Specifics of Heat Pipe Design

The Pipe

Since heat pipes function by heating liquids to a boil within a sealed vessel, pipes must be chosen that can handle high temperatures and pressures. In order to be effective heat exchangers, the pipe should also be as thermally conductive as possible.

The Working Fluid

It is very important that the working fluid and pipe material are compatible with one another. If not, corrosion of the pipe may occur thereby damaging the pipe and decreasing functionality. The production of non-condensable gases may also occur as the fluid reacts with the pipe walls. As explained later, this can affect the convection and therefore the efficiency of the pipe. Table 1 shows a list of common working fluids and the metals that are compatible. This list has been reduced to show only liquids used in the 250-500˚F temperature range. An extended version for high temperatures is available at Thermal Fluids Central [2].

Table 1: Fluid/Metal Compatibility

The Wick

Wicks can be made in a number of ways, each with their own benefits and drawbacks [3]. The three most common methods are using sintered copper or copper mesh along the inside walls or by cutting grooves into the pipe walls. Since the wick will be the primary place where the working fluid condenses, it is best to use a wick that is thermally conductive in order to transfer the heat collected to the exterior of the pipe and then into the surrounding atmosphere.

Internal Atmosphere

Heat pipes work most efficiently if the only gas present on the inside is the vaporized working fluid. Any non-condensable gases within the pipe can impede convection and vaporization of the working fluid. Therefore, most heat pipes are made with a vacuum on the inside. A vacuum also has the effect of increasing the working range of the heat pipe by allowing the working fluid to vaporize at lower temperatures.

Amount of Liquid Needed

 Though a clear idea of how much water to use in the heat pipe has not yet been attained, there are a few important considerations to make sure the most effective amount of water is used.  The pipe will be used to heat up liquid water into water vapor; thus, a dynamic equilibrium will be established.  By definition, dynamic equilibrium is when a reversible reaction reaches a state where the coefficients of the two states of matter remain constant.  It is important to calculate at what values the dynamic equilibrium will be reached because then it will be known how much water the pipe can hold. If too little liquid is present, all of it will vaporize and no equilibrium will exist for convective heat transfer. If too much liquid is present, too much heat will be required to heat the liquid to boiling thereby decreasing the heat pipe's efficiency

An equation that will be essential to the success of using the correct amount of liquid is the Ideal Gas Law Equation:

PV = nRT

Where "P" stands for pressure, "V" stands for volume, "n" stands for the amount of moles used in the system, "R" is the Ideal Gas Law Constant, and "T" stands for temperature.  With this equation, the amount of water needed can be found by solving for "n", then converting this measurement into liters.  A heating curve diagram [figure 3] will also be taken into consideration, to see at which pressure and temperature liquid water will vaporize into water vapor. Given the temperature parameters of the testing set up, this diagram will allow the internal pressure of the pipe to be approximated.

Figure 3