THE 2-PHASE THERMOSYPHON
Let’s examine the thermodynamic process by which a thermosyphon operates. This process is outlined below in a step by step chronology.
Step 1-There is an accumulation of the working fluid in the bottom of the thermosyphon. The most important factor that governs the choice of a working fluid is that it must have an extremely low boiling point. Permafrost soil is typically at a temperature of 31F and consequently the fluid boiling point should be less than 31F. Propane and carbon dioxide have been used, and the Trans-Alaska Pipeline has thermosyphons charged with ammonia. Anyway, heat flow occurs from the soil to the fluid. Once the fluid reaches a critical temperature it boils.
Step 2 –The boiling liquid evaporates into vapor. The vapor rises to the top of the thermosyphon.
Step 3-When the vapor reaches the top, another heat transfer process occurs. This time heat flow occurs from the vapor to the atmosphere. The vapor cools to a critical temperature, at which point it condenses to liquid. The fins facilitate the heat exchange (or “cooling”) process ; they can be likened to cooling fins on a chainsaw or motorcycle engine.
Remember, this process only occurs when the ambient temperature is below freezing. In order for the vapor to condense, it must lose heat. It can only lose heat if the surrounding atmosphere is colder than it.
Step 4 –The condensate drips down the inside walls of the thermosyphon. As it descends, it is cooled further by the atmosphere. The cooled liquid finaly enters the reservoir of working fluid contained in the bottom of the thermosyphon.
Step 5- The process begins again.
Here are a couple of important points:
First, the above process is cyclical and continuous. At any given time during operation, all of the above processes are occurring. Also, it is worth noting that the above process is embodied in the name “two phase thermosyphon.” Heat is moved through two phases; liquid and gas. There are single phase thermosyphons, but they are not widely used.In order to understand the advantages of the two phase thermosyphon versus the single phase thermosyphon, we will have to make a closer examination of the phase change process and its relation to heat transfer.
