Abstract This paper analyzes the failure cases of the project due to the improper choice of the head of circulating pump in the central air-conditioning system project, emphasizes the importance of selecting the head of the circulating pump reasonably and puts forward some methods of choosing, which is of reference value to the design of central air-conditioning system. In the central air-conditioning system, the circulating water pump delivers chilled water in the summer, and the hot water in winter to the air-conditioning terminal device. The engineering design should be chosen according to the water flow of the air conditioning system and the system resistance Of the pump.About HVAC design manual has a detailed design calculation method.The problem is that the actual engineering design, some engineers did not follow the calculation method to design calculations, but by experience, of course, the system and some air-conditioning equipment, accessories, etc. The 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 caused great attention of air-conditioning designers. 2 Theoretical analysis of air conditioning system, the size of the water flow from the load And the temperature difference between supply and return water is determined, the system resistance is obtained through hydraulic calculation.The pump selected by flow and resistance should be in the high-efficiency zone during operation, and its working point is the intersection of pump performance curve and pipeline characteristic curve, as shown in Figure 1, A Point [1]. And the project of choice of the pump often appear two kinds of abnormal Figure 1 Schematic diagram of the pump working point 1) The design is conservative, the actual value of the water system is lower, the estimated system resistance is larger, resulting in increased pumping head lift, the choice of circulating pump head than the design flow rate System resistance is much larger.As shown in Figure 2: Figure 2 different pump operating point changes Figure flow QA is the system design flow at this flow 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 area. 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 for a single high-rise building, the building height of 29m, the pump room is located in the basement of the main building design imported centrifugal centrifugal freezer, cooling capacity of 1163 kW, with 2 circulating pumps, 1 with 1 prepared, pump parameters in Table 1. Table 1 pump nameplate parameters Just started debugging exercise, the pump motor current was found to be excessive, pump water pipe vibration, and abnormal sound pump head is only 0.28 MPa, motor current I = 115A.Analysis of the reasons for the sub-catchment pressure difference is only 0.13MPa, the selected pump head is too large. At this time the pump operating point for low lift large flow, the motor is seriously overloaded [3]; pump cavitation Severe, jitter pipe abnormal sound abnormalities; off small pumps and freezer evaporator inlet and outlet valves to ensure that the evaporator pressure required for the import and export Δp = (92 ± 5) kPa, the pump back to normal work. The data is shown in Table 2 (original pump). Table 2 pump test data can be seen from Table 2, the pump head is 0.48MPa, sub-catchment pressure is 0.10MPa, the evaporator pressure is 0.1MPa, the system resistance is not large, and most of the pump pressure head completely consumed in the off small Of the valve on the solution.Replace a low head pump, 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 operation economy is self-evident .Example 2 B project For a regional air-conditioning, heating and cooling 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 circulating pump After a few minutes, the pump outlet pipe vibration and accompanied by abnormal sound, the refrigerator can not start, the fault shows insufficient water circulating water. Check the system valves have been fully open, all the water filter 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 sump is 0.27MPa, the pressure of the sump is 0.40 MPa. According to the pump performance curve, Head H = 0.14MPa, in the case of full open valve system, the flow rate Q should be greater than 500m3.h-1 was right, and this time the flow indicator self-locking, indicating insufficient flow. Thus, the pump work abnormal. In fact , Such a large system, the pump head H = 0.14MPa is also impossible.Finally identify two reasons: â‘ imported pump inlet water filter, the filter mesh is too small, leading to the pump inlet resistance is too large, Resulting in serious cavitation, deterioration of the pump performance; 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, the pump lift 0.52MPa. If the two pumps are running at the same time, the circulating flow increases, the system resistance will increase. Obviously the pump can not guarantee the normal operation of the system Solution: Replace the filter strainer; re-calculate the hydraulic system, replace the pump. This not only caused great economic loss, but also affected the normal operation of air conditioning system, the lesson is profound. 4 Conclusions 1) air conditioning system is not difficult to choose the circulating pump, but designers should pay enough attention, experience and The combination of theoretical calculation 2) the choice of the pump head can not think of the greater the more insurance, but should pay attention to the economy of operation 3) the current replacement of air conditioning products faster, the selection should be very careful, its performance should be carefully studied. Unit: Department of Environmental Engineering, Yangzhou University, Yangzhou 225009, China 5 References [1] Zhou Mo Ren ed. Fluid mechanics pump and fan. Beijing: China Building Industry Press, 1985.318 [2] (4): 64 ~ 67 [3] Song Yunyu. Shanghai Telecom Building Cooling Water Pump Motor Overload Analysis and Solutions. HVAC, 1995 , (1): 44 ~ 47