Things to Consider When Designing With Heat Pipes
Heat pipes are used very often in particular applications where conventional cooling methods are not suitable. Once the need for heat pipe arises, the most appropriate heat pipe needs to be selected. Often, this is not an easy task and the following needs to be considered.
As an effective heat conductor, heat pipe can be used in situations where a heat source and a heat sink need to be placed apart, to aid heat conduction of a solid, or to aid heat spreading of a plane. However, not every heat pipe is suitable for all applications. For that reason, the following needs to be considered when designing with heat pipes:
- Heat load, or heat to be transported
- Operating temperature
- Pipe material
- Working fluid
- Wick structure
- Length and diameter of the heat pipe
- Contact length at the evaporating zone
- Contact length at the compensating zone
- Effects of bending and flattening of the heat pipe
What materials can be used to construct a heat pipe?
A particular working fluid can only be functional at certain temperature ranges. Also, the particular working fluid needs a compatible vessel material to prevent corrosion or chemical reaction between the fluid and the vessel. Corrosion will damage the vessel and chemical reaction can produce a non-condensable gas.
Table 1 illustrates the typical operating characteristics of heat pipe; from past researches, experiments and from commercial production. For example, the liquid ammonia heat pipe has a temperature range from -70 to +60˚C and is compatible with aluminum, nickel, and stainless steel.
The liquid ammonia heat pipe has been widely used in space and only aluminum vessels are used due to its lighter weight. Water heat pipes, with a temperature range from 5 to 230˚C, are most effective for electronic cooling applications and copper vessels are compatible with water.
Heat pipes are not functional when the temperature of the pipe is lower than the freezing point of the working fluid. Freezing and thawing is a design issue, which may destroy the sealed joint of a heat pipe when placed vertically. Proper engineering and design can overcome this limitation.
What is a wick structure and how does it affect the performance of the heat pipe?
A heat pipe is a vessel whose inner walls are lined up with the wick structure. There are four common wick structures used in commercially produced heat pipes. They include groove, wire mesh, sintered powder metal, and fiber.
The wick structure allows the liquid to travel from one end of the heat pipe to the other via capillary action. Each wick structure has its advantages and disadvantages. There is no perfect wick. Every wick structure has its own capillary limit.
- The groove wick has the lowest capillary limit among the four, but works best under gravity assisted orientations where the condenser is located above the evaporator.
- The wire mesh wick has the most uniform wick, its works under against gravity orientations where the evaporator is located above the condenser.
- The sintered powder metal wick works best under against gravity orientations. Since the sintered powder metal wick is metallically bonded to the pipe wall, its conduction heat transfer from the pipe wall to the wick or vice versa is the best among the four common wicks.
- The fiber wick works best for tight radius bends.
Fig. 1 depicts the performance of four wicks. It can be seen that the groove heat pipe has the lowest capillary limit among the four but works best under gravity-assisted conditions.
How does length and diameter affect the performance of the heat pipe?
The difference in vapor pressure between the condenser and the evaporator governs the rate at which the vapor travels between them. Also, the diameter and the length of the heat pipe affect the rate at which the vapor travels, and therefore need to be considered when designing with heat pipes. Larger cross sectional areas of the heat pipe (i.e. larger diameter of the heat pipe) will allow higher vapor volume to be transported from the evaporator to the condenser. The cross-sectional area of a heat pipe is the direct function of both the sonic and entrainment limit of the heat pipe. However, the operational temperature of the heat pipe also affects the sonic limit of the heat pipe. Figure 2 compares the heat transport for heat pipes with different diameters. One can see, that the heat pipes transport more heat at higher operational temperatures.
The rate at which the working fluid returns from the condenser to the evaporator is governed by capillary limit and is the reciprocal function of the heat pipes length. Longer heat pipe transports less heat than shorter heat pipes. Figure 3 represents the amount of heat a 6mm diameter copper water sintered powder metal wick heat pipe will transport at various lengths and orientations.
How does orientation affect the performance of heat pipes?
A wick structure with a higher capillary limit can transport more working fluid from the condenser to the evaporator against gravity. But as previously mentioned, the sintered powder metal wick heat pipe, with the highest capillary limit, works best under gravity assisted conditions where the evaporator is located above the condenser. Figure 3 shows the effect of gravity on sintered powder metal wick heat pipes.
How is the performance affected by heat pipe bends?
If a heat pipe is bent, the sonic limit and entrainment limit may be reduced in relation to the bend radius and the angle of each bend. If the bend radius is too tight, the wick could be cracked (sintered powder metal), collapsed and pinched off (wire mesh). Therefore, tight bends to a heat pipe may reduce the amount of heat that can be transported. Figure 4 illustrates the experiment results of temperature difference in between the evaporator and the condenser of a 6mm diameter 300 mm long heat pipe, bent from straight to 180˚ U bend at a 30˚ bend interval. The bend radius is Enertron’s recommended minimum bend radius which is 3X the amount of the pipe diameter, or 18 mm. The experiment results prove that if the bend radius is equal or larger than 3X, the bend should not affect the performance.
How is the performance affected by heat pipe flattened?
If a heat pipe is flattened, the sonic limit and entrainment limit will be reduced in relation to the flattened thickness. Therefore, excessive flattening to a heat pipe will reduce the amount of heat that can be transported, or even completely block the vapor passage. The photo at the right shows a sintered powder metal wick heat pipe that was over flattened and had its vapor passage blocked. Figure 5 illustrates the experiment results of temperature difference in between the evaporator and the condenser of a 6mm diameter 300 mm long heat pipe, at round, flattened to 3.5 mm and 2.5 mm thickness. This experiment results prove that proper flattening does not affect the performance but excessive flattening does. In the rule of sum, if the vapor passage thickness is larger than 2mm after flattened, the performance should not be reduced in comparison to the round pipe.
How is the performance affected by heat pipe mean operating temperature?
The mean operating temperature of a heat pipe affects the heat pipe performance. The higher the mean temperature, the better the performance. This is due to the viscosity of the working fluid being lower at higher temperature which enables more working fluid to flow through the wick from the condenser to the evaporator. The working fluid can also be more volatilely changed to gas phase at a higher temperature. Figure 6 illustrates the experimental result from testing a 6mm diameter by 250 mm long copper water sintered powder metal wick heat pipe. 30 watts of heat were inputted into the evaporator, by varying the coolant temperature to the condenser and changing the orientations from -90 degree (against gravity) to +90 degree (gravity assisted), the temperatures at the evaporator and the condenser were recorded and the equivalent thermal conductivity was then calculated for the particular orientation and coolant temperature.
Are heat pipes reliable?
Heat pipes have no moving parts and you are looking at 20+ years MTBF operational. However, care must be given when designing and manufacturing the heat pipe. Two manufacturing factors that can reduce the reliability of the heat pipe: the seal of the pipe and the cleanness of the pipe interval chamber. Any leakage in the heat pipe will eventually fail the pipe. If the internal chamber is not thoroughly clean, when the heat pipe is subjected to heat, the residue will generate non-condensable gas and degrade the pipe performance. Improper bending and flattening of the pipe may also cause the leakage on the pipe seal. There are some external factors that may also shorten the life of a heat pipe such as shock, vibration, force impact, thermal shock, and a corrosive environment.
Based on all the design criteria and limitations of the heat pipes, designing with heat pipes might not be an easy task. You can consult with an Enertron engineer for assistance should you need clarification on any design dilemmas or any other questions regarding heat pipes you might have.
How to select a heat pipe?
- Investigate and determine the following operational parameters
- Heat load and geometry of the heat source
- Possible heat sink location, the distance and orientation relative to the heat source
- Temperature profile of heat source, heat sink and ambient
- Environmental condition (such as existence of corrosive gas)
- Select the pipe material, wick structure, and working fluid. (Consult with an Enertron engineer or original heat pipe manufacturer to select the most appropriate heat pipe)
- Determine the working fluid appropriate for your application
- Select pipe material compatible to the working fluid
- Select wick structure for the operating orientation
- Decide on the protective coating
- Determine the length, size, and shape of the heat pipe (consult with Enertron engineer)