**2. Heat pipe and Thermosyphon manufacturing**

In this section, each step of a heat pipe and a thermosyphon manufacturing is described in detail. The proposed procedure has low-cost and the heat transfer passive devices are easyto-manufacture. The steps are cleaning, assembly, tightness test, evacuation procedure, and filling with the working fluid. These procedures were based on [21–26].

If the operation conditions are favorable to gravity, a thermosyphon can be manufactured. As a result, a capillary structure is not necessary. However, in adverse conditions, a wick should be selected and accommodated in the involucre inner. As mentioned before, the capillary structures can be screen meshes, grooves, sintered powder, among others.

The involucre material, capillary structure (if applicable), and the working fluid depend on the application, and they need to be chemically and mechanically compatible. For thermoelectric cooling, the operation temperature is around 150°C, which makes suitable to use copper and distilled water for involucre and working fluid, respectively.

First of all, the main components of the heat pipe or the thermosyphon have to be prepared. The sintered heat pipe involucre consists of the casing, the closing lids, and the capillary, as shown in **Figure 3**.

#### **2.1. Cleaning process**

The cleaning of the heat pipe or the thermosyphon is necessary to ensure the working fluid wettability, the impurity elimination, and the vacuum quality improvement [27]. Consequently, the heat pipe or thermosyphon components need to be thoroughly cleaned, before the introduction of the working fluid. For this purpose, first, the casing, the closing lids, the capillary, and the capillary structure (if applicable) are cleaned with acetone in order to remove larger dirties. Then, they are thoroughly cleaned with a sulfuric acid solution (H<sup>2</sup> SO4 of 0.1 M). After that, these components are taken to an ultrasonic bath, where they remained immersed in acetone for 30 min. Finally, the cleaning is completed. **Figure 4** shows the cleaning of the sintered heat pipe components in an ultrasonic bath.

**Figure 3.** Sintered heat pipe components.

The heat pipes basically consist of a metal tube sealed with capillary structure internally, which is embedded with a working fluid [12]. This capillary structure can be made of screen meshes, grooves, or sintered media [13]. The metal screen is the most commonly used capillary structure because of availability, ease of construction and good capillary pumping [14]. The grooves, as capillary structure, have a high thermal conductivity and good permeability [15]. The sintered metal wicks are manufactured by packing tiny metal particles between the inner heat pipe wall and a mandrel in powder form [16]. As mentioned earlier, the thermosyphon is a heat pipe assisted by gravity, which means that it has no capillary structure to return the working fluid [17]. Some researches available in the literature about thermal management of thermoelectric cooling used heat pipes and

Thus, in this chapter, manufacturing of low cost and easy-to-manufacture heat pipes and thermosyphon is described in detail, and an experimental evaluation of the thermal performance is accomplished for several different passive devices that can be used for thermal management of thermoelectric cooling. The considered devices were a rod, a thermosyphon, a mesh heat pipe, a grooved heat pipe, and a sintered heat pipe. In order to evaluate the best

In this section, each step of a heat pipe and a thermosyphon manufacturing is described in detail. The proposed procedure has low-cost and the heat transfer passive devices are easyto-manufacture. The steps are cleaning, assembly, tightness test, evacuation procedure, and

If the operation conditions are favorable to gravity, a thermosyphon can be manufactured. As a result, a capillary structure is not necessary. However, in adverse conditions, a wick should be selected and accommodated in the involucre inner. As mentioned before, the capillary

The involucre material, capillary structure (if applicable), and the working fluid depend on the application, and they need to be chemically and mechanically compatible. For thermoelectric cooling, the operation temperature is around 150°C, which makes suitable to use copper

First of all, the main components of the heat pipe or the thermosyphon have to be prepared. The sintered heat pipe involucre consists of the casing, the closing lids, and the capillary, as

The cleaning of the heat pipe or the thermosyphon is necessary to ensure the working fluid wettability, the impurity elimination, and the vacuum quality improvement [27]. Consequently,

passive heat transfer device, their thermal performance was compared.

filling with the working fluid. These procedures were based on [21–26].

structures can be screen meshes, grooves, sintered powder, among others.

and distilled water for involucre and working fluid, respectively.

**2. Heat pipe and Thermosyphon manufacturing**

thermosyphons [4, 5, 18–20].

356 Bringing Thermoelectricity into Reality

shown in **Figure 3**.

**2.1. Cleaning process**

**Figure 4.** Cleaning procedure in the ultrasonic bath.

### **2.2. Assembling of the heat pipes and the thermosyphon**

After the cleaning process, the heat pipe or thermosyphon can be properly assembled. As the wick is inside the casing, the closing lids and the capillary are welded to the tube extremities (**Figure 5**). In the case of copper involucres, the welding process can be performed with the aid of a soldering iron and the parts can be brazed using a tin alloy as filler material.

To perform the filling procedure with the working fluid, a small filling station has to be developed. The filling station is composed of a universal support, a graduated burette (scale of 0.1 mL) with a capacity of 25 mL, and a forceps (**Figure 8**). The burette and the polymeric hose are completely filled with the working fluid. The evacuated heat pipe or thermosyphon is coupled to the burette by the polymeric hose. Make sure that there are no air bubbles in the tube connecting the burette and the heat pipe or thermosyphon. The next step is to carefully open the burette valve. The forceps are carefully opened to drain the working fluid until the heat pipe or thermosyphon was charged with the correct quantity. It is emphasized that at the moment of filling, great care must be taken; otherwise, the heat pipe or thermosyphon vacuum will be lost. If this happens, the entire vacuum process must be performed again. After charging, the capillary is closed with

Heat Pipe and Thermosyphon for Thermal Management of Thermoelectric Cooling

http://dx.doi.org/10.5772/intechopen.76289

359

grip pliers and the capillary end is welded to the completely sealing (**Figure 9**).

**Figure 6.** Low-cost tightness test.

**Figure 7.** Evacuation procedure.

#### **2.3. Tightness test**

A tightness test has to be conducted to verify if there was no flaw in the welding process of the heat pipe or thermosyphon. A manual positive displacement pump, a water container (e.g., a sink full of water), and a polymeric tube are necessary to accomplish a low-cost test (**Figure 6**). The polymeric tube makes the connection between the pump and the capillary tube. The heat pipe or thermosyphon is inserted into the water container and the air is pumped into the tube using the positive displacement pump. If there are any flaws in the solder, bubbles will appear in the water. In case of the presence of bubbles, the heat pipe or thermosyphon has to be disassembled, cleaned, welded, and retested.

#### **2.4. Evacuation procedure**

First, the heat pipe or thermosyphon is connected to a vacuum pump (*Lab*1000™) that can remove some residual liquid from the cleaning process. Then, the heat pipe or thermosyphon is linked to a vacuum pump *EOS Value*™ i260SV by a polymeric hose. This second pump will do the evacuation process, which the internal pressure should reach at least 90 mbar (9 kPa) – **Figure 7**. To make sure the connections do not leak, the polymeric hose is connected to the capillary with high vacuum grease *Dow Corning*™, prior to starting the vacuum process. The evacuation procedure has a duration of at least 8 h. At the end of the procedure, the polymeric hose is sealed with the assistance of a forceps and the vacuum pump is turned off.

#### **2.5. Filling with working fluid**

The amount of working fluid inserted in the heat pipe or thermosyphon is essential for capillary pumping system because the heat transfer depends on that amount of fluid. If there is not enough fluid, the pumping system stop to work and the heat pipe or thermosyphon collapses and as a result, the transferring heat is ceased. As a result, the filling ratio should be carefully chosen. Usually, the filling ratio is based on the evaporator volume.

**Figure 5.** Assembled heat pipe.

To perform the filling procedure with the working fluid, a small filling station has to be developed. The filling station is composed of a universal support, a graduated burette (scale of 0.1 mL) with a capacity of 25 mL, and a forceps (**Figure 8**). The burette and the polymeric hose are completely filled with the working fluid. The evacuated heat pipe or thermosyphon is coupled to the burette by the polymeric hose. Make sure that there are no air bubbles in the tube connecting the burette and the heat pipe or thermosyphon. The next step is to carefully open the burette valve. The forceps are carefully opened to drain the working fluid until the heat pipe or thermosyphon was charged with the correct quantity. It is emphasized that at the moment of filling, great care must be taken; otherwise, the heat pipe or thermosyphon vacuum will be lost. If this happens, the entire vacuum process must be performed again. After charging, the capillary is closed with grip pliers and the capillary end is welded to the completely sealing (**Figure 9**).

**Figure 6.** Low-cost tightness test.

**2.2. Assembling of the heat pipes and the thermosyphon**

be disassembled, cleaned, welded, and retested.

**2.3. Tightness test**

358 Bringing Thermoelectricity into Reality

**2.4. Evacuation procedure**

pump is turned off.

**Figure 5.** Assembled heat pipe.

**2.5. Filling with working fluid**

After the cleaning process, the heat pipe or thermosyphon can be properly assembled. As the wick is inside the casing, the closing lids and the capillary are welded to the tube extremities (**Figure 5**). In the case of copper involucres, the welding process can be performed with the

A tightness test has to be conducted to verify if there was no flaw in the welding process of the heat pipe or thermosyphon. A manual positive displacement pump, a water container (e.g., a sink full of water), and a polymeric tube are necessary to accomplish a low-cost test (**Figure 6**). The polymeric tube makes the connection between the pump and the capillary tube. The heat pipe or thermosyphon is inserted into the water container and the air is pumped into the tube using the positive displacement pump. If there are any flaws in the solder, bubbles will appear in the water. In case of the presence of bubbles, the heat pipe or thermosyphon has to

First, the heat pipe or thermosyphon is connected to a vacuum pump (*Lab*1000™) that can remove some residual liquid from the cleaning process. Then, the heat pipe or thermosyphon is linked to a vacuum pump *EOS Value*™ i260SV by a polymeric hose. This second pump will do the evacuation process, which the internal pressure should reach at least 90 mbar (9 kPa) – **Figure 7**. To make sure the connections do not leak, the polymeric hose is connected to the capillary with high vacuum grease *Dow Corning*™, prior to starting the vacuum process. The evacuation procedure has a duration of at least 8 h. At the end of the procedure, the polymeric hose is sealed with the assistance of a forceps and the vacuum

The amount of working fluid inserted in the heat pipe or thermosyphon is essential for capillary pumping system because the heat transfer depends on that amount of fluid. If there is not enough fluid, the pumping system stop to work and the heat pipe or thermosyphon collapses and as a result, the transferring heat is ceased. As a result, the filling ratio should be carefully

chosen. Usually, the filling ratio is based on the evaporator volume.

aid of a soldering iron and the parts can be brazed using a tin alloy as filler material.

**Figure 7.** Evacuation procedure.

**3. Characteristics of the developed passive devices**

**Characteristics Rod Thermosyphon Heat Pipe**

Evaporator length

Adiabatic section length [mm]

Condenser length

Volume of working fluid [mL]

Capillary structure — No capillary

**Table 1.** Main features of the heat transfer passive devices.

structure

**Figure 10.** Phosphor bronze screen mesh #100. (a) General view (b) micro-scale image.

[mm]

[mm]

The heat pipes and thermosyphon were produced by copper tubes ASTM B-75 Alloy 122 with an outer diameter of 9.45 mm, an inner diameter of 7.75 mm, and a length of 200 mm. The rod was obtained from a full copper bar ASTM B-75 Alloy 122 with the same dimensions of the developed heat pipes and thermosyphon. All the devices had an evaporator of 80 mm

80.0 80.0 80.0 80.0 80.0

20.0 20.0 20.0 20.0 20.0

100 100 100 100 100

— 2.26 2.19 1.73 2.26

Phosphor bronze screen mesh #100

Inner diameter [mm] — 7.75 7.75 6.20 7.75 Outer diameter [mm] 9.45 9.45 9.45 9.45 9.45

Working fluid — Water Water Water Water

Filling ratio [%] — 60 60 60 80

**Mesh Grooved Sintered**

Heat Pipe and Thermosyphon for Thermal Management of Thermoelectric Cooling

http://dx.doi.org/10.5772/intechopen.76289

361

Microgrooves by the wire-EDM

Copper powder sintered

**Figure 8.** Filling station with the heat pipe.

**Figure 9.** Welding the capillary end.
