Pressure Build Up Vaporizer

[duude]

Can someone please explain to me the basics of how a cryogenic vaporizer is designed? I.e., those used for oxygen or nitrogen supply. How do they increase the pressure from the liquid in the tank to the outlet? Isn't there a pressure drop within the piping of the exchanger?

 

[Compositepro]

They do not increase pressure. They simply add heat to to liquid in order to turn it into a gas. Some of this gas may be used to maintain the pressure in the tank. If the gas pressure supplied by the vaporizer is higher than the tank pressure, then there is a pump between the tank and vaporizer. You also have to take the head of liquid in the tank into account.

 

[duude]

Then what's the deal with the 'pressure-raising vaporiser' in this diagram?

http://en.wikipedia.org/wiki/File:Vacuum_Insulated_Evaporator_diagram.svg

 

[Latexman]

The pressure in the vessel follows the vapor pressure/temperature of the contents. It's a little hard to know for sure with this simple PFD, but the vaporizer may also add heat back to the vessel through a thermosiphon effect ( Thermosiphon ). If heat is added to the vessel, the content's temperature increases, and the vapor pressure increases.

Good luck,
Latexman

 

[duude]

Just to make sure I'm getting this right, there is no pressure increase inside the exchanger itself as the volume is not being constrained during flow.

However, if heat is added to the closed vessel then the pressure increases as the ideal gas law would have it. Can the exchanger be operated in batch mode, in a sense having standing liquid on side gaining heat from flowing liquid on the other side, thereby increasing the pressure?

 

[Latexman]

The pressure regulator and end users (To Pipe) may constrain the volume.

Another way to say it is, if heat is added to the closed vessel then the vapor pressure increases as the temperature would have it.

Good luck,
Latexman

 

[Compositepro]

As I said: "They simply add heat to to liquid in order to turn it into a gas. Some of this gas may be used to maintain the pressure in the tank."

In the diagram vapor will be drawn off the top of the tank. If this continues the pressure in the tank will fall and the liquid will get colder as it boils. When the valve above the vaporizer opens it allows vapor to come out of the vaporizer which then results in liquid flowing into the vaporizer by gravity. This boils rapidly and generates lots of vapor. If no vapor were removed from the tank it would eventually explode. When the desired pressure is reached the valve above vaporizer closes and the vapors being generated push the liquid out the bottom of the vaporizer, back into the insulated tank. The exact same effect could be accomplished with an electric heater in the tank.

The ideal gas law is not really relevant here. The change in vapor pressure of the liquid with temperature is. The liquid in the tank is almost always at its boiling point.