Heat Pipe Questions: Understanding Two Phase Cooling and Pressure Changes

[Yarbles]

Hi,
To help me understand two phase cooling etc. I have some simplified heat pipe questions:

If a sealed, vertical pipe, half filled with water has heat applied to its bottom half and the top half is held at 120C, is it safe to say that the inner surface of the bottom half of the pipe will also stay at 120C (assuming it stays under water)?
Once the water reaches 120C, will the pressure remain the same regardless of how much heat is applied?
Finally, what would be the effect of lowering the pressure inside the pipe before any heat is applied (as with real heat pipes)?

Thanks,
Y.

 

[theodoros.mihos]

Very small part of water evaporate up to the saturation vapour pressure to alter the exhaust temperature to 120 C.

 

[Vedward]

At that temperature, about 20% of heat is needed to raise water one unit and 80% is needed to transition from liquid to vapor. Other factors include whether air is in the pipe, insulation, whether heat is transferred along the pipe. In general, to have a steady state solution, there will be a temperature variation through the length of the pipe. In the ideal case, with no heart transfer along the pipe, there could exist a steady state with 120 C constant in the vessel, heart in equals heat out. If heat inb is increased, temp would rise unless heat out is also increased.

 

[Yarbles]

Im a little confused by most of that but 'air in the pipe' got me thinking.
What effect would air have in my simplified pipe above?