Research On Energy Conservation Of Sanitation Suction And Discharge Systems With Pneumatic Vacuum Ejector

Vacuum sanitation is the safe and sound disposal of human excreta to satisfice the primal urge under special living conditions. The vacuum is generated in excreta collection tank by the pneumatic ejector and the propulsive force for sewerage is provided by the difference between the atmospheric pressure behind the sewage and the vacuum ahead. However, a significant portion of the energy consumed by the present vacuum sanitation systems is wasted due to improper settings of the same propulsive force for different amount of excreta. In respect that designers of current systems did not consider the difference in energy supply to collect different amount of excreta, these systems can be improved upon through sewerage force adjustment for energy conservation. To improve the energy efficiency, this research proposes an energy conservation design in vacuum sanitation systems based on pneumatic ejector circuit. In this new design, the sewerage force is adjusted due to the amount of excreta. Theoretical research and experimental study are carried out on the composition of energy-saving vacuum sanitation systems, sewerage-force adjustment technology, vacuum generation process and relationship between the excreta amount and the propulsive force.The measurement of excreta amount and the adjustment of sewerage-force are studied by controlling the compressed air supply. The close-stool of this new sanitation device is specially designed, including outside framework and inside vessel, convenient for testing the excreta amount. The electric signals of volume and mass of the excreta are transmitted to the computer and the sewerage force is adjusted according to the amount of excreta.The pneumatic ejector circuit for the energy-saving sanitation device is studied, and the vacuum degree can be adjusted. According to the features of sanitation, the discharge gas of the ejector must be depurated with auxiliary elements, where the discharge pressure is raised. By theoretical analysis and experimental test, the working efficiency of the ejector is in a satisfying range when the discharge pressure is below the critical value. On the other hand, the vacuum pressure sensors and directional control valves are installed at the vacuum port of the ejector to realize real-time measurement and adjustment. However, the flow resistance of these vacuum elements changes the static flow-rate characteristics and the vacuum response time. To analyze the influence from vacuum resistance on the flow-rate characteristics of the ejector, a new dimensionless parameter Gr* is introduced and the-effect on the suction flow rate is analyzed by mathematical approach, as a useful reference for choosing the vacuum elements in the energy-saving sanitation device.The compressed air consumed by the vacuum sanitation system is measured. When the temperature of the air source is constant, the supplied air flow-rate is proportional to the supply pressure and dimensional parameters of the Laval nozzle. The principle on measuring the suction flow-rate of the ejector is studied and the suction flow-rate of the new energy-saving sanitation device is measured efficiently and precisely. Compared with the conventional measurement method for suction flow-rate, the new measurement apparatus saves more than 80% of the compressed air and the consumed time is only 10% of the current time, with the testing error of 2.3%. Due to the measurement result, the least-square fitting is taken on the experimental data and the fitting coefficients are obtained.The working process of the energy-saving sanitation device is divided into two periods, such as the vacuum generation process in the excreta collection tank and the excreta sewerage process. The pressure response in the excreta collection tank is studied separately due to the two processes. By using the static flow-rate characteristic expression, chamber discharge model and pipe differential model, the vacuum generation process is simulated. Analyzing the dynamic response characteristics of the energy-saving vacuum sanitation device, the simulation models are simplified and the simplified model simulates the vacuum generation process well in this new sanitation device. After that, the pressure response in the excreta sewerage process is proposed and the control procedure is designed according to the relationship between the excreta amount and sewerage force. To verify the effect of energy conservation, experiments are carried out in the artificial excreta collection. The sewerage force and energy consumption change along with the amount of the excreta. When there are solid components in the excreta, the required sewerage force is larger. With proper sewerage force adjustment, this new sanitation device reduces the air consumption and realizes energy conversation. When the volume of artificial excreta varies from 0.2x10-3m3 to 1.6x10-3m3, the vacuum degree varies from 3 kPa to 41 kPa, and the energy consumption changes from 88 J to 1380 J. In the operation of artificial excreta collection, the variable vacuum-degree design saves more than 30 percent of the energy supply.