Frequently Asked Thermal Questions
• What is a heat pipe?
• How does a heat pipe work?
• What are heat pipes used for?
• What is the thermal conductivity of a typical heat pipe?
• What materials can be used to construct a heat pipe?
• What are the primary heat transport limitations of a heat pipe?
• Are heat pipes reliable?
• Are heat pipes expensive?
• Can heat pipes work against gravity?
• What fluids are used in heat pipes?
• How does a water heat pipe work below 100° C?
• Can heat pipes freeze?
• What heat exchanger alarm features can you provide? How?
• How do you seal the core element in the Aavid HXi® series?
• Can the fins in an HX® be coated for environmental protection?
• What is the difference between the HX®, HXi® and HXc® technologies?
• Do you build custom designs?
• Can you provide a computer-generated model of how your heat exchanger will operate in my application?
• Why should we buy a heat exchanger from Aavid?
What is a heat pipe?
A heat pipe is a heat transfer device with an extremely high effective thermal conductivity. Heat pipes are evacuated vessels, typically circular in cross section, which are back-filled with a small quantity of working fluid. They are totally passive systems, with no moving parts, and transfer heat from a heat source to a heat sink with minimal temperature gradients, or to isothermalize surfaces.
How does a heat pipe work?
Through the evaporation and condensation of the working fluid. As heat is input at the evaporator, fluid vaporizes, creating a pressure gradient in the pipe. This forces the vapor to flow along the pipe to the cooler section where it condenses, giving up its latent heat of vaporization.The working fluid is then returned to the evaporator by capillary forces in the porous wick structure or by gravity.
What are heat pipes used for?
Heat pipes are used for a wide variety of applications — anywhere heat must be transferred with a minimum thermal gradient, either to increase the size of the heat sink, to relocate the sink to a remote location or where isothermal surfaces are required. Typical applications include computer processor cooling, isothermal furnace liners and aerospace heat transfer.
What is the thermal conductivity of a typical heat pipe?
Heat pipes do not have a set thermal conductivity like solid materials because they have a two-phase heat transfer. Instead, their effective thermal conductivity improves with length. A 12-inch and a 4-inch heat pipe, each carrying 100 W, will have about the same thermal gradient, so the 12-inch pipe will have the higher effective thermal conductivity. Unlike solid materials, a heat pipe will have its effective thermal conductivity changed with the amount of power being transferred and with the evaporator and condenser sizes. Effective thermal conductivities can range from 10 to 10,000 times (4,000 W/meter·K to 4,000,0000 W/meter·K) the effective thermal conductivity of copper, depending on the length of the heat pipe.
What materials can be used to construct a heat pipe?
The heat pipe wall or shell material selection is driven by compatibility of the working fluid. The heat pipe working fluid is selected based on the operating temperature range of the application. After a working fluid is selected, the heat pipe wall or shell material is selected based on its chemical compatibility with the working fluid to prevent corrosion or chemical reaction between the fluid and the heat pipe wall or shell material. A chemical compatibility problem between the working fluid and wall material within a heat pipe can create a chemical reaction that produces a non-condensable gas. Non-condensable gases within a heat pipe can cause operational failure.
What are the primary heat transport limitations of a heat pipe?
The four basic heat pipe heat transport limitations are:
Capillary limit: This is the maximum capillary pumping pressure of the wick structure to transport the working fluid from the heat pipe condenser to evaporator. The capillary pumping pressure must overcome three basic pressure drops within the heat pipe, namely, vapor pressure drop, liquid pressure drop and gravitational/body force pressure drops.
Boiling limit: The boiling limit occurs when the maximum radial heat flux (W/cm2) is exceeded resulting the rate of working fluid vaporization to exceed the rate at which the liquid condensate is returning from the condenser section of the heat pipe. When the boiling limit is reached, liquid working fluid is not available to absorb heat and the heat pipe goes into a dry-out condition and will not operate.
Sonic limit: The maximum flow rate of the working fluid vapor flow rate traveling from the heat pipe evaporator to condenser. When the vapor flow rate exceeds the sonic velocity, chocked flow is achieved and the heat pipe will not operate isothermal.
Entrainment limit: This occurs when the sheer force of the vapor flowing from the evaporator to the condenser section of the heat pipes at the vapor-wick interface causes liquid droplets to be entrained and carried to the condenser section. Exceeding the entrainment limit may prevent the working fluid from returning from the condenser section to the evaporator section, as a result the heat pipe will not operate.
Are heat pipes reliable?
Yes, mainly because they have no moving parts. They are ideal for applications such as aerospace where maintenance is not feasible. The main cause of heat pipe failures is gas generation in the heat pipe, but this can be completely avoided by proper cleaning and assembly procedures. Aavid is the only heat pipe manufacturer in the world that can claim over 40 years of heat pipe reliability and life test data.
Are heat pipes expensive?
Compared with traditional (and less effective) heat transfer methods such as aluminum extrusions and cast heat sinks, heat pipes can have a higher initial cost. That is why heat pipes are not recommended for applications where cooling can be performed by simple conductive heat sinks. In more demanding applications, however, the overall cost of heat pipes is competitive with other alternatives. The initial cost is also partially offset by improvements in system reliability and increased life of cooler running electronics. In large quantities, the cost of heat pipes drops significantly and often makes them the most economical solution to a cooling application.
Can heat pipes work against gravity?
Yes, this occurs whenever the evaporator is located above the condenser. In these applications the working fluid must be pumped against gravity back to the evaporator. This occurs through wick structures that pump working fluid through capillary pressure developed in the porous wick. The finer the pore radius of a wick structure, the higher against gravity the heat pipe can operate. (Nanoscale wicks are available.)
Not all types of passive heat transfer can operate against gravity. A thermosiphon is similar to a heat pipe but has no wick structure and will only operate gravity-aided.
What fluids are used in heat pipes?
Heat pipe working fluids range from helium and nitrogen for cryogenic heat pipe applications, to liquid metals like sodium and potassium for high-temperature heat dissipation. Some of the more common heat pipe fluids used for electronics cooling operations are ammonia, water, acetone and methanol. Aavid has experience making heat pipes using all of these fluids for cryogenic applications to high temperature (>1,000 °C) applications.
How does a water heat pipe work below 100° C?
Water at atmospheric pressure boils at <100° C, but inside a heat pipe, water is not at atmospheric pressure. The internal pressure of the heat pipe is the saturation pressure of the fluid at the corresponding fluid temperature. The fluid in a heat pipe will boil at any temperature above its freezing point. Therefore, at room temperature (20° C) a water heat pipe is under partial vacuum, and the heat pipe will boil as soon as heat is input.
Can heat pipes freeze?
Yes, heat pipe working fluids, including water, maintain their normal freezing point. Heat pipes will not operate until the temperature rises above the freezing temperature of the fluid. Properly designed heat pipes, however, will not be damaged by freezing or thawing of the working fluid. Aavid has successfully designed, developed and manufactured freeze tolerant heat pipes that have over 20 years of demonstrated and proven field application experience.
What heat exchanger alarm features can you provide? How?
Temperature control, speed control and fan-failure alarms can be integrated into each heat exchanger. These features can be provided by installing a solid-state control board and/or integrating the feature into the fan itself.
How do you seal the core element in the Aavid HXi® Heat Exchanger series?
Aavid uses an RTV sealant to provide a cohesive gasket around both the inner core cassette and the core flange assembly. Each inner core cassette and core flange assembly is subjected to a vacuum test design to simulate NEMA 4 conditions.
Can the fins in an HX® be coated for environmental protection?
Yes, the typical coating is either a hexavalent chromate or a RoHS compliant clear chromate. Coatings such as Herresite or E-Coat can be added to heat exchangers to provide environmental protection to the unit (minimum volumes apply).
What is the difference between the HX®, HXi® and HXc® technologies?
Each technology offers its own merits in regard to size, efficiency, adaptability for customization and power capability. Allow Aavid to review the application in order to recommend the best solution.
Do you build custom designs?
Aavid utilizes a broad scope of technologies to deliver fully optimized, custom solutions as well as our standard offerings.
Can you provide a computer-generated model of how your heat exchanger will operate in my application?
Yes, Aavid can utilize CFD (computational fluid dynamics) programs such as Aavid SmartCFD to model the performance of a heat exchanger within the enclosure.
Why should we buy a heat exchanger from Aavid?
Understanding of the total thermal circuit is crucial to a product's success. Aavid has the capability to design and manufacture thermal management solutions at the component, board, and system levels.
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