**2. Design and manufacture**

The design and manufacture of a thermosyphon is a complex process because there should be considered several parameters such as length, shape, weight and volume of the device, heat load, transport distance, ratio of lengths between evaporator and condenser, acceptable temperature gradients, temperature range of operation, amount of working fluid, service life and safety. Of course, the working fluid thermophysical properties and the manufacturing material properties should be taken into account as key variables.

The authors divide the thermodynamic processes in 2 categories: 1) heat transfer by conduction through the tube wall and 2) heat and mass transfer, by convection inside the two-phase thermosyphon. (Noie, 2005) presented in his work an experimental study of a thermosyphon (980 mm of length and an internal diameter of 25 mm) made of copper and smooth inside, using distilled water as a working fluid. The goal of the study was to obtain the thermal characteristics of the thermosyphon (temperature distribution in the outer wall and all along the tube, boiling heat transfer coefficient and the maximum heat transfer rate), varying 3 parameters: heat supply (100 < Q < 900 W), loading rate (30% ≤ Ψ ≤ 90%) and

From above mentioned, it follows that in order to design high efficiency heat recovery equipment based on two-phase thermosyphons, there should be solved first the main issues inherent to to their manufacture: loading rate, maximum heat transfer rate, and compatibility between material of container and the working fluid. In order to address these issues, an analytical-experimental investigation has been carried out in the Thermal Engineering and Applied Hydraulics Laboratory, for the last four years. Following are

The design and manufacture of a thermosyphon is a complex process because there should be considered several parameters such as length, shape, weight and volume of the device, heat load, transport distance, ratio of lengths between evaporator and condenser, acceptable temperature gradients, temperature range of operation, amount of working fluid, service life and safety. Of course, the working fluid thermophysical properties and the manufacturing

length of the evaporator (varying the length of electrical resistance).

material properties should be taken into account as key variables.

Fig. 1. Two-phase thermosyphon.

presented the results of this investigation.

**2. Design and manufacture** 

Because there are no standards in the public domain for the manufacture and design of twophase thermosyphons is necessary to develop the methodologies to design them. Following are described three methodologies for the calculation of key parameters used in the design and manufacture of two-phase thermosyphons. For the development of these methodologies water was considered as the working fluid.

The first parameter to calculate is the relation between the lengths of the zones of evaporation and condensation, and the total length of the thermosyphon. From this relation it can be obtained the total length of the thermosyphon for certain heat recovery equipment.

For security reasons, one of the most important parameters to be calculated is the working pressure of thermosyphon under different operation conditions and for distinct amounts of working fluid in the thermosyphon. These working pressures of the internal fluid are: the pressure when the device is off, i.e., when the thermosyphon is at room temperature, this is when the device is in non-operating conditions (transportation, storage, etc.), and the operating pressure.

The third parameter is the evaporation rate of the working fluid in the process of loading the thermosyphon. This process is directly related to the procedure of loading the working fluid to the thermosyphon, which was implemented in this research.

#### **2.1 Relations between the lengths of the areas of air and gases and the total length of the thermosyphon**

Heat recovery equipment based on two phase thermosyphons consists of an outer envelope with thermosyphons grouped inside. According to the principle of operation of the thermosyphons (evaporation/condensation of a working fluid) heat is transferred from the evaporator, located at the bottom where the combustion hot gases flow, to the condenser, located at the top, where the fluid to be heated circulates. Thus, the hot gases flowing through the evaporation zone, transferring the heat from that zone to the condensation zone through the thermosyphons. On the other hand, the gases to be heated, for example air, flow in the opposite direction through the condensation zone absorbing the heat dissipated by the thermosyphons.

It can be considered that the efficiency of the thermosyphon is 95%, that is, 95% of heat of combustion gases is transferred into the condensation zone for heating the air. The energy balance for steady state conditions may be expressed as:

$$
\dot{m}\_a c\_{pa} \left( T\_{a,out} - T\_{a,in} \right) = \dot{m}\_g c\_{pg} \left( T\_{g,in} - T\_{g,out} \right) \tag{1}
$$

Applying the continuity equation gives the expression (2).

$$
\rho\_a A\_a v\_a c\_{pa} \left( T\_{a,out} - T\_{a,in} \right) = \rho\_{\mathcal{g}} A\_{\mathcal{g}} v\_{\mathcal{g}} c\_{pg} \left( T\_{\mathcal{g},in} - T\_{\mathcal{g},out} \right) \tag{2}
$$

In addition, the area for the passage of air, *Aa*, is defined as the product of the length of the thermosyphon condensation zone, *lc*, by the width of the passage of air, *a*:

$$A\_{\mathfrak{a}} = l\_{\mathfrak{c}} a \tag{3}$$

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 101

This methodology considers that the velocities of hot gases and air are equal and that the thermosyphon has an efficiency of 95%. In addition, the width of the passage of air is equal to the width of the passage of hot gases, which is the width of the air preheater, *a*. Taking these considerations into account and substituting equations (3) and (4) in equation (2) gives

> 

To determine one of the design parameters sought, which is the ratio between the length of the condensation zone and the total length of the thermosyphon, equations (5) and (6) are

*t g pg g in g out a pa a out a in*

Knowing this, from equation (5) is obtained the relation between the length of the

1 *e c t t l l*

However, the equations developed above, consider only the energy and mass balances, this analysis could lead to violations of the second law of thermodynamics. To be sure that the results do not violate this law, the mathematical models to calculate the entropy production in the system are used. Just for recalling that the total entropy change must be greater than

If the air is considered as an ideal gas and the isobaric heating process is reversible, then the

*S SS*

As an example, a parametric analysis to study the relationship of the lengths of the zones of evaporation and condensation with respect to the total length of the thermosyphon was carried out. The outlet temperatures of both air and combustion gases were varied. The air inlet temperature is assumed to be 25 °C and the combustion gases of 250 °C. The

*a a pa*

*g g pg*

*S mc*

The expression to calculate the total entropy change of the system is as follows:

*S mc*

Similarly for combustion gases the entropy change is expressed as follows:

, , ln *a out*

*T*

*T* 

*a in*

, , ln *g out*

*T*

*T* 

*g in*

*l cT T cT T*

*a c pa a out a in g e pg g in g out lc T T* , , 0 95 . *lc T T* , , (6)

(7)

*l l* (8)

(9)

(10)

*sys g a* (11)

 , , ,, , ,

 

the expression (6)

or equal to zero.

entropy change of air is expressed as follows:

resolved simultaneously and following equation (7) is obtained.

evaporation zone and the total length of the thermosyphon (8).

.

0 95

.

0 95

*l cT T*

*g pg g in g out c*

Similarly, for the passage of hot gases

$$A\_{\mathcal{R}} = l\_{e^\*} a \tag{4}$$

Because the adiabatic zone is negligible, the total length of the thermosyphon is the sum of the lengths of the evaporation and condensation zones (5),

$$I\_t = I\_c + I\_e \tag{5}$$

Fig. 2. Gas-gas type heat transfer equipment. Total, evaporation zone and condensation zone

Because the adiabatic zone is negligible, the total length of the thermosyphon is the sum of

*tce*

Hot gases flow

the lengths of the evaporation and condensation zones (5),

Screen

thermosyphons

characteristic lengths are shown.

Similarly, for the passage of hot gases

*a* Two-phase

Air flow

*lt* 

*A la g e* (4)

*lll* (5)

*lc* 

*le* 

This methodology considers that the velocities of hot gases and air are equal and that the thermosyphon has an efficiency of 95%. In addition, the width of the passage of air is equal to the width of the passage of hot gases, which is the width of the air preheater, *a*. Taking these considerations into account and substituting equations (3) and (4) in equation (2) gives the expression (6)

$$\rho\_d \rho\_c l\_{pa} \left( T\_{a,out} - T\_{a,in} \right) = 0.95 \rho\_g l\_e c\_{pg} \left( T\_{g,in} - T\_{g,out} \right) \tag{6}$$

To determine one of the design parameters sought, which is the ratio between the length of the condensation zone and the total length of the thermosyphon, equations (5) and (6) are resolved simultaneously and following equation (7) is obtained.

$$\frac{l\_c}{l\_t} = \frac{0.95 \rho\_{\mathcal{S}} c\_{pg} \left( T\_{\mathcal{S},in} - T\_{\mathcal{S},out} \right)}{0.95 \rho\_{\mathcal{S}} c\_{pg} \left( T\_{\mathcal{S},in} - T\_{\mathcal{S},out} \right) + \rho\_a c\_{pa} \left( T\_{a,out} - T\_{a,in} \right)} \tag{7}$$

Knowing this, from equation (5) is obtained the relation between the length of the evaporation zone and the total length of the thermosyphon (8).

$$\frac{l\_c}{l\_t} = 1 - \frac{l\_c}{l\_t} \tag{8}$$

However, the equations developed above, consider only the energy and mass balances, this analysis could lead to violations of the second law of thermodynamics. To be sure that the results do not violate this law, the mathematical models to calculate the entropy production in the system are used. Just for recalling that the total entropy change must be greater than or equal to zero.

If the air is considered as an ideal gas and the isobaric heating process is reversible, then the entropy change of air is expressed as follows:

$$
\Delta \dot{S}\_a = \dot{m}\_a c\_{pa} \ln \left( \frac{T\_{a,out}}{T\_{a,in}} \right) \tag{9}
$$

Similarly for combustion gases the entropy change is expressed as follows:

$$
\Delta \dot{S}\_{\mathcal{S}} = \dot{m}\_{\mathcal{S}} c\_{p\mathcal{S}} \ln \left( \frac{T\_{\mathcal{S},out}}{T\_{\mathcal{S},in}} \right) \tag{10}
$$

The expression to calculate the total entropy change of the system is as follows:

$$
\Delta \dot{S}\_{\text{sys}} = \Delta \dot{S}\_{\text{g}} + \Delta \dot{S}\_{a} \tag{11}
$$

As an example, a parametric analysis to study the relationship of the lengths of the zones of evaporation and condensation with respect to the total length of the thermosyphon was carried out. The outlet temperatures of both air and combustion gases were varied. The air inlet temperature is assumed to be 25 °C and the combustion gases of 250 °C. The

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 103

The operating pressure of the thermosyphon is also found in the tables of thermodynamic properties of water (Incropera & Dewitt, 2006), the specific volume of water was calculated

To calculate the operating pressure of the thermosyphon a parametric analysis was performed, where the variable parameter is the loading ratio. The interval of studied loading ratio is 5% to 45%. The geometric characteristics of the thermosyphon were taken as follows: length of 1 m and a diameter of 2.54 cm. The temperature of 20 °C and a pressure of 1 bar were the ambient conditions. It was considered that the maximum operating

Fig. 4. T-s diagram of isochoric operation of the thermosyphon for different loading ratios.

Figure 4 shows in a T-s diagram the isochoric process that takes place in the thermosyphon

As an example applying the described method a loading ratio of 10% is given, and the value of the specific volume of water that was obtained using equation (14) is 0.0100177 m3/kg. Therefore by exposing the thermosyphon at room temperature of 20 °C, after the loading process, the non-operating pressure is equal to 0.0233 bar. Table 1 shows the thermodynamic properties of water inside the thermosyphon. On the other hand, by bringing the thermosyphon to the operating temperature for an isochoric process, an operating pressure equal to 39.7 bar is achieved. Table 1 shows the properties for the

> *s*  kJ/kg ºC

20 0.0233 0.010017 84.21 0.297 0.00015 84.83 5.8805 250 39.7 0.010017 1393.1 3.38 0.1794 1353.3 238.0

Table 1. Properties of the working fluid, water, for 10% loading ratio and non-operating

*x*  (-)

*u*  kJ/kg

*c*  kJ/kg ºC

operating temperature of 250 ºC and the specific volume of 0.0100177 m3/kg.

*H*  kJ/kg

*<sup>f</sup>*, and operating temperature.

with the equation (14),

temperature of the thermosyphon is 250 ºC.

for different loading ratios, from 5% to 45%.

*T*  (ºC)

conditions.

*p*  (bar)

(m3/kg)

investigated interval of the air outlet temperature is 35 ºC to 105 °C. If temperature difference between the entrance and outlet of both air and gas are equal to 70 ºC,

$$T\_{a,out} - T\_{a,in} = T\_{\text{g,in}} - T\_{\text{g,out}} = 70 \text{ °C} \tag{12}$$

so at the exit of the air preheater, outlet temperatures of air and hot gases are equal to *Ta,out* = 95 °C and *Tg,out* = 180 °C. This requires that the ratios of the lengths of the zones must be 42% of *lt*, as shown in Figure 3, and *le* must be 58% of the total length of the thermosyphon according to the equations (7) and (8).

#### **2.2 Operating pressure**

To determine the pressure during operation and non-operation of the thermosyphons, there were considered the loading ratios, and ambient temperature and pressure at which was done the loading process of the thermosyphon. The loading ratio is determined by

$$
\Psi = V\_f / V\_t \tag{13}
$$

After choosing the values of loading ratios, it can be obtained the working fluid volume; in this case is water, and therefore it may also be known the mass, *mf*, of water that fills the thermosyphon. After the loading process of the thermosyphon, this mass of water, *mf*, will fill the total internal volume of the thermosyphon, *Vt*, as steam and liquid, so the specific volume of water can be determined by the expression (14)

$$
\nu\_f = V\_t / m\_f \tag{14}
$$

Upon completion of loading process of the thermosyphon, it is then sealed and exposed again to ambient temperature, therefore, the internal pressure drops to the design parameter called the non-operating pressure. From the tables of thermodynamic properties of water (Incropera & Dewitt, 2006) the value of the non-operating pressure is determined with the values of ambient temperature, *Tamb*, and the specific volume, *f*.

Fig. 3. Length of the condensation zone and total length versus the outlet air temperature, for different outlet temperatures of gases.

investigated interval of the air outlet temperature is 35 ºC to 105 °C. If temperature

so at the exit of the air preheater, outlet temperatures of air and hot gases are equal to *Ta,out* = 95 °C and *Tg,out* = 180 °C. This requires that the ratios of the lengths of the zones must be 42% of *lt*, as shown in Figure 3, and *le* must be 58% of the total length of the thermosyphon

To determine the pressure during operation and non-operation of the thermosyphons, there were considered the loading ratios, and ambient temperature and pressure at which was

After choosing the values of loading ratios, it can be obtained the working fluid volume; in this case is water, and therefore it may also be known the mass, *mf*, of water that fills the thermosyphon. After the loading process of the thermosyphon, this mass of water, *mf*, will fill the total internal volume of the thermosyphon, *Vt*, as steam and liquid, so the specific

Upon completion of loading process of the thermosyphon, it is then sealed and exposed again to ambient temperature, therefore, the internal pressure drops to the design parameter called the non-operating pressure. From the tables of thermodynamic properties of water (Incropera & Dewitt, 2006) the value of the non-operating pressure is determined with the

Fig. 3. Length of the condensation zone and total length versus the outlet air temperature,

done the loading process of the thermosyphon. The loading ratio is determined by

volume of water can be determined by the expression (14)

values of ambient temperature, *Tamb*, and the specific volume,

, ,,, *T TTT a out a in g in g out* 70 ºC (12)

*V V f t* (13)

*f tf V m* (14)

*f*.

difference between the entrance and outlet of both air and gas are equal to 70 ºC,

according to the equations (7) and (8).

**2.2 Operating pressure** 

*lc / lt* [-]

for different outlet temperatures of gases.

The operating pressure of the thermosyphon is also found in the tables of thermodynamic properties of water (Incropera & Dewitt, 2006), the specific volume of water was calculated with the equation (14), *<sup>f</sup>*, and operating temperature.

To calculate the operating pressure of the thermosyphon a parametric analysis was performed, where the variable parameter is the loading ratio. The interval of studied loading ratio is 5% to 45%. The geometric characteristics of the thermosyphon were taken as follows: length of 1 m and a diameter of 2.54 cm. The temperature of 20 °C and a pressure of 1 bar were the ambient conditions. It was considered that the maximum operating temperature of the thermosyphon is 250 ºC.

Fig. 4. T-s diagram of isochoric operation of the thermosyphon for different loading ratios.

Figure 4 shows in a T-s diagram the isochoric process that takes place in the thermosyphon for different loading ratios, from 5% to 45%.

As an example applying the described method a loading ratio of 10% is given, and the value of the specific volume of water that was obtained using equation (14) is 0.0100177 m3/kg. Therefore by exposing the thermosyphon at room temperature of 20 °C, after the loading process, the non-operating pressure is equal to 0.0233 bar. Table 1 shows the thermodynamic properties of water inside the thermosyphon. On the other hand, by bringing the thermosyphon to the operating temperature for an isochoric process, an operating pressure equal to 39.7 bar is achieved. Table 1 shows the properties for the operating temperature of 250 ºC and the specific volume of 0.0100177 m3/kg.


Table 1. Properties of the working fluid, water, for 10% loading ratio and non-operating conditions.

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 105

3 2 Gr *a p eva a g TL* 

From the tables of properties of air, it is possible to get all the properties required to find the values of the parameters above mentioned using the average air temperature, *Tp* (23). This average temperature is obtained from room temperature, *Tamb*, and temperature of the

> 2 *amb op*

*T T*

The developed methodology allows the calculation of the evaporation rate to analytically study the behavior of the flow of water evaporation as a function of ambient conditions that

A parametric analysis was made with the same geometric conditions that the above described analysis, it is considered a thermosyphon with a length of 0.9 m and an inner diameter of 2.15 cm. The ambient conditions were taken as 20 °C and a pressure of 1 atm. The temperature of the thermosyphon during the filling process, *Top*, which takes place during the loading process is 100 °C. The characteristic length of evaporation, *Leva*, is equal to the area divided by the perimeter of the orifice where the steam escapes from the

Figure 5 shows the evaporation mass flow as a function of the temperature for different

Fig. 5. Evaporation mass flow as a function of the ambient temperature and relative

The use of steel and water as a building material and working fluid, respectively, is very attractive in the design of the thermosyphon due to its low cost and high heat transmission (Terdtoon et al. 2001). However, it is well known that the ferrous material is chemically incompatible with water, and a manifestation of this incompatibility is the appearance of

*p*

*T*

occur at the time of the loading process of the thermosyphon.

thermosyphon during the loading process, *Top*.

thermosyphon during the filling process.

humidity.

relative humidity, ranging from 40% to 90%.

**2.4 Control of corrosion in the thermosyphons** 

(22)

(23)

#### **2.3 Evaporation rate**

In the loading procedure developed in this research it is required that part of the working fluid evaporates filling all the internal volume of the device and forcing the non condensable gases to leave from the upper end of the thermosyphon. Thus, in this process a small amount of water escapes, so the initial water mass must be bigger than the desired final mass inside the thermosyphon. Therefore, it is important to determine the time of evaporation of the desired amount of initial fluid. This methodology proposes to know this time by the rate of evaporation or mass flow rate of evaporation from a free water surface (Incropera & Dewitt, 2006), defined by the following equation

$$
\dot{m}\_{\text{eva}} = \overline{h}\_m A\_{\text{eva}} \left( \rho\_{\upsilon, \text{stop}} - \phi \rho\_{\upsilon, \text{satam}} \right) \tag{15}
$$

In the tables of saturated water vapor (Incropera & Dewitt, 2006), it was obtained the saturation density corresponding to room temperature, *ρv,satam*, and the saturation density corresponding to the temperature of the fluid during the loading process, *ρv,satop*.

The area of evaporation surface, *Aeva*, can be obtained from the following expression

$$A\_{\rm eva} = \pi d\_{\rm int}^2 \not\!\!\mathbf{4} \tag{16}$$

To calculate the mass transfer coefficient ( *hm* ) it was used the analogy of heat and mass transfer (Incropera & Dewitt, 2006)

$$
\overline{h}\_m = \overline{h}\_a \Big/ \left(\rho\_a c\_{pa}\right) \left(D\_{a,w}/a\_a\right)^{2/3} \tag{17}
$$

The binary diffusion coefficient is calculated using the following relation

$$D = D\_0 \left( p\_0 / p \right) \left( T / T\_0 \right)^{3/2} \tag{18}$$

To calculate the average coefficient of free convection it was used the definition of Nusselt number, equation (19) (Incropera & Dewitt, 2006)

$$\mathbf{h}\_a = \mathbf{k}\_a \mathbf{N} \mathbf{u} / \mathbf{L}\_{\text{evra}} \tag{19}$$

Where the Nusselt number is given by equation (20) for Rayleigh numbers, Ra ≤ 109 (Incropera & Dewitt, 2006)

$$\text{Nu} = 0.68 + \frac{0.67 \,\text{Ra}^{1/4}}{\left[1 + \left(0.492 \,/\,\text{Pr}\right)^{9/16}\right]^{4/6}} \tag{20}$$

Where the Rayleigh number is defined by equation (21) (Incropera & Dewitt, 2006)

$$\mathbf{Ra} = \mathbf{Gr} \times \mathbf{Pr} \tag{21}$$

Where the Grashof number, Gr, which is the ratio between buoyancy forces and viscous, is defined by the expression (21) (Incropera & Dewitt, 2006)

In the loading procedure developed in this research it is required that part of the working fluid evaporates filling all the internal volume of the device and forcing the non condensable gases to leave from the upper end of the thermosyphon. Thus, in this process a small amount of water escapes, so the initial water mass must be bigger than the desired final mass inside the thermosyphon. Therefore, it is important to determine the time of evaporation of the desired amount of initial fluid. This methodology proposes to know this time by the rate of evaporation or mass flow rate of evaporation from a free water surface

> *m hA eva m eva v satop v satam*

In the tables of saturated water vapor (Incropera & Dewitt, 2006), it was obtained the saturation density corresponding to room temperature, *ρv,satam*, and the saturation density

> <sup>2</sup> <sup>4</sup> *A d eva int*

To calculate the mass transfer coefficient ( *hm* ) it was used the analogy of heat and mass

To calculate the average coefficient of free convection it was used the definition of Nusselt

Where the Nusselt number is given by equation (20) for Rayleigh numbers, Ra ≤ 109

0.67 Ra Nu 0.68

Where the Rayleigh number is defined by equation (21) (Incropera & Dewitt, 2006)

defined by the expression (21) (Incropera & Dewitt, 2006)

1 0.492 /Pr

Where the Grashof number, Gr, which is the ratio between buoyancy forces and viscous, is

1/4

4/6 9/16

*m a a pa a w a hh cD* 

The binary diffusion coefficient is calculated using the following relation

 / ,

  2 3

3 2/ *D D p p TT* 00 0 (18)

*hk L a a eva* Nu (19)

Ra = Gr × Pr (21)

corresponding to the temperature of the fluid during the loading process, *ρv,satop*.

The area of evaporation surface, *Aeva*, can be obtained from the following expression

 

, , (15)

(16)

(17)

(20)

(Incropera & Dewitt, 2006), defined by the following equation

**2.3 Evaporation rate** 

transfer (Incropera & Dewitt, 2006)

(Incropera & Dewitt, 2006)

number, equation (19) (Incropera & Dewitt, 2006)

$$\mathbf{Gr} = \mathbf{g}\mathcal{J}\_a \mathbf{T}\_p \mathbf{L}\_{eva}^3 \Big/ \mathbf{v}\_a^2 \tag{22}$$

From the tables of properties of air, it is possible to get all the properties required to find the values of the parameters above mentioned using the average air temperature, *Tp* (23). This average temperature is obtained from room temperature, *Tamb*, and temperature of the thermosyphon during the loading process, *Top*.

$$T\_p = \frac{\left(T\_{amb} + T\_{op}\right)}{2} \tag{23}$$

The developed methodology allows the calculation of the evaporation rate to analytically study the behavior of the flow of water evaporation as a function of ambient conditions that occur at the time of the loading process of the thermosyphon.

A parametric analysis was made with the same geometric conditions that the above described analysis, it is considered a thermosyphon with a length of 0.9 m and an inner diameter of 2.15 cm. The ambient conditions were taken as 20 °C and a pressure of 1 atm. The temperature of the thermosyphon during the filling process, *Top*, which takes place during the loading process is 100 °C. The characteristic length of evaporation, *Leva*, is equal to the area divided by the perimeter of the orifice where the steam escapes from the thermosyphon during the filling process.

Figure 5 shows the evaporation mass flow as a function of the temperature for different relative humidity, ranging from 40% to 90%.

Fig. 5. Evaporation mass flow as a function of the ambient temperature and relative humidity.

#### **2.4 Control of corrosion in the thermosyphons**

The use of steel and water as a building material and working fluid, respectively, is very attractive in the design of the thermosyphon due to its low cost and high heat transmission (Terdtoon et al. 2001). However, it is well known that the ferrous material is chemically incompatible with water, and a manifestation of this incompatibility is the appearance of

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 107

thermosyphons. This avoids the use of a complex procedure that could cause leakage or

It was also determined that the material of which has to be built the core of the thermosyphon is A-179 carbon steel. This type of material is commercial steel used in the manufacture of heat exchangers and boilers. In order to check if the wall thickness is adequate, i.e., if the tube will withstand the maximal operating pressures, it was used the

> 40 bar 0.0127 m

It was obtained a minimum thickness of 0.341 mm, which is less than the thickness of the commercial tube (1.953 mm) used in the thermosyphons, so it is guaranteed to use it only to the pressures and temperatures planned in this investigation, without the risk of an

The chosen working fluid is distilled water. Distilled water was chosen because of its easy availability, is not dangerous and its thermodynamic and physical properties are well

Table 2 presents the specifications of the thermosyphon designed and implemented in this

Material of the container Carbon Steel A-179. Material of the outer tube and fins Extruded aluminum Working fluid Distilled water

The aim of experimental research is to understand the performance of a thermosyphon when varying the following parameters: amount of working fluid, heat supply and cooling air speed. To achieve this there were designed two experimental facilities for loading the

0.4 1800 bar 0.85 0.4 40 bar

*E p*

known, also allows the use of the thermosyphon in the range of 5 °C to 250 °C.

Total length 900 mm. Evaporator length 525 mm. Adiabatic zone length 25 mm Condenser length 350 mm. Inside diameter of the container 21.5mm Outside diameter of the container 25.4 mm Outer diameter of aluminum tube 27 mm Fin diameter 53.9 mm

Maximum operating pressure 40 bar Maximum operating temperature 250 ºC

working fluid and for testing the efficiency of two-phase thermosyphon.

Table 2. Specifications of the two-phase thermosyphons.

**3. Experimental investigation** 

0.341 mm

(24)

contamination inside the thermosyphon.

*i ext*

*p r <sup>t</sup>* 

*i*

equation (24)

accident.

investigation.

rust. To avoid this effect, there have been used a series of corrosion-inhibiting additives that are added to water, among which is hydrazine hydrate.

For application in thermosyphon tubes, hydrazine hydrate has the following characteristics:


Hydrazine interacts with the iron oxide to form magnetic iron oxide Fe3O4 (magnetite), which is of black color and usually forms in water or steam when there is a deficiency of oxygen and on a layer of Fe2O3 (hematite). This film or barrier that adheres to the metal surface prevents the transport of reactive species of water, or the transport of products outside of this interface, i.e., prevents the flow of electricity. Therefore, the metal-water system is not oxidized or reduced, so the addition of hydrazine can slow and sometimes halt the destruction of the steel.

The addition of hydrazine hydrate to thermosyphon working fluid may have some effect on the heat transfer of the device. Therefore, it was determined experimentally the influence of the amount of hydrazine hydrate on the thermal performance of a thermosyphon. The results of this research are presented in (Carvajal-Mariscal et al., 2011) and it was found that the minimum concentration of hydrazine hydrate, which is necessary to reduce the effects of corrosion, but little changes the thermal performance of the thermosyphon is 50 mg/L.

#### **2.5 Manufacture of the two-phase thermosyphon**

The two-phase thermosyphons used in this investigation were made with A179 carbon steel tubes, with 900 mm long, 25.4 mm external diameter and 21.5 mm inner diameter. According to the calculations using the methodology above mentioned and corrected to standard sizes, was established that the length of the condensation zone should be 40% of the length of the thermosyphon, and the other 60% is the evaporation zone. Because it was chosen the length of the thermosyphon equal to 90 cm, so the evaporation zone has a length of 0.55 m and the length of the condenser is 0.35 m.

Generally, the thermosyphons are built with a lid of the container of the same material at the bottom which is welded to the container. At the top is also welded a cap, which has a concentric orifice where is welded an appendix, which is used to enter the working fluid, and also serves to facilitate the sealing. However, the thermosyphons used in this study were constructed with the following additions: The lower end is fitted with a coupling to use a tapered cap of 3/8 of diameter and with a PTN thread. At the top is installed a needle valve.

Coupling was implemented at the bottom because its diameter allowed the cleaning and loading of the working fluid, the conical cap was chosen to avoid leaks of the working fluid during the testing. The needle type valve was implemented for quick and easy sealing of the

rust. To avoid this effect, there have been used a series of corrosion-inhibiting additives that

For application in thermosyphon tubes, hydrazine hydrate has the following characteristics:

 Pressure and temperature of degradation are higher than those to be taken when operating the thermosyphon in heat recovery processes of medium temperature (up to

Hydrazine interacts with the iron oxide to form magnetic iron oxide Fe3O4 (magnetite), which is of black color and usually forms in water or steam when there is a deficiency of oxygen and on a layer of Fe2O3 (hematite). This film or barrier that adheres to the metal surface prevents the transport of reactive species of water, or the transport of products outside of this interface, i.e., prevents the flow of electricity. Therefore, the metal-water system is not oxidized or reduced, so the addition of hydrazine can slow and sometimes halt

The addition of hydrazine hydrate to thermosyphon working fluid may have some effect on the heat transfer of the device. Therefore, it was determined experimentally the influence of the amount of hydrazine hydrate on the thermal performance of a thermosyphon. The results of this research are presented in (Carvajal-Mariscal et al., 2011) and it was found that the minimum concentration of hydrazine hydrate, which is necessary to reduce the effects of corrosion, but little changes the thermal performance of the thermosyphon is 50 mg/L.

The two-phase thermosyphons used in this investigation were made with A179 carbon steel tubes, with 900 mm long, 25.4 mm external diameter and 21.5 mm inner diameter. According to the calculations using the methodology above mentioned and corrected to standard sizes, was established that the length of the condensation zone should be 40% of the length of the thermosyphon, and the other 60% is the evaporation zone. Because it was chosen the length of the thermosyphon equal to 90 cm, so the evaporation zone has a length

Generally, the thermosyphons are built with a lid of the container of the same material at the bottom which is welded to the container. At the top is also welded a cap, which has a concentric orifice where is welded an appendix, which is used to enter the working fluid, and also serves to facilitate the sealing. However, the thermosyphons used in this study were constructed with the following additions: The lower end is fitted with a coupling to use a tapered cap of 3/8 of diameter and with a PTN thread. At the top is installed a needle

Coupling was implemented at the bottom because its diameter allowed the cleaning and loading of the working fluid, the conical cap was chosen to avoid leaks of the working fluid during the testing. The needle type valve was implemented for quick and easy sealing of the

 It is dosed at a ratio close to 1:1 for the concentration of dissolved oxygen in water. In the form of hydrazine hydrate is considerably reduced the toxicity of pure hydrazine.

are added to water, among which is hydrazine hydrate.

Acts as a passivator by forming a protective layer.

**2.5 Manufacture of the two-phase thermosyphon** 

of 0.55 m and the length of the condenser is 0.35 m.

Generates no solid waste.

the destruction of the steel.

300 ºC).

valve.

thermosyphons. This avoids the use of a complex procedure that could cause leakage or contamination inside the thermosyphon.

It was also determined that the material of which has to be built the core of the thermosyphon is A-179 carbon steel. This type of material is commercial steel used in the manufacture of heat exchangers and boilers. In order to check if the wall thickness is adequate, i.e., if the tube will withstand the maximal operating pressures, it was used the equation (24)

$$t = \frac{p\_i r\_{ext}}{\zeta E + 0.4 p\_i} = \frac{(40 \text{ bar})(0.0127 \text{ m})}{\left[ (1800 \text{ bar})0.85 + 0.4(40 \text{ bar}) \right]} = 0.341 \text{ mm} \tag{24}$$

It was obtained a minimum thickness of 0.341 mm, which is less than the thickness of the commercial tube (1.953 mm) used in the thermosyphons, so it is guaranteed to use it only to the pressures and temperatures planned in this investigation, without the risk of an accident.

The chosen working fluid is distilled water. Distilled water was chosen because of its easy availability, is not dangerous and its thermodynamic and physical properties are well known, also allows the use of the thermosyphon in the range of 5 °C to 250 °C.

Table 2 presents the specifications of the thermosyphon designed and implemented in this investigation.


Table 2. Specifications of the two-phase thermosyphons.

#### **3. Experimental investigation**

The aim of experimental research is to understand the performance of a thermosyphon when varying the following parameters: amount of working fluid, heat supply and cooling air speed. To achieve this there were designed two experimental facilities for loading the working fluid and for testing the efficiency of two-phase thermosyphon.

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 109

scale. The electrical resistances arrangement of the evaporator was isolated with mineral

With this method of loading the inner volume of the thermosyphon fills only with working fluid as vapor or liquid. This ensures that there are no non-condensable gases accumulated in it. The time that takes to perform this procedure depends on the excess fluid that has to be removed, in that way that at the end of the procedure only the desired amount of working

Figure 7 shows a schematic of the experimental setup. It consists of a thermosyphon installed in a wind tunnel. On the section of the evaporator are placed, three electrical resistances of 250 W and 110 V AC. The power source is the same as in the previous

Heat is removed from condenser section by a flow of cooling air. The test section is located on the suction side of the tunnel; the air is forced by an axial ventilator with a 1.12 kW motor. To measure the air temperatures at the inlet and outlet of the test section type "K" thermocouples were used. Moreover, seven K-type thermocouples were placed on the outer surface of the tube in order to measure the temperature distribution along the thermosyphon. The record of these temperatures was achieved by a Cole Parmer data

Fig. 7. Schematic of the experimental setup for thermosyphon performance testing.

sections of the fins were removed to provide a better placement.

Thermocouples were placed from the bottom of the evaporator in the positions of 5, 18, 35.5, 52, 65, 76.5 and 88 cm. These thermocouples were attached firmly to the surface and small

The supply of heat to the evaporator section is calculated using the following relationship:

wool and a stainless steel shell, in order to minimize heat losses.

**3.2 Testing facility for thermosyphon performance** 

fluid stays in the thermosyphon.

installation.

acquisition system.
