The air-cooled heat pump water-cooling unit has been widely used in most parts of China due to its advantages of energy saving, cooling and heating, flexible and convenient use, low initial investment and small space. However, the biggest problem encountered in the heating operation of the air-cooled heat pump unit in winter is the frost on the surface of the evaporator. Due to the formation and growth of the frost layer, the heat transfer resistance between the surface of the evaporator and the air is increased, and the airflow is increased through the evaporator. The flow resistance at the time causes the air flow rate through the evaporator to decrease, and the heat exchange efficiency is significantly reduced, resulting in a decrease in the amount of heat exchange between the air and the evaporator, and the working condition of the heat pump unit is deteriorated or even not working properly. Therefore, the air-cooled heat pump must be defrost at the right time when operating under frost conditions.
At present, the common defrosting method for air-cooled heat pump units is realized by reverse circulation (refrigeration cycle) of the system. However, this traditional four-way valve reversing defrost method has a series of drawbacks: due to the reversal of the four-way valve, the original high and low pressure parts of the refrigeration system are switched, which causes the refrigeration system to "run the oil" phenomenon and reduce the reliability of the system. And service life; when defrosting, the refrigerant absorbs heat from the heating system for defrosting, causing the temperature of the hot water to fluctuate sharply, thus affecting the comfort of the air conditioning system; and from the start of defrosting to the end of defrosting, four The valve must be operated twice, and the system's high and low pressures are also switched 2 times and then re-established balance, which not only causes energy loss but also lengthens the total defrosting process of the system. Therefore, the development of defrosting methods, defrosting control strategies, and new defrosting methods has become one of the research priorities of heat pump air conditioning systems.
However, most of the research at home and abroad has been carried out or improved around the reverse defrosting method, but the various drawbacks inherent in the reverse defrosting method (caused by the four-way valve reversal) cannot be eliminated. Therefore, in order to solve the many drawbacks brought by the traditional four-way valve reversing defrosting method to the air-cooled heat pump system, and improve the overall performance of the unit, it is of great significance to develop a new defrosting method.
To this end, the author developed a new type of defrost DDD sensible defrosting. The sensible heat defrosting method and the reverse defrosting method were compared. The obvious hot defrosting mode of the test table not only solved the various drawbacks of the reverse defrosting method, but also the defrosting time. Great comfort in terms of comfort.
In this paper, the theoretical analysis and experimental research methods will be used to study the energy required for the sensible defrost process, and compare it with the energy required for the reverse defrost process.
1 sensible heat defrosting principle and working process analysis and analysis of traditional defrosting methods, it can be found that the current drawbacks of defrosting methods are mainly attributed to the four-way valve reversing. In this paper, the author proposes a new type of defrosting method DDD sensible defrost.
The sensible heat defrost refers to the bypass circuit before the exhaust pipe of the refrigeration system compressor to the electronic expansion valve, and the high temperature and high pressure exhaust gas of the compressor is directly led to the electronic expansion valve, and then the throttling of the electronic expansion valve The compressor exhaust gas is introduced into the air heat exchanger, and the frost layer on the fin side of the air heat exchanger is removed by the heat of the compressor exhaust gas, and the refrigerant flow rate is controlled by adjusting the electronic expansion valve to ensure that the refrigerant is exchanged in the air. Only sensible heat exchange is performed in the heat exchanger without condensation.
In the defrosting process, the four-way valve does not need to be reversed, so that various problems caused by the four-way valve reversing defrosting have been solved. The heat source for sensible heat defrosting is the work done by the compressor and the heat storage of the compressor casing. The cycle principle of the sensible heat defrosting process is as shown. It can be seen that the sensible heat defrost cycle can be approximated as four processes on the pressure map: process 122 represents the compression of the compressor from start to end, during which the refrigerant is compressed from a low temperature and low pressure gas. The high temperature and high pressure gas; the process 223 indicates that the high temperature and high pressure refrigerant gas is throttled by the electronic expansion valve to become a high temperature and low pressure gas; the process 324 indicates that the high temperature and low pressure refrigerant gas is cooled in the air heat exchanger to release heat, and this is also The process of melting the frost layer on the heat exchanger fins; the process 421 is a process in which the refrigerant is sucked from the compressor to the start of compression, at which time the temperature rises.
2 sensible defrosting method energy analysis and reverse defrosting mode energy consumption 211 evaporator total defrosting process required for defrosting process According to Krabow et al. experimental research, the entire defrosting process, the evaporator surface will appear frosting , melting, wet and dry surface conditions. Therefore, the whole process can also be divided into four stages: preheating stage, melting stage, evaporation stage and drying heating stage.
From the energy point of view, during the defrost process, heat is transferred from the refrigerant to the pipe wall, part of the energy is absorbed by the pipe wall, part of it is transmitted from the pipe wall to melt the frost layer, or the water on the surface of the heat exchanger is evaporated, and a part is emitted to the air. in.
According to the above process, the energy of the defrosting process can be expressed by the equation as Q r = Q s + Q c + Q f + Q v(1) Q s = C dT dt(2) Q c = αc A c( T w - T a) (3) Q f = α w A c ( T w - T f) (4) Q v = R vh fg (5) where Q r is the amount of heat transfer from the lateral wall of the refrigerant, that is, The total heat required for the frost; Q s is the heat stored in the pipe wall; Q c is the convective heat transfer between the pipe wall and the air; Q f is the heat absorbed by the frost layer; Q v is the heat required for the evaporation of water; C is the heat capacity of the pipe wall; T w is the wall temperature, T f is the refrigerant temperature; h fg is the latent heat of vaporization of water; aw and R v are the direct heat transfer coefficient and moisture of the frost layer and the pipe wall, respectively. Rate.
In order to determine the effect and performance of the sensible heat defrosting method, this paper compares the sensible heat defrosting method with the reverse defrosting method under the same conditions. The same condition refers to the same time when the heat pump unit is operated under the same ambient temperature and humidity. In this way, the energy required for defrosting by two kinds of defrosting methods is analyzed under the same conditions of frost formation.
From the representation of the sensible heat defrost cycle schematic diagram and the reverse defrost cycle schematic diagram on the pressure map, it can be seen that when the reverse defrost cycle is run, the refrigerant directly enters the evaporator after being discharged from the compressor, and the sensible heat defrost is operated. When the refrigerant is discharged from the compressor, it enters the evaporator after being throttled by the electronic expansion valve (this causes the refrigerant not only to reduce pressure but also lowers the temperature), and the difference causes the sensible heat to enter the evaporation cycle. Refrigerant temperature ratio reverse
The defrost cycle refrigerant has a lower temperature. By analyzing the two kinds of defrosting methods, in the case of the same degree of frost formation in the evaporator, this paper assumes that: 1 the surface of the evaporator has the same amount of frost; 2 after the frost layer melts, the remaining water on the surface of the evaporator is the same. It can be concluded from the assumption that Q f is equal during the defrosting process, and that Q v is equal by the assumption 2.
According to the previous analysis, the temperature of the refrigerant entering the evaporator during sensible heat defrosting is lower than the temperature of the refrigerant entering the evaporator during reverse defrosting, which can be obtained by formula (2) and formula (3). The Q s and Q c at the time of sensible heat defrosting are smaller than the Q s and Q c at the time of reverse defrosting. Then, the Q r at the time of sensible heat defrosting is smaller than the Q r at the time of reverse defrosting. Therefore, from the above analysis, it can be concluded that the sensible defrost cycle requires less heat than the reverse defrost cycle at the same degree of frosting.
The defrosting energy analysis of the two types of defrosting methods is known from the previous analysis. Different defrosting methods are used, and the defrosting heat required under the same conditions is not equal. Similarly, running different defrosting methods, the composition and source of heat required for defrosting are not the same. It is assumed that the reverse defrost cycle is indicated by the subscript n, and the sensible defrost cycle is indicated by the subscript x. The defrosting energy of the two different defrost cycles is modeled and compared below.
1 Reverse defrosting method In the reverse defrosting process, the heat required for defrosting is mainly composed of two parts: Q n = Q y1 + Q sh (6) where Q y1 is the compressor during reverse defrosting Work; Q sh is the amount of heat extracted from the refrigerant water during reverse defrosting.
Assuming that the reverse defrosting time is t n1 and the average compressor power during the defrosting process is W n1 , then Q y1 = W n1 t n1 (7) The author believes that the defrosting time spent on the defrosting method should be removed from the system. The sum of the time it takes for the frost to end the defrost and restore the system to its pre-defrost state, not just the time it takes to start the defrost to the end of the defrost.
Therefore, after the end of the reverse defrost cycle, the system still needs to run the heating cycle for t n2 to restore the system to the state before the defrost starts, that is, the temperature of the refrigerant water is raised to the temperature before defrost. Therefore, Q sh = Q y2 + Q a2 (8) where Q y2 is the work done by the compressor when the system recovers the defrosting state; Q a2 is the amount of heat that the system draws from the environment during this period.
When the system runs heating cycle to heat the refrigerant water to restore the level before defrost, because the system just switches the four-way valve, it is not stable operation, and the unit heating cycle CO P takes an average of 2 (averaged by the dynamic process of switching the four-way valve of the prototype) Out).
Assume that the recovery time of the reverse defrost system is t n2, and the average power of the compressor during recovery is W n2, then Q y2 = W n2 t n2(9)Q sh = 2 W n2 t n2(10) Reverse defrost The various drawbacks such as "running oil" and other phenomena, the analysis of its roots lies in the reversal of the four-way valve.
During the whole defrosting process, the four-way valve needs to be reversed twice. In each process of reversing, the evaporator and condenser of the system must be exchanged once, and the heat exchanger changes from high temperature and high pressure to low temperature and low pressure. Or from low temperature and low pressure to high temperature and high pressure, the original cycle of the system is destroyed, and the pressure and temperature distribution must be re-established through the compressor work, which causes a large amount of energy loss in the refrigeration system during the commutation process.
Defining the loss of the system four-way valve commutation is Q sun, n, then Q r, n = Q n - Q sun, n Q r, n = Q y1 + Q sh - Q sun, n Q r, n = W n1 t n1 + 2 W n2 t n2 - Q sun, n(11) Analyze the commutation process of the four-way valve of the refrigeration system. When the four-way valve is reversed, the pressure in the evaporator and the condenser are interchanged. exchange.
It can be thought of as a combination of a shutdown process and a startup process for a refrigeration system. The energy loss caused by the two-way four-way valve commutation during the reverse defrosting process is equivalent to the energy loss caused by the two-time shutdown of the refrigeration system. Therefore, this paper studies the energy loss caused by the four-way valve commutation system based on the energy loss method of the air conditioner start-stop process.
In order to study the start-stop loss of the air-conditioning system, Guo Xianming calculated the curve of the cooling capacity and CO P over time during the startup process and the variation of the system cooling capacity with time during the shutdown process by solving the pressure curve obtained by the dynamic process of starting and stopping the system. .
In order to evaluate the energy loss, the energy loss rate R loss is introduced: that is, the ratio of the cooling capacity loss due to the start-stop of the system to the steady-state cooling capacity Q cyc of the system in one cycle, ie
R loss = t on Q cyc -∫t on 0 q cyc q 0 dt +∫t off 0 q cyc q 0 dt t on Q cyc(12) defines T cyc as the system start and stop cycle; T on, T off respectively For the system, the compressor working time and shutdown time in the start and stop cycle; F n = t on /T cyc is the starting ratio, which can be approximated as the ratio of the air conditioning load to the steady state cooling capacity of the air conditioner under the same indoor and outdoor meteorological conditions; = 1 /T cyc is the system start/stop frequency (times/h).
Then, the equation (12) can be written as R loss = 1 -∫t on 0 Q cyc Q 0 dt -∫t off 0 Q cyc Q 0 dt fr F n According to the experiment and calculation, the energy loss coefficient during the start and stop of the system is obtained. R loss as a function of F n and fr.
In the case where F n and fr are known, the system energy loss rate can be derived from the curve, and multiplied by the system cooling capacity in one cycle to obtain the specific energy loss caused by the start and stop of the system.
Reverse defrost 1 time can be equivalent to the system start and stop 2 times, F n and fr can be determined by the defrost cycle and the defrost time, from which the ratio of reverse cycle energy loss can be obtained.
2 sensible heat defrost mode can be seen from the P 2 h of the sensible heat defrost cycle principle, in the sensible heat defrost process, the heat required for defrost is Q x = Q y3 + Q xu (13) Q y3 is the work done by the compressor during sensible heat defrosting; Q xu is the heat storage of the compressor. Since the criteria for considering the defrost time are the time taken by the refrigeration system from the start of the defrost to the end of the defrost and the restoration of the system to the pre-defrost level, the source of the heat of the sensible defrost is only investigated when the defrost time is examined. It is the work done by the compressor, because even if the compressor heat storage provides defrosting heat during the sensible heat defrosting process, the heat is still added back when the system returns to the pre-defrosting level after the defrosting.
Therefore, from the perspective of time, the source of heat for sensible heat defrosting is mainly the work done by the compressor.
Assuming that the reverse defrosting time is t x1 and the average compressor power during the defrosting process is W x1, there is Q x = W x1 t x1 because the temperature and pressure distribution of the condenser and evaporator change during the sensible heat defrosting process. It will also cause a part of the energy loss Q sun, x.
It can be seen from the pressure enthalpy diagram and test of the sensible heat defrosting method that the temperature of the condenser, the pressure of the evaporator and the pressure of the evaporator are small, mainly caused by the temperature distribution change of the evaporator, because this part of the heat calculation is complicated and relatively The total defrost heat is very small, so Q r, x = Q x - Q sun, x = W x1 t x1 - Q sun, x 3 test study 311 test device author in Changzhou Aster Air Conditioning Equipment Co., Ltd. With the assistance, a test prototype was developed based on the sensible heat defrosting scheme. The prototype system flow is as shown. A detailed description of the prototype system can be found in the literature.
312 System Performance Test Bench In order to ensure the accurate and effective test results, the prototype test was conducted at the Refrigeration and Air Conditioning Product Testing Center of Changzhou Aster Air Conditioning Equipment Co., Ltd. The test center was designed and built by the Hefei General Machinery Research Institute.
313 test results and analysis to verify the effect of sensible heat defrosting, and compared with four-way valve reversing defrosting, the authors set different defrosting control procedures for the prototype, at the same degree of frosting (the prototype is the same) The same time was spent in the temperature and humidity environment, and the artificial simulation environment was guaranteed by the product testing center. Two kinds of defrosting methods were compared.
In order to take the four-way valve reversing defrosting and sensible heat defrosting, the temperature curve of the inlet and outlet water of the shell-and-tube heat exchanger is compared. Where T out, T in is the condenser water and inlet water temperature. For better comparison, the absolute time of the two defrosts is translated. For the specific defrosting test process and the comparative analysis of the results, this paper only analyzes the time of the defrosting process.
It can be seen that both start the defrost at 600 s, the reverse defrost ends at 750 s, and the four-way valve switches. However, the condenser outlet temperature returned to the pre-defrost level at 900 s. The sensible heat defrosting ended the defrosting at 820 s and returned to the pre-defrosting level at 880 s.
In reverse defrosting, the refrigerant extracts heat from the hot water supplied by the condenser, causing the temperature of the water to drop sharply. In the case of sensible heat defrosting, the hot water can still be supplied because the refrigeration system does not extract heat from the supplied hot water. Provide a certain amount of heat to the room and lower its temperature.
Therefore, there is a recovery period (60 s) of the condenser outlet water temperature after the completion of the defrosting, that is, compensating for the temperature of the hot water supplied to the air conditioning system during the defrosting period to cause the temperature to decrease. This time has nothing to do with the energy required for defrosting. Therefore, it is not considered in the verification of energy analysis tests.
From the above test, we can get: t n1 = 150 s, t n2 = 150 s, t x1 = 220 s.
At the same time, because in the defrosting process, because the refrigeration system changes drastically, the input power of the compressor also changes greatly, so the power of the compressor is averaged, measured by the test, W n1 ≈ W n2 = 51 kW , W x1 = 45 kW, compressor power W = 60 kW during normal heating. Normally taken heat CO P is 3 (measured by normal heating operation of the unit).
For the reverse defrosting Q r, n = W n1 t n1 + 2 W n2 t n2 - Q sun, n from the previous analysis, the loss caused by the reversing of the four-way valve in the reverse defrosting is equivalent to the refrigeration system in one Loss of 2 shutdowns during the frost cycle.
The defrost cycle taken by the test is 1 h, and the on-off loss curve can be found. The energy loss caused by the four-way valve commutation accounts for 2% of the total system heating energy, then Q r, n = 51× 150 + 2 × 51 × 150 - 60 × 2 % × 3 × 3600 = 9 990 kJ sensible heat defrost Q r, x = Q x - Q sun, x = W x1 t x1 - Q sun, x because of sensible heat The Q sun, x during the defrosting process is mainly caused by the change in the temperature distribution of the evaporator. Since this part of the heat calculation is complicated and the heat relative to the total defrost is small, the calculation is ignored.
Then there is Q r, x = 45×220 = 9 900 kJ, so there are Q r, n≈Q r, x. The above test shows that the same frosting degree (the prototype runs in the same temperature and humidity environment for the same time) Under the author, the authors have verified the theoretical energy analysis of the reverse defrosting method and the sensible heat defrosting method.
From the above analysis, it can be seen that under the test conditions, the sensible heat defrosting time is 220 s, and the reverse defrosting time is 300 s, so it can be obtained that the sensible heat defrosting time is shortened by 2617 % compared with the reverse defrosting time. Defrost The shortening of the time means that the heating capacity is improved. For the sake of comparison, the author defines the hourly heating rate, that is, the ratio of the hourly heating time to the total running time of the unit operation. In the test, a defrost operation cycle for reverse defrosting is 3 900 s, in which the heating time is 3 600 s and the defrosting time is 300 s. In the same time, the sensible heat defrosting can supply more heat for 80 s. Therefore, there is: Δξ = 80 3 900×3 600 3 900 = 2 % means that the sensible heat defrosting method can increase the heating rate per hour by more than 2% compared to the reverse defrosting method.
4 Conclusions 1) In view of the shortcomings of the existing reverse defrosting method, a new defrosting method DDD sensible defrosting was developed, and the mechanism, function and process of sensible heat defrosting method were analyzed.
2) From the perspective of the frost layer, the total energy required for the defrosting process of the evaporator is analyzed. Under the same conditions (the same degree of frosting), the theoretical heat comparison between the sensible heat defrosting method and the reverse defrosting method is carried out. The heat required to obtain a sensible defrosting method is less.
3) From the perspective of the refrigeration system, the defrosting heat composition of the two defrosting methods was studied. Reverse Defrost Mode Because there is a four-way reversal of the four-way valve, the pressure and temperature distribution of the refrigeration system are destroyed and needs to be re-established, so there is a large amount of energy loss. However, there is no four-way valve commutation in the sensible heat defrosting mode, and the energy loss is small.
4) Two kinds of defrosting methods were compared. The test results showed that under the same conditions (the same degree of frosting), the sensible defrosting method was shortened by 2617% compared with the reverse defrosting method. The heat rate is increased by 2%, and the temperature of the refrigerant water fluctuates within 5 °C. Therefore, both the theory and the experiment prove that the sensible heat defrosting method is superior to the reverse defrosting mode in energy saving, defrosting time and comfort.
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