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Tuesday, 10 August 2010

HEAT PIPE

INTRODUCTION

A heat pipe is a device that efficiently transports thermal energy from its one point to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors. A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid. The length of the heat pipe can be divided into three parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container.

Basic components of a heat pipe

The basic components of a heat pipe are
1. The container
2. The working fluid
3. The wick or capillary structure

Container

The function of the container is to isolate the working fluid from the outside environment. It has to be there for leak proof, maintain the pressure differential across the walls, and enable transfer of thermal energy to take place from and into the working fluid.

The prime requirements are:
1. Compatibility (Both with working fluid and External environment)
2. Porosity
3. Wettability
4. Ease of fabrication including welding, machinability and ductility
5. Thermal conductivity
6. Strength to weight ratio

The working fluid

The first consideration in the identification of the working fluid is the operating vapor temperature range. Within the approximate temperature band, several possible working fluids may exist and a variety of characteristics must be examined in order to determine the most acceptable of these fluids for the application considered.

The prime requirements are:
7. Compatibility with wick and wall materials
8. Good thermal stability
9. Wettability of wick and wall materials
10. High latent heat
11. High thermal conductivity
12. Low liquid and vapor viscosities
13. High surface tension

Wick

The wick structure in a heat pipe facilitates liquid return from the evaporator from the condenser. The main purposes of wick are to generate the capillary pressure, and to distribute the liquid around the evaporator section of heat pipe. The commonly used wick structure is a wrapped screen wick.

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