1 question raised 2 Analysis of hydraulic conditions under low load Under normal circumstances, in the final design of the load, the hot water network main line economy than the friction resistance is selected at 60-80 Pa / m. When the flow quality adjustment or heat source staging construction is changed in stages, and the external network determines the pipe diameter according to the final stage, if the flow rate adjustment is adopted in stages, the pump set with different lift and flow rates should be selected. If the economical ratio of 60 to 80 Pa/m is used, the head of the small flow pump at low load is selected. As a result, the circulatory water pump has excessive power selection, high operating power consumption, and unreasonable operating conditions of the system. It can be known from the basic principles of fluid mechanics that the water flow G of the closed loop system has the following relationship with the pressure loss ΔP of the calculated pipe section: ΔP=kG2 (1) (2) Where: ΔPp is the pressure difference at the beginning and end of the pipe section, Pa; G is the medium circulation flow, m3/s; k is the comprehensive resistance characteristic of the pipeline, kg/m7; λ is the resistance coefficient along the path; The sum of the coefficients; L is the length of the pipe, m; d is the inner diameter of the pipe, m; Ï is the fluid density, kg/m3. It can be seen from formula (1) that when the pipe network is laid according to the final pipe diameter, as long as the valve opening degree is not changed, the enthalpy is constant, and the fluid of a certain density and temperature is transported (for the liquid, when the temperature and pressure do not change much) When the density is considered constant, the comprehensive resistance characteristic of the pipeline is constant. The resistance loss ΔP of the pipe network system is determined only by the circulating water flow rate G through the pipeline, and the pressure drop change increases or decreases with the square of the flow rate change. Therefore, if the design flow rate of the pipeline is G1, the pressure loss under the design condition is ΔP1, and the actual flow rate during the initial stage of heating or the construction of the heat source is G2, and the corresponding pressure loss is ΔP2. (3) Example 1. The design flow rate of the hot water network of a central heating boiler room in a city is 1 200 m3/h, and the heat network zui far loop single length (calculated length) is 4 300 m. The heat supply load of the heat source is the final design load. One-third of the two 20 t/h steam furnaces are put into operation, and the corresponding circulating water volume of the hot water supply network is 400 m3/h. The external pipe network is built for heating at the final heat load. At the end of the original design, 2 sets (1 standby) flow rate 1 200 m3/h, lift 80 m, motor power 355 kW water pump; 2 low flow load 400 m3/h, lift 50 m, motor power 75 kW pump . Analysis: The economical ratio of the main line of the hot water network is 70 Pa/m, then the unfavorable loop pressure drop ΔPmax=4 300×2×70=0.6 MPa=60 m head The system is a two-stage heat exchange, and the heat exchanger has a single-stage resistance of 0.05 MPa. Heat exchanger (two-stage) total resistance: 2 × 0.05 MPa = 0.1 MPa = 10 m head, then the final cycle pump head is: ΔP  final = 1.15 × (60 + 10) = 80 m head In the near future, the actual total circulating water volume is 400 m3/h, and the pressure drop ΔP2 = (400/1 200) 2 × 800 = 88.9 kPa is obtained according to formula (2). The analysis shows that since the circulating water volume in the near future is only 1/3 of the final period, the corresponding total pressure drop is only 1/9 of the final period, the flow rate in the pipeline is very low, and the pipeline resistance is small. Obviously, the original design has a significantly larger head of 50 m at low loads. If considering the factors such as sufficient margin, run two steam furnaces at low load, select two flow rates of 200 m3/h, and lift 20 m pumps, then the corresponding motor power is 18.5 kW, and the power saving is obvious. Example 2. If the quality adjustment of the flow rate is changed in stages, the large and small pumps are used in combination with the configuration scheme to run the large-flow water pump during the severe cold period, and the small-flow water pump is operated in the initial and final cold periods. Generally, the small-flow water pump is the circulating water volume for the large-flow water pump. %～70%, take 65%, still in the above case, the circulating water volume in the cold period is 1 200 m3/h. The circulating water volume in the initial and final cold periods = 0.65×1200=780 m3/h: ΔP2 = (780/1 200) 2 × 800 = 338 kPa At low load, 2 pumps with a flow rate of 400 m3/h, a head of 50 m and a motor power of 75 kW can be used. Power saving at low load: (335-75 × 2) / 335 = 57.7 %. 3 Conclusion For low-load working conditions, it is not easy to select the circulating pump head for low load according to the economical ratio of the hot water network main line 60-80 Pa/m, and the specific conditions should be analyzed to determine the low load of the heating network. The head and flow of the circulating water pump are not only conducive to power saving, but also avoid unreasonable operating conditions of large flow and low temperature difference, and ensure the heating quality. Meltblown Nonwoven, Meltblown Nonwoven Fabric, Melt Blown Nonwoven Fabric Ningbo DOKEE Medical Technology Co., Ltd. , https://www.dokeemedical.com
In the hot water network heating system, the correct choice of circulating water pump not only relates to the economics of the heating network operation, but also affects the heating quality. At present, in the case that the hot water network heating system has not popularized the speed-regulating water pump, the central and medium-sized hot water nets are mostly adapted to the influence of the outdoor atmospheric temperature change during the heating period on the design of the heating room. Change the quality of the flow in stages. The quality adjustment of the flow rate in stages is superior to the central quality regulation in terms of energy saving. In addition, when the final construction scale of the heating boiler house is determined, the external network diameter is determined according to the final load due to the short-term heat load shortage and insufficient construction funds, and the heat source is constructed in stages. In the initial stage of heating, construction of the heat source stage, when the actual circulating flow of the heating network is less than the low load of the designed circulating flow, how to select the corresponding circulating pump head should be carefully analyzed and reasonably determined.