First, analysis of central air-conditioning operation control method Central air-conditioning system design is based first on outdoor weather parameters and indoor air-conditioning design parameters to calculate the cooling load, according to the characteristics of zoning structure, according to product samples select the appropriate equipment, combined into a system. However, most of the air conditioning system is working under dissatisfaction with the load. Uncontrollable workload in the control mode is not reasonable, the system will be greatly reduced energy efficiency. At present, the operation mode of the air conditioning system is feng shui system, which is mainly the valve (manual and automatic valve regulation). The main engine utilizes the unloading mode. These methods sacrifice the energy consumption of the resistance to meet the end load requirements, resulting in the high operation cost. If variable frequency control is adopted, the energy transfer and transportation links are controlled by VWV and VAV, so that the transmission and transport are coupled and the optimal temperature difference is replaced, the power is only 30-60% of that of other control systems, And energy efficiency is double effect, because the demand for cooling host energy consumption at the same time decline. The main machine adopts inverter energy-saving control to keep the physical quantity (such as temperature difference, pressure, etc.) of the refrigerant movement under design conditions and save energy compared with other load-transfer modes. For example, YORK YT centrifugal chiller unit is equipped with frequency conversion unit Under partial load energy efficiency can be reduced to 0.2kw / cold ton, showing that the inverter control mode in the air conditioning system, the application prospect is very broad. In the past due to price reasons, the application of frequency conversion technology in the central air conditioning system is more difficult to promote. In frequency conversion technology, computer automation and control technology is very mature today, with this technology and HVAC expertise, it is not an expensive technology, its economy in low-power air-conditioning are affordable, the central air-conditioning system In the more should not be a problem: (1) central air conditioning running longer, more prominent energy-saving issues; (2) inverter control in the entire system accounted for the proportion of the cost is not high; (3) inverter controller capacity, Unit price per kilowatt is lower. The central air conditioning system using the inverter is feasible, the investment recovery is generally 6 to 12 months to 10 years, the frequency controller life, its net income in 10 times more than the amount of investment. Second, the central air-conditioning speed energy-saving principles Refrigerator through the compressor will compress the refrigerant into a liquid back to the evaporator and chilled water heat exchange, chilled water refrigeration, chilled water pump chilled water to the fan coil cooling coil In the cold wind blower to cool the purpose. The evaporated refrigerant releases heat in the condenser to exchange heat with the cooling circulating water, the cooling water brought by the cooling water pump brings the cooling water to the cooling water tower to be spray-cooled by the water tower fan, Between the heat exchange, the heat emitted to the atmosphere. The old operating system is "star-delta conversion start" full-pressure operation, when the air conditioning unit at full load, the system can not start smoothly when starting the motor, start the grid impact, frequent start for a long time will cause the motor Insulation decreases, the motor temperature is too high, in the operation can not be effectively based on the wards and office needs, the temperature is effectively adjusted, only the maximum frequency feed, which will inevitably result in a waste of energy. After the transformation by frequency conversion, according to the room's cooling and heating needs automatically adjust the cooling pump and cooling pump flow to achieve the purpose of energy saving. (1) Since the current cooling water circulation pump operates at full power frequency, the ambient temperature is low and the return water temperature of the cooling water is low during the early and late phases of the refrigeration cycle, causing the lithium bromide to crystallize, resulting in the decrease of efficiency and even protection of the air conditioning unit. The use of variable frequency constant temperature control, the backwater temperature is effectively controlled, will greatly improve the efficiency of air conditioning units, to save energy (2) Since the chilled water circulation pump is also full power frequency operation, but not according to the requirements of the indoor temperature automatically adjust the flow , And through the frequency conversion refrigeration pump according to the outdoor temperature and indoor temperature requirements can automatically adjust the flow, improve efficiency, to save energy. (3) Reduce the air conditioner start-up, stop the impact on the power supply and system to reduce air-conditioning on / off the impact on the power grid, due to the larger circulating pump power frequency starting / stopping pump, the impact on the grid larger, Affect the operation of other equipment. Frequency control, the pump to achieve soft start, soft stop, the current is less than the rated current, no impact on the power grid. Reducing the water draining effect of the circulating water when stopping the pump can stop the water draining effect when the pump is stopped and eliminate the impact of the water draining on the air conditioning system pipe network due to the soft stop of the frequency conversion and the process of stopping the pump. (4) reduce the failure rate of equipment After the use of variable frequency control, the circulating pump most of the time work below the rated power, which will effectively reduce the failure rate of equipment and reduce equipment maintenance and repair. (5) to improve the degree of automation equipment Circulation pump overload, over-current protection of the chiller cooling water, chilled water temperature automatic control, to ensure the safe and efficient operation of the unit. In summary, the central air conditioning circulating pump using frequency control has obvious economic benefits, the transformation of the system is necessary. Third, the central air conditioning frequency conversion transformation program design: 3.1 cooling water circulating pump frequency conversion program 3.1.1 frequency control method, a professional cooling pump variable frequency control theory - constant temperature control 3.1.2 variable frequency energy-saving control principle variable frequency control system based on the freezer The temperature difference between the backwater and the effluent, changing the rotation speed of the cooling water circulating pump, that is, changing the flow of the cooling water, so as to keep the temperature difference between the backwater and the effluent of the freezer constant. Pump power consumption and speed is the cubic relationship, that Pαn3n - the speed of the cooling pump It can be seen, the pump power consumption in the ideal state, with the chiller is proportional to the cooling capacity, that PαQQ--, cold water Cooling capacity of the unit 3.1.4 Control system functions 1. Cooling water, backwater temperature detection, display 2. Constant temperature difference automatic control 3. Energy-saving rate: up to 20% 4. Operation pump and standby pump switch 5. Cooling water over Temperature difference, under pressure alarm 6. Cooling water pump overload alarm 3.2 Freezing water circulating pump variable frequency transformation program 3.2.1 Frequency control method 1, Closed-loop control of chilled water pump system [1], Closed-loop control of chilled water pump system in cooling mode The program guarantees The end of the device chilled water flow supply, determine the minimum operating frequency of a freezer inverter, set it to the lower limit frequency and locked, frequency conversion chilled water pump frequency regulator is installed in the chilled water system through the main backwash Of the temperature sensor to detect the temperature of the chilled water return, and then set by the temperature controller to control the inverter frequency change, the control method is: Promise up frequency greater than the set temperature. 2], closed loop control of chilled water pump system in heating mode This mode is the control scheme of the chilled water pump system when the heat pump is running (ie heating) in the central air conditioner. In the same way as the cooling mode control scheme, the minimum operating frequency of a freezer pump inverter is determined while ensuring the supply of the chilled water flow to the endmost equipment, which is set as the lower limit frequency and locked. The frequency of the chilled water pump is adjusted by The temperature sensor installed on the backwater main of the chilled water system detects the backwater temperature of the chilled water, and then controls the frequency change of the frequency inverter through the temperature set by the temperature controller. The difference is: when the temperature of the frozen return water is less than the set temperature, the frequency is increased steplessly. When the temperature of the cold water returned by the temperature sensing is higher, the output frequency of the frequency converter is lower. 3.2.2 Control System Function 1. Chilled water / hot water outlet, return water temperature detection, display 2. Constant temperature control / temperature control optional 3. Operation pump and standby pump switch 4. Freeze pump and hot water pump switch control 5 Underwater temperature difference, under pressure alarm 6. Circulating pump overload alarm According to the above program analysis, combined with the actual pump operation, the central air conditioning system is now running a great energy-saving space, the saving rate can reach about 20%, energy-saving transformation Is very valuable. 3.2.2 Control System Structure 1. Inverter: An energy-efficient F frequency inverter from POWTRANTECHNOLOGY, whose software maximizes motor power factor and motor efficiency. 2. Temperature and pressure transmitter: The system uses high-precision temperature transmitter to monitor the pipe network to ensure the normal operation of the system within the required temperature range. 3. Control instruments: the British instrument, control of high precision, low failure rate. 4. Control System: The Beijing central air-conditioning company dedicated control system, with good operability, enabling remote monitoring. 5. Low-voltage electrical appliances: air switches, contactors, relays, buttons, etc. are imported products to ensure the safe and reliable system operation.