| Hydraulic pipe network is one of the most important components. In practical engineering, the water loop in central air conditioning system is becoming one of the most energy-consuming, problem-production and inconsequence-running parts. Based on the acquired scientific achievements, a further study on the characteristic of the pipe network is made, the second order partial derivative of fluid's flow rate with pipe's impedance is obtained. A first order and second order partial derivative of fluid's flow rate with pump's frequency is obtained, too. Therefore, variety of fluid's flow rate due to impedance change of certain branch or pump's frequency could be expanded to Taylor's second progression, which is more accurate and is validated by experiment study. In addition, the hydraulic stability of primary only chilled water system using common differential pressure control strategy is studied. Both theory and experiment prove the hydraulic stability referred to above equals zero.On the basis of the further study of hydraulic characteristics, a kind of optimal control strategy is introduced, that is when load of a certain branch drops, the electrical modulating valve is reduced while decreasing pump's frequency whose minimum value is to keep other branches still working in the prior equilibrium state. calculation indicates that this new optimal control strategy could further save the pump's energy of chilled water system.At last, according to theory of minimum energy consumption of the pipe network, minimum energy will be consumed by the chilled water loop when one or several valves are fully opened in an ideal primary-only chilled water system is theoretically proved. Consequently, an evaluation index PCOP(Pump's Coefficient Of Performance) with obvious physical meaning was introduced. The PCOP of the chilled water loop using constant differential pressure control strategy is discussed and an example is given in the end.
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