Analysis of Pressurized Cylinder: Flashing to Steam or Hot Water?

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Discussion Overview

The discussion revolves around the behavior of water in a pressurized cylinder when a drain valve is opened, specifically whether the water will flash to steam, remain as hot water, or a combination of both. The context includes calculations related to enthalpy and phase changes under varying pressures and temperatures.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant poses a question about the state of water (steam, hot water, or both) when a pressurized cylinder is opened to the atmosphere at a specific temperature and pressure.
  • Another participant provides a calculation for the percentage of flash steam using enthalpy values and latent heat of vaporization, suggesting that approximately 22% of the water should flash to steam.
  • A different participant questions the units used in the calculations and presents their own enthalpy values, resulting in a calculation that indicates about 12% steam.
  • One participant emphasizes the importance of using the correct enthalpy values at the specified pressures rather than temperatures, seeking verification from others.

Areas of Agreement / Disagreement

Participants express differing calculations and interpretations regarding the percentage of steam produced, indicating a lack of consensus on the correct values and methods to use.

Contextual Notes

There are uncertainties regarding the appropriate enthalpy values to use based on pressure versus temperature, as well as variations in ambient temperature assumptions affecting the calculations.

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I have a question for you all, say you have a pressurized cylinder with water in it, it is at 200*F and at a pressure of 300psig. Now you open a drain valve on the bottom to the atomosphere.

Will it flash to stream or just come out as hot water or a combo of the two?

What equations and info do tyou use to figure this out?

Thanks!
 
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Here is a calculation for flash steam:

% flash steam = (100 * (hf1 - hf2)) / hfg2

where:
hf1 = enthalpy of liquid at pressure p1
hf2 = enthalpy pf liquid at pressure p2
hfg2 = latent heat of vaporization at pressure p2

This calculation is from ASHRAE Systems 2004.

Now if I'm interpretting this correctly, that equals

% flash steam = (100 *(393.84 - 180.07)) / 970.3 =
% flash steam = (100 *(213.7)) / 970.3 =
% flash steam = 21377.00 / 970.3 = 22.03%

So approximately 22% should flash to steam.

Do not try this experiment at home!
 
what units are you using? I used 80*F as the ambient temp.

I got hf1=169 BTU/lb, hf2=49 BTU/lb and hfg2 970 BTU/lb.

so I came up with 12% steam.
 
Sat. Liquid Enthalpy hf of 393.84 at 300 psi Abs Pressure and 180.07 at atmospheric pressure of 14.696 psi Abs Pressure.

I'm not positive that I am right, but the formula calls for the hf at the given pressure, not the given temperature. Maybe someone else can verify this.
 

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