Heat Pipes

The Heat Pipe is a sealed, self-contained, component which utilises the latent heat of vaporisation of a liquid for absorption and transfer of thermal energy from one place to another.

It is a passive component and requires no external driving force other than the thermal energy to be transferred.

How a Heat Pipe works

The boiling point of any liquid is dependent upon the (atmospheric) pressure being exerted upon it - e.g. Water normally boils at 100oC at sea level, but on top of Mount Everest it will boil at 75oC because of reduced atmospheric pressure. If the water were placed in an absolute vacuum, it would continue to boil at temperatures as low as 0.01oC.

Another interesting effect is the way some materials will soak up liquids e.g. Blotting paper and ink or a hurricane lamp wick and the paraffin fuel.

Hence if we take a tube, line its inside wall with an absorbent material, soak the absorbent material with a liquid, and remove all the air from within the tube we will then have a liquid in contact with the whole of the inside of the tube which is capable of boiling at any temperature above its freezing point.

If we now apply heat to the tube, the liquid (working fluid) will start to boil. As it boils it will generate vapour and in so doing absorb the applied heat.

Four effects will then occur:-

1. As the working fluid boils, its level within the absorbent material (wick) will be depleted.
2. The vapour generated will create a local excess pressure and will tend to flow down the tube (as it has an open internal structure) to equalise the pressure.
3. When the vapour meets a cooler section of the tube, it will tend to condense down and release the thermal energy it absorbed and also creating an excess of liquid on the wick.
4. The absorbent properties of the wick will encourage this excess working fluid to spread evenly throughout itself thereby returning the excess fluid from the cooled end back to the heat input end hence replacing the fluid boiled off in stage 1.

As noted in (2) above, the inside of the Heat Pipe has an unobstructed flow as far as the vapour is concerned and hence the pressure will be substantially uniform. Therefore the temperature will also be uniform - i.e. The Heat Pipe is essentially isothermal over its whole length.

The Heat Pipe has thus absorbed the applied heat at its 'evaporator', returned it along its length as vapour, released the applied heat at its 'condenser' and finally transferred the condensed working fluid back to the evaporator (by utilising the surface tension of the working fluid and the associated capillary pumping effect of it) within the wick . All this is accomplished at a virtually uniform temperature.