Heat Pipe Technology Explained
Heat pipes are used as a highly efficient method of transporting heat from one location to another, generally to remove excess heat from a process environment. Heat transfer is optimised by engaging latent heat, much like refrigeration, but without the disadvantage of moving parts. Most often used in steady state or cyclic scenarios.
Sterling Thermal Technology (Sterling TT) is a leading heat exchanger manufacturer. With our industry knowledge, we have put together this guide on heat pipe technology. Looking at how heat pipes work, the technology behind them, and their various applications.
We use heat pipe technology in our compact air to air cooler, Avantair, designed for industrial applications.
What are heat pipes?
A heat pipe is a sealed device that transfers heat from one end of the heat pipe to expel it at the other. It uses the evaporation and condensation of a fluid within the pipes (a process called phase transition) to transfer heat in a way that is highly efficient.
Heat pipe technology explained
Heat pipe technology relies on the evaporation and condensation of a fluid, called phase transition. This is coupled with a process of capillary action. The design is key to enabling this.
The design of the pipe is the first essential part of heat pipe technology. It has three aspects: an envelope (or casing), a fluid and a wick. There are variations used in different applications, but this is the core design.
The wick encases a sealed vacuum at the centre of the heat pipe.
The fluid is contained within the wick.
The envelope is the external casing of the pipe.
Diagram source (public domain): https://commons.wikimedia.org/wiki/File:Heat_Pipe_Mechanism.svg
The heat transfer process
The technology of the design allows the heat pipe to harness the fluid’s heat vaporisation properties, leading to a highly efficient heat transfer process.
The fluid moves through the pipe’s wick from the low temperature end (the condenser) towards the high temperature end (the evaporator).
At the high temperature end, heat enters the system, while at the low temperature end, heat leaves the system.
Heat enters the heat pipe and causes the fluid to evaporate into gaseous form.
The gas moves through the vapour cavity at the centre of the heat pipe. As it reaches the colder end, it condenses. This releases the latent heat in the gas and it becomes a fluid again.
Capillary action then moves the fluid back to the hot end of the heat pipe, allowing the process to continually repeat.
The heat pipe can transfer heat energy from a hot environment and expel it, effectively cooling the device of which it’s a part.
A passive process
A heat pipe uses a passive process. Phase transition (evaporating and condensing) and capillary action keeps the heat pipe working for as long as there’s a big difference between temperatures at the hot and cold ends of the heat pipe.
Once the temperature difference decreases (often if the heat source has been cooled effectively and isn’t generating more excessive heat), then the process naturally stops. If the heat increases again, it restarts.
Heat pipes are an extremely efficient piece of heat transfer technology.
They have a high thermal conductivity that can theoretically be between 4,000 and 100,000 W/m K, depending on the design. To put this in perspective, solid copper has a thermal conductivity of just under 400 W/m K.
It means heat can be transferred extremely quickly.
Different heat pipe technologies
While the above describes the core design and process of all heat pipes, there are variations that are used in different contexts and applications. These are just some of them:
Flat heat pipes
Flat heat pipes, also known as vapour chamber heat pipes, are very similar to conventional, tubular heat pipes. They contain a working fluid, a sealed hollow vessel and use capillary action. However, they also use a support structure to allow for clamping without the tube collapsing.
They are commonly used in gaming laptops to allow the laptop to be of a more sleek design while being able to remove the CPU’s high heat output.
They are also used in other contexts where a high heat input is applied to a relatively small evaporator.
Oscillating heat pipes (OHPs)
These are a variation of heat pipes that use pressure-driven, two-phase fluid flow technology. The advantage is that they can be built with different materials and fluid, and in different shapes and sizes, than what’s possible with traditional heat pipe technology.
Loop heat pipe (LHP) designs
While a conventional heat pipe uses a counter-current flow between the fluid and gas, a loop heat pipe uses a co-current flow. It was originally designed for use in spacecraft as changes in gravity fields don’t affect it.
Thermosyphon technology enables the heat pipe to be much longer than is possible with a traditional model. It uses gravity to aid the movement of the liquid, with the evaporator positioned below the condenser.
While a typical standard heat pipe might be less than 30cm long, a thermosyphon could be several meters.
Heat pipe applications
Heat pipes are used in a wide range of domestic, commercial and industrial settings.
Some industrial applications include:
Motor and generator cooling
Heat pipe technology is often utilised when cooling motors or generators, particularly in industrial applications. It provides an extremely high performing heat exchanger design.
For example, our air to air cooler: Avantair. It uses thermosyphon heat pipes to efficiently transfer heat in typically challenging environments such as refineries, offshore applications, liquefied natural gas plants and TLPs (Deepwater Tension Leg Platforms).
It’s a unique heat exchanger that allows for a more compact air to air cooler than with some conventional heat exchangers.
Heat pipes can be used to pre-heat air in various industrial applications, improving the efficiency of next-stage processes.
For example, in thermal power industries, heat pipes can be used as preheaters to warm air that will be then used in a boiler’s combustion of fuel. It enhances the overall efficiency of the process.
Domestic applications of heat pipes include:
The central processing unit (CPU) of a computer or laptop typically generates a lot of excess heat, particularly when the computer is performing high demand operations. Heat pipe technology is utilised here to provide an effective cooling solution that prevents the computer from overheating and not working.
Flat heat pipes are often used to allow for a sleeker device that still effectively transfers heat away from the CPU.
Heat pipes have been used to manage moisture and humidity in air conditioning systems through the integration into a heat exchanger. The evaporator in the heat pipe acts as a ‘pre-cooler’. It cools outdoor air before it reaches the primary air conditioner, enhancing the effectiveness of the cooling coil.
Read frequently asked questions relating to heat pipe technology.
Are there alternatives to heat pipes?
There are many ways to transfer heat in different applications, but heat pipes are considered one of the most efficient in certain contexts. They have the advantage of very high thermal conductivity, a passive process, low-cost manufacturing, and isothermic properties.
What liquid is used in heat pipes?
Different liquids can be used according to the application of the heat pipe, but water is perhaps the most common in the heat exchanger industry as a whole. At Sterling TT, we most commonly use R134a.
Ammonia and nitrogen are used in low temperature heat pipe operations, while caesium, potassium, NaK and sodium are often used in high temperature applications.
How effective are heat pipes?
Heat pipes are extremely effective due to their very high thermal conductivity. The exact number varies but can be as high as 100,000 W/m K. Compare this to solid copper pipes (390 W/m K) or solid aluminium (200 W/m K).
Contact Sterling TT for more information or to submit your enquiry