Abstract: Through the analysis of the failures of circulating pump in the central air-conditioning system project, the failure cases of the project are analyzed, the importance of selecting the head of the circulating pump is reasonably emphasized, and some methods of selection are put forward, which have reference value to the design of central air conditioning . [Keywords:] central air conditioning, pump lift, reasonable choice Abstract the 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 problem is raised in the central air conditioning system, the circulating pump in the summer to send chilled water, hot water in winter to the air conditioning terminal device engineering design should be based on air conditioning system water flow and the system Resistance selection performance of the pump.HAVA design manual has a detailed design calculation method.The problem is that the actual engineering design, some engineers did not calculate the design calculation, but by the experience of course, the system and some air conditioners The lack of careful study of new products such as equipment and accessories has resulted in the failure of the selected pump to meet the requirements or the resulting increase in operating costs and even the pump can not function properly, The designer 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 a high efficiency area, the working point Is the intersection of pump performance curve and pipeline characteristic curve, as shown in Figure 1, point A [1], while the project selected the pump often appear two kinds of abnormal situation.Figure 1 schematic diagram of the pump working point 1) design more conservative, water The actual value of the system flow rate is lower, the larger the estimated system resistance, resulting in increased pumping head lift, so that the choice of circulating pump head design flow rate is much greater than the system resistance shown in Figure 2: Figure 2 different pumps work Point change diagram 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. Pump actual HA. The actual pump performance curve is not â… , but â…¡, the operating point for the B, the flow QBA, and B is not pumping high efficiency area. Obviously this is more adverse than the first case. The only way to solve the problem is to change the pump only. 3 Engineering Example Example 1 A project is a single high-rise building with a building height of 29m and a pump house located in the basement of the main building. The design chooses one imported Carrier centrifugal freezer, 1163 kW, equipped with 2 circulating pumps, 1 with 1 prepared, pump parameters in Table 1. Table 1 pump nameplate parameters Pump Type Parameters H / MPa Q / m3.h-1 N / kW I / A IS150-125-400 50 200 45 84 At the beginning of the debugging exercise, we found that the pump motor current is too large, the pump outlet pipe vibration is abnormal, and there is abnormal sound. Pump head is only 0.28MPa, the motor current I = 115A. Analysis of the reasons for the manifold differential pressure Only 0.13MPa, the selected pump head is too large. At this point the pump operating point for low lift large flow, the motor is seriously overloaded [3]; pump cavitation serious pipe wobbles abnormal sound abnormalities; off the pump and freezer evaporation Import and export valves, to ensure that the evaporator pressure drop required for the import and export Δp = (92 ± 5) kPa, the pump back to normal work at this time the test data shown in Table 2 (original pump) .Table 2 pump test data test items imported Pressure / MPa outlet pressure / MPa manifold pressure / MPa manifold pressure / MPa motor current / A new original pump 0.3 0.3 0.78 0.53 0.31 0.31 0.41 0.41 82 40 As can be seen from Table 2, the pump head is 0.48MPa, water diversion Pressure drop is 0.10MPa, evaporator pressure is 0.1MPa, system resistance Is not large, and most of the pump pressure head is completely consumed in the small valve. Solution: replace a low head pump, the test data as shown in Table 2 (new pump) .Comparison Table 2 data, the motor current reduced from 82A to 40A , Its operating economy is self-evident Example 2 project B is a regional air-conditioning, cooling and heating stations to a number of buildings centralized cooling and heating.Designed 2907kW freezer 2, circulating pump 3, 2 with 1 prepared System debugging, open a freezer, a circulating pump .A few minutes later, the pump outlet pipe vibration and abnormal sound, the refrigerator can not start, the fault shows insufficient circulating water chilled water .Inspection system valves have been fully open, the water All the filters were cleaned, the system exhaust more thoroughly. Nameplate parameters: Q is 500m3.h-1, H is 0.475MPa, N is 90kW; Test data: evaporator inlet and outlet pressure 0.02MPa, pump import and export pressure 0.14 MPa, water collector pressure 0.27MPa, water separator pressure 0.40 MPa. According to the pump performance curve, when the head H = 0.14MPa, in the case of the system valve is fully open, the flow Q should be greater than 500m3.h-1 was right, At this point flow indicator self-locking, showing the flow In fact, such a large system, the pump head H = 0.14MPa is also impossible.Finally identified for two reasons: â‘ imported pump inlet water filter, filter The mesh is too small, leading to the resistance of the pump inlet is too large, resulting in serious cavitation, deterioration of the pump performance; â‘¡ pump flow and lift are very small, the system resistance is larger. Remove the brush filter screen operation, The differential pressure of the water collector reaches 0.45MPa, the lift of the pump is 0.52MPa. If the two pumps are running at the same time, the circulation flow will increase and the system resistance will increase. Obviously, the pump can not guarantee the normal operation of the system. Network; the system re-hydraulic calculation, replacement of the pump, which not only caused a greater economic loss, but also affect the normal operation of air-conditioning system, the lesson is profound. 4 Conclusion 1) air conditioning system is not difficult to choose circulating pump, To attract enough attention of designers, experience and theoretical calculations should be combined. 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. Product replacement faster, the selection should be very careful, its performance should be carefully studied.
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