Heat Pipes

Heat pipes are amazing devices.  A physicist or a mathematician would call them elegant.  An elegant solution to a problem in physics or math is one that is simpler,  than the previous best solution usually because it needs less steps than any previous solution.  Heat pipes definitely are in this category.  In an engineering sense they are elegant.

Picture this.  You have a heat source  down low, say at ground level, and you want to transfer this heat to somewhere up high.  Heat pipes can transfer heat downward but are far better than transferring heat upward.  You may want to do this because you want the heat in the higher location, say in a water tank, or you may want to do it because you want to get rid of the heat in the lower location, say from an engine or bank of batteries.  Let's take the very simplest example.

Say you have a trough-form parabolic mirror that tracks the sun, at ground level.  The trough pivots around a simple galvanized black-painted pipe that is located at its focal line.  Above the mirror the pipe continues up into a water tank.  The pipe can twist and turn anyway you want it but it always slopes upward as you go toward the water tank.  

Once the pipe is formed, you seal off the bottom by screwing on the cap and soldering, brazing or epoxying it shut.  It must be air tight.  You put in the appropriate* amount of water, filling it from the top.  You then boil the water by heating the bottom of the pipe until the steam has driven out all the air from the pipe.  You seal the top of the pipe air tight.

*more about this below

Just a tad of physics here before we go on.  The boiling point of water depends on the pressure under which the water is held.  You are probably familiar with pressure cookers.  Typically they hold pressure up to one extra atmosphere (14.7psi) for a total of 29.4psi.  The pressure relief valve allows pressure above this level to escape so you don't end up with a bomb in your kitchen.  At this pressure, water boils at  124.5⁰C.  So your food is heated to above the usual 100⁰C that you achieve if you boil or steam it.  That is why it cooks faster.

Conversely, if you lower the pressure, water boils at a lower temperature.  In fact, if you put some water into a vacuum resistant glass flask and draw a vacuum on it, you will see the water boil and if your vacuum is sufficient, the drawing away of latent heat (540cal per gram evaporated), will cause the remaining water to freeze.

You probably see where this is going.  As you heat the bottom of the pipe, water will boil.  With no air to retard the molecules of water vapor, they flash up to the top of the pipe virtually instantly, (or to the sides of the pipe if you haven't lagged <insulated>) it.  At the top where the pipe is in contact with the water in your water cylinder, the water will condense, giving up its latent heat.  The water will then run down the pipe to be heated again and evaporated.  

Since there is no air in the pipe to provide pressure boiling will occur at very low temperatures and in fact will occur as long as the bottom of the pipe is hotter than the top.  The pressure in the pipe is controlled by the temperature of the top of the pipe.  In our case, the temperature of the water in the hot water cylinder.

If your water cylinder is already at 100⁰C, there will be one atmosphere of pressure in the pipe.  Here is where we get to safety.

If you completely filled, or nearly completely filled the pipe with your working fluid (it doesn't have to be water)*, by the time that you had turned all the fluid into vapor, the pressure would be tremendous and the pipe would likely burst.  The quantity of water you put in the pipe also controls the maximum pressure that you will have in the heat pipe.  It will be the pressure achieved when all the fluid is vapor.  

* Water is a useful working fluid because of it's very high latent heat of evaporation.

And in fact you don't want the whole pipe filled with fluid. You want the pipe above your heat source empty.  This will allow a virtual instantaneous transfer of the latent-heat-carrying-vapor to the heat sink where it will condense and trickle back down the heat pipe.

As I mentioned, heat pipes can be used either to take heat from a source to where you want it or to remove heat from somewhere you don't want it to a heat sink.