Abstract: By analyzing the failure cases of the circulating pump in the central air-conditioning system project, the paper analyzes the failure cases of the project, emphasizes the importance of choosing the head of the circulating pump reasonably and puts forward some methods of choosing, which have reference value to the design of the central air-conditioning system. Keywords: central air conditioning, pump head, reasonable choice Abstract This article analyzes the cases of engineering failures resulted from the improper selection of circulating water pump head in central air conditioning system, emphasizes the importance of reasonable selection, and further advances approaches for the selection In the central air-conditioning system, the circulating water pump delivers chilled water in summer and the hot water in winter to the air-conditioning terminal, and the engineering design should be selected according to the air flow and the system resistance of the air conditioning system Good pump. HVAC design manual has a detailed design and calculation methods. The problem is that the actual engineering design, some engineers did not follow the calculation method to calculate the design, but by experience of course, the system and some air-conditioning equipment, accessories Lack of careful study of new products, resulting in the choice of the pump can not meet the requirements, or cause increased operating costs, or even pump does not work, which had to cause air-conditioning designers Attaches great importance .2 Theoretical analysis of air conditioning system, the size of the flow of water by the load and the temperature difference between supply and return to determine the resistance of the system through hydraulic calculations obtained by the flow and resistance of the pump selection, operation should be in the high efficiency area, the working point for the pump performance The intersection of the curve and the pipe characteristic curve, as shown in Figure 1, point A [1], while the project selected pump often appear two kinds of abnormal situation.Figure 1 pump working point diagram 1) design more conservative, the actual flow rate of water system The lower the value, the larger the estimated resistance of the system, resulting in increased pumping head lift, so that the choice of circulating pump head design flow rate greater than the system resistance shown in Figure 2: Figure 2 different pump operating point changes Flow QA is the system design flow at which pump head can be HB. The actual choice of pump head HS. In order to ensure the QA, we must change the pipeline characteristics, that is, through the valve off the import and export of small pumps, the pipeline characteristic curve from â… to â…¡. Obviously, ΔP = HB-HA completely through the valve throttling, which is very uneconomical Of the project, but also to avoid the situation in the project, if the same set of pumps work in winter, due to smaller flow, throttling more serious, more uneconomical, or even cause pump instability [2]. 2) Self-confidence, the resistance of air-conditioning system is estimated to be small, the selected pump head less than the design flow system resistance shown in Figure 3: Figure 3 pump line characteristics and the actual operating point design A, pump flow QA, For the HA. The actual pump performance curve is not â… , but â…¡, the operating point for the B, flow QBA, and B is not in the pump high-efficiency zone. Obviously this is more adverse than the first case. The only way to solve Only to replace the pump.3 Project Example 1 A project is a single high-rise building, the building height of 29m, the pump room is located in the basement of the main building.It is designed to import a Carrier centrifugal freezers, a cooling capacity of 1163 kW, with 2 Taiwan circulating pump, 1 with 1 prepared, pump parameters in Table 1. Table 1 pump nameplate parameters Pump Type Parameters H / MPaQ / m3.h-1N / kWI / AIS150-125-400502004584 Just started debugging exercise, the pump motor current Too large, the pump outlet vibration powerful, and there is abnormal sound.Pump head is only 0.28MPa, motor current I = 115A.Analysis of the reasons for the sub-catchment pressure is only 0.13MPa, the selected pump head is too large. Pump working point for the low lift large flow, the motor is seriously overloaded [3]; serious cavitation pump, pipe jitter, abnormal sound; off the pump and freezer evaporator inlet and outlet valves to ensure that the pressure required for the import and export of the evaporator The difference Δp = (92 ± 5) kPa, so that the pump back to normal work. Test data at this time as shown in Table 2 (the original pump.) Table 2 Pump Test Data Test items inlet pressure / MPa outlet pressure / MPa manifold pressure / MPa points Water pressure / MPa motor current / A Original pump new pump 0.30.30.780.530.310.310.410.418240 From Table 2 shows that the pump head is 0.48MPa, manifold differential pressure is 0.10MPa, the evaporator pressure is 0.1MPa, System resistance is not large, and most of the pump pressure head completely consumed Turn off the small valve on the solution.Replacement of a low head pump, the test data as shown in Table 2 (new pump) .Comparison of data in Table 2, the motor current is reduced from 82A to 40A, its operating economy is self-evident. B project is a regional air-conditioning, cooling and heating stations to a number of buildings focused on cooling, heating design 2 2907kW freezer 2, circulating pump 3, 2 with 1. Preparation of a freezer system debugging, a Circulation pump .A few minutes later, the pump outlet pipe vibration and accompanied by abnormal sound, the refrigerator can not start, the fault shows insufficient frozen water circulating water.Inspection system valves have been fully open, water filter all clean, the system exhaust more thoroughly. Nameplate parameters: Q is 500m3.h-1, H is 0.475MPa, N is 90kW; Test data: the pressure difference between the inlet and outlet of the evaporator is 0.02MPa, the pressure difference between the inlet and outlet of the pump is 0.14MPa, the pressure of the water collector is 0.27MPa, Pressure 0.40 MPa. According to the pump performance curve, when the head H = 0.14MPa, the system valve is fully open, the flow Q should be greater than 500m3.h-1 was right, and this time the flow indicator self-locking, indicating insufficient flow Thus, pumps work abnormally, in fact, so large System, the head of the pump H = 0.14MPa is also impossible.Finally identified for two reasons: â‘ pump inlet installed a brush water filter, the filter mesh is too small, causing the pump inlet resistance is too large, resulting in severe cavitation , The pump performance deterioration; â‘¡ pump flow and lift are very small, the system resistance larger. Remove the brush filter filter operation, points, manifold pressure difference of 0.45MPa, pump head is 0.52MPa. Obviously the pump can not guarantee the normal operation of the system. Solution: Replace the filter of the water filter; re-calculate the hydraulic system and replace the pump, which will not only result in the increase of circulating flow but also increase the system resistance Large economic losses, but also affect the normal operation of air-conditioning system, the lesson is profound. 4 Conclusion 1) air-conditioning system, circulating pump selection is not difficult, but designers should pay enough attention to experience combined with the theoretical calculations. 2) The choice of the pump head can not think of the more the more insurance, and should pay attention to the economy of operation. 3) The current replacement of air-conditioning products faster, the selection should be very careful to recognize its performance the study.