High temperature centrifugal chiller and its characteristics
Industrial chiller: / Deng'S Tian Xudong 1, Liu Hua 2, Zhang Zhiping 2, Li Hongbo 2 Guava Leaf Extract,Psidium Guajava Extract,Healthy Guava Leaf Extract,Pure Guava Leaf Extract NANJING ZELANG MEDICAL TECHONOLOGY CO., LTD , https://www.hawfitness.com
(1. Hefei General Machinery Research Institute, Hefei 230088, China; 2. Zhuhai Gree Electric Appliance Co., Ltd., Zhuhai 519070, China)
Abstract: High-temperature chillers are important equipment in temperature and humidity independent control air conditioning systems, and are also large equipment often used in industrial production. This paper explores the characteristics of high-temperature centrifugal chillers and their variable load operation, and provides reference for the design and industrial application of temperature and humidity independent control air conditioning systems.
Key words: independent control of temperature and humidity; centrifuge; high temperature chiller; IPLV; partial load CLC number: TB65 Document code: A do:i 10. 3969/.j issn. 1005-0329. 2009. 10. 013
1 Introduction Water-cooled chillers are central air-conditioning cooling equipment commonly used in large public buildings, while centrifugal chillers are well received by users because of their high energy efficiency and large unit cooling capacity. As the energy consumption of air conditioning systems accounts for a large proportion of building energy consumption, reducing the energy consumption of air conditioning systems has become a hot topic in the refrigeration and air conditioning industry in the context of high energy crisis and high energy consumption and consumption reduction. Many strategies have been proposed. Temperature and humidity independent control is a new idea put forward in system optimization design. The method adopts the temperature and humidity independent control technology to improve the energy efficiency index of the chiller by increasing the effluent temperature of the chiller, and changes the working mode of the entire air conditioning system by using the dry end heat exchange mode. These measures can reduce the air conditioning from a system perspective. The energy consumption of the system. In the chiller itself, the centrifugal chiller is different from other types of chillers. The higher the effluent temperature requires higher evaporation temperature, and the high evaporation temperature has many effects on the centrifugal compressor. According to the normal standard conditions (7 °C effluent) Temperature) The compressor [1] designed to operate at higher evaporating temperatures may cause the energy-efficient operating characteristics of the centrifugal chiller to be insufficient due to changes in system flow rate and pressure ratio. research.
This paper introduces the characteristics of high-temperature centrifugal chillers and their variable operating conditions, and provides technical reference for designers of temperature and humidity independent control air conditioning systems.
2 Characteristics of centrifugal chillers at high water temperature The design of a centrifugal chiller with a 4000 kW refrigerant for R134a is taken as an example: when the cold water effluent temperature is 16 ° C, the evaporation temperature is 14 ° C, and the condensing temperature is 37 ° C. The cold and superheat degree is 2 °C, the efficiency of the compressor and the efficiency of the motor are taken as 0.8 and 0.94 respectively. The calculation is analyzed by the NIST software of the National Institute of Standards and Technology of the United States. The COP value of the unit can reach 8. 01.
In order to obtain the data of the product designed according to the standard working conditions under changing working conditions, the existing centrifugal chiller was tested, and one actual 4000kW refrigerant was used as the R134a centrifugal chiller in the standard tester. Conditions (cold water inlet temperature 12 ° C, outlet water temperature 7 ° C; cooling water inlet temperature 30 ° C, outlet water temperature 35 ° C) and high water temperature conditions (cold water inlet temperature 21 ° C, outlet water temperature 16 ° C; cooling water into The water temperature was 30 ° C and the outlet water temperature was 35 ° C. The test was carried out [1], and the test results are shown in Table 1.
From the test data in the table, it is not difficult to see that the chiller assembled according to the centrifugal compressor designed under standard working conditions does not significantly increase the cooling capacity when working at high water temperature, and the input power decreases. The COP value of the unit has been increased by about 10%, which is far from the theoretical calculation.
The reason for this can be analyzed from the characteristics of the centrifugal refrigeration compressor. Centrifugal compressor is driven by the motor to drive the rotor to rotate. The refrigerant vapor from the evaporator enters the impeller through the suction chamber. The impeller rotates at high speed. The blades on the impeller drive the gas and generate a certain centrifugal force. The gas is thrown out from the center of the impeller to the outer periphery. After this movement of the gas, the velocity is increased and the pressure is increased, which is the result of the conversion of mechanical energy acting on the impeller. The gas leaves the impeller and enters the diffuser. As the diffuser passage area increases, the gas is decelerated and pressurized, and its kinetic energy is converted into pressure energy. After the compressed refrigerant vapor flows out of the diffuser, it is collected by the volute and then sent to the condenser through the exhaust pipe, thus completing the compression of the refrigerant [2].
When the centrifugal refrigeration compressor is operated at a high evaporating temperature, the compression ratio of the compressor is smaller than that of the standard design condition. When the chiller is discharged at 7 ° C, the designed suction pressure of the unit is 0.35 MPa, and the exhaust pressure is 0. 937MPa, the pressure ratio is approximated. When the unit is operated at a cold water outlet temperature of 16 ° C (the condensing temperature is constant), the suction pressure of the unit is 0. 473MPa, the exhaust pressure is 0. 937MPa, the pressure ratio is similar. 9%。 The compression ratio is reduced by 25.9%. Figure 1 is a performance curve of a centrifugal refrigeration compressor. It can be seen from the figure that in the working area of ​​the centrifugal compressor, when the pressure ratio is reduced by 25%, the flow rate is increased by nearly one time. With such a large flow change, the flow path design of the original unit's impeller and diffuser will inevitably block the increase of the flow rate, so that the flow rate cannot reach the rated value, and the performance of the machine group will be affected.
In addition, at a fixed speed, the pressure ratio has a certain correspondence with the efficiency of the compressor. Figure 2 shows the relationship between the pressure ratio of the centrifugal compressor and the stage efficiency at a fixed speed.
The efficiency of the compressor is reduced to only 0.7 after the change of the pressure ratio from 2.7 to 2.0. about. Therefore, when the compression ratio of the compressor changes, if the design speed of the compressor is not adjusted, the performance improvement of the unit will be greatly affected.
3 Improvement of centrifugal chiller 3.1 Adjustment of compressor design For the above analysis, the compressor in the centrifugal chiller for high water temperature should be designed and optimized according to the new design conditions to make the unit energy efficiency. Further improvement.
(1) Improved flow area obstruction means that the airflow at the outlet of the flow channel reaches a critical state. At this time, the volumetric flow rate of the gas is already a large value of Zui, and the flow back pressure of the compressor can be reduced, and the flow rate cannot be increased any more. Called the "blocking" condition. Increasing the area of ​​the flow channel can improve the flow capacity of the refrigerant. However, for the central air-conditioning chiller, most of the time the unit is running under partial load, so the refrigerant flow rate will also vary with the state of the unit operation. Changes occur, and the fixed runner area also affects the operation of the unit. For this reason, the diffuser with variable flow section should be designed in the unit design to adapt to the change of flow, so as to meet the operation requirements of the unit under different load conditions. Figure 3 shows the design of a variable section diffuser.
(2) Adjustment of design speed In order to make the unit still have higher efficiency under the changed pressure ratio, it is necessary to adjust the design speed of the compressor. It can be seen from the curve display of Fig. 2 that the speed of the compressor has a certain correspondence with the high efficiency point of the Zui. When the speed decreases, the curve of the pressure ratio is gradually shifted to the left, and the shape of the curve is basically unchanged. The span begins to compress. According to the analysis and calculation, when the pressure ratio is 2.0, the compressor speed is reduced by 20%, and higher efficiency is obtained.
3. 2 Performance test According to the above technical analysis, Zhuhai Gree Electric Appliance Co., Ltd. designed and manufactured a centrifugal high-temperature chiller with 4000kW, refrigerant R134a and cold water outlet temperature of 16 °C, and the chiller according to high cold water. Outlet water temperature conditions (cold water inlet temperature 21 ° C, outlet water temperature 16 ° C; cooling water inlet temperature 30 ° C, outlet water temperature 35 ° C) and national standard GB/T 18430. 1 IPLV test conditions into the performance test, test results As shown in Figure 4.
From the test data, it can be seen that the energy efficiency index COP value of the unit is 7.7, and the energy efficiency index of the centrifugal chiller is increased by 33.2%, which is close to the calculation index of the design software; When the effluent temperature of the cold water is 16 ° C, the IPLV value calculated according to the national standard GB/T 18430. 1 is 8.1 [1]. Table 2 shows the results of the partial load test.
The test results show that the improved high water temperature chiller has a significant improvement in energy efficiency compared with the standard unit, which is close to the theoretical calculation of the software. When it is used in an air conditioning system with independent temperature and humidity control, Significantly reduce the energy consumption of the system.
4 Change trend of COP value The central air-conditioning system is designed according to the large load of zui. In actual operation, the chillers are basically operated under different working conditions and partial load conditions. Therefore, the product needs to be The variable operating conditions and the unloading operation were tested.
The variable operating conditions of the chiller include changes in the temperature of the cooling water inlet water, changes in the set value of the cold water outlet temperature, and operation of the unit under partial load due to changes in the air conditioning load itself. In practical applications, it may be a combination of one or more of the above conditions. The variable operating conditions and partial load characteristics of the chiller can reflect the product's product adaptability and COP value.
4.1 Variable cooling water condition test During the operation of the centrifugal chiller, if the temperature of the cooling water changes, it will directly affect the cooling capacity and energy efficiency characteristics of the unit. When the cooling water temperature is lowered, the cooling unit will obtain a lower condensing temperature, which will increase the cooling capacity of the unit and improve the efficiency of the unit [4]. During the test, the inlet temperature of the cold water was kept at 21 ° C, and the outlet water temperature was 16 ° C. The inlet temperature of the cooling water was gradually increased from 26 ° C to 35 ° C. The measured COP value was as shown in FIG. 5 .
4. 2 Cold water outlet water temperature test The rise of the cold water outlet temperature also has a great impact on the unit's cooling capacity and energy efficiency. When the temperature of the cold water rises, it is more conducive to evaporation, which will enable the unit to obtain a higher evaporation temperature and increase the cooling capacity of the unit [4]. During the test, the inlet temperature of the cooling water was kept at 30 ° C, the outlet water temperature was 35 ° C, and the cold water outlet temperature was raised from 12 ° C to 20 ° C. The performance parameters of the tested chillers are shown in Figure 6. When the outlet water temperature is lower than 12 °C, the flow rate of the refrigerant becomes smaller, which may cause the unit to work close to the critical point of surge, which may cause abnormality in the unit, which should be noted in actual operation.
4. 3 Partial load test When the cold water outlet temperature is constant, the performance parameter of the unit changes with the load change. The characteristic is that the water temperature of the water-cooled chiller is kept at 16 °C, and the load of the manual adjustment unit is gradually decreased from 100%. The test unit COP Changes in values ​​and cooling capacity. The temperature of the cooling water was kept at 30 ° C during the test. The results of the water test are shown in Fig. 7.
It can be seen from the figure that the COP value of the chiller decreased by 42.6% when the chiller compressor operating load was reduced from 100% to 25% in the case of a chiller with a constant effluent of 16 °C. In the actual application process, the control system of the chiller is usually set to a fixed outlet temperature. When the actual air conditioning load is reduced, the automatic control system of the unit will be automatically unloaded, reducing the load ratio of the compressor and becoming a partial load operation. , which leads to a decline in COP. This shows that in the work of energy saving and consumption reduction, it is not enough to pay attention only to the COP value under the full load condition of the unit itself, and it is necessary to pay attention to the total energy consumption of the entire air conditioning system.
It is worth noting that due to the working characteristics of the centrifugal compressor, when the chiller is running due to changes in operating conditions, the refrigerant flow rate is reduced, it is possible to make the operating point of the unit close to the surge threshold of the compressor [3], This affects the safety of the unit operation, which requires the anti-surge control to be considered in the design of the unit.
Once the centrifugal refrigeration unit is in operation, it should immediately take adjustment measures to reduce the outlet pressure or increase the inlet flow. Pressure ratio and load are the two major factors that affect surge. When the load is getting smaller and smaller, when it reaches a certain limit point, surge will occur, or when the pressure ratio is too large, it will also occur. Vibration. Generally, the safe working area can be set in the control system. When the operating point of the unit is close to the critical area, the unit operating point is kept away from the critical area by adjusting the flow or pressure to achieve the purpose of protection.
5 Conclusions (1) When the centrifugal chiller is operated at the high cold water outlet temperature, the compressor itself needs to be redesigned.
(2) When the centrifugal chiller works under partial load, its energy efficiency index will be significantly reduced. When designing the air conditioning system, attention should be paid to the system's load configuration and control optimization to minimize the energy consumption of the entire air conditioning system;
(3) When designing the air conditioning system, the air conditioning system should be avoided to operate at a lower load ratio. However, due to the central air conditioning system during operation, part of the load operation of the chiller is inevitable. Therefore, by improving the chiller in the part The performance of the load runtime and the control strategy of the optimized air conditioning system are also very important.
references
[1] GB/T 18430. 1-2007 Steam compression cycle cold water (heat pump) unit Part 1: Cold water (heat pump) units for industrial or commercial use and similar [S].
[2] Xia Bin. Principles of Operation and Control of Centrifugal Compressors [M]. Beijing: Mechanical Industry Press, 1988.
[3] Ma Xiaohua. Anti-surge control of large centrifugal compressors [J]. West Coal Chemical Industry, 2008.
[4] Wu Yezheng, Han Baoqi. Refrigeration Principles and Equipment [M]. Xi'an: Xi'an Jiaotong University Press, 1997.