The air-cooled heat pump air conditioning unit is an energy-saving air conditioning device that does not pollute the environment, can be cooled or heated, and has a high energy efficiency ratio. It does not require a separate machine room and has a small footprint. At present, many developed countries have vigorously developed heat pump technology according to their actual national conditions, and have formed their own characteristics. Since the 1990s have been widely used in China, the use of the region has been rapidly promoted from south to north.
However, defrosting has always been a headache for heat pump users when heating in winter in cold and high humidity areas.
In the winter operation of the air-cooled heat pump unit, when the temperature of the outdoor heat exchanger coil is lower than the dew point temperature, condensed water is generated on the surface, and the condensed water is frosted once it is lower than 0 °C. If the surface of the heat exchange coil is frosted due to low temperature, it will inevitably reduce the efficiency of heat exchange. Although the condensed water forms ice when frosting, the total amount of latent heat exchange is equal, but the ice layer on the coil not only reduces the heat exchange area, but also reduces the amount of air that can pass. As the frosting thickens, the heat exchange efficiency of the air and the coil, that is, the heat exchange efficiency of the air and the refrigerant, becomes worse and worse. The frost layer also increases the resistance of the air through the fins, changing the geometry of the coil and ultimately reducing the air volume of the unit.
This is a vicious cycle. Once the coil is frosted, the heat exchange area is reduced, the air volume is reduced, and the evaporation of the system continues, so the heat exchange required can only be maintained by lowering the evaporation temperature. Decreasing the evaporation temperature will further reduce the surface temperature of the coil, increase the temperature between the air inlet and outlet, and finally increase the thickness of the frost layer. In severe cases, the coil will freeze and the unit will not operate.
2 Defrost method and principle The defrost process usually consists of three stages: the first stage fan stops, so that the condensation temperature rises as soon as possible to defrost, the second stage frost gradually melts, and the fan does not open, so as to avoid heat loss. In the air around the heat exchanger. The third stage of the fan is turned on, mainly to make the frost that has melted into water all evaporate and dry.
Sun Yuqing et al. introduced the nucleation theory, crystal dynamics theory and meteorological theory for the complex heat and mass transfer problem of frosting, which is unsteady, phase transition and moving boundary, and established a more accurate physical and mathematical model. Studies have been conducted to inhibit frost formation.
Theoretical studies and experiments have shown that the high hydrophobic coating on the surface of the heat exchanger reduces the surface energy between it and the water vapor, increases the contact angle, and is effective for suppressing frost formation. In addition, in order to suppress frost formation, the humid air flowing into the heat exchanger can be purified, and when necessary, the wind speed can be increased, so that ice crystals or supercooled water droplets formed in the gas phase can pass through the heat exchanger wall as quickly as possible. Based on some research results at home and abroad, defrosting is nothing more than electric heating, hot gas defrosting, reversing, and hot water defrosting.
2.1 electric heating defrosting electric heating method is the simplest method of defrosting. The idea is to install an appropriate power resistor on the outdoor heat exchanger. When the frost on the outdoor heat exchanger is severe, the electrical switch is turned on, and the heating wire is energized and heated to melt. A common electric heating defrost circuit control diagram is shown.
The CT is a defrost time controller with a single pole double throw switch and an electric motor that drives the drive gear for timing. In the heating operation condition, the contacts 2, 3 are turned on, and the liquid supply solenoid valve coil YV is energized. The high-temperature and high-pressure steam coming out of the compressor enters the indoor condenser, liquefies into a medium-temperature high-pressure liquid, is throttled by an expansion valve (or capillary tube), is converted into a low-pressure low-temperature vapor-liquid two-phase mixture, and then enters the surface of the heat pump through the liquid supply solenoid valve. The heat exchanger evaporates and absorbs heat, and finally converts it into a low-pressure low-temperature vapor and is withdrawn by the compressor to complete a heating cycle. When the defrost time arrives, the contacts 2, 3 are open and 2, 4 are turned on. When the liquid supply solenoid valve coil YV is de-energized, the compressor suction pressure drops. When it is lowered to the lower limit of the pressure controller, the pressure controller contact trips, the AC contactor coil IKM loses power, and the refrigeration cycle stops running. And because the AC contactor coil 2 KM is powered, its contacts close the electric heating wire to heat the defrosting. When the defrost ends, the contacts 2, 4 are turned off, 2, 3 are turned on, and the heating cycle resumes running heating.
2.2 Reversing method When the outdoor heat exchanger has frost, which affects the normal heat exchange effect, the heat pump can be converted into a cooling process through the four-way reversing valve by using the two-way reversing valve of the heat pump, so that the heat pump absorbs heat from the indoor Discharge to an outdoor heat exchanger to melt the frost on the outdoor heat exchanger. This method does not require any additional equipment, just let the four-way reversing valve act when defrosting is required. But it also brought problems. 1 During the defrosting period, the indoor temperature drops, affecting comfort. 2 Four-way reversing valve has frequent movements, high noise and easy wear. 3 For units with higher power, due to their large diameter, they cannot be equipped with four-way valves, and a conversion line with 8 valves is specially designed.
Yuan Xiuling et al. have theoretically and experimentally proved that the use of thermal expansion valves is more efficient than capillary tube throttling when using the commutation method for defrosting. The bypass copper tube is better than the thermal expansion valve. In order to study the effect of different throttling mechanisms on the reverse cycle defrosting time, a bypass copper tube with an outer diameter of 22 mm and a thermal expansion valve were used as the throttling mechanism during defrosting, respectively. An experimental study was carried out on a 55 kW air-cooled heat pump water chiller unit.
The results show that the defrosting time of the bypass copper pipe system is shortened by 1.5 min compared with the thermal expansion valve system, in which the defrost time is shortened by 1.3 min and the drainage time is shortened by 0.2 min.
2.3 Hot Fluoride Defrosting Method If a bistable solenoid valve is indirectly connected from the exhaust port of the compressor to the pipe of the condenser and the pipe of the liquid supply solenoid valve to the chiller in the refrigeration system, the high temperature in the compressor can be utilized. High-pressure refrigerant (based on fluoride) is defrosted as shown.
The bistable solenoid valve has a signal input and a signal output. It has two working states: "on" and "off". When the forward pulse signal is input, the bistable solenoid valve is in a "off" state.
Conversely, when the reverse pulse signal is input, it is in an "on" state. When the temperature measured by the temperature sensing element of the CT reaches the defrost temperature, the outdoor fan stops and the liquid supply solenoid valve closes. At the same time, the thermostat applies a reverse pulse electrical signal to it, and the bistable solenoid valve opens, bypassing the high pressure gas of the refrigeration system and sending it to the outdoor heat exchanger for defrost. The high temperature and high pressure gas is exothermicly cooled in the chiller and sucked back by the compressor. At this time, the frost layer condensed on the surface of the outdoor heat exchanger is heated by the heat released from the high-temperature and high-pressure gas inside the surface heat exchanger, and melts into water at the contact with the metal surface, and the adhesion is lowered. The frost layer falls into the water tray by its own weight and is discharged from the drain pipe of the water tray. Achieve defrosting purposes. After the end of the defrost, the thermostat sends a signal, the bistable solenoid valve is closed, the defrost circuit is disconnected, and the system returns to the cooling condition to complete a defrost cycle. In this way, the compressor avoids the frequent drawbacks of opening and stopping, and reduces the energy consumption. The whole system has few self-control components and low failure rate, and the structure of the air cooler is simple, easy to manufacture and can be reduced in size. Reduced costs, improved product quality, and safe and reliable work. In addition, in order to prevent the compressor from inhaling high temperature and high pressure gas after the completion of the defrost, a regenerator can be installed in the suction circuit.
2.4 Improvement of hot fluorine defrosting D hot gas defrosting method Hot gas bypass defrosting is through the bypass circuit, the high temperature exhaust gas of the compressor is directly introduced into the outdoor heat exchanger to defrost. During the defrosting process, the indoor and outdoor heat exchanger fans stop running, and the heat source of the defrosting is the power consumed by the compressor and the heat storage of the compressor casing. The basic principle is similar to the above thermal fluorine defrosting, as shown in Figure 3. Only its control mechanism is more precise and complicated, and it can achieve defrosting on demand, avoiding excessive pumping from the compressor and reducing the efficiency of the compressor.
A gas-liquid separator is used here, and the separated liquid is stored in the separator, and the gas is heated by the compressor to be used for defrost. Thus, as the defrosting progresses, the liquid refrigerant continuously returns to the gas-liquid separator, and the mass of the refrigerant stored in the coil of the outdoor heat exchanger is gradually reduced, so that the temperature of the refrigerant gradually rises, and the defrosting effect is enhanced. .
2.5 Freon auxiliary heater In places where the outdoor air temperature is low, an auxiliary heater is required due to insufficient heat supply of the heat pump in winter. The common method is to provide a heater at the air outlet of the indoor unit. This method not only has low heat transfer efficiency, poor safety performance, but also has a long defrosting time and a large indoor temperature drop. The Freon heater can obviously overcome the above defects. The indoor side heat exchanger is divided into two parts, and a Freon auxiliary heater is added in the middle. When the heat pump is running in winter, the high-temperature Cleone gas discharged from the compressor enters the front part of the indoor heat exchanger. Condensate into a liquid. At this time, the portion of the liquid is again evaporated into a gas by heating by the Freon heater, and then enters the latter half of the indoor heat exchanger. In this way, according to the whole indoor heat exchanger, the heat absorbed by the outdoor heat exchanger of the heat pump is transmitted to the air-conditioned room together with the heat generated by the Freon heater, which complements the insufficient heat supply due to the low outdoor environment temperature. . The related literature describes the test on the KFRd-70LW heat pump air conditioner, which has a good auxiliary heating effect, and the defrosting time is reduced from 3min to 1min (outdoor temperature -1 °C); from 10min to 3min (outdoor temperature -7) °C).
3 Defrost time According to the relevant literature excerpts, and after two years of on-site tracking test, it was found that the defrost loss accounted for 10.2% of the total energy loss of the heat pump, and due to the problem of the defrost control method, about 27% of the defrost function is Frosting on the surface of the fins is not serious and does not require defrosting to enter the defrost cycle. Some of the methods currently used have more or less problems, such as the occurrence of redundant defrosting actions, or the need to even send signals when defrosting is required. Therefore, in order to truly achieve the purpose of energy saving, in addition to selecting a good defrost method, we must also design a good control method, accurately determine the defrost timing, and issue a defrost signal in time.
At present, the widely used indicators for determining the defrosting moment have the following categories: The first type only has simple control time. For example, when the early heat pump is operated in winter, it is mandatory to set the defrost cycle to 30 minutes or 40 minutes. The shortcoming of this method is obvious. The system does not have frost on the surface of the outdoor heat exchanger. The defrosting starts when the time is up, and the energy waste is very large, which does not meet the principle of energy saving. The second type uses the outlet air temperature of the coil as the control index. Usually, a critical temperature T is set first. When the outlet air temperature is lower than this temperature, the timing starts. If the temperature does not rise after 30 minutes, the frost is considered to be accumulated. Serious, start defrosting. This method is operability, but the experience is too heavy, the setting of T is relatively critical, the T value is too high, the defrosting is frequent, the system fluctuates greatly, and the T value is low, the defrosting is not timely, and the frosting time is too long. Also reduce the efficiency of the system. At the same time, different regions, different outdoor climatic conditions, T values ​​must be changed accordingly, not easy to master. The third category uses the pressure difference as the control amount. When the outdoor heat exchanger is seriously frosted, the coil will be blocked, causing an increase in the pressure inside the pipeline; however, the measurement accuracy of the general pressure gauge is insufficient, and the use of high-precision pressure components increases the cost. The fourth category uses fan current as an indicator. Because when the outdoor heat exchanger is seriously frosted, the resistance of the fan increases, and the corresponding performance is reflected in the fluctuation of the fan current. However, there are many factors causing the current fluctuation, which is used as an indicator for determining the defrost time, which is likely to cause false alarms. . The fifth category is judged by the flow rate of the refrigerant in the coil. The flow rate of the refrigerant in the coil is monitored and the difference between the refrigerant and the frost-free coil is compared. When the difference is greater than a certain value, the defrosting condition is started. This method is indeed feasible through two years of monitoring by Strong (1988). In the sixth category, the temperature gradient of the outdoor coil is used as a criterion.
The latter two are recognized in many literatures and are more practical.
The best defrosting time control and the maximum average heat supply control defrosting method proposed in the literature are theoretically new, but it is difficult to implement. Personally think: using the idea of ​​self-adjusting fuzzy defrosting control and the basic structure of the system, determining the difference between indoor and outdoor atmospheric temperature, relative humidity and the rate of change of fin temperature as the input domain, the fuzzification and blurring of the input amount The reasoning method is to realize the simulation of fuzzy defrosting control on the high position machine. The defrosting process is compared with the experimental data by this method, and the judgment result is in good agreement with the actual situation. Compared with the conventional defrosting method, this method not only prolongs the heating working time, reduces the number of defrosting times and the defrosting loss, but also improves the working performance and feasibility of the unit.
4 Conclusion Wuhan is located in Central China, the three towns across the two rivers, the temperature difference between winter and summer is particularly large.
Summer is hot, known as the "fire stove"; winter is relatively cold, humidity is large, but it does not belong to the heating area. Therefore, residents have dual requirements for refrigeration and heating. Heat pump use is very common, and real estate developers are mostly willing to use air-cooled heat pump air conditioners. At present, the small wall-mounted type for household use and the units up to hundreds of KW are used; the potential market for air-cooled heat pumps is very optimistic. However, there are still many problems in the defrosting process of air-cooled heat pump air conditioning units, mainly including: inspiratory and exhaust pressure changes are drastic, the impact on the compressor is large, and the amount of refrigerant in the system is large; the evaporator and the condenser Frequent conversion, destroying the normal operation of the unit, so each defrosting requires a long process to restore the unit to normal operation; the four-way reversing valve moves frequently, affecting its life; due to the beginning of the defrost stage In the pressure decay process, some systems will stop due to the pressure decay to the low pressure protection value; the water supply temperature changes greatly during the defrosting process. These problems have seriously affected the reliability of the unit's defrosting operation. Therefore, it is necessary to further study the defrost mechanism and propose a new defrost method to gradually solve the above problems.
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