**4. Heat pipe heat exchanger (HPHX)**

A wide range of research investigations had been done to explain and understand the thermal performance and operational behavior of heat pipes technology when it is used as a heat exchanger for recovering the heat that is wasted in HVAC and other engineering applications. The literature studied many factors that affected the operating behavior and thermal performance of heat pipe heat exchanger (HPHX) like working fluid, heat input, inlet air temperature, and velocity, pipes geometry, and arrangement style.

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*Heat Pipes Heat Exchanger for HVAC Applications DOI: http://dx.doi.org/10.5772/intechopen.95530*

HPHX.

enhancement [17].

**4.1 Review of heat pipe heat exchanger (HPHX)**

tion was heated by air with temperature (100–250) <sup>o</sup>

This part will review the literature that investigated the recovering heat by using

The heat pipe heat exchanger with staggered and inline arrays arrangement was investigated [15]. The heat pipe included sections with a height of 150 mm for the evaporator and condenser and 50 mm for the adiabatic zone. The heat pipe was fabricated from steel with a 20 mm inner diameter. They used 48 heat pipes put in an arrangement of eight rows. The results showed the effect of tube configuration, and the rate of heat transfer will increase by using a staggered configuration. **Noie** [16] study experimentally and theoretically the influence of heat input, air temperature, and velocity on the performance of thermosyphon HPHX under steady-state operating conditions. The heat exchanger is comprised of six rows of 90 finned thermosyphon. The heat pipe had a length of 600 mm for the evaporator and condenser section and 100 mm for the adiabatic section and filled partially with water by 60% (from evaporator volume) filling ratio. The evaporator sec-

and the air was used to cool the condenser section at 25°C. The effectiveness-NTU method was used in the simulation program was developed to predict the outlet temperature. The results showed that the experimental and theoretical results were close. The results suggested avoiding the equal air velocity at the evaporator and

By using a heat exchanger with eight rows of thermosyphon in an HVAC system,

Yau (2007) investigated experimentally the effect of the inlet air conditions on the sensible heat ratio (SHR). The results concluded that the SHR decreased by using HPHX with the HVAC system. These meant that the cooling coil capability for removing moisture with HPHX was enhancing, and it strongly recommended to install heat pipe heat exchangers with an HVAC system for moisture removal

El-Baky and Mohamed [18] examined a heat pipe heat exchanger to recover heat by connecting it with ducts of inlet and return air streams. They investigated

liquid back from the condenser to the evaporator section, the heat pipes have 4 layers of the brass screen with a 0.125 mm wire diameter. The findings manifest that the effectiveness will be high as the inlet air temperature was near to the operating temperature of working fluid inside the heat pipe. The results revealed that increasing the inlet air temperature led to enhance the heat transfer rate and effectiveness

A loop type of heat pipe (LHP) was integrated into the window type airconditioning system [19] to introduce and perform a possible reheat process in the system. This configuration presented an enhancement in the COP of the system with a reduction in energy consumption. The author concluded that the loop heat pipe could de enhance the thermal performance for the large capacity units due to

Yau and Ahmadzadehtalatapeh [20] carry out an experimental investigation to study the thermal performance of a horizontal heat pipe heat exchanger. They used two rows of copper heat pipe filled partially with R134a as working fluid. They examined the influence of the inlet air velocity and variation of inlet air temperature (27–35°C). The mathematical simulation was used to predict the thermal resistance for one heat pipe and then compared with experimental findings that gave a good agreement between the experimental and theoretical results with increasing air velocity. The findings deduced that the sensible effectiveness for heat

condenser zones duo to the minimum effectiveness of heat pipe [16].

the influence of flow rate ratio, and inlet air temperature (32–40) <sup>o</sup>

for the evaporator and condenser section.

the double effect LHP in the system.

pipe decreased with increasing air velocity.

C & velocity (0.5–5.5) m/s,

C. To help the

*Heat Transfer - Design, Experimentation and Applications*

**3.2 Advantages of oscillating heat pipe (OHP)**

of the working fluid [11].

condenser region.

traditional heat pipe:

motion.

not interfere inside the OHP.

effectively produce thin films.

**4. Heat pipe heat exchanger (HPHX)**

ity, pipes geometry, and arrangement style.

change (mass transfer).

increases.

At nucleation locations, dispersed bubbles generate and coalesce to grow in size as bubbles are continuously heated. The formation and growth of dispersed bubbles happen continuously when supplying the heat at the evaporator zone because the temperature of the inner wall of the tube was higher than the saturated temperature

As the condenser cools, the pressure reduces, and a condensation process of bubbles accrues. This process pumps the working fluid again to the evaporator region and continuous between both sections and resulting in oscillating motion. The cycle gets completed in this way, and the same cycle is repeated again and again for heat transportation from the evaporator region to the

OHP is a highly effective thermal conductivity, thus resulting in a high level of temperature uniformity from the evaporator to the condenser. The operation of OHP can start at a low temperature to a high temperature, where the liquid and vapor phases can coexist. The manufacturing of OHP is almost by any shape. The OHP has some advantages and unique operating characteristics in comparison to a

1.Part of the heat input at the evaporator zone of OHP will convert to the kinetic energy of the working fluid to maintain the oscillating/pulsating

2.The liquid and vapor phases move in the same direction, and both phases do

3.The evaporating and condensing heat transfer is significantly enhanced because liquid plugs that thermally driven in capillary tubes or channels that

4.Oscillating/pulsating flows of working fluid inside capillary tubes/channels significantly improve the heat transfer by forced convection and the phase

5.The heat transferability of the OHP dramatically grows as the input power

A wide range of research investigations had been done to explain and understand the thermal performance and operational behavior of heat pipes technology when it is used as a heat exchanger for recovering the heat that is wasted in HVAC and other engineering applications. The literature studied many factors that affected the operating behavior and thermal performance of heat pipe heat exchanger (HPHX) like working fluid, heat input, inlet air temperature, and veloc-

6.Wickless formations and low manufacturing expense.

7.The design of OHP can be independent of gravity [14].

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