Abstract: SX Z8-2040 hm2 central air conditioner is an air conditioner in a clothing factory. Its cooling capacity is designed according to the hot summer and the maximum human flow, as is the matching refrigeration and cooling water pump motor. As we all know, China has four distinct seasons. As far as Guangdong province is concerned, it is hot for about four months, and the relative temperature is low for the other eight months, plus the temperature difference between day and night. (Clothing factory has night shift) For central air conditioning, there will be some excess cooling capacity, which will lead to unstable indoor temperature. The water pump belongs to the quadratic law load, and the power factor and efficiency are very low when running at full power frequency. In addition, the motor configuration is too large, resulting in a great waste of energy. On the other hand, because the water pump starts at Y-△ and runs at full power frequency, it causes great mechanical wear. Backwater water hammer is generated during shutdown, which causes wear and damage of water stop valve and water pump. If the refrigeration pump and cooling pump are changed to variable frequency speed regulation, the temperature difference PID closed-loop control is configured. It can reduce the speed of water pump, improve the starting performance, simplify the circuit and stop inertia. The above improvements can reduce mechanical wear rate and electrical failure rate, eliminate water hammer phenomenon, and more importantly, save energy. However, in today's increasingly tense energy situation in the world, it is of great practical and far-reaching historical significance to carry out energy-saving transformation of central air conditioning.

Key words: central air conditioning, variable frequency speed regulation, temperature difference control, PID, energy saving. Paper content: (1) Basic components of central air-conditioning system The central air-conditioning system consists of three parts: refrigeration system, cooling water circulation system and chilled water circulation system. 1, refrigeration system (refrigeration unit) The refrigeration unit is the heart of central air conditioning and the source of refrigeration. It consists of a compressor and a condenser. Its function is to reduce the circulating backwater to 7- 10℃ through the "internal heat exchange" of the refrigeration unit. The model is SX Z8-2040 hm2, and the Chinese full name is: steam double-effect lithium bromide absorption refrigeration unit, with a refrigeration capacity of 2040KW and a cold water flow rate of 350 cubic meters per hour. 2. The cooling water circulation system consists of cooling pump, cooling water tower, cooling fan and pipeline. Its function is to use the cooling pump to pressurize, send cooling water to the refrigeration unit for continuous circulation, and take away the heat released by the refrigeration unit (heat generated by mechanical movement and internal heat exchange). 3. The chilled water circulation system consists of chilled pump, pipeline, bellows and fan. The chilled water "frozen" from the refrigeration unit is pressurized by the refrigeration pump and transported to the bellows of each user, and the cold air evaporated by the evaporator in the bellows is taken away by the fan. (2) Thermistors and thermocouples for temperature control are equipped with temperature control protection circuits, and the touch screen displays observation. (3) Traction system 1, refrigeration unit traction system: compressor and unit, with power distribution of 6. 25KW, in which * * * is 5. There are two 5KW electric pumps, and the compressor is driven by hot steam. 2. Cooling pump drive system: two 55KW water pump motors, y-△ start, one for use and one for standby. 3. Refrigeration pump drive system: two 55KW water pump motors, y-△ start, one for use and one for standby. 4. Fan drive system: a 22KW water-cooled fan and several 4KW fans. (4) Basic considerations for system transformation 1. In order to achieve the purpose of energy saving, the water pump is a secondary load, generally an elastic load, which has strong contractibility and considerable energy saving potential. The resistance moment of water pump is proportional to the quadratic power of rotation speed, so the resistance moment at low speed is far less than the rated torque. When running under the rated voltage of power frequency, the effective torque of the pump is quite different from the load torque, which is the mechanical characteristic of the pump load, such as a big horse-drawn car, with low technical coefficient and low efficiency. A is the mechanical characteristic curve of pump load running at rated voltage of power frequency. When the load torque is equal to the rated torque TLN of the motor, the rated working point is N and the speed is nN. When the load torque decreases to TLQ, the operating point moves to Q and the speed increases to nq. As mentioned above, the power factor and efficiency at this time are very low.

B frequency conversion step-down operation A frequency conversion speed regulation under rated voltage can adjust the motor speed and effective torque according to the ratio of U/F, reduce the voltage and frequency borne by the motor and make the effective torque of the motor close to the load torque. Figure 4-2 b shows the mechanical characteristic curve of the pump after depressurization. The effective torque of the motor is TME, which is very close to the load torque TLQ. Then the power factor and efficiency are in the best state, the current is reduced and the voltage is also reduced. We know: P=UICOS￠ According to this formula, with the decrease of output voltage and current, the output power naturally decreases, achieving the purpose of energy saving. 2. As mentioned above, the external heat exchange of the central air conditioning system is completed by two circulation systems, namely the cooling water circulation system and the chilled water circulation system. As we know, the speed of water pump motor is proportional to the speed of circulating water, and the whole process

The speed of heat exchange of motor, water pump, cooling pump, circulating system, frequency converter-+power supply, given temperature difference transmitter and temperature sensor, and central air conditioning system is also proportional to the speed of circulating water. If the flow rate of circulating water is controlled according to the temperature of backwater and influent, the speed of heat exchange is controlled. According to this principle, the cooling pump and freezing pump can control the speed of the motor by frequency conversion and built-in PID intelligent speed regulation according to the temperature. This is a reasonable control mode. High temperature indicates that the heat released by the air conditioning system is increasing, so the speed of the water pump motor should be increased, otherwise it can reduce the speed and save energy. (5) The specific transformation scheme of the system 1. The control of chilled water outlet temperature by chilled water circulation system is the result of "freezing" of refrigeration unit, which is relatively stable. Therefore, only the return water temperature is enough to reflect the indoor temperature. Therefore, the variable frequency speed regulation system of refrigeration pump can be controlled according to the return water temperature. High backwater temperature means high room temperature, so it is necessary to increase the speed of freezing pump and speed up the circulation of frozen water. On the contrary, low backwater temperature means low room temperature, which can reduce the rotation speed of freezing pump and slow down the circulation speed of frozen water. 2, cooling water circulation system control Because the water temperature of the cooling tower changes with the ambient temperature, its side can not accurately reflect the heat generated inside the refrigeration unit. Therefore, the speed of cooling pump should be controlled according to the temperature of backwater and inflow, so as to realize the constant temperature difference control of backwater and inflow and rationalize the frequency conversion speed regulation of motor. The large temperature difference indicates that the refrigeration unit produces a large amount of heat and the internal heat exchange speed should be accelerated. The rotating speed of cooling pump should be increased to improve the circulating speed of cooling water. Small temperature difference means that the refrigeration unit generates less heat, so the rotation speed of the refrigeration pump can be reduced to reduce the circulation speed of cooling water. 3. Constant temperature (degree) difference controls the chilled water circulation system, and only the return water temperature is enough to reflect the speed of external heat exchange. Pt 100 platinum resistor can be used with E series thermostat. The temperature of return water is converted into electrical signal through thermistor and thermostat, and the output current is 4-20 mA, which can be used as the feedback signal of frequency converter and compared with the given letter. In the cooling water circulation system, the water temperature of the water tower changes with the ambient temperature. The heat transfer rate that cannot be reflected unilaterally must be based on the backwater temperature and the inlet water temperature. Two temperature difference transmitters Pt 100 platinum resistors are used together, and the temperature difference between backwater and influent is converted into electrical signals through thermal resistors and temperature difference controllers. The output current is 4-20mA, which is used as the feedback signal of the frequency converter and compared with the given signal. Determine the rotation of the water pump. (VI) Parameter setting of frequency converter and system control principle 1, TVF2455 9952= 1 data initialization 9906=2 PID application macro related parameter setting, which is designed for closed-loop control system and suitable for pressure, temperature and flow control. PID application macro includes the following contents: input signal, output signal, input U/I selection start/stop (DI 1 D 15) analog output frequency conversion. Simulate a given (AI 1) frequency. Output the actual value of frequency conversion ai10-10v (AI2). Ai20- 10V control mode (DI2) relay output 1 fault output or 4-20mA allowed operation (DI6) relay output 2 constant speed operation1001=1= (di65438+0) start/stop.

Input AI 1 given 120 1=4 4=DI3 multi-speed output 1205=50 multi-speed 4 given corresponding DI3 unit HZ 140 1=4 4= fault pull-in relay output/kloc. Variable 2 102= 1 stop function 1= inertia stop 2008=50 maximum frequency unit HZ2007=28 minimum frequency unit HZ4405= 1 deviation value inversion 1= inverse 2202=8 acceleration time unit S26005. Ai2 and feedback signal (4-20MA). Di6-Allow operation. DI 1— Go. Di2- Manual/Automatic (closed PID control). DI3—- Constant speed operation. KM relay-fault introduction. When the water pump is just started, there is no temperature difference between the thermal resistance Rt 1 and Rt2, because the inlet and return water have the same temperature. The temperature difference transmitter has only a small output, and the frequency converter is placed in the manual position. At this time, KI 1 KI4 KI6 turns off the inverter and runs at a constant speed. After 20 minutes, the temperature difference between the water inlet and the water outlet of the cooling water pipe, and the temperature difference transmitter outputs a deviation signal of 4-20mA according to the temperature difference value as the feedback signal of the frequency converter. When KI4 is turned off and KI2 is turned on, the frequency converter enters the automatic PID closed-loop control link, compares the analog given voltage with the feedback signal to obtain the deviation value, and then outputs the analog given frequency signal to control the frequency of the cooling pump motor, thus controlling the motor speed. The large temperature difference indicates that "heat exchange is accelerated" and the motor speed is accelerated. When the temperature difference is small, the motor speed can be slowed down. On the other hand, because the frequency converter is set to 2602=2, it can make full use of the outstanding characteristics of frequency regulation and frequency modulation of frequency converter. Make the U/F ratio in the best state, and then the effective torque and the load torque are very close to achieve the purpose of energy saving. (VII) The operation effect of changing to frequency conversion speed regulation After nearly one year of operation, the user's response is that it has already returned to its original value in half a year. If the average energy saving is 30% and the power is 1 10KW, the energy saving is at least 30 degrees per hour, and the expected effect is achieved. The specific points are as follows: 1. By observing the cooling pump's rotating speed, the highest frequency is 42HZ and the lowest frequency is 28HZ. Energy saving is about 35%. The rotation speed of the refrigeration pump is reduced to the highest frequency of 46HZ and the lowest frequency of 35HZ. Energy saving is about 25%. 2. According to the calculation of 16 hours per day, energy can be saved in one year: 172800 kWh. 3. The control circuit is simplified and the electrical failure rate is reduced. 4. The temperature control effect is good, and the indoor temperature is relatively stable. 5, the motor speed drops, and the mechanical wear is obviously reduced. Implement inertial shutdown to eliminate water hammer phenomenon.