**1. Introduction**

96 Heat Exchangers – Basics Design Applications

Wan Daud, W.R. (2008). Fluidized Bed Dryers-Recent Advances. *Advanced Powder* 

Due to high fuel prices, it has become necessary to investigate new methods for saving and more efficient use of energy, emphasizing the use of energy remaining in the waste gases of combustion equipment. For this reason, in the last five decades there has been an important technological development in heat transfer equipment, to promote changes in configuration and applying heat transfer systems with high effectiveness. One example is the use of twophase thermosyphons (Azada et al., 1985; Faghri, 1995; Gershuni et al., 2004; Noie, 2005; Peterson, 1994; Reay, 1981).

A two-phase thermosyphon is a device that is used for heat transfer; this process occurs inside it as a cycle of evaporation and condensation of a working fluid (Faghri, 1995; Peterson, 1994). This device is easily constructed, has no moving parts inside and works individually. The two-phase thermosyphon consists of: condensation, evaporation and adiabatic zones (Figure 1). Operation starts when heat is supplied to the evaporator zone, so a portion of the fluid evaporates, taking the latent heat of evaporation inside the tube up to the condenser section. In this last section, vapor condenses and transfers its latent heat of condensation to the surroundings. The condensate runs down as a film on the inner wall of the tube with the aid of gravity.

There have been conducted several investigations in the field of thermosyphon design and development. The authors of reference (Park et al., 2002) studied the heat transfer characteristics depending on the amount of working fluid and when the operation limits occur. The two-phase element was made of copper and as working fluid FC-72 (C14 F14) was used. The thermosyphon was subjected to a heat supply in the range of 50-600 W and with 10-70% load rate. For the convection coefficients in the condenser and in the evaporator, the authors used the theory of Nusselt and Roshenow respectively. They found that the operation limits manifest in different forms depending on the loading rate of the fluid. For small loading rates (Ψ = 10%) the drying limit occurs in the evaporator, while for high loading rates (Ψ = 50%) is the flooding limit that appears. In the first case, evaporator temperature increases from the evaporator bottom; in the second case the evaporator temperature increases at the top of the evaporator. These conclusions were made by observing the temperature distribution along the thermosyphon. Moreover, (Zuo & Faghri, 2002) conducted an analytical and experimental research on the thermodynamic behavior of the working fluid in a thermosyphon and a heat pipe, using a temperature-entropy diagram.

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 99

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

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

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

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

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

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

> 

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*:

*mc T T mc T T a pa a out a in g pg g in g out* , , ,, (1)

*a a a pa a out a in g g g pg g in g out Avc T T Avc T T* , , , , (2)

*A la a c* (3)

methodologies water was considered as the working fluid.

to the thermosyphon, which was implemented in this research.

balance for steady state conditions may be expressed as:

Applying the continuity equation gives the expression (2).

operating pressure.

**the thermosyphon** 

the thermosyphons.

Fig. 1. Two-phase thermosyphon.

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

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 presented the results of this investigation.
