1 interface progressive freezing system test device
1.1 Test device During the construction process of this system, it is found that the normal four-way valve in the original system scheme will have a commutation abnormality when working. The reason is that the high and low pressure difference during the operation of the system is small, which is insufficient to overcome the frictional resistance when the main spool moves, and the main spool valve is in the intermediate position state, thereby causing the four-way valve to be reversed. Therefore, the original system was modified to a heat pump system using two compressors, the four-way valve was omitted, and a solenoid valve was added to the suction side of each compressor to prevent the exhaust of one compressor from being operated against the other. The station caused an impact.
The improved interface progressive freezing system mainly has 3 compressors, 2 icing water tanks, throttling components and so on. The system installation diagram is shown in Figure 1. The three compressors form three sets of heat pump systems. The second phase of the system is Han Qiang, et al.: The experimental study of the progressive ice-melting process of the interface is the SHW33TC4-U compressor produced by Shanghai Hitachi Electric Co., Ltd. (HIGHLY) with power of 15,075,075 respectively. kW, using R22 as a refrigerant.
1) Auxiliary refrigeration system refrigerant absorbs heat from high-level icing water tank, and the surface of the straight-light tube heat exchanger in the high-level icing water tank freezes. The refrigerant gas after absorbing heat is compressed by the compressor and then enters the outdoor condenser to release heat. After passing through the accumulator, drying the filter, the solenoid valve enters the throttling valve to throttle, and finally enters the high-level icing tank.
2) Heat pump cycle 1 The refrigerant absorbs heat from the low-level icing tank, and the surface of the straight-light tube heat exchanger in the low-level icing tank freezes. The refrigerant gas after the heat is absorbed by the compressor and enters the high-level icing tank to be discharged. After the heat is passed through the check valve, the solenoid valve, the accumulator, the drying filter, the check valve, the throttling valve is throttled, and finally the low-level icing tank is entered.
3) Heat pump cycle 2 The refrigerant absorbs heat from the high-level icing tank, and the surface of the straight-light tube heat exchanger in the high-level icing tank freezes. The refrigerant gas after the heat is absorbed by the compressor and then enters the low-level icing tank to be discharged. After the heat is passed through the check valve, the accumulator, the drying filter, the solenoid valve, the check valve, the throttling valve is throttled, and finally the high-level icing tank is entered.
1.2 Interface progressive freezing system working process 1) When the system starts, the raw material solution is first added to the high-level icing water tank, the auxiliary refrigeration system is opened, and the ice layer is formed in the high-level icing water tank through the evaporator of the system, until the ice layer reaches a certain thickness After that, the auxiliary refrigeration system is turned off, and the concentrated solution discharge valve of the high-level icing tank is opened, so that the unfrozen concentrated solution automatically flows into the low-level icing tank from the high-level icing tank by gravity.
2) After the concentrated solution is finished, start the ice layer spray washing system, remove the solute on the surface of the ice layer with a small amount of water, and wait until the flushing liquid flows out of the high level icing water tank, discharge it to the low level icing tank, and close the high level icing. The concentrated solution discharge valve of the water tank starts the heat pump cycle 1. The system absorbs heat from the concentrated solution flowing into the low-level icing tank to form an ice layer for further concentration, and sends the obtained heat to the high-level icing tank. In the heat exchanger, the ice layer of the high freezing water tank is melted to form molten water.
3) After the ice layer of the high-level icing tank is completely melted, the heat pump circulation subsystem is turned off, and the melt water valve of the high-level icing tank is opened, the concentrated solution discharge valve of the low-level icing tank, the molten water flows into the melting water tank, and the concentrated solution is discharged. To the concentrated solution tank.
4) After the concentrated solution of the low-level icing tank is finished, start the ice-washing subsystem, rinse the solute on the surface of the ice layer with a small amount of water, and wait until the rinsing liquid flows out of the low-level icing tank and discharge it to the concentrated solution tank. Close the melt water valve of the high-level icing tank and the concentrated solution drain valve of the low-level icing tank.
5) Open the feed solution pump, and the feed solution absorbs the cold amount of the concentrated solution and the molten water through the concentrated solution tank and the melting water tank, and after entering the high-level icing water tank after pre-cooling, after the required liquid level height is reached, the feed solution is closed. The pump starts the heat pump cycle 2, and the heat exchanger in the high-level icing tank is converted into the evaporator of the heat pump system, which absorbs heat from the solution of the high-level icing tank to form an ice layer, and at the same time, the heat exchanger in the low-level icing tank As a condenser of the heat pump system, heat is transferred to the ice layer of the low-level icing tank to melt it to produce molten water.
6) After the ice layer of the low-level icing tank is completely melted, turn off the heat pump cycle 2, open the melt water discharge valve of the low-level icing tank, and let the melt water flow from the low-level icing tank into the melt water tank.
7) After all the molten water in the low-level icing tank has flowed out, close the melt water discharge valve and open the concentrated solution discharge valve of the high-level water tank, so that the unfrozen concentrated solution automatically flows into the low-level icing water tank from the high-level icing water tank by gravity.
8) After the concentrated solution is finished, start the ice layer washing subsystem, remove the solute on the surface of the high-level icing tank ice layer with a small amount of water, and after all the rinsing liquid flows out of the high-level water tank, close the concentrated solution discharge valve of the high-level water tank to start the heat pump. Cycle 1, the system will absorb heat from the concentrated solution of the low-level icing tank to form an ice layer, and at the same time send the heat obtained to the heat exchanger of the high-level icing tank, so that the ice layer of the high-level icing tank melts and forms Melt the water, so cycle back and forth.
2 Test data processing and analysis The experimental conditions of the progressive freezing system of this interface: initial temperature of the solution 5, the ambient temperature of the experimental device 8. In the test, the test temperature parameters are platinum resistance and the national secondary standard mercury thermometer; the thickness of the ice layer is measured by the vernier caliper Evaporation, condensation pressure is measured by a pressure gauge. All test instruments and meters are calibrated before measurement to ensure the accuracy of the measured data.
2.1 Auxiliary refrigeration system performance curve of auxiliary refrigeration system obtained through three sets of tests.
It can be seen that after 5 minutes of system operation, the aqueous solution in the high freezing water tank begins to freeze. As the ice layer is formed and grows on the heat exchange tube, the solute is excluded from the solid phase measurement to the liquid phase measurement at the solid-liquid phase interface. When starting to freeze, the heat transfer resistance is small, because the thickness of the ice layer is small, and the heat transfer resistance is small, so the system COP is large, which is 5; as the thickness of the ice layer increases, the heat transfer resistance increases, the system COP Rapidly reduce, after 20 minutes of system operation, the COP slowly decreases; after 40 minutes of system operation, the COP is rapidly reduced to 2. The average COP of the auxiliary refrigeration system is 35. It can be seen that when the auxiliary refrigeration system starts to freeze, the heat transfer resistance is small. The icing rate is faster; as the ice layer becomes thicker, the heat transfer resistance increases and the icing rate decreases rapidly; when the icing thickness reaches about 35 mm, the icing rate decreases slowly; when the icing thickness reaches about 5 mm, The icing rate drops again rapidly.
It can be seen that the ice thickness at the same time has a small difference, and the test data has good repeatability, which verifies the reliability of the auxiliary refrigeration system.
2.2 Heat pump cycle 1) Heat pump cycle 1 performance curve obtained through 3 sets of tests.
It can be seen that after the heat pump cycle 1 is operated for 5 minutes, the low-level icing tank starts to form an ice layer along the surface of the heat exchange tube, and the ice layer on the heat exchange tube in the high-level icing tank begins to melt. In the low-level icing tank, when the icing starts, the heat transfer resistance is small, the icing rate is faster, and the system COP is larger, 95. As the ice layer becomes thicker, the heat transfer heat resistance increases, and the system COP increases rapidly. Reduced to 7; after 30 minutes of system operation, the COP slowly decreases; after the system runs to 70 minutes, the COP is rapidly reduced to 5. The average COP of the heat pump cycle is 62. It can be seen that when the heat pump cycle 1 starts to freeze, the heat transfer resistance is higher. Small, the icing rate is faster; as the ice layer becomes thicker, the heat transfer resistance increases and the icing rate decreases rapidly; when the icing thickness reaches about 35 mm, the icing rate decreases slowly; when the icing thickness reaches about 5 mm The icing rate drops again rapidly.
It can be seen that the heat pump cycle 1 has the same ice thickness, COP, and icing rate at the same time, and the reliability of the heat pump cycle 1 is verified.
2) The pressure change curve of the heat pump cycle 1 system is seen; the actual COP is compared with the Carnot cycle COP.
It can be seen that the heat pump cycle COP and the Carnot cycle COP cycle are quite different, because the system uses a common compressor, so the efficiency is lower at low pressure ratio operation. The heat pump cycle COP is about twice that of the auxiliary refrigeration system, because the system combines the evaporator, the condenser and the interface progressive freezing system to realize the function exchange between the evaporator and the condenser, and remove the solid wall ice layer. And recycling the amount of cold released during the melting of the ice layer, greatly reducing the energy consumption of the freeze concentration process.
When the surface of the heat exchange tube is melted, the heat is transferred from the inside to the outside of the tube. Therefore, the inner ice layer melts first, and the melt water flows slowly along the channel formed inside the ice layer. After the system runs for 20 minutes, the channel formed on the inner side of the ice layer has been It is obvious that some of the faster melting ice layer has been detached from the heat exchange tube; at this time, the system high pressure starts to rise significantly, and it is necessary to start the ice layer washing and spraying system to accelerate the melting of the ice layer and the ice layer to fall off. The heat exchange tube after the supply provides cooling capacity.
Heat pump cycle 2 is similar to heat pump cycle 1 and will not be described here.
2.3 Error Analysis The error in this system is mainly generated by the compressor. The reason is that the system uses a common compressor, so it is less efficient in low-pressure ratio operation, such as a specially designed compressor that operates at low pressure ratio. The coefficient of performance will be greatly improved.
3 conclusions
In this paper, an interface progressive freezing test system using heat pump energy-saving technology is proposed. The test data is reproducible and the reliability of the test system is verified. Through the experimental research, the following conclusions are drawn:
1) When the system begins to freeze, the system has a large COP and a high icing rate due to the small thermal resistance. As the thermal resistance increases, the COP decreases rapidly and the icing rate decreases rapidly. When the thickness of the ice layer reaches a certain level The COP and icing rate changes tend to be flat; as the ice layer thickness increases again, the COP and icing rates decrease again rapidly.
2) The specially designed compressor operating at low pressure ratio can greatly improve the performance coefficient of the system.
3) Increasing the tube discharge density, reducing the thickness of the icing or using a ribbed heat exchanger can significantly increase the coefficient of performance of the system.
4) The ice washing and spraying system can greatly improve the performance coefficient of the system.
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