[Introduction] Air-cooled heat pump hot and cold water units since the 90's in our country has been widely used, the use of geographical rapid promotion from south to north. The advantage of such a unit is a machine with two, to improve the unit utilization. Cooling, the condenser with air-cooled, eliminating the need for water-cooled chiller cooling water system; heating with heat pump operation mode, energy-saving and without using the boiler caused by the pollution of the environment; unit installation, easy to set, can be set On the roof without having to set up a special room and so on. With the development of economy in our country, the power of our country is getting more and more nervous while the proportion of power consumption of air-conditioners in the summer is increasing, prompting the country to raise the energy-saving requirements of the air-conditioners. In the process of rebuilding the air conditioners, Electronic expansion valve to replace the thermal expansion valve has become a trend, and electronic expansion valve with a wide range of flow control, high control accuracy and suitable for circuit control and other characteristics of the load changes can change the flow of the system to ensure that the evaporator is very small The stability of the work under the export of superheat, the air conditioner to start and change load dynamic characteristics greatly improved. 1 Experimental device and test method MH008 type air-cooled heat pump chiller unit prototype with nominal cooling capacity of 18.40KW was used. The air-cooled heat pump hot and cold water was measured under various air conditioning conditions [7.8] as specified in the standard. Cooling units, heating capacity and energy efficiency of units under different throttling devices. Temperature and temperature sensors are installed at the inlet and the outlet of the water-side heat exchanger in the system, and turbine flow meters are set on the inlet pipelines. The arrangement of these measuring points provides specific measured values ​​for analyzing the thermal performance of the system. Due to different water temperatures in different seasons, cooling towers and electric heaters in the cooling water system are used to regulate the water temperature at the inlet and outlet. 2 test system works Test 4 cut-off valve to achieve the thermal expansion valve and electronic expansion valve switch between the use of thermal expansion valve as a throttle element, the shut-off valve 5 and 13 closed, 4 and 10 open; and with Electronic expansion valve as a throttle element, the shut-off valve 4 and 10 closed, 5 and 13 open. At the electronic expansion valve, we connected two capillaries in parallel with the aim of making additional adjustments when the capacity of the electronic expansion valve is not sufficient. 1 scroll compressor 2 four-way valve 3 air-side heat exchanger 4, 5, 10, 13, 25, 26, 27, 28 shut-off valve 14, 15, 16, 17 one-way valve 6 19, 21 Drying filter 9, 18 Reservoir 11 Plate heat exchanger 7, 22 Sight glass 8 Thermal expansion valve 20 Electronic expansion valve 23, 24 Capillary 12 Gas-liquid separator 29 Intake pipe 30 outlet pipe 3 test results and analysis According to the standard JB / T4329--1997 we tested the system respectively in the thermal expansion valve and electronic expansion valve under the action of refrigeration at maximum load conditions, the standard refrigeration conditions, low temperature work Condition of cooling capacity and energy efficiency ratio; heating in the maximum load conditions, standard heating conditions and defrost condition heating and energy efficiency ratio. Under the same operating conditions, the system cooling capacity and heating capacity in the use of electronic expansion valve as a throttling element than the thermal expansion valve, especially when heating, large more, are about 3000 watts, In the cooling is also more than 1000 watts. Energy efficiency in the same conditions have also been a corresponding increase, indicating that the use of electronic expansion valve to replace the traditional thermal expansion valve to improve system performance, make the system more energy efficient. Considering the effective use of energy, the second law of thermodynamics (exergy analysis) evaluates the thermodynamic cycle from an energy perspective. Revealed the conversion, transmission, utilization and loss of energy in the fire, so as to find an improved solution to improve the thermal economy of the entire circulation system. Through the exergy analysis of the small air-cooled chiller-heater unit system, the paper explores the conditions and laws of the exergy losses of the system components and provides reference for the improvement of the system weakness. The process of system operation loss is calculated as follows: Compression process: Δecom = w + ein-eout (1) Condensation process: Δecon = ein-eout (2) throttling process: Δethr = ein-eout (3) Evaporation process: Δeeva = ein- eout + Eq0 / qm (4) where ein is the emissivity value at the inlet of each process; eout is the emissivity value at the exit of each process; w is the compressor work, Eq0 is the cooling Agent from the low temperature heat source into the heat, qm refrigerant refrigerant flow. The following table is the electronic expansion valve and thermal expansion valve as a throttling element of the system during the exergy loss as a percentage of the total loss of the four processes. Table 1 Emissions Loss Calculation Results of Each Process Maximum Load of Refrigeration Cooling Standard Low Temperature Heating Maximum Load Heating Standard Defrosting Conditions Compression Exergy Loss Proportion (%) Thermal Expansion Valve 32.42 39.06 35.75 36.45 37.98 38.42 Electronic Expansion Valve 22.9 16.95 14.07 19.92 20.5 17.57 Proportion of Exergy Loss in Condensation Process (%) Thermal Expansion Valve 35.6155.08 36.9162.94 41.3466.33 41.8261.77 43.9663.79 42.967.2 Electronic Expansion Valve Exhaust Evaporation Loss Proportion (%) Thermal Expansion Valve 18.8615.33 12.1514.17 12.9413.44 10.1113.58 9.9212.61 11.6111.14 Electronic Expansion Valve Exhaustion ratio of throttling process (%) Thermal Expansion Valve 13.116.69 11.875.94 9.976.16 11.624.73 8.153.11 7.084.10 Electronic Expansion Valve As can be seen from Table 1, with the electronic expansion valve, the loss of throttling process is smaller than that of the thermal expansion valve, and the condensation loss in four processes Accounting for the largest proportion of the way to reduce losses is to use enhanced heat transfer to reduce heat transfer refrigerant and cooling medium temperature difference, you can use efficient heat transfer screw pipe to increase the cooling medium flow rate, less Dirt and grease resistance and other methods, can also be recycled thermal condensation heat domestic hot water generation and other measures to reduce the condenser with a loss. The loss in the throttling process is due to the irreversible throttling caused by frictional resistance during the flow of the viscous fluid. The adjustment sensitivity of the electronic expansion valve is higher than that of the thermal expansion valve, so the loss is low. So in the future the transformation of the system can match the two systems of the system to reduce the exergy loss and improve the efficiency of the system. 4 Conclusion The use of electronic expansion valve to replace the thermal expansion valve, the system performance testing and energy efficiency analysis results show that: 1) In the same cooling and heating conditions, the system has been enhanced cooling and heating capacity, reflected in the cooling capacity And heating system increases, the system's energy efficiency has also been a corresponding increase, to achieve the purpose of energy saving. 2) The use of electronic expansion valve makes the system performance has been improved, mainly because the electronic expansion valve through the sensor parameters collected by the drive plate drive valve opening and closing, the reaction faster than the thermal expansion valve, it is particularly suitable The use of heat pump units with drastic changes in working conditions. In addition, the temperature sensing components of the electronic expansion valve are thermocouples or thermal resistances that more accurately reflect the changes in superheat than the temperature sensing packages. Therefore, the temperature of the electronic expansion valve is low, Superheat regulation than the thermal expansion valve convenient. 3) In addition, during the cooling and heating process, the loss of condensation process is the largest, so the system performance can be further improved by adjusting the match between the two devices and reducing the exergy loss during the condensation process in the future. While the system water side heat exchanger in the heat pump is downstream, and the heat pump in the heating, the plate heat exchanger load than when cooling large, so if the way downstream should be able to improve the system performance. At the same time can also use frequency conversion technology, changing the compressor speed to adjust the system in variable operating conditions, the refrigerant flow in the system, the unit at higher efficiency. References 1. He Farming. Electronic expansion valve and thermal expansion valve comparison. World Shipping, October 2004 2. Xue Xuan, Gu Zhongping, et al. Application of Electronic Expansion Valve in Small Central Air Conditioner. "Refrigeration and Air Conditioning", April 2002 3. Tan Lei et al. Comparison of Electronic Expansion Valve and Thermal Expansion Valve in Air - cooled Heat Pump Unit. Refrigeration Technology, No.4, 2001. 4. Zhang Zhiyou. "Refrigeration principle and equipment." Beijing, Machinery Industry Press 5. Zhang Long, Sun Chang and so on. Exergy Analysis of Subcooled Small Ice Storage System. Energy Efficiency Technologies, April 2005 6. Mohsen Farzad, Dennis Lo, neal. System Performance Characteristics of An Air Conditioner over a Range of Charging Conditions [J]. Int.J.Ref.1991,147. JB / T4329--1997 volumetric cold water (heat pump) unit standard performance conditions. 8. GB / T10870-2001 volumetric and centrifugal cold water (heat pump) unit performance test methods
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