Central air conditioning system circulating pump lift a reasonable choice

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, a 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.
Keywords central air conditioning; water pump head; proper selection

1 The question 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 system resistance to choose a good pump performance HVAC design manual has a detailed design calculations Method.The problem lies in the actual project 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 and other new products lack of careful study, resulting in the selected pump can not meet Requirements, or increase operating costs, or even pump does not work, which had caused the air conditioner designers attach great importance.

2 theoretical analysis

The size of water flow in air conditioning system is determined by the temperature difference between load and water supply and return, and the system resistance is calculated by hydraulic calculation.Water pump selected by flow and resistance should be in high efficiency area during operation, and the working point is the pump performance curve and pipeline characteristic curve Of the intersection, as shown in Figure 1 A [1], while the project selected pump often appear two kinds of abnormal situation.


Figure 1 pump working point diagram

1) The design is more conservative, the actual value of the water system is lower, the estimated resistance of the system is larger, resulting in pump head lift increased, the choice of circulating pump head design flow rate greater than the system resistance. 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 Ⅰ to Ⅱ. Obviously, ΔP = HB-HA completely through the valve throttling, which is very uneconomical , But also the project to avoid the situation, if the same set of pump operation in winter, due to smaller flow, throttling more serious, more uneconomical, or even cause pump instability [2].

2) The design is too self-confident, the air-conditioning system resistance is estimated to be small, the selected pump head less than the design flow under the system resistance shown in Figure 3:


Figure 3 pump pipe characteristics curve and the actual working point

The design work point is A, the pump flow is QA, the lift is HA. The actual pump of water pump is not â… , but â…¡, the working characteristic curve of the pump is not â…¡, the operating point is B, the flow rate QBA, Obviously this is more unfavorable than the first case, and the only solution is to replace the pump only.

3 Project Example 1 Project A is a single- storey 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 with a cooling capacity of 1163 kW and 2 circulating pumps, 1 with 1 preparation, pump parameters in Table 1.

Table 1 pump nameplate parameters

Pump model 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, it was 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, motor current I = 115A.Analysis of the reasons for the sub-catchment pressure is 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]; serious pump cavitation, pipe jitter, abnormal sound; off the pump and freezer evaporator inlet and outlet Valve, to ensure that the pressure required for the import and export of the evaporator Δp ​​= (92 ± 5) kPa, so that the pump back to normal work at this time the test data in Table 2 (the original pump).

Table 2 Pump Test Data

Test items imported pressure / MPa
Outlet pressure / MPa
Water collector pressure / MPa
Water separator pressure / MPa
Motor current / A

The original pump new pump 0.3
0.3
0.78
0.53
0.31
0.31
0.41
0.41
82
40


Table 2 shows that the pump head is 0.48MPa, manifold differential pressure is 0.10MPa, the evaporator pressure is 0.1MPa, the system resistance is not large, but most of the pump head pressure is completely consumed in the small valve.

The solution: replace a low-lift pump, test data as shown in Table 2 (new pump) .Comparison of Table 2 data, the motor current is reduced from 82A to 40A, its operating economy is self-evident.
Example 2 B works for a regional air conditioning, cooling and heating station to a plurality of buildings was concentrated cooling, heating design 2907kW refrigerating machine 2, three circulating pump, with an apparatus 2. Open a chiller system debugging, A circulating pump .A few minutes later, the pump outlet pipe vibration and accompanied by abnormal sound, the refrigerator can not start, the fault shows insufficient water circulating water.Inspection system valves have been fully open, the water filter all clean, the system exhaust The data of the nameplate: 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, Water pressure 0.40 MPa. According to the pump performance curve, when the head H = 0.14MPa, the system valve in the case of full open, flow Q should be greater than 500m3.h-1 was right, and this time the flow indicator self-locking display 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, causing the pump inlet resistance 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 filter operation, points, manifold pressure difference of 0.45MPa, The head 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.

Solution: Replace the filter strainer, re-calculate the hydraulic system, replace the pump, which not only causes great economic loss, but also affects the normal operation of the air-conditioning system. The lesson is profound.

4 Conclusion

1) Air conditioning system is not difficult to choose the circulating pump, but designers should pay enough attention to the experience to be combined with theoretical calculations.
2) The selection 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 cautious, its performance should be carefully studied.

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