INTRODUCTION heat pipe thermal conductivity with excellent performance, secondary Jianbi heat exchangers, heat flux to adjust the heat transfer equipment and other general do not have the advantages of energy, which in the industrial heat transfer and recovery of waste heat, and other aspects of access to wider use in metallurgical, chemical, Building materials, power and other industries there are many successful examples [1,2]. In recent years, the heat pipe heat transfer technology applications in the construction of energy-saving research is the ascendant, but this kind of research are a lot of case studies, did not have a strong wider significance. This paper aims to construction of energy flow and thermal characteristics in the construction of energy-saving technology in the application and the application of heat pipe ways and means for a more systematic analysis, for the practical application of the methodology to provide general guidance.
1 construction waste heat (cold) can flow characteristics of the construction can flow analysis shows that, despite the inflow of building a variety of energy, but through different channels the energy conversion, will eventually exhaust gas, waste water, building envelope from the form of heat. Building waste heat (cold) emissions have the following features, these features are hot and the application of heat exchangers in line.
(1) the proper temperature, and thermal (cold) is larger. For air-conditioned buildings, the new wind replacement gas from the indoor temperature and humidity close to temperature and humidity, large buildings, exhaust heat (cold) up to the total load of 30% to 40%.
(2) in time easier to achieve balance. Construction waste heat emissions to a certain extent cyclical, and the waste heat exhaust ventilation air conditioning (cold) and the processing of new wind energy demand in time fully synchronized.
(3) waste heat emissions and energy use close to the location.
(4) heat (cold) and the energy requirements of the grade rather close.
(5) the use of waste heat and pollution caused by the heat exchange equipment likely to cause little effect on the corrosion, but the factory and a hospital operating theatres such as exhaust ventilation and air conditioning need to consider this issue.
Currently, the recycling of construction waste heat means more or less as shown in Figure 1.
2 heat pipe technology in the construction of energy-saving
2.1 natural ventilation thermal (cold) recovery ventilation ducts and risers for the natural ventilation system, most did not take heat recovery measures, because the installation of conventional heat exchanger, a loss may be due to pressure caused by excessive ventilation failure. Heat pipe heat exchanger with excellent performance and flexibility of the heat and the structure of the form, in the ducts and risers for natural ventilation duct system is used effectively. Tests showed that the wind speed, wind 0.5 m / s, heat recovery efficiency of 50 per cent of the cases, the cause of the pressure loss of about 1 Pa [3].
Heat pipe heat exchangers used in natural ventilation system of energy recovery, the overall performance of the finned form of heat pipe arrangement, the row number, air speed and other factors constraining each other. 随着风速的提高，其热交换效率降低而阻力增大。 With the wind speed increase, its heat exchange efficiency and reduce the resistance increased. For natural ventilation duct system, the proposed heat exchanger winds of not more than 1 m / s;-finned, blinds-finned than the cone, needle-shaped fin good effect of heat transfer and pressure loss of small, mainly because the former fin - More contact with the wall, while the latter will lead to increased airflow disturbance; pipeline for the wind speed 0.5 m / s, a single-fin or blinds Pai Ping-finned heat pipe, heat exchange efficiency up to 40 percent, double Proximity to 70 percent, with the heat pipe row after the increase, not increase the efficiency of heat transfer, but the pressure loss significantly increased heat pipe unit and are staggered with rectangular, although the heat exchange efficiency increased slightly, but the increase was more pressure loss Notable [4,5].
2.2 on exhaust heat (cold) recovery of public buildings is large in scale, also ventilation, such as hospital operating theatres sometimes requires ventilation frequency of 40 or more times per hour, while the large-scale construction, exhaust the energy taken away by about 30% of the total load to 40 percent of the energy recovery potential.
Through separate heat pipe heat exchanger, air-conditioning systems can make use of the exhaust pretreatment new wind energy, by 30 percent if the Xinfeng Liang, energy-saving air-conditioning systems will enable more than 7 percent; With the cold and hot air temperature and the increase of new wind than the Increase in energy-saving effect will be even more significant. 。 Tests showed that cold and hot air temperature difference as long as 3 ℃ to energy recovery. Accordingly, China's Shanghai, Nanjing and other middle and lower reaches of the Yangtze River in the summer air-conditioned "cooling" of the recovery up to 1500 h over time, according to meteorological parameters, three years early recyclable equipment investment costs [6].
Use of waste heat recovery on the exhaust or waste to pretreatment new cold wind, more all-in the form of heat exchangers, heat pipe heat exchanger can only heat recovery, in the new wind and return air heat exchange in the process of recovery than the efficiency All heat exchangers relatively small, but the greater the scope of [7].
2.3 residential air-conditioning does not change the basic energy-saving air-conditioner on the basis of the existing configuration and the heat pipe heat exchanger unit, composed of heat pipe - Air-conditioner combination system, the winter recovering exhaust heat, reduce air-conditioner load, to achieve energy-saving Purpose, and in summer can increase refrigeration capacity and air-conditioning system Qushi ability to fully or partially cancel further heat load and improve comfort [8].
The study of the topic at home and abroad is in the ascendant, the researchers in different conditions are obtained encouraging results. Wu, who received the results: System cooling capacity increased by 20% to 32.7%, relative humidity in the air supply <70% of the conditions, the heat pipe heat exchanger alternative to thermometers, relative humidity in the air supply ≥ 70% Under the conditions, it may be necessary to support hot [9]. Mcfarland, who pointed out: For the standard temperature for 22 ℃, relative humidity of 50 percent of the air-conditioned room conditions, the heat - combined system desiccant air-conditioners increased 62 percent capacity, reduce the heat load to 20 percent, the system potential efficiency (Latent energy-efficiency ) Increased 90 percent; further, considering the standard air conditioner up to 10 a life of more than heat pipe - an annual operating system combination of air conditioner 1000 h, the system can calculate the additional heat pipe heat exchanger investment recovery period (Simple payback peri-od) For the 4 a, which is also fully in the economic feasibility of [10].
Heat pipe - the combination of air conditioner system optimal design and optimize the operation is very important [11]. In addition, the air resistance increases, the need to appropriately increase air-conditioner fan pressure head, equipment costs, which have increased the overall size.
2.4 in the Solar Project of the heat pipe itself has many characteristics, such as temperature, heat flux can change, thermal diodes, and other characteristics, so that it works in the application of solar energy is extremely broad prospects.
2.4.1 solar water heaters
Vacuum tube heat pipe collector with anti-Ganshao, anti-Donglie to prevent heat transfer reverse losses caused by thermal characteristics, making solar water heaters in a wide range of application to regional climate, heat pipe-type solar collector to become a collector An important form. According to statistics [12], as at the end of 2000, sales of solar water heaters in China, 6.1 million m2, the output value of 5 billion yuan, solar water heaters maintain the existing volume of more than 26 million m2, is the world's largest production and marketing power.
The heat pipe solar water heaters, mainly from several areas to improve the efficiency of water heaters [13 to 16]:
① determine the best collector of the pitch, balanced system cost, efficiency between the technical and economic balance;
② to take water temperature and intensity of solar radiation or a combination of temperature control strategy and determine the best corresponding control parameters;
③ determine the appropriate diameter than the high-water tanks, water volume and collector area ratio, and other structural parameters, to determine the optimal operating parameters, such as recycling rate, to maintain the internal temperature gradient of water tanks, as far as possible, first of all access to high-temperature water;
④ the vacuum tube heat pipe-type water heaters, heat pipe condensation appropriate extension of the relative length, or a ring-shaped structure, such as condensation of the last paragraph of the form of increased condensation of the heat transfer area, can strengthen the heat of condensation of water tanks and between - Heat.
Through these improvements, water heater can raise the overall efficiency of 20 percent or more.
The solar hot water supply is another effective form of heat pump water heaters, solar-assisted [17], all-weather application.
2.4.2 solar air-conditioning
Yourenzhichu solar air-conditioning systems, the more solar radiation is strong, the higher the temperature of the environment, at this time of the solar air-conditioning cooling capacity will be strong, and this is the ideal state of natural harmony.
As technology, cost and other reasons, the general absorption of solar energy air-conditioning and refrigeration adsorption refrigeration technology. The existing system equipment is relatively large, relatively high investment costs, the performance by the constraints of climate factors. However, vacuum-tube heat pipe technology has been relatively mature, could be the work of solar collectors temperature from 70 ℃ to 120 ℃, greatly improve the collector's thermal properties, solar air-conditioning to provide better technical basis, solar air-conditioning Enticing prospect. Rushan City, Shandong Province to build a solar energy absorption air conditioning and heating integrated system [18]. The system uses a vacuum heat pipe collector of 2160, the total area of lighting 540 m2, the body heat of 364 m2, cooling and heating power of 100 kW, air conditioning, heating construction area of 1000 m2, with hot water for the life of 32 m3 / d . Test results show that: solar collectors on the average efficiency of air-conditioning in summer when more than 40 percent, when the winter heating up to 35 percent, the transition season to provide hot water at up to 50 percent or more of lithium bromide absorption chillers COP Values up to 0.70, the whole system of cooling efficiency up to 20 percent. In addition, the system through the auxiliary boilers, all-weather operation.
Shanghai Jiaotong University developed the solar absorption of ice-making machine prototype, solar collectors per square metre per day ice-5 ~ 7 kg; 1998, a proposed absorption of solar energy heating and cooling of the combined cycle Local device, using 2 m2 of solar collectors, the daily production of 90 ℃ hot water 60 kg, ice-making 5 kg [19].
2.5 heat pipe heating system in recent years, some heat pipe radiator gradually emerged and developed into a hot radiator.
Freon 11 respectively, water, methanol acetone for the heat pipe to test the quality showed that the methanol or acetone for the heat of the radiator of the best properties [20].
HTM contrast ordinary water radiators, radiator heat pipe has the following advantages:
① surface temperature uniform, Jebrail, who experiments show that: For 90 ℃ and 72 ℃ the HTM water, different from ordinary radiator surface temperature difference between the maximum were 16.1 ℃ and 12.2 ℃; and the same surface area Heat pipe radiator to heat methanol to the quality of the maximum temperature was 2.87 ℃ and 5.68 ℃, acetone for the heat pipe to the maximum temperature difference of 0.25 ℃ respectively, and 2.10 ℃;
② no ordinary radiators prone to the problem of corrosion;
③ requirements greatly reduce the amount of media, can greatly save transmission power consumption, simplify the transmission pipeline system;
④ simple portable system, without water pressure constraints, ease of installation.
Heat pipe radiator for central heating system, there are also some practical engineering issues to be further improved:
① heat pipe radiator of the second heat exchanger, mainly depends on its ability to heat and heat medium of the heat resistance, and enhanced heat medium heat pipe heat transfer between the quality is the key to improving efficiency, compared ordinary water HTM Radiators, heat transfer coefficient slightly lower;
② heat pipe from the stability of started to work, the negative pressure to produce positive pressure from the large-span pressure to stop work, the shell of the need to maintain a certain negative pressure, the production process requirements as compared with ordinary radiator high;
③ heat of the boiling point of saturation pressure, toxic, flammable, and the compatibility of shell material, thermal heat transfer, and other factors and parameters on the physical properties of heat pipe radiator and practical applicability of the performance plays a decisive role, to Careful consideration.
2.6 heat pipe to explore new energy-saving technologies
2.6.1 geothermal resources development in China's abundant geothermal resources in recent years is the fastest-growing, low-temperature geothermal use, such as heating, bathing, medical, tourism, planting and breeding industries. National geothermal heating has grown to 8 million m2, which reached 5 million in Tianjin m2, with the emphasis on the environment, the Beijing area are to strengthen the use of geothermal planning [12].
Zhao Yuchun and the solar heat pipe in the greenhouse by the application of economic analysis: greenhouse agriculture, the fuel cost of the total cost of 20% to 32.2%, the use of heat pipe technology, the use of solar energy and geothermal can greatly reduce greenhouse agriculture The operation and management costs, enhance economic efficiency [21].
Riffat, who proposed a new type of heat pipe / jet refrigeration model [22]. The refrigeration system performance COP value can be comparable with the absorption refrigeration, but the structure is very compact, efficient use of solar and other forms of low-grade 80 ℃ about the heat. Methanol for the quality, performance was significantly higher than that for the quality of water when the temperature in 80 ℃, evaporation temperature of 5 ℃, condensing temperature at 35 ℃, the former than the latter 70 percent of methanol as refrigerants, Condensation of 35 ℃ to passive air cooling, single-stage injection system COP value of 0.7, two-stage system can further improve performance; condensing temperature performance of the system, is greater than the temperature in temperature and evaporation of its impact, Therefore, should focus on consideration of the condensation of the enhanced heat transfer.
2.6.3 use of low-grade industrial waste heat in a stable low-grade industrial waste heat of the circumstances, the use of heat pipe technology use of industrial waste heat as the heat absorption refrigeration, to improve energy efficiency and reduce environmental pollution purpose [23].
2.6.4 in food processing applications in food processing, use plug-in heat pipe can be greatly shortened and thaw frozen foods of the time required to reduce food surface water evaporation, thereby reducing energy consumption and maintain uniform food quality and reduce Bacterial contamination [24].
3 Conclusion (1) construction of energy flow analysis shows that a great potential for energy recovery, heat pipe heat transfer technology suitable for building waste-to-energy and energy conservation; heat pipe heat exchanger with high efficiency, compact structure, diversity in form, layout flexibility, and applicability, and other superior Sexual.
(2) the use of waste heat recovery on the exhaust or waste to pretreatment new cold wind, and the entire heat exchangers, heat pipe heat exchanger heat exchange efficiency slightly lower, slightly higher pressure loss, but the scope of temperature, does not exist Cross-contamination in the air ducts and risers, natural ventilation system, the heat pipe heat exchanger with heat exchanger pressure than the regular loss of small, highly efficient heat transfer to the superiority; heat pipe heat exchanger with the room air conditioner combination, can improve the air Handling capacity, saving energy consumption, improve comfort, but the need to increase fan pressure head.
。 (3) heat pipe technology in solar energy, geothermal energy and other forms of low-grade heat advantage in the use of more and more attention has been widespread use of solar water heaters, solar air-conditioning believe that the practical and popular will also not far off.
。 (4) heat pipe radiator surface temperature is uniform, saving metal consumption, reduce heat medium volume, Anti-oxidation corrosion, and facilitate the installation of such advantages, but there are also some practical engineering issues to be further improved.
(5) heat pipe heat transfer technology used in the construction of energy recycling and energy conservation, many engineering problems have yet to be systematic and in-depth study of its economy has yet to be more practical test.
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