Research On Efficiency Security Technology For Parallel Water Pumps Variable Frequency Operation

The conveying energy consumption of the dynamic conveying equipment such aswater pump occupies large proportion in the total energy consumption of the HVACsystem. Generally, the operation mode of pump with variable frequency control is usedin order to reduce the energy consumption. But for the reasons of the improperconfiguration and control method in the condition of multi-pumps variable frequencyoperation, not only the efficiency of the operation is lower with higher energyconsumption, but the stability of the variable water volume system is also impacted.In the view of above problems, new variable frequency control strategies for thewater pumps in HVAC system, which include constant-terminal-resistance-rangecontrol method and efficiency-based pump number switching control method, arepresented in this paper. The water pumps can stay in a high efficiency condition with thecombination of these two control methods. The characteristics and suitability of thesetwo methods are presented by theory analysis and the control strategy is given in thispaper.First of all, the characteristics of the variable frequency control is analyzed. Theenergy-conserving principles of the variable frequency control method is introduced andthe available range of the variable frequency pump is researched. With study of factorsin pipes and equipment, the lowest rotating ratio is defined and the optimal operatingrange of the variable frequency pumps is worked out by researching the equal efficiencycurve. Combined with the calculating models of the pump performance, frequencyconverter and the motor, the calculating models of the total energy consumption andenergy saving ratio is determined.According to the operation characteristics of the variable frequency pumps, acontrol method with constant terminal resistance is presented and considering the actualcondition, the design value of the terminal resistance characteristic is determined. Bymodel building, the performance of the regulating valve and the pumps is analyzed andcalculated under this control method. Considering the actual fluctuation of the user’sload, the constant-terminal-resistance-range control is presented, and through the furtheranalysis, an advanced control method is given. Compared with the constant pressuredifference control, the influence factors of the energy saving ratio using constant-terminal-resistance-range control method is studied, and the case verification isconducted. With the research on the changes of terminal resistance under differentutilization ratio, the applicable conditions of the constant-terminal-resistance-rangecontrol are proposed.Under the constant-terminal-resistance control and constant pressure differencecontrol, the change law of the pump operation condition with the traditional pumpnumber switching method is presented by theory analysis. According to the speed rangeof the variable frequency pump, the reasonable configuration mode of the numbers ofthe pumps and cooling and heating sources is given. The disadvantages of the traditionalpump number switching method are pointed out and the efficiency-based pump numberswitching technology is proposed, based on the constant-terminal-resistance controlmethod. Several key technologies of the efficiency-based pump number switchingcontrol method are analyzed, such as the influence on the pump number switching bythe constant-terminal-resistance-range and range of the high efficiency, the selection ofthe pump number and the suitability of this switching technology under constantpressure difference control. Through these analysis the advanced switching method isput forward.In the last section, the process of the combination automatic control strategy withconstant-terminal-resistance-range method and efficiency-based pump numberswitching method is given.