Study On Pipeline Characteristics And Optimal Control Strategy With Variable Differential Pressure In Secondary Pump System

The centralized air-conditioning system of public buildings has enormous potential for energy conservation.The stable operation and optimal control of the chilling or chilled water system are the key to energy conservation and emission reduction.In the variable flow control of an air-conditioning chilled water system,the pump speed is usually adjusted with reference to the differential pressure to cope with changes in the end load.In this paper,a precise hydraulic model for the primary-second variable flow system is established.The influence of the position of the pressure difference monitoring point on the flow distribution of the pipe network under the opening rate of 90% snap-off valve is studied.The results show that the closer the user is to the pressure difference monitoring point,the better the hydraulic stability is.If the monitoring point is set at ends of Z1,when Z1 is closed,the hydraulic imbalance of Z1 and Z10 will be 2.3% and 12.6%.Assuming that the terminal uses a continuously controlled two-way valve,the cause,development,and solution to the problem of “bypass reverse flow” when the chilled water flow demand is greater than the primary side supply are studied.The hydraulic characteristics,heat transfer characteristics and pressure difference control mechanism of the secondary pump system under partial load are studied.The results show that the minimum supply-return water pressure difference decreases with the decrease of flow ratio,when the flow ratio is reduced to 40%,the minimum supply-return water pressure difference is reduced to 15.7% of the design condition.when the end load is allocated,with the decrease of the frequency,it will go through the hydraulic adjustable zone and the hydraulic imbalance zone,and the supply-return water pressure difference will be reduced from the overpressure area to the unpressurized area,the hydraulic imbalance occurs gradually in each user from the most disadvantageous branch to the near cold source side.The secondary side shows "large temperature difference,small flow",and because of bypass mixing flow,the primary shows side "small temperature difference,large flow".For example,when the flow ratio is 60%,the secondary return water temperature rises to 13.9℃,and the secondary return water temperature is only 11.2℃.In this paper,the optimal control strategy of variable pressure difference setting value is put forward,and it is pointed out that the load distribution form has important influence on the energy saving effect of the variable differential pressure control,and the highest energy-saving rate,up to 28%,when the load is uniformly distributed.The characteristics of the operation of the typical parallel pump control strategy are studied.the results show that,in condition of low load rate and low supply-return water differential pressure and under the single variable frequency pump with power frequency pump sequencing control,the flow of the frequency conversion pump is too low and easy to cavitation;Multi-pump synchronous frequency conversion with quantity control combined with variable differential pressure control has the best energy-saving effect.When primary pump adopts flow ratio control and the secondary pump is based on variable differential pressure control,the energy saving of the primary and secondary pump is up to 77.5% and 56.7% under 60% flow rate of the terminal.