Preliminary study on a new type of heat pipe indir

An initial exploration of a new type of heat pipe indirect evaporative cooler

1 introduction

in 1942, an automotive engineer in the United States invented the heat pipe. In the early 1960s, it was used by the Atomic Energy Association and the National Aerospace Department to cool spacecraft and nuclear reactors. In the 1970s, the heat pipe heat exchanger, as a heat recovery device in the fresh air system, finally showed its advantages in the HVAC industry. Heat pipe is a heat transfer element that realizes heat transfer by relying on the phase change of its internal working liquid. It has the following basic characteristics: ⑴ high thermal conductivity. ⑵ excellent isothermal property. ⑶ variability of heat flux. (4) transferability of heat flow direction. (5) thermal diode and thermal switch performance. (6) constant temperature characteristics (controllable heat pipe). (7) environmental adaptability. Compared with conventional heat exchange technology, the reason why heat pipe technology can continue to be welcomed by the engineering community is that it has the following important characteristics: ⑴ heat pipe heat exchange equipment is safer, more reliable and can operate continuously for a long time than conventional equipment. (2) temperature adjustability of heat pipe wall. ⑶ the structural location of the cold and hot sections shall be arranged flexibly. (4) the heat pipe heat exchange equipment has high efficiency and remarkable energy-saving effect

with the continuous improvement of scientific level, the research and application fields of heat pipe will also be expanded. The development of new energy, chip cooling of electronic devices, CPU cooling of notebook computers, cooling of high-power transistors, silicon controlled components, circuit control boards, and the development of high-efficiency heat and mass transfer equipment in the fields of chemical industry, power, metallurgy, glass, light industry, ceramics, refrigeration and air conditioning will promote the further development of heat pipe technology. Especially in the refrigeration and air conditioning industry, due to the small temperature difference between cold and hot fluids, heat pipe technology can better reflect its advantages, making it one of the practical technical foundations to realize low energy consumption, high efficiency, diversity of cold and heat sources, and take the road of green air conditioning

2 application of heat pipe technology in HVAC

2.1 application of heat pipe technology in air conditioning heat recovery

energy recovery of air conditioning system is one of the contents of air conditioning energy conservation. In commercial buildings, most of the air conditioning return air is sent to the air conditioning room as supply air after cooling and reheating, while the return air of other parts is discharged. At the same time, a large amount of fresh air enters the air-conditioned room after treatment. Because the new and return air need to be cold and heat treated, the exhaust takes away a large amount of energy. Therefore, the research on heat recovery of the air-conditioning system is of great significance to the energy conservation of the air-conditioning system. At present, the domestic devices for recovering exhaust energy are mainly total heat exchangers, and the corresponding runner heat exchangers, plate fin heat exchangers and coil heat ring heat exchangers have been developed. Although the recovery efficiency of this kind of equipment is high, the manufacturing process is complex, the cost is high, and there is cross pollution between the inlet and exhaust air. It is not suitable for hospitals and air-conditioned rooms that produce harmful gases. If sensible heat exchangers are used, the temperature difference between the inlet and exhaust air is small (generally Δ T=10 ℃), which is bound to increase the heat exchange area, but also brings a lot of inconvenience to the air duct connection. Heat pipe will be widely used in heat recovery and heat control of air conditioning system because of its fast heat transfer speed, small temperature drop, simple structure and easy control. The application of heat pipe heat exchanger in air conditioning energy saving has attracted extensive attention abroad. Taking the preheating of heating air with an exhaust volume of 35000m3/min as an example, the exhaust temperature can be reduced from 40 ℃ to 22 ℃, the fresh air temperature can be preheated from 0 ℃ to 8 ℃, and the recovered heat is 459.976kj/h

2.2 application of heat pipe technology in room air conditioners

when the existing room air conditioners are used in wet areas, due to their small dehumidification capacity, the corresponding latent heat load removed is also small, while in wet areas, the latent heat load accounts for a large proportion of the total heat load. Therefore, the air conditioning effect in the room is not ideal, and the indoor comfortable environment cannot be created well. In view of the problem that the latent heat load accounts for a large proportion of the total heat load of air conditioning in humid areas, literature (3) proposes to add gravity heat pipe heat exchanger without changing the compressor, condenser, evaporator and capillary of the original configuration of the room air conditioner. This can significantly increase the dehumidification capacity of the air conditioner and make the air outlet humidity of the air conditioner appropriate. As a result, the dehumidification capacity of the air conditioner can be increased by 30% - 40%. The refrigerating capacity and power consumption of the air conditioner are basically unchanged, and the heat exchange area of the heat pipe heat exchanger required is not more than twice that of the evaporator. The arrangement of heat pipe heat exchanger on the air conditioner is feasible, and will not increase the total volume of the air conditioner much

the combined system of heat pipe and air conditioner is used to recover the exhaust heat in winter, reduce the load of air conditioner and achieve the purpose of energy saving. In summer, the cooling capacity and dehumidification capacity of the air conditioning system can be improved, and the reheat load can be completely or partially cancelled, so as to save energy consumption of the system and achieve the purpose of improving comfort. The air conditioner is simple in structure and easy to manufacture. The heat exchanger integrating heat pipe and evaporator can be easily used in window type and cabinet type air conditioners and dehumidifiers. From the perspective of human comfort, heat pipe air conditioning can meet the requirements of human comfort with more comfortable air supply

2.3 heat pipe heating system

with the development of multi-storey buildings, this has brought problems to the pressure bearing capacity of existing steel radiators. In order to improve the pressure bearing capacity and solve oxygen corrosion, in recent years, some forms of heat pipe radiators have emerged in succession and become a hot spot in the development of radiators. Compared with ordinary water heat medium radiators, heat pipe radiators have the following characteristics: ⑴ the surface temperature is uniform. ⑵ oxidation corrosion is easy to occur without ordinary radiator. ⑶ the amount of heat medium required is greatly reduced, which can greatly save the transmission power consumption and simplify the transmission pipeline system. (4) it is not restricted by water pressure, convenient and flexible installation and less maintenance workload

in addition, because the heat pipe itself has many characteristics, such as temperature uniformity, heat flux can change, thermal diode characteristics, etc., it has a very wide application prospect in the utilization of solar energy and the development of geothermal resources. Heat pipe collector has become an important form of solar heat collection, so the development of solar air conditioning also has broad application prospects. The combination of heat pipe technology and ground source heat pump can make the ground source heat pump technology develop its strengths and circumvent its weaknesses, save investment, have higher efficiency and stronger adaptability

3 working principle of heat pipe and heat pipe heat exchanger

a typical heat pipe is composed of a shell, a wick and an end cap. Fill the vacuum pipe with appropriate working fluid, stick the wick close to the inner wall of the pipe, and then seal both ends to form a heat pipe. The heat pipe is both an evaporator and a condenser, as shown in Figure 1. The end that absorbs heat from the heat flow is the evaporation section. The working medium absorbs the latent heat, evaporates and vaporizes, flows to the end of the cold fluid, that is, the condensation section releases heat and liquefies, and flows back to the evaporation section by capillary force, and automatically completes the cycle. The heat pipe heat exchanger is a device that is assembled by these single heat pipes, and the evaporation section and the condensation section are separated by a partition in the middle. The heat pipe heat exchanger does not need external power to promote the circulation of the working fluid, This is one of its main advantages

Figure 1 heat pipe and heat pipe heat exchanger

4 new heat pipe heat exchanger structure and its working principle

4.1 structure

heat pipe can be roughly divided into three different situations according to different structural forms: integral wick heat pipe, integral thermosyphon (gravity heat pipe), and separated heat pipe

the integral wick heat pipe is the general form of heat pipe. Generally speaking, the heat pipe refers to this kind of heat pipe. The condensate flows back by the capillary action of the wick, and it can work in weightlessness without the action of gravity field. This is its working characteristics. As an energy recovery device, liquid wick heat pipe heat exchanger can recover cooling capacity in summer and heat in winter at the same time. There is no need to change the air flow direction and pipe layout

the characteristics of gravity heat pipe are: heat transfer is unidirectional, condensate flows back by gravity, and the condensation section must be placed on the evaporation section. Because it has no wick, its structure is simpler, it is easy to manufacture, and its working performance is not lower than that of wick heat pipe. The applied heat energy of air conditioning ventilation is low-level heat energy, and most of the heat pipes used are gravity low-temperature heat pipes. Therefore, when the heat pipe heat exchanger is used as a heat recovery system or a cold recovery system, the gravity heat pipe heat exchanger should be preferred wherever possible

the separated heat pipe is used for energy recovery of air-conditioning system exhaust. Ingenious use of the characteristics of the separated heat pipe can not only avoid the complex pipeline design caused by large flow gas migration, but also effectively recover the low-grade energy in the exhaust, reduce the refrigeration/heat of refrigeration/heating equipment, so as to achieve the purpose of energy conservation. At the same time, there are many pipe fittings of separated heat pipes that are in collusion with each other. Once a leak occurs somewhere, it will lead to the loss of the function of the whole row of components or the whole heat exchanger, which should be taken seriously in design and application

the heat pipe heat exchanger is composed of many single heat pipes. When the heat pipe is running, the head-on wind speed is generally limited to the range of m/s. Too high wind speed will lead to excessive pressure drop and increased power consumption. Too low wind speed will lead to the reduction of heat transfer coefficient outside the pipe and the failure of heat transfer performance of the pipe. The heat pipe heat exchanger for air conditioning belongs to the gas gas heat pipe heat exchanger. In order to improve the heat transfer coefficient of gas, the method of adding fins to the tubes is often adopted, which can greatly improve the heat transfer capacity and reduce the number of heat pipes required. Table 1 gives the specifications commonly used in air conditioning and industry for design reference. Table 1 common parameters of finned tubes

remarks: 23 is the non-ferrous metal flue gas side, generally

the structure selection of this new type of heat pipe indirect evaporative cooler proposed in this paper should be based on the actual working environment and combined with various aspects, such as efficiency, the market value of plastic machinery orders in the fourth quarter of the United States increased by 3 percentage points compared with the third quarter of 2013, accounting for ground area, cost, maintenance, layout of water spraying devices, etc., so as to select the optimal layout method

4.2 working principle

indirect evaporative cooling is achieved through various types of heat exchangers, mainly including: ⑴ plate ⑵ tube type. Here, the heat pipe heat exchanger is proposed to be used in the indirect evaporative cooling technology to form a heat pipe indirect evaporative cooler, and one end of the heat pipe is coated with water absorbing material and equipped with a spray device. The schematic diagram of this new type of heat pipe indirect evaporative cooler is shown in Figure 2

take summer as an example. In summer, the indoor temperature is lower than the outdoor temperature. The outdoor fresh air (primary air) flows through channel 1, that is, the evaporation section, which promotes the evaporation of the working fluid and takes away the latent heat of vaporization. The indoor exhaust (secondary air) flows through channel 2, that is, the condensation section. The steam flows into the condensation section due to the saturation pressure difference between the evaporation section and the condensation section, and releases the latent heat at the same time. In this way, the continuous circulation realizes the dry cooling effect on the outdoor fresh air side

in the condensing section, the water spraying device sprays water to the heat pipe. In order to improve the evaporation speed and strengthen the heat exchange efficiency, the water absorbing material is coated in the condensing section of the heat pipe, so as to increase the contact area. If water absorbing materials with strong moisture absorption and desorption capacity and good moisture retention performance are used, the saturated steam adjacent to the water film can be quickly discharged, the evaporation rate of the condensation section can be accelerated, and the heat energy can be released faster. Due to the moisture retention of the water absorbing material, there is no need to continuously spray water to the condensation section of the heat pipe, which greatly reduces the power consumption

in winter, the indoor exhaust temperature is higher than the outdoor fresh air, so the indoor exhaust can be used to preheat the outdoor fresh air. Because the heat pipe (heat exchanger except gravity type) is reversible, the same pipe can be used to preheat and precooling in winter and summer, without modifying the pipe or changing the air flow