Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

Abstract

Heat pipes and two‐phase thermosyphon systems are passive heat transfer systems that employ a two‐phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (105 W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air‐conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air‐to‐air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m3/h, evaporator inlet‐air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (Ce/Cc) of 1 and 1.5. For HPHE's steady‐state operation, a mathematical model for heat‐transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet‐air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (Ce/Cc) of 1.5. The predicted and experimental values of condenser outlet‐air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet‐air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.

Review of Recent Applications of Heat Pipe Heat Exchanger Use for Waste Heat Recovery

“…nuclear reactor [3,11], the ceramic industry [4][5][6]12], solar collector [13,14], coal to liquid process [15,16], HVAC systems [17][18][19][20][21][22], heat recovery system [23][24][25][26][27][28][29], gas city gate station [30], steel industry [31,32], dyeing and printing industry [33], the…”

“…nuclear reactor [3,11], the ceramic industry [4][5][6]12], solar collector [13,14], coal to liquid process [15,16], HVAC systems [17][18][19][20][21][22], heat recovery system [23][24][25][26][27][28][29], gas city gate station [30], steel industry [31,32], dyeing and printing industry [33], the solar collector [38][39][40], photovoltaic panel [41], steel industry [42,43] pressure reduction stations [34], heat recovery of coal-fired flue gas [35], aluminum industry [36], combine harvester [37] Flat-plate heat pipe Flat-plate heat pipes typically have thinner cross-sections, making them suitable for heat conduction within limited spaces.…”

“…Due to their lack of wick structure, simplicity of production, and low cost, thermosyphon heat exchangers are widely used in HVAC systems. In the literature [17][18][19][20][21][22], thermosyphon heat exchangers have been employed to transfer the heat from the discharged hot air to the fresh air entering the system, reducing the energy consumption of HVAC systems. To further reduce energy consumption, U-type HPHXs have been proposed for HVAC systems that require both cooling and reheating.…”

“…Since refrigerant R134a has been widely used in domestic and industrial applications, an alternative working fluid must be found. Sharma et al [18] conducted experimental studies on the thermal performance of air-air HPHXs using R22 and R407c as refrigerants under different evaporator inlet airflow rates and temperatures. The results showed that the heat transfer rate was proportional to the evaporator inlet air temperature and flow rate.…”