Applied Thermal Engineering 30 (2010) 77–84

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate /apthermeng

Review

A review on the application of horizontal heat pipe heat exchangers in airconditioning systems in the tropics

Y.H. Yau, M. Ahmadzadehtalatapeh *

Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 May 2009Accepted 21 July 2009Available online 26 July 2009

Keywords:Air conditioning systemsDehumidification enhancementHorizontal heat pipe heat exchangersTropical climates

1359-4311/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.applthermaleng.2009.07.011

* Corresponding author. Tel.: +60 3 79675210; fax:E-mail address: m_ahmadzadeh56@yahoo.com (M

A literature review on the application of horizontal heat pipe heat exchangers for air conditioning in trop-ical climates was conducted. This paper focused on the energy saving and dehumidification enhancementaspects of horizontal heat pipe heat exchangers. The related papers were grouped into three main cate-gories and a summary of experimental and theoretical studies was made. It was revealed that althoughthere are a number of valuable researches on the impact of heat pipe heat exchangers on the energy con-sumption and dehumidification enhancement of air conditioning systems in the tropics, but only limitedresearch work on the application of horizontal configuration heat pipe heat exchangers in air condition-ing systems has been carried out in these regions. Therefore, it needs more research to deepen the under-standing of the benefits of this heat recovery device in the air conditioning systems. On the basis ofresults obtained from the reviewed research studies, the application of horizontal heat pipe heatexchangers in terms of energy saving, dehumidification enhancement and condensate drainage is recom-mended for the tropics.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 772. Application of horizontal configuration HPHXs for energy recovery purposes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783. Application of HPHXs in tropical, sub-tropical, hot and humid climates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794. Studies on the influence of inclination angle on the HPHXs performance in tropical HVAC systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

1. Introduction

As an efficient heat exchanger, heat pipe heat exchangers(HPHXs) are playing a considerable role in different fields includingair conditioning systems. A heat pipe heat exchanger is a heattransfer device in which the latent heat of vaporization is utilizedto transfer heat over a long distance with a corresponding smalltemperature difference. It consists of individual closed tubes thatare filled with a proper working fluid. In operation, the workingfluid evaporates at the evaporation section and condenses overthe other end of the tube as shown in Fig. 1. The condensed fluidreturns back to the evaporator section through the capillary actionof the wick or gravitational force in the thermosyphon heat pipes.

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+60 3 79675317.. Ahmadzadehtalatapeh).

The advantage of using a heat pipe over the other ordinarymethods to heat transfer is that a heat pipe can have an extremelyhigh thermal conductance in steady state operation. Hence, it cantransfer a high amount of heat over a relatively long length witha comparatively small temperature difference. A heat pipe withliquid metallic working fluid can have thermal conductance muchmore than the best solid metallic conductor such as silver and cop-per. Moreover, simplicity of design and manufacturing, small tem-perature drops, wide temperature application range and the abilityto control and transport high heat rates at various temperature lev-els are the unique characteristics of heat pipes.

In recent times, HPHXs in different forms and designs havefound a wide variety of application including the heating, ventila-tion and air conditioning (HVAC) systems. There is considerable lit-erature on the application of HPHXs, but only literatures on theapplication of horizontal HPHXs in air conditioning systems in

Nomenclature

AC air conditionerET effective temperatureHP heat pipeHVAC heating, ventilation and air conditioningHPHX heat pipe heat exchanger

PMV predicted mean voteSIECHP semi-indirect evaporative cooler and heat pipeSHR sensible heat ratioTRNSYS transient systems simulation program

78 Y.H. Yau, M. Ahmadzadehtalatapeh / Applied Thermal Engineering 30 (2010) 77–84

tropical climates are reviewed in the present paper. This literaturesurvey has been separated into three sections, which focused inturn on the application of horizontal configuration HPHXs forenergy recovery purposes, application of HPHXs in tropical, sub-tropical, hot and humid climates and the studies on the influenceof inclination angle on the HPHXs performance in tropical HVACsystems.

2. Application of horizontal configuration HPHXs for energyrecovery purposes

Since 1970, HPHXs have been extensively applied in manyindustries including HVAC systems. There are several types of heatexchangers available for heat recovery in the HVAC systems, butHPHXs have a number of significant advantages over the other typeof heat exchangers such as lower initial cost, lower maintenancecosts, and lower operating costs. The application of HPHXs in coldclimates was widely recognized and by improving heat pipes, it ispossible to use heat pipes in different climates such as in the trop-ical climates.

One of the most interesting functions of HPHXs is to increasethe dehumidification capacity of the conventional air conditioningsystems. In a conventional air conditioning system, the humidity iscontrolled by cooling the supply air stream below its dew pointtemperature. The cold air is then reheated to a temperature thatis suitable for the conditioned space. An experimental investigationwas carried out by McFarland et al. [1] to determine the effect of aheat pipe on the performance of a conventional residential air con-ditioning system as illustrated in Fig. 2. In this study, the influenceof heat pipe on the moisture removal, the amount of auxiliaryreheat required to maintain the room conditions, and the latent en-ergy efficiency ratio of the air conditioning system were examined.The system was operated at three configurations: with the heatpipe installed, a conventional system without the heat pipe andthe air flow subject only to typical restrictions, and a damperedsystem with the heat pipe removed but dampering added to returnthe airflow to the same value as when the heat pipe was installed.It was demonstrated that for the nominal room conditions of 22� Cand 50% relative humidity, the heat pipe increased the moisture

Fig. 1. Heat pipe.

removal by 62%, decreased the amount of reheat energy requiredby 20%, and increased the latent energy efficiency ratio by 90%.He declared that the application of HPHX in the conventional airconditioning system could be an efficient technique to controlthe humidity and reduce the amount of reheat energy required.

Abd El-Baky and Mohamed [2] also stated that by the applica-tion of a HPHX between two streams of fresh and return air in anair conditioning system, the incoming fresh air could be cooleddown. Ratios of mass flow rate between return and fresh air of 1,1.5, and 2.3 were tested to validate the heat transfer and the tem-perature change of fresh air as shown in Fig. 3. During the tests,fresh air inlet temperature was controlled in the range of 32–40� C, while the inlet return air temperature was kept constant atabout 26� C. The optimum effectiveness evaluated in accordanceto thermo-economical optimization method found in Soylemez[3] and compared with the experimental data. According to theexperimental results, the temperature changes of fresh and returnair were increased with the increase of fresh air inlet temperature.The effectiveness and heat transfer of both the evaporator and con-denser sections were also increased up to 48%, while the inlet freshair temperature was increased up to 40� C. Furthermore, theenthalpy ratio between the heat recovery and conventional airmixing was increased to about 85% with the increase of fresh airinlet temperature. Optimum effectiveness was obtained at freshair inlet temperature near the fluid operating temperature of theheat pipes.

As a heat exchanger, the horizontal HPHXs were also used innaturally ventilated buildings. For example, Riffat and Gan [4] ex-plored the effectiveness of HPHXs for the naturally ventilatedbuildings. In this research, the performance of three types of heatpipe heat recovery units was tested in a two-zone chamber witha horizontal partition. The first heat recovery consisted of a bankof seven externally finned heat pipes, the second had cylindricalspine fins and the third was made of 2-rows of staggered heatpipes. The CFD modeling was used for pressure loss characteristicsof the units. According to the experimental results, the air velocityhad a significant influence on the effectiveness of heat pipe heatrecovery units. It was also found that at the same velocity, the heat

Fig. 2. Cross section of the air conditioning system.

Fig. 3. Test rig (horizontal configuration HPHX).

Y.H. Yau, M. Ahmadzadehtalatapeh / Applied Thermal Engineering 30 (2010) 77–84 79

recovery was between 16% and 17% more efficient using two banksof heat pipes with plain fins than using one bank. Based on the CFDmodeling results, at the velocity of 1 m/s, the predictive pressureloss coefficient for a 2-row in line six pipes was 3.3 compared with4.2 for the staggered pipes and 3.7 for in line seven smaller heatpipes. It was recommended that in naturally ventilated low risebuildings without the wind effect, the design mean air velocityshould be less than 1 m/s. In a similar study, Shao and Riffat [5]investigated the flow loss caused by a HPHX in a natural ventila-tion stack. Experimental results showed that for a heat recoveryefficiency of 50% and stack flow velocity of 0.5 m/s, the pressureloss across an existing type of heat pipe was about 1 Pascal. A com-putational fluid dynamic simulation was used for the investigationof pressure and flow loss that caused by the HPHX. Simulation out-comes showed that the heat pipes located at the bottom of thestack produced greater insertion flow loss than those located atthe top. Moreover, it was observed that the temperature of heatpipe has a small effect on the flow loss performance of the heatexchangers. In another study, Riffat and Zhu [6] introduced a math-ematical model for a new technique of indirect evaporative coolerusing porous ceramic and horizontal configuration heat pipe. Thesystem had two separate air passages, one for indoor air (air pas-sage 1) and another for outdoor air (air passage 2). In this system,the heat applied to one end of the pipe (evaporation section) inpassage 1 makes the liquid working fluid evaporates and theresulting vapor travels to the cooled end (condenser, or the cera-mic container), where it condenses as illustrated in Fig. 4. In orderto develop the mathematical model, the vapor flow inside the heatpipe was assumed as incompressible and laminar due to the lowvelocity. According to the results, a high cooling capacity couldbe achieved under the conditions of dry and windy weather. More-over, the indoor air velocity should be set at about 0.6 m/s toachieve high efficiency. It was also recommended that to improvethe cooler performance, the conductivity between the heat pipecondenser and ceramic container should be increased.

Fig. 4. Diagram of the indirect evaporative cooler.

Saman [7] studied the possible application of a HPHX for indi-rect evaporator cooling with heat recovery for the fresh air pre-heating. In this study, the thermal performance of a HPHXconsisting of 48 wickless heat pipes arranged in 6-row was tested.The dry bulb temperature and relative humidity of both air streamswere controlled in a test chamber and monitored before and afterthe HPHX. The wetting arrangement of the condenser section, flowrate of the two flows, original temperature of the main flow, andthe inclination angle of the wickless heat pipes were consideredin this study. The findings indicated that the cooling performancefactor in the range of 0.55–0.84 increased to the range of 0.65–0.88 with the increase of the evaporator inlet temperature from27� C to 41.5� C. In terms of effectiveness, it was improved fromthe range of 0.25–0.66 to the range of 0.27–0.83 with the increaseof the evaporator inlet temperature from 27� C to 41.5� C. This indi-cated that the application of HPHX for this purpose decreased thefresh air temperature by a number of degrees below the tempera-ture drop achieved when using dry air alone.

In a separate research, Martin et al. [8] examined the thermalcomfort provided by a combined recovery equipment (SIECHP),consisting of a ceramic semi-indirect evaporative cooler (SIEC)and a heat pipe to energy recovery at low temperature in an airconditioning system. For this aim, Fanger’s predicted mean vote(PMV), draught rate, and vertical air temperature difference wereused as the thermal comfort criteria. A complete factorial designwas developed to analyze the total heat recovery by the SIECHPsystem in heating mode and cooling mode. The contribution of air-flow and temperature in heating mode and cooling mode was ana-lyzed. It was found that the contribution factor of airflow has a lowvalue in the heating mode whose percentage was close to 3%, whileits contribute in cooling mode was about 18%. In contrast with theflow rate, the contribution of temperature in the heating mode wasthe highest with about 95% and for the cooling mode was 80%.Based on the experimental results, the SIECHP system allowed apracticable energy exchange between the supply air stream andthe return air stream, improving the operation of the air condition-ing system.

In another study, Martinez et al. [9] designed a mixed energyrecovery system consisting of two heat pipes and indirect evapora-tive recuperators for air conditioning. The energy characterizationof the mixed energy recovery system was performed by means ofthe experimental design techniques. As a main result, it was foundthat by application of the mixed energy recovery system in the air–air conditioning installations consisting of two heat pipes and indi-rect evaporative systems, part of the energy from the return airflow could be recovered, thus improving energy efficiency andreducing environmental impact.

3. Application of HPHXs in tropical, sub-tropical, hot and humidclimates

In line with energy management and building design consider-ations [10–19], the application of HPHXs in order to control thetemperature and humidity level in conditioned spaces in tropicalclimates have been studied by a number of researches. Two novelapplications of HPHX in HVAC systems operating in Tampa, Floridawere reported by Beckwith [20]. It was observed that by the appli-cation of a HPHX, the moisture removal capability was increasedand the operation costs significantly were lowered. Moreover, thevariation of one of the systems was applied to save power by pro-viding the heat recovery between exhaust and make-up air flow ofbuilding and for maintaining comfort by providing free terminalreheat for the supply air flow in indirect zones within a building.The controlled wrap-around HPHX pre-cooled the air andenhanced the moisture removal capability of the cooling coil. A

Fig. 6. Configuration of the air conditioning system with a two-phase heat recoveryloop.

80 Y.H. Yau, M. Ahmadzadehtalatapeh / Applied Thermal Engineering 30 (2010) 77–84

schematic of the wrap-around HPHX is shown in Fig. 5. At the con-denser side, the heat was transferred to the overcooled air, leavingthe cooling coil which was operated at 35.2 kW of cooling load and3.68 g/s of moisture removal without the HPHX. With the HPHX in-stalled, the cooling coil operated at 42.6 kW of cooling load and5.25 g/s of moisture removal. The HPHX supplied 7.1 kW of freereheat and both cooling and reheat power consumptions were de-creased in comparison to the conventional reheat system. Thesame principle, i.e., using a HPHX for dehumidification enhance-ment in an air conditioning system was studied by Yang [21].The return air was pre-cooled in the HPHX and the cooling capabil-ity of the original air conditioning system was redistributed and asa result the latent cooling capacity of the system was improved.The mathematical modeling of the HPHX was developed to deter-mine the pre-cooling degree, required for particular weather andHVAC load conditions. It was pointed out that the sensible heatratio was reduced from 0.65 to 0.6. Furthermore, the experimentalresults showed that the moisture removal capability of the systemwas increased about 8.4%. Similarly, dehumidifying characteristicof HPHXs in humid climates was investigated by Abtahi et al.[22]. By using the HPHX between the warm return air and coldsupply air, heat recovery was achieved with supply air reheatand return air pre-cooled. Pre-cooling lowered the sensible coolingfraction and therefore, improved the moisture removal capacity ofthe system. An analytical overview of the HPHX was also explainedin this study. The analysis involved thermal modeling of the wholesystem and also individual components such as the fin-tubeassembly, the pipe-wick interface and the fluid loop conductance.It was recommended that boiling mechanism and heat transfercoefficients of the air side were the main factors affecting the per-formance. The fin efficiency for a high heat transfer coefficient hap-pening in liquid film falling cases were in the range of 97–98%. Inaddition, for nucleate boiling in the range of 1000–3000 w/m2 k,the fin efficiency decreased significantly to less than 39%.

Mathur [23] analyzed the performance enhancement of anexisting 5-ton (17.7 kW) air conditioning system with an air-to-air two-phase natural circulation heat recovery loop heat exchan-ger as indicated in Fig. 6. A simulation program was developed topredict the performance of two-phase natural circulation heatrecovery loop with an air stream that could be condensed. The per-formance of the system was simulated for hot and humid climateswhich were typical of Southeastern of the United States (summerdesign data for Gainesville, Florida). According to the simulationresults, by retrofitting the air conditioning system with the two-phase heat recovery loop, the operational saving was $933.6/yearand $2334/year with and without considering the peak demand,respectively. It was found that the two-phase heat recovery loopwould pay for itself in less than a year. Mathur [24] also attemptedto enhance the performance of the same system by employing twotechnologies including a two-phase natural circulation heat recov-ery loop and an evaporatively cooled condenser. A computer pro-

Fig. 5. Wrap-around HPHX.

gram capable of simulating the performance of the airconditioning system with the above two technologies was devel-oped and simulated for hot and humid climates, which were typi-cal of Southeastern of United State. The results showed that byretrofitting the existing 5-ton air conditioning system with thetwo-phase natural circulation heat recovery loop and the evapo-ratively cooled condenser, the system performance was increasedup to 84.4%. According to this investigation, a combination of theabove two retrofits would pay for itself in 0.71 year. Wan et al.[25] also examined the effect of a loop heat pipe air handling coilon the energy consumption in a central air conditioning systemwith return air for an office building. Based on the results, the airconditioning system installed with HPHX could save cooling andreheating energy. In the temperature range of 22–26� C indoordesign and 50% relative humidity, the rate of energy saving inthe office building was 23.5–25.7% for cooling load and 38.1–40.9% for total energy consumption. The rate of energy saving ofthe heat pipe air conditioning system was increased with the in-crease of the indoor design temperature and decrease of indoor rel-ative humidity. The study demonstrated that by employing a HPHXin an air conditioning system, the energy consumption could besignificantly reduced and the indoor thermal comfort and air qual-ity also could be improved.

The concept of indirect evaporative cooling employed with anair-to-air HPHX system to increase the pre-cooling effectivenessof the heat exchanger was investigated by Mathur [26]. Severaltypes of evaporative cooling systems which may be used withHPHXs were investigated. Based on the findings, the cooling andpower saving would be 38% and 43%, respectively. Moreover, anannual peak demand saving of $2444 would be achieved if theindirect evaporative system was used with the HPHX. The sameconcept was utilized with the application of direct–indirect evapo-rative cooling system in conjunction with the HPHX [27]. The sys-tem with and without evaporative cooling was simulated and theresults showed that by the application of an evaporative cooling,the pre-cooling of the air was improved, since the supply air tem-perature has been lowered to 19.2� C from 28.2� C. Consequently,saving about 36% and 37% in cooling tonnage and power costs,respectively and $3949 per year at peak demand could beachieved.

A HPHX was tested for dehumidification enhancement and en-ergy saving for an air conditioning system in the sub-tropical Flor-ida climate [28]. By retrofitting the system with the installation ofa HPHX between the warm return air and cold supply air, the heattransferred from the evaporator side to the condenser side. There-fore, the return air was pre-cooled at the evaporator and movednear to the dew point temperature. Consequently, the dehumidifi-cation capability of the system was improved from 22% to 42% forthe inlet temperature of 27� C and the relative humidity of 50%. Interms of energy saving, the results revealed the average saving of75% over the 1985 to 1986. In another research in sub-tropical

Fig. 7. The thermosyphon heat exchanger test rig.

Fig. 8. Simple diagram for a HPHX installed with an air conditioning system.

Y.H. Yau, M. Ahmadzadehtalatapeh / Applied Thermal Engineering 30 (2010) 77–84 81

climate, two methods to lower the energy consumption and peakdemand in the buildings that needed to be maintained at 50% rel-ative humidity and below were investigated at an art museum inSt. Petersburg, Florida [29]. The first method applied the alterna-tive indoor fan cycling strategies and the second method employedthe HPHX in the system. By applying both methods, the moistureremoval capability of the current HVAC system was improvedand significant amount of energy was saved. All indoor fans ini-tially operated continuously in the most business buildings regard-less of the compressor mode. When the compressor turned off, themoisture from the chilled water coil and drain pad was moved intothe supply air because of the contact air flow. Indoor fan controlwas then switched to the AUTO mode, when the fans turned onand off with compressor operation. For the temperature controlledzones inside the museum, the AUTO fan mode improved the stabil-ity of humidity level in a zone in which humidity control wasexisted. With this method, 12% decrease in the annual energy con-sumption was achieved.

In line with the temperature considerations in conditionedspaces, the comfort control also has been explored by a numberof researches. For instance, the impact of comfort control on theair conditioner energy consumption in a humid climate was exam-ined by Henderson et al. [30]. In this study, the impact of control-ling an air conditioning (AC) system to maintain constant comfortinstead of constant temperature was explored. Three differentindices of thermal comfort were employed in the analysis: neweffective temperature (ET*) from Gagge et al. [31], predicted meanvote (PMV) from Fanger [32], and a modified PMV (PMV*) fromGagge et al. [33]. A building simulation model was used to simulatethe impact of comfort control on energy use in a typical residencein Miami (as the representative of a humid coastal climate) andAtlanta (as the representative of a warmer, humid inland climate)using three different AC systems: a high sensible heat ratio (SHR)AC, a medium-SHR AC, and a heat-pipe-assisted AC. Under normaltemperature control, the heat pipe AC (with low SHR) used 14.6%more energy, while the high-SHR AC used 3.1% less than the con-ventional, medium-SHR AC. By considering the impact of comfortcontrol, the power cost of adding heat pipes was reduced from14.6% to 5.5% (using PMV*).

Because of simplicity in design and manufacturing of thermosy-phon heat pipes (gravity-assisted heat pipes), most of the research-ers employed this type of HPHX in their studies. For example, a3-row thermosyphon HPHX was studied using the Hilton Air-Con-ditioning laboratory Unit in RMIT [34] as illustrated in Fig. 7. Thetest conditions in the research could be considered as the represen-tative of a humid area in Australia in summer. Three aspects ofHPHX, including temperature effectiveness, capability of energyrecovery and relative humidity control of supplied air was consid-ered in this study. According to experimental results, the coolingcapability for the system was improved by 20–32.7%. It was alsofound that where air supply was needed below 70% relativehumidity, the condenser of the HPHX could be used as a reheaterto replace the conventional reheating coil to control relativehumidity. On the basis of the results, the application of this typeof heat exchanger instead of conventional reheat coils resultingin energy saving and enhancing the cooling capability of the cool-ing coils was suggested.

The effectiveness of HPHXs was studied in the naturally venti-lated tropical buildings [35]. The thermal performance of a wick-less thermosyphon HPHX was simulated using a computerprogram and the temperature distribution across the HPHX, itsthermal performance, and overall effectiveness of HPHX were con-sidered in the research. Yau and Tucker [36] also studied the over-all effectiveness of a wet 6-row wickless HPHX for a tropicalbuilding HVAC system using a computer simulation program. Theeffects of inclination angle and inlet evaporator relative humidity

on the heat transfer coefficient for the HPHX were determined byusing the TRANSYS software.

The impact of parameters such as air conditioner cooling coiland the heat pipe physical characteristics as well as the conditionsof the air entering and leaving the system on the performance andenergy saving of a HVAC system installed with a HPHX was re-ported by Budaiwi and Abdou [37]. A simple mathematical modeldescribing the overall system performance at various entering andleaving air conditions incorporating the parameters of heat pipeand air conditioner was formulated for this study. Proper selectionof both the heat pipe and the cooling coil characteristics was foundto be necessary for a satisfactory performance under the givenoperating conditions.

Yau [38] conducted a research on the influence of HPHXs on theenthalpy change in a tropical air conditioning system. The tests andsimulations conducted with/without an 8-row heat pipe in an airconditioning system. It was demonstrated that the cooling capabil-ity of the cooling water coil was enhanced with the HPHX installed.Yau [39] again studied the influence of three key parameters of in-let air state, namely dry bulb temperature, relative humidity andair velocity on the sensible heat ratio (SHR) of the 8-row thermosy-phon HPHX. The HPHX evaporator section functions as a pre-coolerand the condenser section as reheating coil as illustrated in Figs. 8and 9. By applying the HPHX, the overall SHR of the HVAC systemwas reduced from the maximum of 0.688 to the minimum of 0.188as the evaporator inlet dry bulb temperature was increased. Fur-thermore, it was shown that SHR was reduced from the maximumof 0.856 to the minimum of 0.188 as the evaporator inlet relativehumidity was increased. It was recommended that tropical HVACsystems should be installed with HPHXs for dehumidificationenhancement.

Because of restrictions in mixing return air with fresh air in hos-pitals, operating rooms and laboratories, the HVAC systems forthese places are found to be energy inefficient. Therefore, the appli-cation of a thermosyphon HPHX as a waste heat recovery device

Fig. 11. Schematic diagram for the air conditioning system (base case).

Fig. 9. Schematic diagram for the test rig.

82 Y.H. Yau, M. Ahmadzadehtalatapeh / Applied Thermal Engineering 30 (2010) 77–84

was investigated for the surgery rooms in the hospitals [40]. TheHPHX was designed and constructed for this purpose. In this re-search, the characteristic design and heat transfer limitations ofsingle heat pipes for three types of wick and three working fluidswere investigated. After obtaining the appropriate heat flux, theair-to-air heat pipe was designed and tested. Since the HPHX wasdesigned to use in the surgery rooms, the tests were run underlow temperature of 15–55� C operating conditions. From theexperimental results, using finned pipes, increasing the numberof rows, and complete insulation of the test rig have a significanteffect on the increasing of HPHXs effectiveness. In a similar re-search regarding the application of HPHXs for the HVAC systemsfor the surgery rooms, Yau [41] simulated the use of double HPHXfor reducing the energy consumption of treating ventilation air inan operating theatre located in the tropical climate as shown inFig. 10. A complete empirical transient system simulation programmodel was assembled to estimate the air states and the entire typ-ical meteorological year energy consumption of an operating the-atre located in Kuala Lumpur, Malaysia. According to thesimulation results, by installation of double HPHX in the HVAC sys-tem, the total amount of 51,000 kWh energy would be saved in ayear operation. In addition, the dehumidification capability of cool-ing coil could be increased from 0.24 to 0.54. Based on the simula-tion results, the application of double heat pipe heat exchangers inthe air handler unit in tropical climates was recommended by theauthor.

Literature review on the application of horizontal configurationHPHXs in heating, ventilation and air conditioning systems revealsthat the research works on the application of horizontal configura-tion HPHXs in tropical buildings are limited and hardly found tothe knowledge of authors.

Mathur [42] conducted a research on the controlling spacehumidity with a HPHX. In this study, the performance of a 5-ton(17.6 kW) air conditioning system with an energy efficiency ratioof 8 has been simulated by retrofitting it with a 6-row horizontalconfiguration HPHX to improve the cooling and dehumidification

Fig. 10. Diagram of two heat pipes in air conditioning system.

as illustrated in Figs. 11 and 12. The performance of the air condi-tioning system was simulated for hot and humid climates, whichwas typical of Southeastern United States. The weather data forDallas [43] was used for the study. The results showed that by ret-rofitting the existing system with the HPHX, additional moisture of0.134 kg/min would be removed from the cooling coil. The addi-tional cooling and dehumidification increased in the performanceby 96% or energy efficiency ratio from 8 to 15.7. It was stated thatfor the system studied, the HPHX retrofit would pay for itself inless than a year.

The effect of a HPHX on the energy consumption and the peakdemand of an existing air conditioning system were investigatedfor a year round operation [44]. A detailed performance studywas carried out by using the climate conditions for St. Louis, Mis-souri for the operation of the HVAC system. A 6-row horizontalconfiguration HPHX was used for the simulation purpose. Duringthe summer, analysis was conducted with and without indirectevaporation cooling and the results stated that by employing aHPHX in the system 22.1 kW of electrical energy could be saved.It was also found that saving in peak demand with indirect evapo-rative cooling is $2452/year and for total operation saving is$6946.23/year. From the simulation results, the retrofit systemcould pay for itself in 0.84 year. Furthermore, in order to calculatethe true energy saving, Mathur [45] studied the effect of a 6-rowhorizontal configuration HPHX in the energy consumption of anair conditioning system using the BIN weather data [46]. A simula-tion program that could predict the energy saving was developedfor this purpose. Investigation was carried out for 33 cities inUSA with widely different climatic conditions. For the cities withcolder climates such as Chicago, the yearly operating cost savingis $14,148.72/year and for the cities, where the climate is mild suchas Miami, the yearly operating cost saving is $8,121.2/year. Theeconomical analysis indicated that a simple retrofit on an existingHVAC system could pay for itself in less than 1 year.

4. Studies on the influence of inclination angle on the HPHXsperformance in tropical HVAC systems

The effect of inclination angle on the performance of HPHXs wasreported in a number of literatures. For instance, Beckert andHerwig [47] carried out a research on the impact of inclination

Fig. 12. Schematic diagram for the air conditioning system with the retrofit.

Y.H. Yau, M. Ahmadzadehtalatapeh / Applied Thermal Engineering 30 (2010) 77–84 83

on the heat transfer rate of an air-to-air HPHX applied in an airconditioning system. A set of two-phase closed thermosyphonswas investigated to find out how far they can be inclined withoutconsiderable decrease in the heat transfer rate. It was found thateven in a nearly horizontal position (up to ±6� with the horizontal)the overall performance is still good enough. It was observed thatturning the set of pipes by an angle of about 12� from �6� to +6�with respect to the horizontal will switch the overall heat transferfrom one direction to the other. It was also observed that in an airconditioning system, where the thermosyphon heat pipes operateas a heat exchanger between the outside channel and the exhaustair, it can be used in a position of �6� to preheat the fresh air inwinter and in a position of +6� to pre-cool the fresh air in summer.Similarly, the effect of tilt angle on the overall heat transfer of twotypes of heat pipes, one with wick and another with no wick wasstudied by Said and Akash [48]. The heat pipes were positionedat different tilt angles of 30�, 60�, and 90� with the horizontal. Itwas revealed that for all the cases, the performance of the heat pipewas improved when wick was used. In terms of overall heat trans-fer coefficients, the wickless heat pipes had a value in the range of4000–5000 W/m2 �C and for wicked heat pipes it was improved tothe range of 6000–7000 W/m2 �C. For the tilt angles of 30�, 60�, and90� (with respect to the horizontal), the amount of enhancement,when wick was used in a heat pipe, was about 55%, 25%, and70%, respectively. Loh et al. [49] also investigated the effect oforientation angle on the performance of three heat pipes wickstructure. It was found that heat source orientation and gravityhad less influence on sintered powder metal heat pipes. Accordingto the experimental results, employing the mesh and groove heatpipe with evaporator is on the top of the condenser was not sug-gested. Furthermore, Foot et al. [50] explained that inclinationangle had no significant effect on the performance of a plate heatpipe solar panel. In contrast, Guo et al. [51] studies showed thatheat pipe effectiveness can be affected by inclination angle. All ofthese investigations were for conditions in which the surface con-densation was not a concern.

Condensation forming on the extended fin surfaces of HPHXs isexpected in high humid climates. As a result, the performance ofthe HPHX is expected to be influenced by any condensation form-ing on the fin surfaces. Literature review revealed that the conden-sation forming on the fin surfaces of a HPHX in high humidcondition and its effect on the HPHX performance has received lit-tle attention. Kim et al. [52] carried out a research on the effect ofair inlet humidity on the air side heat transfer and pressure dropfor an inclined brazed aluminum heat exchanger as shown inFig. 13. The heat exchanger was studied at three different inclinedangles namely 14�, 45�, and 67� (with respect to vertical position).The inlet air humidity was in the range of 60–90%, while the inlettemperature was controlled at 12 �C. It was observed that the ef-fect of air inlet humidity on the heat transfer and pressure dropcharacteristics of heat exchanger was negligible for the inclinationangle of less than 45�. Similarly, Yau [53] claimed that the impactof condensate layer on the performance of a thermosyphon HPHXoperating in a high humid condition was negligible. In the tiltedangle of 30� with the horizontal configuration, the gravitationalforce would be expected to enhance drainage of any condensateon the fin surfaces in evaporator section. Therefore, the effective-

Fig. 13. Schematic diagram of the heat exchanger installation.

ness of the HPHX could be predicted to be better than the verticalconfiguration, but the results revealed that the condensation form-ing on the extended fin surfaces has a negligible effect on the effec-tiveness of the HPHX studied. For instance, for mass flux of 1.3 kg/m2 s and nominal evaporative relative humidity of 80%, the sensi-ble effectiveness was in the range of 0.40–0.50 and 0.41–0.49 forthe vertical configuration and tilted angle of 30�, respectively.

In terms of condensate layer, because of the fins orientation inthe horizontal configuration HPHXs, this configuration HPHXscan be anticipated to lessen negative impact of any condensatelayer on the HPHX effectiveness. Moreover, the space that an airconditioning system occupies in a building is one of the major con-cerns in the building industry. In other words, if the air condition-ing system occupies less space, it will be more economical.Nowadays, the building industry focuses on more benefit of spacesin buildings than before, especially in countries with high popula-tion density. Since the horizontal configuration HPHXs is smaller invertical dimension and this could save in space of building whenthey installed, it seems that the employment of horizontal config-uration HPHXs can be more economical than vertical configurationHPHXs from this point of view.

5. Conclusions

A literature review was carried out on the application of HPHXsin HVAC systems in tropical climates. This review testifies thatHPHXs in both configurations, i.e., vertical (thermosyphon) andhorizontal configuration, can be implemented as an efficientenergy recovery unit in air conditioning systems to remove heator coolness and dehumidification purposes. Moreover, literaturereview on the application of horizontal configuration HPHXs in-stalled with air conditioning systems in tropical climates revealsthat although the humidity control is one of the main concernsin indoor comfort quality in tropical climates, but investigationsregarding the application of horizontal configuration HPHXs fordehumidification enhancement and energy saving purposes in airconditioning systems are limited and most of the studies have beencarried out in sub-tropical climates. Therefore, further investiga-tions on the subject such as optimization of working fluid, impactof rows on the overall performance, and effect of different indoortemperature and relative humidity on the energy saving and dehu-midification enhancement should be made to deepen the under-standing of the benefits of this heat recovery device in areas astropical climates such as Malaysia, Singapore and Thailand. Basedon this literature survey, for condensate drainage considerations,the horizontal configuration HPHXs are recommended especiallyin tropical climates.

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