Due to the changes of the four seasons, when the rain, snow, daytime and night, the outside temperature is different, the heat load of the central air conditioner is much lower than the design load during most of the time. In other words, the central air conditioner actually operates under a low load condition most of the time. According to statistics, the full-load operating time does not exceed 10-20 hours per year.
Practice has proved that in the central air conditioning system (cooling pump and refrigeration pump) access variable frequency system, the use of variable frequency technology to change the motor speed to adjust the flow and pressure changes used to replace the valve control flow, can achieve significant energy savings.
â— Energy saving principle:
Generally, the design temperature difference of the central air-conditioning chilled water is 5~7°C, that is, it works under the conditions of low temperature difference and large flow, thereby increasing the energy loss of the pipeline system and wasting the energy of pump operation. We know that: chilled water and cooling water carry away the heat × flow × temperature difference, while the transmission medium is water, and its specific heat is constant. In this way, we can appropriately increase the temperature difference, reduce the flow rate, and also reduce the rotational speed, so that the product is constant, so that the same amount of heat is taken away from the chilled water and the cooling water, and the purpose of energy saving can be achieved.
1) Simple and safe wiring method, the power saver is connected in series on the main line of the compressor, and the energy output of the energy saver is directly supplied through the contactor;
2) Does not change the original protection and control functions of the compressor and does not change the air volume;
3) Soft-start flexible operation mode, no start-up surge current, reduced wear, reduced heat generation, extended service life;
4) Temperature closed-loop control adaptive system, automatic tracking and adjustment of output power, minimum power consumption, improve the operating efficiency;
5) Overload protection and phase loss protection;
6) It does not change the original operation mode of the air conditioner, and has an optional manual/automatic operation mode, which is convenient for maintenance and repair.
7) Energy-saving rate: 20%-50%.
â— Scope of application: In many shopping malls, supermarkets, office buildings, factories and many other places.
Case: Central air-conditioning energy-saving power-saving control system
Central air-conditioning is an indispensable facility for modern property buildings, large-scale factories, and hotel shopping centers. However, due to the large power and energy consumption of the central air-conditioning, and the existence of a “big horse-drawn car†on the design, the electricity fee for central air-conditioning is one of the users. Huge expenses. Central air-conditioning provides refrigeration for factories, shopping malls, office buildings, entertainment venues, hotel restaurants, and accommodation buildings. It maintains a constant temperature in the entire building, but due to changes in the season and day and night, and other factors, the demand for cooling capacity of buildings has obvious changes in demand. However, traditional central air conditioners cannot monitor environmental changes and adjust their own energy consumption. In addition, the motor power design of the process design has a considerable amount of excess, that is, the flow rate and head of the pump are greater than the actual needs, so the company's development and production are added. Frequency conversion energy-saving devices are very necessary and have obvious power-saving effects. With the maturity and development of frequency conversion technology, “One day's electricity costs two days of electricity†is no longer a fantasy. The frequency conversion energy-saving reform of central air-conditioning is a shortcut to reduce costs and increase efficiency.
Frequency control of cooling water system
1 The main basis of control The basic situation The inlet temperature of the cooling water is the temperature of the water in the cooling tower, which depends on the ambient temperature and the working condition of the cooling fan; the return temperature mainly depends on the heating condition of the freezing machine, but also The water temperature is related.
temperature control
In the control, there are two basic conditions: If the return water temperature is too high, it will affect the cooling effect of the frozen host. In order to protect the frozen host, protective tripping must be performed when the temperature of the return water exceeds a certain value. Under normal circumstances, the return water temperature must not exceed 37 degrees. Therefore, it is desirable to determine the flow rate of cooling water based on the return water temperature. Even if the temperature of the inlet and return water is low, the cooling water is not allowed to stop flowing. Therefore, when implementing variable frequency speed regulation, the inverter needs to preset a lower limit frequency. Taken together, that is: when the return water temperature is lower, the cooling pump runs at the lower limit speed; when the return water temperature is higher, the cooling pump speed gradually increases, and when the return water temperature rises to a certain setting For values ​​(eg 35 degrees), further measures should be taken; either increase the number of cooling pumps running, or increase the number of tower cooling fans running. Temperature control
The temperature difference can reflect the heating condition of the refrigerating machine, and the “temperature difference†回t between the return temperature T0 and the intake T i reflecting the cooling effect, because the size of the temperature difference reflects the heat taken by the cooling water from the freezing machine, so It is advisable to use the temperature difference ∆t as the main basis for control, and to control the temperature difference through frequency conversion. Namely: the temperature difference is large, indicating that the host generates more heat, should increase the cooling pump speed, accelerate the cooling water cycle, on the contrary, the temperature difference is small, indicating that the host generates less heat, can properly reduce the cooling pump speed, slow down the cooling water cycle . The actual operation shows that it is appropriate to control the temperature difference in the range of 3 to 5 degrees.
Temperature and inlet temperature control
Since the inlet water temperature changes with the ambient temperature, it is not the best policy to keep the temperature difference constant. Because, when we use the variable frequency speed control system, we must consider not only the cooling effect, but also the energy saving effect. Specifically, then: the temperature difference is set low, the average speed of the water pump increases, affecting the energy-saving effect: the temperature difference is set high, and when the inlet water temperature is high, it will affect the cooling effect. Practice shows that it is advisable to adjust the temperature difference at any time according to the inlet temperature. Namely: When the inlet water temperature is low, it should mainly focus on the energy-saving effect, and the control temperature difference can be appropriately higher; and when the inlet water temperature is high, the cooling effect must be ensured, and the control temperature difference should be lower.
2 Control Schemes According to the situation described above, there are many kinds of control schemes for cooling pumps using variable frequency speed regulation. Considering the comprehensive effect of energy saving and cooling, we use temperature difference control as the main method, and the return water temperature control is supplemented to control the cooling water. system. Use one frequency converter to switch and control one or more than one motor, the concrete way is: Collect the cooling water inlet and outlet water temperature with the sensor, PID changes the temperature difference quantity to the analog quantity and feeds back to the central processing unit, then the central processor controls the frequency conversion Frequency. When the difference in temperature is not large enough, the flow of cooling water can be appropriately reduced. At this time, the CPU will output the inverter to the set low frequency value, the motor speed will be slowed down and the flow of water will be reduced. When the temperature difference is high, the refrigeration unit has more The heat needs to be taken away. At this time, the CPU outputs the inverter to a set higher frequency value. The motor speed is increased, the water flow is increased, and more heat is taken away. If the return water temperature of the cooling water exceeds 32°C (it can be set according to the actual situation), the inverter will run at a higher frequency in priority, which can provide appropriate flow according to the real-time needs of the system, without wasting electrical energy.
Frequency control of chilled water system
1 The main basis of control In the frequency conversion speed regulation scheme of the chilled water system, there are mainly two kinds of control basis proposed:
1) Differential pressure control, that is, the pressure difference between the outlet pressure and the return pressure is used as a control basis, and the basic consideration is to maintain sufficient pressure on the highest floor of chilled water. There are two problems with this approach: There is no factor that takes into account changes in the temperature of the environment, that is, the amount of heat taken by the chilled water has nothing to do with the room temperature, which is clearly not reasonable.
2) Control of temperature or temperature difference Strictly speaking, the difference between the return temperature and the outlet temperature of the freezing machine indicates that the amount of chilled water taken away from the room should be used as a control basis. However, due to the fact that the temperature of the effluent from the freezing machine is generally stable, in fact, it is only necessary to control it according to the return water temperature. In order to ensure that there is enough pressure on the highest floor, a pressure gauge is attached to the return pipe. If the return pressure is lower than the specified value, the motor speed will not decrease.
2 control plan Synthesize the above-mentioned analysis, there are two kinds of control schemes that can be improved: The pressure difference is controlled by the main temperature as auxiliary The pressure difference signal is the feedback signal, carry on constant pressure difference control. The return water temperature signal is used as the target signal, so that the target value of the pressure difference can be properly adjusted according to the return temperature within a certain range. That is, when the room temperature is lower, the target value of the pressure difference is appropriately decreased, the average rotation speed of the refrigeration pump is reduced, and the energy saving effect is improved. In this way, both the environmental temperature factor and the energy saving effect are improved. The temperature (difference) is dominated by the main differential pressure. The temperature (or temperature difference) signal is used as the feedback signal, constant temperature (difference) control is performed, and the differential pressure signal is used as the target signal. That is, when the pressure difference is high, it means that the load is heavy, the target signal should be appropriately increased, and the average rotation speed of the freezing pump should be increased to ensure that the highest floor has sufficient pressure. For the chilled water system we use full closed loop temperature control. Use one inverter to switch one or more than one frozen motor. The specific method is as follows: Under the premise of ensuring the chilled water flow of the chiller, the minimum operating frequency of a chilled pump frequency converter is determined and it can be set as the lower limit frequency. The frequency regulation of the pump motor is measured by the temperature sensor installed on the system pipe. The thermostat compares it with the setpoint. When the temperature of the frozen return water is greater than the set value, the inverter outputs the upper limit frequency, and the water pump motor runs at a high speed; when the frozen return water temperature is less than the set temperature, the motor works with the set frequency curve.
Signal conversion temperature signal conversion In general, due to the lower temperature, the change range is not large, so the temperature sensor with platinum resistance (Pt100) is appropriate, the signal conversion we directly use the AL808 temperature difference PID, not only the temperature difference signal conversion 0- 10V standard analog signal, and can display backwater temperature, inlet temperature, temperature difference is very convenient to use.
The main functions are: a) Frequency conversion / power frequency switching function If the conversion 1 switch is placed in the "stop" position, the operation of the original control cabinet will not be changed. If the frequency conversion operation is required, the conversion 1 switch must be placed in "1". Bit or “2†bit b) Closed-loop fully automatic running function will set the switch 2 to “closed loop†position, and the “closed loop running†indicator light will be on. At this time, the running frequency of the inverter is automatically given by the PID without manual adjustment. c) Open-loop adjustment function Place change-over switch 2 in the “open-loop†position, and the “open-loop operation†indicator light is on. At this time, the operating frequency of the inverter is set manually by adjusting the potentiometer.
From the operating situation, after the frequency conversion energy-saving reform: 1) The energy-saving effect is significant; 2) The soft-start is realized, the motor starting current is greatly reduced, and the impact on the power grid when the motor is started is avoided; 3) The equipment runs more smoothly and eliminates Water Hammer effect at start-up and shutdown; 4) Automatic closed-loop control is achieved, automation level is improved, operation is safe and reliable, unattended.
Shanghai Xingbai Automation Technology Co., Ltd. provides you with a comprehensive power saving program. Welcome to contact us at Fax http:// Mr. Wang Liu