A heat pipe is an extremely efficient thermal conductor. It can transfer large quantities of heat over a long distance essentially at a constant temperature. It is typically a sealed copper or aluminum tube containing a wick structure on its inner surface and a small amount of working fluid at its saturation state. The fluid absorbs heat and vaporizes at hot spots and condenses and releases heat at cool spots. As the process goes on, heat is transferred from the hot spots to the cool spots.

Because a heat pipe has no moving parts, it is a highly reliable device with demonstrated lifespan of over 20 years. The reliability depends on the manufacturing process and the purity of the materials. At MyHeatSinks, all heat pipes are 100% individually tested to ensure functionalities and reliability.

Quick History

Heat pipes were thoroughly studied by Los Alamos National Laboratory in 1960s. Los Alamos physicist George Grover demonstrated his invention in 1963 being the first to use the term “heat pipe”. Due to the low weight and high heat flux, heat pipes began to gain interests. Beginning with NASA, heat pipes were applied to satellites for thermal equilibrium. From satellites, a wider range of applications can be found using heat pipes such as air conditioning, engine cooling, and electronics cooling.

Wick Structure and Working Fluid

The performance of a heat pipe is mainly determined by its wick structure, which performs 3 primary functions: First, to allow the backflow of the liquid from the condenser section to the evaporator section; Second, to allow the heat transfer between the inner wall and the fluid; Third, to provide room for the fluid to change phase. We currently provide heat pipes with 3 different types of wick structures – the sintered wick, the micro-groove wick and the compound wick. The sintered wick allows high heat flux and wide working angle and is recommended for most electronic applications. Micro-groove wick offers light weight and low cost, but its working angle is limited and often gravity dependent. Compound wick combines the features of both sintered and micro-groove wicks and is preferable in some applications.

The most common working fluids used in heat pipes include water, ammonia, acetone and methanol. In moderate temperature range, water is the ideal working fluid due to its high latent heat and proper boiling point. For low temperature applications, ammonia, acetone and methanol can be used.

Wall Thickness

Most common thickness for heat pipes is 0.3mm. For heat pipes that require milling, the thickness is about 0.4 to 0.5mm. This helps to achieve better flatness after flattening. For ultra-thin heat pipes, the wall thickness is about 0.2mm. Ultra-thin heat pipes can be used in phones or wearable devices.

Fluid Filling, Vacuum, Sealing

Heat pipes can be filled by partially filling with distilled water (or other type of working fluid) and then heated up until the water boils and displaces the air. The heat pipe is then sealed while hot. The typical water amount is about 0.5 to 1.0g and the typical vacuum pressure is about 0.1 Torr.

Q_max increases when vacuum pressure reduces. The maximum heat transport with NCG:

Standard Dimensions

For standard heat pipes, we offer round and flat copper heat pipes with H₂O distilled water working fluid. Standard diameters include: 4mm, 5mm, 6mm, 8mm, and 10mm.

Diameter ØD	4 mm	5 mm	6 mm	8 mm	10 mm
Length L	80-600 mm	80-600 mm	80-600 mm	80-600 mm	80-600 mm
Length L1	1-3 mm	2-4 mm	3-5 mm	4-7 mm	5-8 mm
Length L2	7-9 mm	8-10 mm	9-11 mm	11-13 mm	13-15 mm

Large Diameter Heat Pipes

The largest diameter we currently make is 12.7 mm (0.5 inch). The heat pipes have micro-groove wick and are designed for high-power LED applications and solar systems.

Reliability Test

No.	Test Item	Test Conditions	Sampling Ratio	Purpose
1	High Temp. Aging Test	Ambient Temp. 210 C for 12 hr	100%	Leakage check & aging
2	Thermal Response Test	Insert 1/3 – ½ length of pipe into 50°C water. The temp of other end shall rise to standard value in 25 seconds.	100%	Vacuum & leakage check
3	Qmax Test	Heating Length = 25-35mm, Test temp = 60°C	100%	To measure the max. heat transfer rate
4	Rth Thermal Resistance Test	Fix heat transfer rate and measure temp. difference of heat pipes	>1pcs/2hr	To ensure thermal resistance of each pipe be lower than spec
5	Accelerated Life Test	140 C for 1000hr. Performance decrease by less than 7%	By case	To predict life of heat pipe at certain operating temp.
6	Continuous Life Test	Continuous testing at normal operating conditions	By case	To measure actual life of heat pipe until failure occurs
7	Thermal Cycling Test	Temp. varies from -30 C to 120 C in 10hr, 600 cycles	By case	To measure performance variation after thermal cycling

Heat Pipe Performance Tester (LW – 9354)

To evaluate the performance of heat pipes, we have a state-of-the-art heat pipe tester provided from Long Win Science and Technology Corporation. It is equipped with a water-cooler circulating system, heating control system, attitude adjustment device, measurement system, data acquisition, and test analysis. With the attitude adjustment device, heat pipes can be tested at different angles and directions to evaluate the performance of heat pipes given different testing conditions. For more info about our tester, please visit our Heat Pipe Tester page.

Tests Available:

1. Thermal Performance Steady-State Test
2. Thermal Resistance Measurement at different Angles
3. Thermal Response Rate Measurements

Principle

The principle of the heat pipe testing equipment is to establish a simulated heating source at one end of the heat pipe, and set a cooling device at the other end of the heat pipe to simulate the heat sink. The tester includes a hot side heater for the hot end of the heat pipe and a cold side with a water-cooled cooler for the cold end of the heat pipe. We can then measure the temperature difference between the hot and cold ends of the heat pipe (∆T=Thavg − Tcavg).

Steady–State Test and Curve

Step 1:

Place the test object between the heating end and the condensation end.

Step 2:

Input heat from a controllable heat source and remove the heat from the water cooling through the specimen.

Step 3:

Measure the heating end, the reference temperature of the condensing end and the temperature of other points (Tc1, Tc2, Thc, Thb) on the heat pipe.

Effective Thermal Conductivity

With the heat absorbed by the heat pipe (Q), the thermal resistance (R) and equivalent thermal conductivity (K) of the heat pipe can be calculated.
Q: Heat flux
A: Sectional Area of test object
ΔX: Distance from the heating end to the condensation end
ΔT: Difference between the reference temperature of the heating end and the reference temperature of the condensation end

Heat Transfer Capacity

Thermal Resistance of Heat Pipes Example

Heat pipes with larger diameters transfer more heat than heat pipes with smaller diameters. Shorter length heat pipes can also transfer more heat than heat pipes with longer lengths.

Thermal Response Rate Example

Tt: Water Temperature
Tc: Heat Pipe Temperature

The thermostatic water is set to 65°C The initial temperature of the heat pipe is approximately ~20°C. After about7 seconds, the hot end temperature rises to about ~55°C and approaches steady state.

Benefits of Heat Pipes

Heat pipes can transfer large amounts of heat over a long distance at a constant temperature. There is a very low temperature difference across its length. Typically, less than 2°C (compared to larger ΔT on copper or aluminum tubes). Heat pipes also have highly effective thermal conductivity focused along the axis. With low maintenance and long life expectancy, heat pipes are highly reliable with a lifespan of 20 years.

RoHS Compliance

Our heat pipes are in compliance with the European Union Directive 2002/95/EC on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS Directive).

Restricted Substances	Symbol	Limit
Lead	Pb	<1000ppm
Mercury	Hg	<1000ppm
Cadmium	Cd	<100ppm
Hexavalent Chromium	Cr6+	<1000ppm
Polybrominated Biphenyls	PBB	<1000ppm
Polybrominated Diphenyl Ethers	PBDE	<1000ppm
Bis (2-ethylhexyl) Phthalate	DEHP	<1000ppm
Butyl Benzyl Phthalate	BBP	<1000ppm
Dibutyl Phthalate	DBP	<1000ppm
Diisobutyl Phthalate	DIBP	<1000ppm

Characteristics and Features

High Thermal Conductivity – A heat pipe has much lower thermal resistance and much higher thermal conductivity than any solid conductors such as silver, copper and aluminum, enabling it to transfer heat more efficiently and evenly in several orders. For example, a 6mm diameter heat pipe with sintered wick can transfer up to 120W heat.

Excellent Isothermal Performance – Due to the small pressure drop of saturated vapor from the evaporator section to the condenser section, a heat pipe can maintain almost a constant temperature over its full length.

Reciprocity of Heat Flow Direction – The circulation inside a sintered heat pipe is driven by capillary force, instead of gravity. So either end can be the evaporator section or the condenser section.

Adaptability to Environment – Due to the various wick structures, we can supply heat pipes that are suitable and reliable to meet any environmental challenges.

Surface Treatment – Our heat pipes are treated by an anti-oxidation process. They can also be nickel plated or anodized to increase its weldability or heat radiation capability.

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