Frequency conversion energy-saving control method of central air-conditioning water circulation system

Frequency conversion energy-saving control method of central air-conditioning water circulation system

The energy consumption of the air-conditioning water system accounts for a large part of the entire air-conditioning system. Therefore, it is of great significance to study the energy saving of the air-conditioning water system. Inverter air conditioners using inverter control technology can achieve fast, energy-saving and comfortable temperature control effects. This article first analyzes the composition and working principle of the air-conditioning water system, and discusses the energy-saving control of the variable flow of cold water, and finally puts forward some key designs to make the pump work in the high-efficiency area.

Through the frequency converter, according to the needs of the air terminal, the heating, cooling and dehumidification operation modes can be automatically selected according to the ambient temperature, so that the room can reach the required temperature quickly in a short time, and at low speed, low energy consumption, and high performance With small temperature fluctuations, adjust the working frequency of the refrigerant water pump and cooling water pump, and change the refrigerant water volume and cooling water volume in the system to achieve the purpose of energy saving. The variable-flow energy-saving control mode is also called central air-conditioning energy-saving control. This article focuses on the principle analysis and relevant design points of the frequency conversion energy-saving control of the central air-conditioning water circulation system.

　　 1. The working principle of air conditioning water system

　　 The main equipment of the air conditioning water system includes: refrigeration units, cooling water pumps, chilled water pumps, cooling towers, fan coils and other air conditioning terminal equipment. The air conditioning water system is a complex system, and the components are interrelated and affect each other.

　　 1. Fan-coil water system

The composition and working principle of fan coils. Fan coils are mainly composed of fans, coils, air filters, motors, and room temperature control devices. Fans often use forward multi-wing centrifugal fans or cross-flow fans. It is a finned coil heat exchanger. The main function of the fan coil water system is to transport and distribute cold fluid to meet the load requirements of the terminal equipment or unit. The configuration should have sufficient conveying capacity, economically and reasonably select water pumps, pipes and pipe diameters, have good hydraulic stability, and should facilitate the operation adjustment of the air-conditioning system when the load changes, so as to achieve the energy-saving operation requirements of the air-conditioning system, and It is easy to manage, repair and maintain.

　　 2. Working principle of chiller

　　 When the natural cold source cannot meet the air conditioning needs, artificial refrigeration is used. There are mainly the following:

　　1) Vapor compression refrigeration

　　 The composition and working process of vapor compression refrigeration system: The vapor compression refrigeration system mainly consists of four main equipments: refrigeration compressor, condenser, expansion valve and evaporator, which are connected by pipelines to form a closed circulation system.

　　2) Absorption refrigeration

Absorption refrigeration is the same as vapor refrigeration. It uses the physical characteristics of liquid to absorb heat when vaporizing. The difference is that vapor compression refrigeration uses mechanical energy as compensation, while absorption refrigeration uses consumption. The thermal energy is compensated, so that the heat is transferred from the low temperature pyrogen to the high temperature heat source.

　　3, cooling tower

　　 The heat released during the cooling and condensation process of the refrigerant in the condenser is generally taken away by air and water. Condensers using air as the cooling medium are mostly used in small refrigeration systems and water-deficient areas. The condensers of large and medium-sized refrigeration systems mostly use water as the cooling medium. The cooling return water of the refrigeration device is sprayed from the upper part of the cooling tower to the filling layer in the tower to increase the contact area and contact time between water and air. The cooled water flows from the filling layer to the lower collecting pool and passes through the circulation The water pump is sent back to the refrigeration unit for recycling. The top of the cooling tower is equipped with a ventilator to make air pass through the packing layer from bottom to top at a certain flow rate to enhance the evaporative cooling effect of water.

　 2. Energy-saving control of cold water variable flow

　　 1. Main control measures of variable flow system

After the cold water flow changes, the water system needs to adjust the piping system and the water pump in order to achieve a stable balance under the new flow, that is, adjust the valve opening and the pump speed. Different control strategies will produce different control effects. , And the energy consumption of the water pump is also different, the following are respectively compared. The control modes of variable flow operation of cold water system mainly include temperature difference control method and pressure difference control method.

　　1) Temperature difference control method

　　The temperature sensor measures the temperature difference on the mains supply and return water. Compared with the set value, a large temperature difference increases the water flow of the cold water pump, and a small temperature difference decreases the water flow. When the load drops, the flow will decrease accordingly. Through the temperature difference controller and frequency converter, the speed of the water pump is reduced to reduce the flow. As the total water volume of the system decreases, the water flow of the end devices of the air-conditioned rooms with similar functions will be reduced proportionally to adapt to changes in load. It is suitable for the situation where the system is small, the room functions are relatively simple, and the overall energy consumption changes are consistent, such as Fan-coil system is widely used in the current air-conditioning system in our country. The water circuit of the fan coil system is basically not controlled, or is controlled by a three-way valve or solenoid valve. The system pressure difference is almost unchanged at part load, which makes it difficult to collect the pressure difference signal. Therefore, the fan coil system adopts the pressure difference control to be accurate The performance is poor, and the system uses the most temperature difference control.

　　2) Differential pressure control method

　　 Differential pressure control uses the change of the measured point differential pressure to control the water supply volume of the pump. The pressure transmission speed is high, so the differential pressure control responds quickly. At present, the main pipe constant pressure differential is used in the cold water system. The dry pipe constant pressure difference control only replaces the temperature sensor with a pressure sensor. The end load is reduced and the flow of chilled water is turned off to increase the pressure difference on the mains supply and return water. The controller will control the water pump to reduce the water flow and maintain the supply. The pressure difference of the return water main pipe remains unchanged, so that for the other ends where the load does not change, the pressure difference between the supply and return water of the main pipe remains unchanged, and the pressure difference between the supply and return water of each branch basically remains the same (due to the flow rate Reduced, the resistance loss on the main pipe is reduced, and the pressure difference on the branch pipe is slightly increased), ensuring that there is sufficient water flow at each end.

　　2, the principle of variable flow system

　　 The most important purpose of the air conditioning cold water system is to provide energy exchange for each terminal device of the air conditioning system. How to save energy as much as possible while meeting this requirement is constantly improving in the development of the cold water system. In the process of the development and improvement of the cold water system, new problems are always encountered, such as the cold water temperature difference is too small, the water system resistance loss is too large, the pipe network hydraulic imbalance, etc., if the temperature difference between the supply and return water is kept constant, then The demand for the flow of chilled water drops, and the energy consumption of the pump can be reduced by reducing the transport of chilled water. This is the variable flow technology.

　　 Three, plan design

As far as the current general renovation design is concerned, temperature control is generally adopted. During the renovation of the central air conditioning system, the original central air conditioning system is maintained, and heat pump units, plate heat exchangers, heat storage tanks and Hot water tanks, circulating pumps, regulating valves and other equipment have been added, and a frequency conversion speed regulating intelligent control energy-saving workstation with general functions has also been developed.

　　1) Control of chilled water pump

　　 The chilled water pump motor should adopt soft start. The starting frequency system of the chilled water pump motor is set to 45Hz. To protect the safe operation of the air-conditioning system, the minimum operating frequency of the cold and warm water system is set to 30 Hz.

　　 Set the upper limit alarm output signal on the temperature difference regulator. When the terminal load changes suddenly and the temperature difference reaches 5℃, the frequency is directly switched to 45Hz. Increase the pump output to the maximum flow rate to improve the tracking speed of the load. After the chilled water pump is started, according to the control parameter value output by the intelligent controller, the operating frequency of the chilled water pump inverter is adjusted, the speed of the chilled water pump is controlled, and the flow of chilled water is dynamically adjusted to make the temperature of the chilled water supply and return close to the intelligent controller. Set the optimal value. Summer: The rated supply temperature of chilled water is 7℃, the rated return temperature is 12℃, and the temperature difference △T=5℃; in winter: the cold and warm water supply temperature is 50℃, the return water temperature is 45℃, and the temperature difference △T=5℃.

　　2) Control of cooling water pump

After the cooling water pump is started, the operating frequency of the cooling water pump inverter is adjusted according to the control parameter value output by the intelligent controller, the rotation speed of the cooling water pump is controlled, and the flow rate of the cooling water is dynamically adjusted to make the inlet and outlet temperature of the cooling water approach the setting of the intelligent controller. The optimal value.

　　3) Control of cooling tower fan

When the fan is started, after the cooling tower fan frequency is set to a high limit of 45Hz and kept running for 30 minutes, the system will adjust the fan speed according to the input value of the cooling water inlet and outlet temperature sensor of the air conditioner to make the cooling water inlet temperature approach the set value. So as to ensure that the central air-conditioning host is in the best operating state at any time. In order to realize the energy-saving operation of the cooling tower fan and the air-conditioning host under the best working conditions.