure at -20? With the help of tables?
 
Last edited by a moderator:
Is it read directly from the table? would it be 399.6 kPa?
 
No idea, I don't have these tables. But yes, you should read it directly from the table.
 
Last edited by a moderator:
So, how would I use this information to find the initial pressure?

Could I just use the ideal gas? P1/T1 = P2/T2 ?
 

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