Heat Pipe

heat pipe

 The characteristic of Heat Pipe

Heat pipe is the most effective het transfer component nowadays. Especially, it can transmit heat if great distance through a small section without any outside power. 

Axial thermal conductivity of heat pipe is far in excess of any known metal. Thermal conductivity of some metals is as shown in the following table:

Material
Thermal conductivity W/(m.℃)
Material
Thermal conductivity W/ (m.℃)
heat pipe
2*104~1.6*106
Pure aluminum
236
silver
428
Beryllium
218
pure copper
401
Aluminum alloy
150~210
gold
318
Carbon steel
47.6

The working principle of heat pipe
As the diagram shows: Inside the heat pipe is a vacuum closed cavity of 1.3*(10-1~10-4)Pa, and filled with working liquid, When one side is heated, the liquid evaporate in the core, the steam flow to the other side under slim pressure and emit heat condensing to liquid, and the liquid flow to evaporate side be capillarity along the porous material, on such endless cycle, the heat is transformed from on side to the other side.

 heat pipe structure and principle

What is heat pipe?
Heat pipe is an element whose heat conductivity efficiency is particularly high. It uses phase change theory and capillarity action, making its own heat transfer efficiency is several thousands of tines higher than the same material metal. Phase change heat transfer coefficient of water is shown as the following table:

Kinds of phase change heat transfer
Heat transfer coefficient W/(m2.℃)
Water's boiling
2500~5000
Steam condensation
6000~400000
Organic steam condensation
500~2000

From an academic perspective, the research indications are various, but none of them can be used as the specific indications, there is no significance for the heat pipe application, Which indications shall we care about for the solar heat pipe?
Life time and performance of the solar water heater is the concern that customers care, it is also the core issue of the countries that promote the use of solar water heater focus on. No matter which kind of certification, including the European Solar Keymark certification, the United states SRCC USEC certification, the Switzerland SPF certification, the Germany’s Blue Engel certification, or the China’s Gold soar certification, without exception make specific requirement about the life time and performance, although the test method and indications are different.
Solar heat pipe the heat transfer component of the solar water heat. Its lifetime and heat transfer efficiency have direct impact on the lifetime and gaining heat efficiency. Therefore, when we choose solar heat pipe we should be concerned about the following indications:
1. Lifetime
Lifetime is the first consideration of solar heat pipe, it mainly focus on the pipe does not transfer heat or heat transfer efficiency decay.
⑴Form of Expression: Heat transfer power
⑵Test method: test high temperature performance → test heat transfer power
⑶Judge base:

Power
Q ≥120W
Q ≥120W≥80W
Q ≥80W
Result
Qualified
Not qualified
Failure

⑷The curve diagram of solar heat pipe.(see picture1: the decay curve of solar heat pipe)

2. High-temperature resistance properties:
High-temperature resistance properties are the base of choosing heat pipe and test preparative, it mainly represent on the heat pipe burst, failure or decay under high temperature.
It represents on the solar heat pipe preventing blast under high temperature, the parameter is:

Wall Thickness
0.5mm
0.5mm
0.5mm
Allowable
200℃
220℃
250℃

It represents on solar heat pipe failure or decay under high temperature (see the test method and judge base), it proves the solar heat pipe exists problems as following:
    1. The working media is not appropriate
    2. Working Media and shell is incompatible.
    3. Solar heat pipe exist chronic leakage
    4. Impurity exists shell or working media.

3. Low-temperature resistance properties:
IC-CN antifreeze technology, change the working media icing sequence, so it will not frost cracking under -30℃.

4. Heat transfer power
Heat transfer Power is an important indication of judging heat pipe performance, it reflects the heat loss during the heat transfer procedure from the side, The larger the transmission power, the less the heat loss, accordingly, the solar collector can gin more heat.

5. Isothermal performance
Isothermal performance is the auxiliary evaluation indications of solar heat pipe performance. The heat pipe that tested under high temperature can he judged if exist non-condensable gas inside it. By the isothermal performance testing. Accordingly, we can check if the shell and working media is appropriate.
6. Starting time

 

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