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Volume 1: Heat Pipe Basics 101

Join Enertron, Inc. as we delve into the fundamental basics of heat pipe technology. ----------------------------------------------------------------------------------------------------------------- What is heat pipe? Heat pipe is a heat superconductor. Heat pipes consist of: Container Vessel (typically a pipe), Wick Structure, and a Working Fluid. Heat pipes are made by vacuuming and evacuating any air within the pipe. Afterwards, the pipe is charged with working fluid. This working fluid then evenly spreads into the wick structure within the pipe. When heat is inputted into one end of the heat pipe, the working fluid begins to boil and absorb large amounts of heat using only a small amount of working fluid. During this process, the phase change to vapor occurs with no change to the temperature inside the heat pipe vessel. The heat generated throughout this phase change is called Latent Heat of Evaporation. The Vapor then travels at Sonic Speed from the evaporator end to the other end of the pipe. When the vapor flow meets cold at the other end of the pipe, it releases heat and condenses back to liquid form. This end of the pipe is called the condenser end and the heat generated throughout this phase change is called Latent Heat of Condensation. The condensed liquid then travels back to the origin end of the pipe via the wick structure. Overall, a heat pipe cycle has four phases with each of these phases playing a role in the heat pipe's ability to transport heat from one end of the pipe to the other. The four phases are: Evaporating, Vapor Transporting, Condensing, and Liquid Returning. The Evaporating & Condensation phenomena can be seen in our everyday tasks. Simple everyday events such as, boiling water in a pot, or witnessing the climate cycle evaporate water from lakes, and then having the vapor rise to form a cloud, are all examples of the Evaporating & Condensation phases. When the water vapor within the cloud rises to the point that it meets cold enough air, it condenses back to liquid form and drops to the ground as either rain, snow, or hail. This same weather pattern can be found, on a smaller scale, within a heat pipe vessel, where a heat pipe uses a small amount of working fluid to transport a large amount of heat while maintaining a very small temperature difference between the evaporator and condenser. Latent Heat: Let us take a look at water as an example using Latent Heat. The Latent Heat of water is 539 calories. Evaporating 1 gram of water into steam requires 539 calories of heat. Or, in reverse, condensing 1 gram of steam back into water requires ejecting 539 calories of heat. Both evaporating or condensing are the physical phenomena of phase change. The power of Latent Heat can be felt when you rub alcohol onto your skin and feel that cold rush once the alcohol evaporates. Similarly, you can also feel Latent Heat when you come out of a swimming pool and feel a chill breeze from the wind blowing. Sensible Heat: Let us take a look at the same water example again, only this time using sensible heat. The sensible heat of water is 1 Calorie per degree C per gram. This means that, to increase 1 degree C in 1 gram of water, you will need 1 calorie of heat. Or, in reverse, to lower 1 degree C in 1 gram of water, you will need to eject 1 calorie of heat. It is quite easy to observe Latent Heat or Sensible Heat in action. You can view these in action by immersing a thermometer into a pot of water, turning on the stove, and then viewing as the temperature reading rises until the water begins to boil and turn into steam. When a matter changes its phase but no change in temperature occurs, we call it Latent Heat in Action. On the other hand, when a matter's temperature is changing but no phase has been changed, we call it Sensible Heat in Action. With the exception of diamond and graphite, Copper has the highest thermal conductivity over all matters. The thermal conductivity of copper is 400 watts per cubic meter. A heat pipe's effective thermal conductance is 20,000 watts per cubic meter; making it 50 times more powerful than copper. The reason for heat pipe being so effective can be attributed to Latent Heat. There are five operating limits governing a heat pipe's performance. These five limits affect how much heat can be transported from the evaporator to the condenser under any given condition. The limits are: Viscous Limit, Sonic Limit, Entrainment Limit, Capillary Limit, and Boiling Limit.