Experimental Investigation of Air-to-Air Counter Flow Heat Pipe Heat Exchanger for Heat Recovery System

IARJSET ISSN (Online) 2393-8021 ISSN (Print) 2394-1588 International Advanced Research Journal in Science, Engineering and Technology ISO 3297:2007 Certified Vol. 4, Issue 3, March 2017 Experimental Investigation of Air-to-Air Counter Flow Heat Pipe Heat Exchanger for Heat Recovery System Manisha Rathod Mechanical Engineering Department, Trinity College of Engineering and Research, Pune, India Abstract: This paper deals with the waste heat recovery using heat pipe heat exchanger. Heat pipe is a device which carries the heat from high temperature region & ejects it to the cold or low temperature region. The advantage of using a heat pipe over other conventional methods is that large quantities of heat can be transported through a small cross- sectional area over a considerable distance with no additional power input to the system. The heat pipe heat exchangers are used in heat recovery applications to cool the incoming fresh air. Two streams of fresh and return air are connected with heat pipe heat exchanger to investigate the thermal performance and effectiveness of heat recovery system. The aim of this project work is to investigate the thermal performance and effectiveness of heat pipe heat exchanger for heat recovery applications by measuring the temperature difference of warm and cold air through the evaporator and condenser side. The hot-air temperature increased from 60 to 900C; the heat-transfer rate increased slightly. The mass flow rate of hot air increased from 0.30997, 0.4643, 0.4889, 0.508 kg/s led to a slight increase in effectiveness. As the hot-air temperature increases from 60 to 90 0C, the effectiveness slightly increased. Keywords: Heat pipe; Heat pipe Heat exchanger; Heat recovery. I. INTRODUCTION In the light of an ever increasing demand for energy, the pipe heat exchanger for heat recovery in hospital and need for energy savings has become an important laboratories, where the air must be changed up to 40 times economic consideration. One means of saving energy is to per hour, with the characteristic design and heat transfer recover a portion of the energy in a warm waste stream limitations of single heat pipes for three working fluids and then to use the recovered energy to preheat another and three types of wick have been investigated. El-Baky colder stream. Heat pipe heat exchanger for heat recovery and Mohamed [6] used heat pipe heat exchanger to equipment are aimed for recovering sensible heat and they recover heat in air conditioning applications to cool the are recommended for systems in which inlet and return air incoming fresh air. Martinez et al. [7] designed a mixed should not be mixed such as surgery rooms in hospitals air-energy recovery system, consisting of two heat pipes and chemical and biological laboratories. The advantage and indirect evaporative recuperators. The experimental of using heat pipes over conventional methods is that large set-up used is described and the proposed energy recovery quantities of heat can be transported through a small cross- system is characterized.S. Rittidech et al. [8] studied the sectional area over a considerable distance with no CEOHP air-preheater consisted of two main parts, i.e. the additional power input to the system, (except for the fans rectangular house casing and the CEOHP. The house to drive the airstreams) together with simplicity of design casing was designed to be suitable for the CEOHP. The and ease of manufacture, [1]; less pressure drop of fluid; inside house casing divided the CEOHP into three parts, advanced maintainability; high reliability; simpler i.e. the evaporator, the adiabatic section and condenser structure and smaller volume. Gravity assisted heat pipe, section. Meena [9] used a CLOHP/CV air-preheater for for its special characters of without wicks, has found recovering the waste heat from the drying cycle to reduce numerous applications in heat recovery systems in terrene. the relative humidity and achieve energy thrift. HPHXs are suitable for energy recovery in AC systems in Ahmadzadehtalatapeh and Yau [10] studied the effect of tropical areas where the inlet fresh air at high temperature heat pipe heat exchangers on the existing air conditioning could be pre-cooled before it reaches the cooling coil. A system of a hospital ward located ina tropical region.It was HPHX is a heat exchanger consisting of externally finned found that by applying the new design, a considerable tubes filled with a proper refrigerant (i.e. working fluid). amount of energy and power could be saved.Yau[11] There are two heat transfer sections in HPHXs, i.e. the developed an 8-row thermosyphon-based heat pipe heat evaporator and the condenser sections for the heat exchanger (HPHX) for tropical building HVAC exchange between the two air streams.[2,3] systemsfor dehumidification enhancement.Hagens et Yang et al.[4] have studied the feasibility of using heat al.[12] done measurements and predictions of a heat pipe pipe heat exchangers for heating applying automotive heat exchanger with two filling ratios of R134a, varying exhaust gas. Noieet at. [5] designed and constructed heat air flow rate and input temperature, and suggested that a Copyright to IARJSET DOI 10.17148/IARJSET.2017.4312 54  IARJSET ISSN (Online) 2393-8021 ISSN (Print) 2394-1588 International Advanced Research Journal in Science, Engineering and Technology ISO 3297:2007 Certified Vol. 4, Issue 3, March 2017 heat pipe equipped heat exchanger is a good alternative for each heat pipe = 22.4 mm, Ltubes air–air exchangers in process conditions when air–water = 0.6 m cooling is impossible, typically in warmer countries. 3 Type and dimensions thickness = 0.4 Laubscher and Dobson [13] developed a novel heat pipe of fins Aluminum plate mm heat exchanger which circumvents the need for an fin, intermediate coolant loop, using Dowtherm-A as working 4 Heat pipe arrangement SL = 77 mm, ST = fluid. Staggered, 90 mm 5 Number of rows NL = 10, NT = 4 II. EXPERIMENTAL SET UP 6 Total number of the N = 40 heat pipes In order to study thermal performance of the heat pipe heat 7 Heat pipe SS/water exchanger, a test rig was constructed. The schematic material/working fluid diagram of the test rig is shown in fig 2.1 and actual test Design/Construction cold side hot side rig is shown in fig 2.2 Details The test rig consists of 8 Type of Fluid Air A heat pipe heat exchanger Air A centrifugal blower 9 Design inlet Atmospheric 80- Electrical heaters (with total capacity of 20 kW) temperature 1000C Thermocouples 10 Design air flow rate 0.1-0.6 0.1- Temperature monitor and two rectangular air ducts; one 0.6kg/sec for the hot air and the other for cold air. 11 Type of Fins on HP Circular The flow rates of hot and cold air were determined by 12 Method of fin measuring the velocity of air using a digital flow meter. attachment Shrink fit 13 Fin Density 5fpi 14 No. of HP rows along the flow 10 15 No. of HP rows across the flow 4 16 Total no. of HP with water as working fluid 40 17 Type and size of inlet and outlet of Heat Exchanger Rectangular Fig. 2.1: Experimental set up 18 HP tube Material SS-304 19 Fin material SS-304 20 Support structure Carbon steel 21 Carbon Casing material steel,IS2062 III. EXPERIMENTAL RESULTS AND DISCUSSIONS A series of tests was performed in order to investigate the characteristics of the heat pipe heat exchanger. The readings were taken by varying mass flow rate of hot air from 8.5 m/s to 20 m/s and the air velocity of cold air was 19 m/s and maintained constant throughout the Fig 2.2: Actual experimental set up experimentation. The readings were taken by varying hot air temperatures from 650C to 900C, the results are The physical parameter of HPHE are given in table 2.1 discussed in this section. Table 2.1 physical parameter of heat pipe heat Fig 3.1 shows the effect of heat input on heat transfer rate exchanger to condenser. In this figure X axis represent the heat input and Y axis represents the heat transfer to condenser. From Sr. no Description Specifications fig the increase in heat input to the evaporator section 1 Physical dimensions of 770mm(length) causes the increase in heat transfer to the condenser the heat exchanger 420mm(width) section. The heat transfer rate is maximumfor the 900mm(height) maximum heat input. For the 5.1 kW of heat input heat 2 Physical dimensions of Do = 25.4 mm, Di transfer to the condenser section is 3.51 kW. Copyright to IARJSET DOI 10.17148/IARJSET.2017.4312 55  IARJSET ISSN (Online) 2393-8021 ISSN (Print) 2394-1588 International Advanced Research Journal in Science, Engineering and Technology ISO 3297:2007 Certified Vol. 4, Issue 3, March 2017 Figure 3.3 shows the effect of mass flow rate on effectiveness. In this figure X axis represents the mass flow rate and the Y axis represents the effectiveness of heat exchanger. The effectiveness of heat exchanger increases with increase in mass flow rate. When the mass flow rate increases the heat input is increased. And the effectiveness is depends on the heat input and heat transferred to condenser. With increase in mass flow rate the heat input is increased and it causes to increase in effectiveness of the heat exchanger. The maximum effectiveness is 0.62 at minimum mass flow rate 0.5077 Kg/S. Fig 3.1: Effect of heat input on heat transfer rate The effect of change in Reynolds number on heat transfer Figure 3.2 shows the effect of evaporator heat input on coefficient at evaporator side is shown in figure 3.4. effectiveness for different mass flow rate. The heat input is varied by varying the hot air inlet temperature. As the heat input to the evaporator increases the effectiveness of the heat exchanger increases. Re Vs h 70 H.T. coefficient at evaporator side 60 50 40 30 300000 400000 500000 600000 700000 Reynolds number Fig 3.4 Effect of Reynolds no on heat transfer coefficient at evaporator side Fig 3.2: Effect of heat input on effectiveness As Reynolds number increases the heat transfer coefficient increases. Nusselt number is a function of Reynolds The effectiveness is maximum for the maximum heat number and Prandlt number. input and the maximum mass flow rate. As the effectivness is depend on the heat input and the heat For turbulent flow transfer to condenser. 𝑄 = 𝑚𝐶𝑃 (𝑇𝑖 − 𝑇𝑜 ) this equation Nu= 0.037*(Re)0.8*(pr)0.33 shows that heat transfer is depend on the mass flow rate therefore when mass flow rate increases heat transfer rate For laminar flow, increases and it causes increase in effectiveness. We got Nu= 0.664*(Re)0.5*(pr)0.33 maximum effectivness 0.62 at maximum mass flow rate of 0.5077 kg/S and at heat input of 5.103kW These equation shows as Reynolds number increases the Nusselt number increases. But Nusselt number is a function of heat transfer coefficient. ℎ𝐿 Nu= 𝐾 Therefore from this equation when Nusselt number increases the heat transfer coefficient increases. The maximum heat transfer coefficient obtained at maximum Reynolds number. 59.84 W/mK heat transfer coefficient obtained at the Reynolds number 6.9*105. The effect of change in Reynolds number on Nusselt number is shown in figure 3.5. In this figure X axis shows the Reynolds number and Y axis shows the Nusselt Fig 3.3 Effect of mass flow rate on effectiveness number. Copyright to IARJSET DOI 10.17148/IARJSET.2017.4312 56  IARJSET ISSN (Online) 2393-8021 ISSN (Print) 2394-1588 International Advanced Research Journal in Science, Engineering and Technology ISO 3297:2007 Certified Vol. 4, Issue 3, March 2017 v. Increase in Reynolds number increases the heat Re Vs Nu transfer coefficient increases. The 1501 Nusselt number obtained at 6.9*105 Reynolds number. 1500 REFERENCES Nusselt nuber 1300 [1] Jay M. Ochterbeck, Heat Pipes, chapter 16, 1181-1184. 1100 [2] David Reav, Peter Kew, Heat pipes, 5 th edition, Pergamon Press, Oxford, UK, (2006). [3] Faghri A., Heat pipe science and technology, Taylor and Francis 900 Washington DC, USA, (1995). [4] Cengel, Heat and mass transfer, McGraw hill education, (2002) 700 [5] Yang F, Yuan X, Lin G. Waste heat recovery using heat pipe heat exchanger for heating automobile using exhaust gas. Applied 300000 400000 500000 600000 700000 Thermal Engineering 2003; 23:367–72. [6] Noie-Baghban S, Majideian G. Waste heat recovery using heat pipe Reynolds number heat exchanger (HPHE) for surgery rooms in hospitals. Applied Thermal Engineering 2000; 20:1271–82. [7] Abd El-Baky MA, Mohamed MM. Heat pipe heat exchanger for Fig 3.5 Effect of change in Reynolds number on Nusselt heat recovery in airnconditioning. Applied Thermal Engineering Number 2007; 27:795–801. [8] Martinez FJR, Plasencia MAA, Gomez EV, Diez FV, Martin RH. Design and experimental study of mixed energy recovery system, As Reynolds number increases the heat transfer coefficient heat pipe and indirect evaporative equipment for air conditioning. increases. Nusselt number is a function of Reynolds Energy and Buildings 2003; 35:1021–30. number and Prandlt number. [9] Rittidech S, Dangeton W, Soponronnarit S. Closed-ended For turbulent flow oscillating heat pipe (CEOHP) air-preheater for energy thrift in a dryer. Applied Energy 2005; 81:198–208. Nu= 0.037*(Re)0.8*(pr)0.33 [10] Meena P, Rittidech S, Poomsa-ad N. Closed-loop oscillating heat- For laminar flow, pipe with check valves (CLOHP/CVs) air-preheater for reducing Nu= 0.664*(Re)0.5*(pr)0.33 relative humidity in drying systems. Applied Energy 2007; 84:363– These equation shows as Reynolds number increases the 73. Nusselt number increases. The 1501 Nusselt number [11] M. Ahmadzadehtalatapeh, Y.H. Yau, The application of heat pipe heat exchangers to improve the air quality and reduce the energy obtained at 6.9*105 Reynolds number. consumption of the air conditioning system in a hospital ward—A full year model simulation, Energy and Buildings 43 (2011) 2344– IV. CONCLUSIONS 2355. [12] Yat H. Yau, Application of a heat pipe heat exchanger to dehumidification enhancement in a HVAC system for tropical In the present work, Heat pipe heat exchanger with 40 heat climates—a baseline performance characteristics study, pipe is used to investigate the thermal performance of heat International Journal of Thermal Sciences 46 (2007) 164–171. pipe heat exchanger. In this variation of heat transfer to the [13] H. Hagens, F.L.A. Ganzevles, C.W.M. van der Geld, M.H.M. Grooten, Air heat exchangers with long heat pipes: Experiments condenser is investigated with the varying heat input. and predictions, Applied Thermal Engineering 27 (2007) 2426– Effect of heat input and mass flow rate on the 2434. effectiveness for different mass flow rate is also studied. Variation of Nusselt number and heat transfer coefficient with the varying Reynolds number is reported. The main conclusions are summarized as follows: i. The increase in heat input to the evaporator section causes the increase in heat transfer to the condenser section. The heat transfer rate is maximum for the maximum heat input. For the 5.1 kW of heat input heat transfer to the condenser section is 3.51 kW. ii. As the heat input to the evaporator increases the effectiveness of the heat exchanger increases. We got maximum effectivness 0.62 at maximum mass flow rate of 0.5077 kg/S and at heat input of 5.103kW. iii. The effectiveness of heat exchanger increases with increase in mass flow rate. The maximum effectiveness is 0.62 at minimum mass flow rate 0.5077 Kg/S. iv. Increase in Reynolds number increases the heat transfer coefficient increases. 59.84 W/mK heat transfer coefficient obtained at the Reynolds number 6.9*105. Copyright to IARJSET DOI 10.17148/IARJSET.2017.4312 57