Improvement of throttle device of air - cooled heat pump unit and exergy analysis of its system

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.

Figure 1 Experimental test device

Figure 1 for the experimental study of the schematic diagram of the test device. The nominal cooling capacity of 18.40KW MH008-type air-cooled heat pump hot and cold water unit prototype, in the standard conditions for a variety of air-conditioning [ 7 . 8 ] under the conditions of the measured air-cooled heat pump chiller unit in different throttle cooling capacity, heating capacity and energy efficiency. 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.

In the test, four shut-off valves are used to switch between the thermal expansion valve and the electronic expansion valve. When the thermal expansion valve is used as a throttling element, the shut-off valves 5 and 13 are closed and valves 4 and 10 are opened, while the electronic expansion valve is used as the section Flow elements, 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 replenishing the electronic expansion valve.

1 scroll compressor; 2 four-way valve; 3 air side heat exchanger; 4,5,10,13,25,26,27,28 cut-off valve;
14, 15, 16, 17 Check valve, 6, 19, 21 Drying filter, 9, 18 Reservoir, 11 Plate heat exchanger, 7, 22 Sight glass,
8 Thermostatic expansion valve 20 Electronic expansion valve 23, 24 Capillary 12 Gas-liquid separator 29 Water inlet pipe 30 Outlet pipe Figure 2 System schematic

According to the standard JB / T4329--1997, we test the system respectively in the thermal expansion valve and the electronic expansion valve under the action of refrigeration at maximum load conditions, standard cooling conditions, low temperature conditions of the cooling capacity and energy efficiency ratio; heating At the maximum load conditions, the standard heating conditions and defrost conditions heating and energy efficiency, the following is a comparison of test data.

Figure 3 Comparison of cooling capacity Figure 4 Comparison of cooling energy efficiency

Figure 5 Comparison of heating system Figure 6 heating energy efficiency comparison chart

From Fig. 3 and Fig. 4, it can be seen that under the same operating conditions, the cooling capacity and heating capacity of the system are larger than those of the thermal expansion valve when the electronic expansion valve is used as a throttling element. In particular, when heating, , Are about 3,000 watts, also in the cooling more than 1000 watts. It can be seen from Figs. 5 and 6 that the energy efficiency is also correspondingly improved under the same operating conditions, which shows that replacing the traditional thermal expansion valve with the electronic expansion valve improves the system performance and makes 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.

System operation in the loss of each process is as follows:

Compression process: Δe com = w + e in -e out (1)

Condensation process: Δe con = e in -e out (2)

Throttling process: Δe thr = e in -e out (3)

Evaporation process: Δe eva = e in - e out + E q0 / q m (4)

Where e in is the emissivity value at the inlet of each process; e out is the emissivity value at the exit of each process; w is the compressor work, E q0 is the temperature of the refrigerant introduced from the low temperature heat source heat exergy, q m is the flow rate of refrigerant of the refrigerant.

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.

As can be seen from Table 1, the use of electronic expansion valve, the throttling process than the use of thermal expansion valve when the smaller, in the four processes, the largest proportion of condensation loss, to reduce losses through the use of enhanced heat transfer , Reduce the refrigerant and cooling medium heat transfer temperature difference, you can use efficient heat transfer screw pipe to improve the cooling medium flow rate, reducing dirt and grease thermal resistance and other methods, you can also use the recovery of condensed heat to generate domestic hot water and other ways to reduce the condenser Exergy 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.

The use of electronic expansion valve to replace the thermal expansion valve, the system performance testing and energy efficiency analysis results show that:

1) Under the same cooling and heating conditions, the cooling and heating capacity of the system has been enhanced, showing an increase in cooling capacity and heating capacity, and a corresponding increase in the system's energy efficiency ratio to achieve energy saving purpose.

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.

What method? Electronic expansion valve and thermal expansion valve comparison. World Shipping, October 2004

2. Xue 汀 Ting ⒐ crisp declining caries? 缱 excellent 蛘 head г ⌒ ⌒ 醒 Oxybutadiene omethoate smart mechanical fatigue τ ether ether? Tons of 溆 肟 oxime Qiao Ao fine? April 002

3. Talk about Lei and so on. Comparison of Electronic Expansion Valve and Thermal Expansion Valve in Air - cooled Heat Pump Unit. Refrigeration Technology, No.4, 2001.

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5. Zhang Long, Sun Chang and so on. Exergy Analysis of Subcooled Small Ice Storage System. Energy Saving Technology, 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.1991,14

7. 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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