Research On Correction Control Of Central Air Conditioning Chilled Water System

As an important part of the centralized air conditioning system,the transmission and distribution system of the chilled water will directly affect the energy consumption of the air conditioning system.According to the literature review,it is found that the problem of "large flow and small temperature difference" generally exists in the air-conditioning water system.Compared with the "small flow and large temperature difference" characteristic of the heat flow at the independent end,the overall characteristic curve of actual chilled water has deviated significantly.Scholars have studied the deviation problem of the air conditioning water system in actual operation.However,due to the complex form of the air conditioning system,the large number of terminals,and the influence of indoor and outdoor factors,the operating characteristics of the air conditioning water system are difficult to obtain if it just based on the theoretical derivation method.Simulation methods is a suitable method.However,the existing simulations lack of accurate calculations on the coupling between building thermal characteristics and air conditioning hydraulic characteristics.The simulation results cannot reflect the actual air conditioning system operation and the dynamic change of room temperature so they cannot reflect the operating characteristics of the air conditioning water system.Based on the thermal coupling between the room and the air-conditioning terminal equipment as well as the hydraulic coupling between the room and multiple air-conditioning terminals,this paper establishes a short-step(5 minute)air-conditioning terminal and a coupling calculation thermal model of the building room,and develops an air-conditioning chilled water system simulation program.This simulation is in order to achieve a condition which is similar to the actual automatic control conditions and to simulate the dynamic change process of room temperature during air conditioning cooling process.In order to achieve these goals,I establish a water network model of all air conditioning terminals whith the water system to simulate the branch flow,node pressure and other key parameters in order to realize the joint simulation of all factors and air conditioning chilled water system.According to the air conditioning cooling capacity and the amount of water required by the chilled water system,the operating characteristics of the air conditioning water system and the power consumption of the transmission and distribution system of chilled water are obtained.The experiment was carried out through the central air-conditioning water system test bench.The thermal model of the simulated building was constructed based on the actual test bench construction and the program was used to simulate all important factors.Then I compare the experimental results with the simulation results to verify the accuracy of the model.Besides,compared with the traditional hourly time step model used for building thermal simulation,the existing model can reflect the actual operating conditions and energy consumption of the chilled water transmission and distribution system better.A simulation model is constructed,which simulates the control and adjustment process of air conditioning cooling condition under different control methods.This simulation is based on end-on-off adjustment and end-on-end continuous adjustment water pump in one simulation day in summer.There are different load rates between the terminals of the air conditioning system and the hydraulic coupling between the terminals,ect.These inevitable factors in actual operation will cause the "large flow and small temperature difference" phenomenon of the air conditioning water system.These factors make the air conditioning water system characteristic curve deviated from the ideal characteristics.By changing the terminal and the pump control mode,the operating conditions of the water system can be improved.Besides,the operating energy consumption deviation of the transmission and distribution system can be reduced.And the purpose of correcting the deviation of the centralized air conditioning chilled water system can be achieved.Further,using the simulation program of the air conditioning water system,I study the key factors including set value of pressure difference of pump,terminal air volume,outlet water temperature of the chiller,etc.These factors may cause a deviation between the air conditioning water system characteristic curve and the design condition of the air conditioning water system characteristic curve.Different setting values of pressure difference will not affect the change trend of the air conditioning water system characteristic curve.When the air volume is insufficient and the chiller outlet temperature rises,the coil will have insufficient cooling capacity,resulting in "large flow and small temperature difference" phenomenon.There is a deviation between the characteristic curve of the air-conditioning water system and the characteristic curve of the air-conditioning water system at the design working condition.Besides,it will increase the energy consumption of the transmission and distribution system.On the basis of recognizing these problems,a method to avoid deviation of the overall characteristics of the air conditioning water system during operation is proposed.On the air conditioning water system test bench,I carried out the pump deflection control experiment of the end water valve on and off adjustment water system under five different working conditions.The proportional value P and integral value I used by the pump flow PI controller were adjusted to make the air conditioning water system have better control effect.The experiment shows that the total flow rate of the pump frequency conversion operation is less than the total flow rate of the pump under the corresponding working conditions.And the pump frequency conversion operation realizes the correction control of the frozen water flow rate of the pump power frequency operation.The effectiveness of the proposed method is verified.