Frequently Asked Questions

What is a thermosyphon heat pipe?

A heat pipe is a heat transfer device in the form of a tube, evacuated and hermetically sealed containing a working fluid and vapor phase. A cluster or system of heat pipes will make up a heat pipe heat exchanger that requires no moving parts. They are completely passive and considered to be isothermal in nature meaning that from one end of the heat pipe to the other, it will transfer heat from a heat source to a heat sink with insignificant temperature degradation.

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How does a heat pipe work?

Internal to the heat pipe is a working fluid that as one end of the heat pipe is heated by the hot source, the liquid turns to a vapor absorbing the latent heat of vaporization. The hot vapor flows to the colder end of the tube where it condenses and gives up the latent heat. The condensed liquid then flows by gravity back to the source end of the tube only to repeat the process again.  Since the latent heat of evaporation is typically very large, considerable quantities of heat can be transported with a very small temperature differential from one end of the heat pipe (evaporating end or source) to the other (condensing end or sink).

Thermosyphons will also act much like a diode in that heat will only travel in one direction.  Should heat be applied to the condenser end, there is no working fluid to evaporate; therefor there will be neither vaporization nor heat transfer to the evaporator.

In addition, the various working fluids have operational temperature ranges where they will start and stop there process.  As an example, when utilizing sodium (Na) as a working fluid, it does not change to its vapor phase until it reaches 550°C. Of course, the internal vacuum plays a significant role for all working fluids.

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What are heat pipes used for?

The main objective of any heat exchanger is to return the most energy from a source waste heat stream and to transfer the maximum or calculated amount of energy possible back into a process or a source recovery energy stream.  By performing very efficiently and effectively two objectives can be met.  The source energy stream can be cooled to a desired temperature and/or the sink can be heated to a desired or controlled temperature.

Typical applications include very small electronic devices to very large utility grade power generating applications in the many megawatts. Thermosyphon systems can then be thought of as completely scalable in dimension.

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What is the thermal conductivity of a typical thermosyphon heat pipe?

Heat pipes, due to the latent heat or dual-phase heat transfer process, do not have a fixed thermal conductivity like solid materials. Although, their effective thermal conductivity can be in a range of 10-10,000 times the thermal conductivity of copper. This will depend upon many factors such as the internal working fluid, the physical geometry of the pipe including the material. However, the largest contributor will be the length as the longer the heat pipe is, the higher the effective thermal conductivity.

This can simply be explained by using an example of a 4-inch pipe and a 12-inch pipe, each having the equal capacity of 100 watts. However, as each will have the same thermal gradient, the 12-inch pipe will have the higher effective thermal conductivity.

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What materials are used to construct a heat pipe?

The longevity and usefulness of a heat pipe will be determined by the selection and compatibility of the container metals and the required welding materials making up its construction.  The pipe components used will be dictated by the working fluids employed, the chemical make-up and temperatures of the source and sink energy streams to the heat pipes.  For the heat pipe design, the container metal, working fluid(s) and the incoming energy stream chemical make-up must be considered or the performance can be significantly degraded by either internal pipe material erosion or by a chemical reaction with the working fluid that produces a non-condensable gas.

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Are heat pipes reliable?

Heat pipes have resulted in a highly reliable technology as they were developed to be operated passively and without moving parts.  Additionally, there are a number of publications and reports relating to heat pipes provided over the last few decades with many produced from various professional organizations. Due to the potential use of heat pipe technology suitable for a great many applications, there has been a significant amount of research and development performed proving their effectiveness.

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How do heat pipes compare in cost to classical heat exchangers?

In years past, when heat pipes were manufactured manually, one heat pipe at a time or “bench” produced, heat pipe heat exchangers were seen to have a higher initial cost.  This was especially noteworthy when compared with classical or traditional heat transfer designs constructed of aluminum or cast heat sinks.  Even at that time, from a life cycle cost standpoint, heat pipes would prove to be more cost effective due to more favorable maintenance costs, improvements in system reliability and increased lifespan.  Today, with the advent of patented manufacturing processes, heat pipes, with all of their advantages over their traditional rivals, offer a very competitive cost structure based on a kW for kW basis.  This becomes even more apparent when reviewing more demanding applications that consist of energy streams that are very hot or dirty or the combination there of.  In addition, as the application grows in kW size, the cost per kW can drop significantly and often times making them the logical solution from an economical perspective.  As an example, when reviewing the power industry, many heat pipe air preheaters are considered for new and retrofit boilers.

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Can heat pipes work against gravity?

Yes, there are heat pipes that will work against gravity. These heat pipes will rely on internal wicks or grooved structures to move the vapor from one end of the heat pipe to the other.  The advantage to this style of heat pipe is that they can work in any orientation.  However, there are limitations to this style of heat pipe when compared to a gravity assisted thermosyphon heat pipe. Some limitations can be cost, pipe length, freeze recovery and other situations of increasing temperatures.

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What fluids are used in heat pipes?

One of the major advantages of heat pipes is their ability to utilize various fluids with each having different properties that will favor the particular application.  For instance, each heat pipe application has an individual temperature range in which each heat pipe is required to operate within.  The benefit is the ability to best match the efficiency of the approaching energy stream temperatures of the application and best match the metal make-up of the heat pipe.  In addition, from a temperature standpoint, as the input energy stream temperature changes on its path through the heat exchanger giving up its heat, the working fluids can be altered to reflect the best efficient working fluid required.  The various temperature ranges are:

Cryogenic Temperature Range

Cryogenic heat pipes operate between -270 to -73 °C.  Working fluids can include helium, argon, oxygen, and krypton. The amount of heat that can be transferred for cryogenic heat pipes is relatively low due to the small heats of vaporization and high viscosities of the working fluids.  We typically do not work within this range.

Low Temperature Range

The low temperature range will fall from -73 to 150°C.  Many heat pipe applications will fall within this range. Commonly used fluids are ammonia, acetone and Freon compounds.

Intermediate Temperature Range

The more common working fluids in the medium temperature range, 150 to 480°C, are water, Toluene, Naphthalene, and Dowtherm-A.  Water, which is perhaps the most widely used working fluid, has good thermos-physical properties such as large heat of vaporization and surface tension.

High Temperature Range

The high temperature range (480°C and above) will include sodium, lithium, cesium, silver and a sodium-potassium compound (NaK).  The heat transport rates for liquid-metal heat pipes are generally much higher than those in the other temperature ranges because the surface tension coefficients, latent heats of vaporization, and thermal conductivities of liquid metals are exceptionally high.

How does a water heat pipe work below 100° C?

Under normal circumstances, at 100°C, water at atmospheric pressure will boil.  However, when evacuated and encapsulated within a heat pipe, water is no longer at atmospheric pressure.  This means that the internal pressure of the heat pipe will be the saturation pressure of the chosen working fluid at the equivalent temperature.  When the working fluid is at any temperature above its freezing point, it will begin to boil.  As an example, a heat pipe utilizing water at room temperature (20°C) and at a partial vacuum can boil as soon as any heat is applied to the evaporator end.

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Will heat pipes freeze?

With water being used as the most common working fluid, it still maintains its normal freezing detail. Properly designed heat pipes can operate after thousands of freeze/thaw cycles with no ill effects.  Without the primary thermal energy flowing, heat pipes utilizing water for example for its working fluid, will, depending upon the internal pressure of the heat pipe, begin to freeze once the temperatures fall below 0°C with conduction being through the heat pipe wall as being the primary force of heat removal. When the surrounding heat source starts back up from the frozen conditions and the temperatures of the heat pipes begin to approach their designed working temperature , the heat pipes will again begin to operate. This same process is duplicated with other normally liquid working fluids as well as solid metal working fluids such as lead or silver. So heat pipes are thought of as freeze / thaw tolerant.

If required and depending upon the constraints of the application, in order to alleviate frozen startup issues, a dissimilar working fluid that is compatible with the required metal container and is not prone to freezing can be utilized within the first rows of the heat exchanger to accelerate the thawing process.

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What heat exchanger features can be provided?

As no application is the same, there are options to Heat Pipe Heat Exchangers available to meet or exceed most conditions generally found in any heat exchanger application.

• Easy access maintenance panels are generally included that will allow for in situ cleaning procedures should they be required.

• For conditions of extremely high particulate count, feature options can include built-in cleaning assistance devices such as partition plate vibrators, sonic horns or compressed air blow-offs.

• Condensation drains

• Particulate traps

• Temperature and mass flow diverters or dampers

• Future anticipated additional capacity can be accommodated with added partition plate connection points

• Multiple energy streams can be utilized

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Are custom designs available?

Unless a client has designated a particular design as a standard within their process, AMSEnergy designs each Heat Pipe Heat Exchanger to meet the performance requirements prescribed by the specific application.

Due to the modern manufacturing process, we can build our custom designs in a very competitive nature with other heat exchanger technologies.  This also allows the ability to supply very scalable units to fit most space constraints presented.

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Can a computer-generated model of how a heat exchanger will operate for a given application be supplied?

AMSEnergy has access to CFD (computational fluid dynamics) programs such as Flotherm, Autodesk and Solidworks to model the performance of a Heat Pipe Heat Exchanger.

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Why buy a Heat Pipe Heat Exchanger from AMSEnergy?

Not only does our team have a collective number of decades of experience of building Heat Pipe Heat Exchangers for many different heat transfer applications, the manufacturing process allows us to build heat pipes with precise control and efficiency allowing for a cost efficient and effective thermal management system.

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