1 The question is raised Hot water network heating system, the correct choice of circulating water pump, not only involves the economy of heating network operation, but also affect the quality of heating. At present, in the hot water heating system has not yet universal speed pump case, the large and medium-sized hot water network, in order to adapt to the outdoor air temperature changes in heating design of heating buildings at room temperature, the use of more central quality control and Change the quality of the flow in stages. The qualitative adjustment of phased flow changes is widely adopted in terms of operational savings over central conditioning. In addition, when the final building scale of the heating boiler room is determined, it is often determined by the final load due to lack of heat load and insufficient construction funds in the near future, while the heat source is constructed in stages. In the early heating, the end of the heat source or staging, the actual circulating network heat network is less than the design of low-load circulation conditions, the corresponding circulation pump head how to choose, should be carefully analyzed and reasonably determined. 2 low load hydraulic conditions analysis Under normal circumstances, the final design load, the hot water main line economic friction resistance by 60-80 Pa / m selection. When using a phased change of flow quality control or heat source staging construction, while the external network according to the final determination of diameter, if the phased change of flow regulation, should choose the head and flow rate of the pump group. If the economy of 60 ~ 80 Pa / m than the friction to select low-load pump head when the small flow. As a result of circulating pump power consumption is too large, high power consumption, system operating conditions and other ills. From the basic principles of fluid mechanics we can see that the closed-loop system of water flow G and its calculation of pipe pressure loss ΔP has the following relationship: ΔP = kG2 (1) (2) Where: ΔPp is the pressure difference between the beginning and the end of the pipe segment, Pa; G is the medium circulation flow rate, m3 / s; k is the comprehensive resistance characteristic number of the pipeline, kg / m7; λ is the drag coefficient along the pipeline; Σζ is the local resistance Coefficient; L is the pipe length, m; d is the pipe diameter, m; Ï is the fluid density, kg / m3. From equation (1), it can be seen that when the pipe network is finished with the final pipe diameter, as long as the valve opening is not changed, that is, ζ is not changed, the fluid with a certain density and temperature is transferred (when the temperature and pressure do not change much When it can be considered as a constant density) its pipe resistance characteristics of a constant. Resistance loss of pipe network system ΔP only depends on circulating water flow through pipelines G, and pressure drop changes with the flow rate changes in a square relationship. Therefore, assuming that the final design flow of pipeline is G1 and the pressure loss under design conditions is ΔP1, the actual flow rate during the initial, final or heat staging of heating is G2 and the corresponding pressure loss is ΔP2, then: (3) Example 1. The final design flow rate of a city heating boiler water heating network is 1 200 m3 / h and the farthest loop length (calculated length) is 4 300 m. The heat source heat load is the final design load Of the 1/3, put into operation two 20 t / h steam furnace heat exchanger, heating water network corresponding circulating water is 400 m3 / h. External pipe network by the end of the heat load once built heating. The original design of the final selection of 2 (1 prepared) flow 1 200 m3 / h, lift 80 m, 355 kW motor power pump; recently selected low load two flow 400 m3 / h, lift 50 m, motor power 75 kW pump . Analysis: hot water main line economy than the friction resistance of 70 Pa / m, the most adverse pressure drop loop ΔPmax = 4 300 × 2 × 70 = 0.6 MPa = 60 m head The system is a two-stage heat exchanger, take heat exchanger single-stage resistance loss 0.05 MPa. Heat exchanger (two) total resistance loss: 2 × 0.05 MPa = 0.1 MPa = 10 m head, the end of the circulating pump head is: ΔP • Final = 1.15 × (60 + 10) = 80 m head The recent actual total circulating water volume of 400 m3 / h, according to equation (2) find the pressure drop ΔP2 = (400/1 200) 2 × 800 = 88.9 kPa Analysis shows that the recent cycle of water is only 1/3 of the final, the corresponding total pressure drop is only 1/9 of the final, the pipeline flow is very low, pipeline resistance is very small. Obviously, the original design selected at low load 50m was significantly larger head. If we consider leaving sufficient margin and other factors, according to the low load operation of two steam furnace, choose two flow 200 m3 / h, head 20 m pump, the corresponding motor power is 18.5 kW, the power is obvious. Example 2. If the quality control of the flow is changed in phases, the large and small pumps are used to match and configure the combination scheme to operate the large-flow pump in the cold period and to run the small-flow pump in the early and the late stage. Generally, take the small-flow pump circulating water as the large-flow pump % ~ 70%, whichever is 65%, still above Circulating water amount in cold period 1 200 m3 / h Circulating water amount in early and end of cold period = 0.65 × 1200 = 780 m3 / h Then: ΔP2 = (780/1 200) 2 × 800 = 338 kPa Low load can be used when two flow 400 m3 / h, lift 50 m, motor power 75 kW pump. Low-load energy-saving: (335-75 × 2) /335=57.7%. 3 Conclusion For low load condition, it is not easy to select the head of circulating pump with low load more than 60-80 Pa / m of economy of the main line of hot water network. The specific conditions should be analyzed concretely to reasonably determine the heat load of low load When the circulating pump head, flow, not only conducive to energy saving, but also to avoid large flow, low temperature unreasonable operating conditions, to ensure the quality of heating.