Energy-saving application of frequency conversion technology in central air-conditioning system

　　Under the background of my country's rapid economic development, the market demand for central air conditioners has shown a strong growth trend due to the rapid development of real estate. Attracted by the ever-increasing market capacity, more and more manufacturers have joined the field of commercial central air conditioners. The application of energy-saving technology in central air conditioning systems is of great significance to improving the automation level of central air conditioners, reducing energy consumption, reducing the impact on the power grid, and extending the service life of machinery and pipe networks.

　　Central air conditioning systems have been widely used in industrial and civil fields. According to statistics, the power consumption of central air conditioning accounts for more than 70% of the total power consumption of various buildings, so the energy-saving transformation of central air conditioning is particularly important. During the design, the central air conditioning system must be designed according to the hottest weather and the heaviest load, and a 10-20% design margin is left. In fact, most of the time, the air conditioning will not run at full load, so there is a large surplus, so the energy-saving potential is large. Among them, the refrigeration host can automatically load or unload according to the load change, but the refrigeration water pump and the cooling water pump cannot make corresponding adjustments according to the load change, which is a great waste.

　　Central air conditioning is mainly composed of fans and water pumps. The use of variable frequency speed regulation technology can not only maintain the room temperature of the shopping mall at the desired state, making people feel comfortable and satisfied, but more importantly, its energy-saving effect is as high as more than 30%, which can bring good economic benefits.

　　The composition and working principle of central air conditioning system

　　It is mainly composed of a refrigerator, a cooling water circulation system, a chilled water circulation system, a fan coil system and a cooling water tower. Its working principle is shown in the figure below:

　　The refrigerator compresses the refrigerant into liquid through the compressor and sends it to the evaporator for heat exchange with the chilled water to cool the chilled water. The chilled water pump sends the chilled water to the cooling coils at the air outlets of each fan, and the fan blows cold air to achieve the purpose of cooling. The evaporated refrigerant releases heat in the condenser and exchanges heat with the cooling circulating water. The cooling water pump pumps the cooling water with heat to the heat dissipation water tower, where the water tower fan sprays it for cooling, exchanges heat with the atmosphere, and dissipates the heat to the atmosphere.

　　The refrigerated water pump and cooling water pump cannot be adjusted accordingly with the load changes, resulting in great waste.

　　The flow and pressure difference of the water pump system are achieved by valve and bypass regulation. Therefore, there are inevitably large interception losses and large flow, high pressure and low temperature differences (the water pump still runs at close to full power when the load decreases), which not only wastes a lot of electric energy, but also causes the central air conditioner to fail to achieve reasonable results at the end. In order to solve these problems, the water pump needs to adjust the water flow as the load changes and close the bypass. In addition, the starting current of the water pump motor is 3 to 4 times its rated current, which has an adverse effect on energy consumption and electrical life. In order to save energy and costs, the water pump system needs to be transformed. After market investigation and understanding, mature inverters are used to achieve energy saving and extend the service life of motors, contactors and mechanical parts, bearings, valves, and pipelines.

　　The inverter can adjust the motor speed according to the load changes of the chilled water pump and the cooling water pump (even including the cooling tower fan), so as to make the chilled water pump and the cooling water pump make corresponding adjustments while ensuring the normal operation of the central air-conditioning system, so as to achieve energy saving. When the speed of the water pump motor decreases, the electric energy absorbed by the motor from the power grid will be greatly reduced.

　　The inverter control system should be set to low-speed start and full-speed operation at the first start, so that the chilled water system can be fully exchanged for a period of time, and then the frequency can be infinitely adjusted according to the chilled return water temperature. The inverter output frequency is obtained by detecting the return water temperature signal and the temperature setting value through PID calculation. The system temperature can be controlled within the required range by directly setting the inverter parameters.

　　　　1. Necessity of energy-saving transformation of water pumps

　　Central air conditioning is a major electricity consumer in buildings, accounting for about 60% of the annual electricity bill. Therefore, energy-saving transformation of central air conditioning is particularly important.

　　During the design process, the central air-conditioning system must be designed for the hottest weather and heaviest load, and a 10-20% design margin must be left. However, in reality, the air-conditioning does not operate at full load most of the time, so there is a large surplus, so the potential for energy saving is relatively large. Among them, the refrigeration host can be loaded or unloaded according to the load changes, but the chilled water pump and the cooling water pump cannot be adjusted accordingly with the load changes, resulting in a great waste.

　　The flow and pressure difference of the water pump system are achieved by valve and bypass regulation. Therefore, there are inevitably large interception losses and large flow, high pressure and low temperature differences, which not only waste a lot of electricity, but also cause the central air conditioning to fail to achieve reasonable results at the end. In order to solve these problems, the water pump needs to adjust the water flow as the load changes and close the bypass.

　　Because the water pump adopts the Y-△ starting method, the starting current of the motor is 3 to 4 times of its rated current. The starting current of a 90KW motor will reach 500A. Under such a large current impact, the service life of the contactor and motor is greatly reduced. At the same time, the mechanical impact during startup and the water sag when the pump is stopped can easily cause damage to mechanical parts, bearings, valves, pipes, etc., thereby increasing the maintenance workload and the cost of spare parts.

　　In summary, in order to save energy and costs, the water pump system needs to be transformed. After market research and understanding, mature frequency converters are used to achieve this, so as to achieve energy saving and extend the service life of motors, contactors and mechanical parts, bearings, valves, and pipelines.

　　This is because the inverter can adjust the speed of the water pump motor according to the load changes of the chilled water pump and the cooling water pump, and make the chilled water pump and the cooling water pump make corresponding adjustments while ensuring the normal operation of the central air-conditioning system to achieve energy saving. When the speed of the water pump motor decreases, the electric energy absorbed by the motor from the power grid will be greatly reduced.

　　2. Water Pump Energy Saving Renovation Scheme

　　Central air conditioning systems are usually divided into two systems: chilled (medium) water and cooling water. The left half is the chilled (medium) water system, and the right half is the cooling water system. According to the latest domestic and foreign information, and through the investigation of successful examples of closed-loop control transformation in central air conditioning water pump systems, most of the energy-saving transformation solutions for water pump systems are now implemented using frequency converters.

　　1. Closed-loop control of refrigeration (medium) water pump system

　　〔1〕Closed-loop control of chilled water pump system in cooling mode

　　This solution determines the minimum operating frequency of a chilled pump inverter while ensuring the chilled water flow supply to the terminal equipment, sets it as the lower limit frequency and locks it. The frequency regulation of the variable frequency chilled water pump is achieved by detecting the chilled water return temperature through a temperature sensor installed on the return water pipe of the chilled water system, and then controlling the frequency increase or decrease of the inverter through the temperature set by the temperature controller. The control method is: when the chilled water return temperature is greater than the set temperature, the frequency is infinitely increased.

　　〔2〕Closed-loop control of the chilled water pump system in heating mode

　　This mode is a control scheme for the chilled water pump system when the heat pump in the central air conditioner is running (i.e. heating). Like the refrigeration mode control scheme, while ensuring the chilled water flow supply of the terminal equipment, determine the minimum operating frequency of a chilled pump inverter, set it to the lower limit frequency and lock it. The frequency regulation of the variable frequency chilled water pump is to detect the chilled water return temperature through the temperature sensor installed on the return water pipe of the chilled water system, and then control the frequency increase or decrease of the inverter through the temperature set by the temperature controller. The difference is: when the chilled water return temperature is lower than the set temperature, the frequency is infinitely increased. The higher the chilled water return temperature detected by the temperature sensor, the lower the output frequency of the inverter.

　　2. Closed-loop control of cooling water system

　　目前，在冷却水系统进行改造的方案最为常见，节电效果也较为显著。该方案同样在保证冷却塔有一定的冷却水流出的情况下，通过控制变频器的输出频率来调节冷却水流量，当中中央空调冷却水出水温度低时，减少冷却水流量；当中中央空调冷却水出水温度高时，加大冷却水流量，从而达到在保证中中央空调机组正常工作的前提下达到节能增效的目的。

　　Most of the existing control methods first determine the minimum operating frequency of a cooling pump inverter, set it as the lower limit frequency and lock it. The frequency of the variable frequency cooling water pump is adjusted by taking the inlet and outlet water temperature difference of the cooling pipe and the outlet water temperature signal. When the inlet and outlet water temperature difference is greater than the set value, the frequency is infinitely adjusted up. When the inlet and outlet water temperature difference is less than the set value, the frequency is infinitely adjusted down. At the same time, when the cooling water outlet temperature is higher than the set value, the frequency is preferentially infinitely adjusted up. When the cooling water outlet temperature is lower than the set value, the frequency is adjusted according to the temperature difference. The greater the inlet and outlet water temperature difference, the higher the inverter output frequency; the smaller the inlet and outlet water temperature difference, the lower the inverter output frequency.