Air Flowrate Monitoring And Predictive Control Method Of VAV System Based On Damper Mechanical Characteristics

With the development of building control technology and the continuous improvement of building energy saving requirements.variable air volume(VAV)air conditioning system is more and more widely used in all kinds of public buildings,but there are also the following urgent problems to be solved in the actual operation of VAV system.First of all,the real-time measurement of VAV terminal air volume is the prerequisite for the high-performance control of VAV box.The existing VAV terminal air volume measurement methods can not meet the demand of terminal air volume measurement at low flowrate and uneven flow distribution.which leads to the failure of real-time on-demand control at low-load condictions.It not only consumes a lot of energy consumption,but also reduces the indoor thermal comfort.Secondly.most of the existing VAV system air volume control methods are based on the operation and debugging experience and lack of accurate dynamic mathematical model as a support,resulting in rough system control process and large air volume fluctuation,which also affects the further improvement of system operation efficiency.Therefore,in order to achieve wide range airflow measurement,and on this basis to achieve the stable control of VAV system with low energy consumption,this paper proposes a novel airflow measurement and control method of VAV air conditioning system by damper mechanical characteristics.Firstly,the mechanical flow mathematical model of terminal damper in VAV air-conditioning system is established,and the characteristics of the key parameters in the model are studied.Then,the air volume prediction control model and method of VAV air supply network are established by the advantage of accurate monitoring of terminal air flowrate in VAV air-conditioning system.The specific research works are as follows:(1)After analyzing the resistance characteristics of VAV air supply network,this paper proposed a novel method of air flowrate measurement by using the pressure difference or torque characteristics of terminal damper,including the establishment of mathematical model and structural design of the differential pressure type and the torque type airflow sensors.This method could improve the air flowrate measurement performance at low flowrate.Two airflow sensor performance evaluation indexes(output sensitivity and output range ratio)were proposed,and the feasibility of the novel damper airflow measurement method was verified by using the above two indexes.(2)This paper established the mechanical characteristics simulation model of damper airflow sensor based on computational fluid dynamics(CFD)and finite element method(FEM).The change rules of key parameters such as opening,air flowrate,pressure difference,pressure loss,torque and flow coefficient in the damper mathematical model were studied,which provided theoretical support for guiding the selection of test equipment.This paper established the research and development experimental system of the damper airflow sensor for the partial verification of simulation results and the measurement performance of the damper airflow sensor.The measurement uncertainty of the air flowrate test-bed was less than ± 1.38%.(3)For the novel differential pressure damper airflow sensor.this paper studied the flowrate coefficient C,differential pressure and pressure loss characteristics of the damper under different conditions of opening and Reynolds number by simulations.Based on the simulation results,a prototype of damper airflow sensor was designed and developed.The simulation results and measurement error of the prototype were partially verified by experiments.The study results showed that the flowrate coefficient C was basically constant at different opening and Reynolds number,and the maximum control error of the prototype was less than ±5%.(4)This paper proposed the novel torque damper airflow sensor to replace the point measurement by surface measurement of differential pressure between upstream and downstream of damper disc,which further solved the problem of flow measurement under nonideal conditions.The influence of different damper opening on torque flowrate coefficient K and the influence of upstream non-ideal installation conditions(90° elbow)on torque measurement were studied by simulations.Based on the simulation results,the torque sensor and airflow sensor were designed and developed,and the flowrate coefficient K and measurement error of the prototype were measured by experiments.The results showed that.the torque flowrate coefficient K decreases with the increase of opening,and the influence of non-ideal installation conditions on torque measurement was less than 1.88%.The measured error of the damper airflow sensor was within±5%.(5)This paper established the simulation model of VAV duct network equipment and proposed two damper opening prediction algorithms under target demand.The two algorithms were verified by the simulation model.The first algorithm was based on the physical model of duct network,which needed only the network topology and a small amount of sample data to realize the high-precision prediction of damper opening.The second algorithm was based on the black box model.which did not need any duct network information,but needed a certain amount of sample data to train the model,and the prediction accuracy was slightly lower than the first one.(6)This paper proposed a novel total air flowrate control method based on the prediction of terminal damper opening(damper opening prediction method),and compared the control stability and energy saving effect of static pressure control method and damper opening prediction method by simulations.The simulation results showed that the damper opening prediction method had good stability,and could reduce the unstable control time by 50%,and saved 21.6%energy compared with the constant static pressure control method.The application performance of the novel damper airflow sensor in VAV air conditioning system was analyzed based on the simulation results.Compared with the pitot tube airflow sensor,the output sensitivity and output range ratio of the damper airflow sensor were optimized by 5.3 times and 15.7 times respectively.When the set value of the lower load limit was adjusted from 30%of the rated load to 10%,the energy saving rate of the novel damper airflow sensor could reach 10.6%.