# A heat pipe is a simple device which can be used to transfer heat over

1. Oct 20, 2011

### db725

1. The problem statement, all variables and given/known data

A heat pipe is a simple device which can be used to transfer heat over considerable distances using only small temperature differences. It consists of a sealed tube filled with an appropriate fluid. The dryness fraction of the fluid is usually about 0.1. If the tube is then supported with one end above the other, and the lower end heated, then that part of working substance which is liquid collects there and boils. If the upper end of the tube is cooled, then the vapour condenses there, and the condensate is returned by gravity to the lower end. The result is a heat transfer from the bottom to the top of the tube, with no temperature differences.

a. If a heat pipe filled with ammonia were to be used to transfer heat from tightly grouped electronic components to remote heat sinks, what would be their internal pressure?

i) at -10°C (equipment turned off in a cold climate)
ii) at 20°C
iii) at 50°C (maximum operating temperature)

b. Would it be possible to replace the ammonia with:

i) Water?
ii) Carbon Dioxide?

c. What would happen if an inadequate quantity of the working substances was put into the heat pipe?

2. Relevant equations

Data available from steam tables?

3. The attempt at a solution

Tried to sketch the heating process on a p-v diagram. I am not sure where to start with this question. Can't really visualize the process. Any help would be greatly appreciated :)

2. Oct 20, 2011

### LawrenceC

Re: thermodynamics

Think of a sealed tube of some sort in a vertical position. The bottom of it has liquid ammonia or some other suitable fluid. The top part of the tube is embedded in a heat sink, probably something with fins. The initial pressure in the sealed tube is such that the ammonia will boil at needed temperature. The boiling ammonia rises in the tube. But the top end of the tube is at the heat sink so the vapor condenses there. When it condenses it gives up its heat of vaporization and rolls down the sides of the tube back to the rest of the liquid that is boiling. The constant boiling and releasing of the heat of vaporization creates the heat transfer that is done at constant temperature.

Hopes this helps.

3. Oct 21, 2011