Finished heat pipe
Compared with the traditional air-cooled radiator, the heat pipe radiator has doubled the efficiency and broken the air-cooled limit. The heat pipe can also design the radiator into any shape without worrying about interference with other accessories. The high efficiency of heat pipe in heat transfer also allows designers to achieve ideal heat dissipation performance by sticking a thin aluminum sheet on the outer wall of the heat pipe without a lot of expensive copper materials.
A large number of CPU radiators use heat pipes.
Heat pipes are increasingly used in PC radiators. At present, more than 15% of CPU radiators shipped by major radiator manufacturers use heat pipes, especially high-end products. In order to make users more familiar with heat pipes, we have summarized some materials, hoping to give reference to readers who are keen on technology.
Brief introduction of heat pipe principle
The principle of heat pipe technology is actually very simple, that is, using evaporation and condensation of working fluid to transfer heat. Vacuum the inside of the copper tube and inject working fluid. During the evaporation-condensation phase change process, the fluid circulates repeatedly inside, continuously transferring heat from the hot end to the cold end, thus forming a heat transfer process in which heat is transferred from one end of the tube to the other. About the deeper principle of heat pipe, there have been many words on the Internet, so I won't go into details here. Interested readers can check this information themselves. This paper focuses on the knowledge and application of heat pipe technology.
Schematic diagram of heat pipe principle
At present, there are five main heat pipe manufacturers in the world, namely AVC/ Ye Qiang/Guhe /TC/ Huake/Fujikura; The application of heat pipe in PC field is led by AVC, with a share of about 30%. The heat pipe manufacturing technologies of these enterprises are all from Japan, and there is no difference in essence. As far as the specific heat pipe manufacturing process is concerned, it is complicated and requires hundreds of processes. However, with the development of industrialization, the mass production of heat pipes is not difficult and the price is getting lower and lower. Heat pipes, that is, sealed containers with outer walls, are generally made of steel, aluminum and copper, and most of the heat pipes in the radiator for PC are made of copper. The heat pipe specifications of PC radiator are mostly 6mm in diameter, and some products with 8mm in diameter are used, but the quantity is very small. Kyushu Fengshen once introduced the radiator of 8mm heat pipe.
Kyushu Shen Feng 8mm heat pipe radiator
The length of finished heat pipe is generally between 100 ~ 300 mm, and the price of a heat pipe radiator is only a few dollars, so the cost of heat pipe radiator is not much higher than that of traditional air-cooled products.
The process of liquid condensation will adopt the capillary principle, so the capillary structure is the core of a qualified heat pipe product. It has three main functions: first, it provides a channel for the liquid at the condensation end to flow back to the evaporation end; Secondly, it provides a channel for heat conduction between the inner wall and the liquid/vapor; Thirdly, it provides pores necessary for liquid and gas to generate capillary pressure. Capillary structure can be divided into four types: screen, groove, powder sintering and fiber. On the PC radiator, most of them are grooves and powder sintered structures, and powder (sintered heat pipe) accounts for 80%; Groove (slotted heat pipe) accounts for 20%.
Screen structure
fiber structure
Trench structure
Sintered structure
Grooved heat pipe is a kind of capillary structure of heat pipe, which is relatively simple to manufacture. The cost is 2/3 of that of general sintered heat pipes. Trough heat pipe is convenient to produce, but its disadvantages are obvious. Grooved heat pipes have high requirements on the depth and width of grooves and strong directionality. When there is a big bend in the heat pipe, the directivity of the groove becomes a fatal defect, which leads to a great decrease in thermal conductivity. However, the production process of sintered heat pipe is relatively complicated and the cost is relatively high. Heat pipe sintering requires high quality and purity of copper powder, particle size of single copper powder, sintering temperature and sintering uniformity. Therefore, it is not easy to manufacture excellent sintered heat pipes. The thermal conductivity of sintered heat pipes manufactured by different processes and costs is also different.
At present, there are some cheap heat pipe radiators on the market, including some graphics card radiators. Although heat pipes are used, the outer wall is often made of aluminum, and it is almost impossible to use powder sintering process for internal capillary process, so the performance will be inferior to that of high-end heat pipes. When shopping, we can't place too much hope on the heat dissipation performance of this product.
The length of heat pipe is generally 100 mm ~ 300 mm, but PC radiator may not be able to use such a long heat pipe. At this time, it is necessary to cut it off, then seal the cutting head and let the heat pipe work again. The cutting position is called tail-shrinking. The tail end does not conduct heat, so it is also called invalid end.
Heyi
Tail-shrinking two
Shrinkage can't transfer heat, so it can't participate in the heat transfer path. However, in order to reduce the manufacturing cost, some radiators often make invalid necking. For example, at the bottom of heat absorption, the invalid end occupies a long section, which does not help to play the real efficiency of heat pipe; Or the tail end is buried in the heat sink, and the heat sink connected with the invalid end will not get the heat transferred by the heat pipe. Fortunately, we haven't seen this kind of jerry-built radiator product. In Tt's Mini Typhoon products, we will see the situation as shown in the following figure, but careful analysis should not affect the heat conduction, but this kind of heat pipe is really rarely used.
The tail end occupies a large part of the heat absorbing bottom.
Although the heat pipe has unparalleled thermal conductivity, if the heat can't be dissipated, it can't play a role in heat dissipation, which requires the technology of combining heat pipe with fin.
Welding:
The most common connection process between heat pipe and fin is welding, which has low interfacial thermal resistance but high cost. For example, when aluminum fin is welded to copper heat pipe, it is necessary to plate nickel on the surface of heat pipe before welding with aluminum fin. An obvious feature of the process of welding heat pipes is that there are welding holes above the heat pipes.
Wear-resistant fins:
Passing through the fin means that the heat pipe passes through the fin directly by mechanical means. This process has low cost and simple process, but it has high technical requirements for the process itself. Otherwise, it is easy to make the contact between the heat pipe and the fin not close, resulting in high interfacial thermal resistance. The cross-sectional thermal resistance of heat pipe and fin is almost equal to that of welding, but the cost can be greatly reduced. In fact, fin piercing technology is a patented technology of AVC, which makes AVC radiator not only have strong heat dissipation performance, but also maintain a relatively low price.
As I said just now, there is basically no difference in performance between welding and fin piercing. However, in terms of cost, welding each heat pipe will be about 1 USD higher than passing through fins, so the price of heat pipe radiator with welding process is generally higher. Users should determine the purchase direction according to their own spending power.
The heat pipe has the best heat transfer efficiency in the straight-through state. But in fact, heat pipes are often bent. After bending, the heat transfer performance will decrease in different degrees, which is also closely related to the process quality.
Heat pipe bending die
Attention must be paid to the bending of heat pipes: try to keep the diameter unchanged or change little in the bending part. If there is serious deformation, such as the original cylindrical outer wall becomes flat, the thermal conductivity will be greatly reduced, because excessive deformation will lead to partial interruption of the capillary structure inside the heat pipe.
Other types of heat pipes
Heat pipes are not necessarily tubular. There are some flat heat pipes in the notebook. According to different applications, the heat pipe can also be designed into various shapes, which is also a prominent advantage of the heat pipe itself.
In the initial stage of water-cooled radiator, some manufacturers predict that it will be water-cooled in the future. But after such a long time, water cooling is still used by only a few players and has not entered the mainstream. Although water cooling is still dominant in heat dissipation performance, it is expensive, occupies a large space, and water (or other alternative liquids) will have the problem of deterioration and oxidation of internal materials. In addition to the shortcomings of water cooling itself, another reason for its decline is the emergence of heat pipes. When the heat pipe enters the PC field, there is a breakthrough in the heat dissipation technology of heat transfer materials, which makes people give up water cooling, so the space for water cooling gradually becomes smaller and will gradually fade out of the market in the future, and air cooling will still be the main product in the future.
Another important factor is that the rising speed of CPU calorific value has slowed down. It is predicted that there will be no monster PC CPU with TDP power exceeding 130W in the next three years, and the current heat pipe air cooling technology can meet the cooling requirements of CPU. With the increase of heat pipe production capacity and the maturity of technology, we expect that the price of heat pipe radiator will be further lowered, thus entering the low-end market and being selected by more users.