After 30 years of development, heat pipe technology has entered the practical stage. It has many unique advantages that other heat transfer technologies do not have: excellent heat transfer efficiency and reliability, isolation, low resistance, small size, controllability and back separation. Therefore, heat pipe technology has been effectively applied in more and more fields. The application of heat pipe technology in chemical industry, building materials, metallurgy, power engineering, bioengineering and other fields in China is in the international leading position, and its application in the field of heating and air conditioning is also developing rapidly. Sustainable development has become the development direction of the whole society. Heating and air conditioning is a big energy consumer, and the development of its technology will naturally take the road of sustainable development. Because of the excellent heat transfer performance of heat pipe technology, it has been widely used in the recycling of low-grade energy such as waste heat, waste heat, solar energy and geothermal energy, and has become one of the practical technical foundations for realizing low energy consumption, high efficiency, diversified cold and heat sources and green air conditioning. It has broad development prospects in realizing the harmonious coexistence and sustainable development between man and nature.
Several characteristics of double heat pipe technology
Compared with conventional heat exchangers, heat pipe heat exchangers with heat pipes as heat transfer units have many unique advantages:
2. 1 High heat transfer efficiency.
Because the heat pipe heat exchanger takes the heat pipe as the heat transfer unit, the heat pipe has a high thermal conductivity, and compared with metals such as silver, copper and aluminum, the heat pipe per unit weight can transfer several orders of magnitude more heat. The efficiency of heat pipe heat exchanger can often reach more than 80%, which can effectively use almost unlimited low-grade energy such as industrial waste heat, solar energy and geothermal energy, and recover the heat energy with small temperature difference between supply and return air of heating and air conditioning system.
2.2 Heat pipe wall temperature adjustability
The temperature of heat pipe wall can be adjusted, which is very important in low-temperature waste heat recovery or heat exchange, because the temperature of heat pipe wall can be adjusted above the dew point of low-temperature fluid through proper heat flow transformation, thus preventing dew point corrosion and ensuring long-term operation of equipment.
The adjustability of heat pipe wall temperature has another significance for heating and air conditioning systems. In the air supply and return system, by adjusting the wall temperature, the cold end temperature is lower than the dew point temperature of cold fluid, which can not only recover sensible heat, but also enhance dehumidification and reduce latent heat load, thus greatly improving the effect of heating and air conditioning system.
2.3 Constant temperature characteristics
The development of variable heat pipe (VCHP) can realize the constant temperature characteristics of cold and heat sources under off-design conditions. It can not only keep the temperature of condensation section or heat sink unchanged when the temperature of heat load or heat source changes greatly, but also be successfully used to keep the temperature of heat pipe or heat source unchanged when the temperature of heat load or heat sink changes. This characteristic of heat pipe makes it attractive in heating and air conditioning engineering that other heat exchange methods do not have. For example, the exhaust gas emitted by the automobile heats the passenger compartment, and the heat load of the heat source and the exhaust temperature vary greatly with the working conditions of the automobile, but the temperature of the condensation section or heat sink is required to remain unchanged. Heat pipe liquid control technology can solve this problem.
2.4 strong adaptability
The structure and location of cold and hot sections are flexible, which can realize the separation of Huiyuan. The structural design and position arrangement of the heated part and the exothermic part of the heat exchange equipment composed of heat pipes are very flexible. The separation distance of Huiyuan can be determined according to the actual needs and the performance of the adopted heat pipe, which can be from tens of centimeters to 100 meters, and zero leakage of hot and cold fluids can be realized. It is especially suitable for waste heat and waste heat recovery in toxic environment in heating and air conditioning engineering.
In addition, the heat pipe heat exchanger has the characteristics of safety, reliability, low resistance and unidirectional heat conduction (thermal diode). The heat pipe heat exchanger has almost no mechanical obstacles and is a secondary wall heat exchange. Generally, the heat pipe will not be destroyed in the evaporation section and the condensation section at the same time, and the reliability of equipment operation is greatly enhanced. Heat pipe heat exchanger is very suitable for recovering waste heat from various continuous production processes as a heat source for heating and air conditioning projects. The principle of thermal diode has very important application value in solar energy, ground source energy utilization and heat recovery engineering of air conditioning system.
Application of three heat pipes technology in heating and air conditioning engineering
The heat pipe was officially invented in the United States from 1964. After more than 30 years of development, heat pipe heat transfer technology has been widely used because it has many unique advantages that conventional heat transfer technology does not have. The focus of application has shifted from aerospace to the ground, and from industrial applications to civilian products. After more than 20 years of efforts, the industrial application of heat pipe technology in China has reached the international advanced level, and its application in the field of heating and air conditioning has also made great progress [4 ~ 8].
3. 1 Ventilation and air conditioning project using heat pipe technology to recover heat energy
Commercial buildings and public institutions generally have high-power ventilation systems. The fresh air coming in from the outdoor is first treated to make its temperature and humidity within the range specified in the design, and then the conditioned air discharged from the building structure is constantly replaced. In this air replacement process, the energy consumption is quite large. Therefore, it is of great economic benefit to recover the heat energy in the exhaust of air conditioning equipment by some method. Heat pipe heat exchanger has the characteristics of high efficiency, isolation performance and thermal diode function, which is very suitable for recovering heat energy from air conditioning exhaust.
If the fresh air volume of air conditioning system is considered as 30% of the air supply volume, the split thermosyphon heat pipe cold and heat recovery device can save energy by more than 7%. Experiments show that as long as the temperature difference between hot and cold air exceeds 3℃, energy can be recovered. Accordingly, the cold recovery time of air conditioning in the middle and lower reaches of the Yangtze River in Shanghai, Nanjing and other areas in China can reach more than 1500h in summer. According to meteorological parameters, the initial investment cost of equipment can be recovered within three years.