A method for automatic identification of unsteady critical point for a ventilator air performance test system

Abstract

An automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system is disclosed. This method requires only an upgrade to the existing ventilation fan air performance testing system, controlling three variables related to the system's resistance adjustment. The program adds automatic steady-state determination and peak detection to the data acquisition process, automatically determining the critical steady state based on fluctuations in upper and lower limit peak values, thus improving testing efficiency. This invention effectively identifies non-steady-state critical states of airflow during ventilation fan air performance testing, improving product design and avoiding the selection of such conditions for on-site applications, thus preventing abnormal phenomena and associated maintenance costs. It also solves the industry's difficulty in determining critical non-steady-state conditions during fan testing.

Description

Technical Field

[0001] This invention relates to an automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system. Background Technology

[0002] A ventilation fan air performance testing system can be used to test and record static pressure at different flow rates, and plot the corresponding static pressure-flow curves to provide data reference for the practical application of the fan. The system includes a ventilation chamber containing a nozzle wall with a front and rear flow equalization orifice plate on either side. A differential pressure gauge is located near the nozzle wall to measure the pressure difference between the two sides. The fan under test is installed at the inlet of the ventilation chamber, and a valve and an auxiliary fan are installed at the outlet. The walls of the ventilation chamber also have multiple static pressure measurement points, each equipped with a static pressure measuring instrument. For example, the invention patent with authorization announcement number CN100575909C can be considered an example of an existing ventilation fan air performance testing system.

[0003] Centrifugal fans are typically used in medium to high flow rate areas (high-efficiency areas). Due to inherent product structure or design issues, airflow instability can occur during testing. Current performance testing methods for fans include automated and manual testing. These methods, using constant static pressure or constant flow rate control by adjusting the testing system, often overlook airflow fluctuations caused by auxiliary fans at low flow rates and valves at small openings. These factors can easily affect airflow stability during testing, potentially widening the unsteady-state critical region. Current unsteady-state assessments are usually based on subjective human judgment. However, different testing equipment and control programs may slightly alter the actual unsteady-state characteristics of the product. The testing process requires highly experienced personnel, and subjective human judgment cannot guarantee accurate identification. The inability to accurately identify critical regions increases the difficulty for engineers in product design improvements and introduces the risk of misjudgment in after-sales improvement services during actual engineering applications. Summary of the Invention

[0004] To address the shortcomings mentioned above, this invention provides an automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system.

[0005] To achieve the above objectives, the present invention provides an automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system, comprising the following steps:

[0006] s1. After the tested fan is connected to the test system, the test system runs the tested fan and the auxiliary fan, controls the valve to open 100% and keeps a set number of nozzles on the nozzle wall in the open state. By adjusting the speed of the auxiliary fan, the flow velocity at the open nozzles is kept within the range of 30-35m / s, so that the static pressure value P measured by the static pressure measuring instrument is 0Pa.

[0007] s2. Under static pressure of 0Pa, the test system collects and records the flow rate pressure difference ΔP fluctuation data measured by the differential pressure gauge within three minutes, and calculates the maximum flow rate under static pressure of 0Pa based on the value of the flow rate pressure difference ΔP.

[0008] s3. Divide the maximum flow rate into intervals of 10% to determine the flow rate measurement points of 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, and 0%.

[0009] s4. Adjust the flow rate by first controlling the auxiliary fan speed and then controlling the valve opening degree, so that the flow rate is adjusted from the highest to the above percentage flow measurement points in sequence, until the test system determines that the adjusted percentage flow measurement point has reached a stable state.

[0010] s5. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed, until the test system returns to the unstable state. The test system identifies and records the unsteady critical point A. If the test system cannot return to the unstable state, the test system determines the unsteady critical point A to be 100% of the maximum flow rate.

[0011] s6. Continue the test of the next percentage flow point in the order of step s3 until the test system determines that the adjusted percentage flow measurement point has returned to an unstable state;

[0012] s7. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed, until the test system returns to a stable state. The test system identifies and records the unsteady critical point B.

[0013] s8. The value of the unsteady critical point A should be greater than 70% of the maximum flow rate, while the value of the unsteady critical point B should be less than 50% of the maximum flow rate.

[0014] s9. Determine the actual operating area of ​​the ventilation fan product between the unsteady critical point B and the unsteady critical point A.

[0015] Furthermore, in step s1, the tested fan operates at the design speed, and the static pressure 0Pa is the arithmetic mean of multiple test data of the static pressure measuring instrument within three minutes after the valve opening degree and the auxiliary fan speed have stabilized.

[0016] Furthermore, in steps s4 to s7, the testing system determines whether a certain measurement point has reached a stable state. The flow rate at the measurement point is processed separately according to whether it is less than 90% of the maximum flow rate. If the flow rate at the measurement point is less than 90% of the maximum flow rate, based on the upper and lower peak values ​​of the collected static pressure fluctuations, the fluctuation range of the static pressure value P measured by the static pressure measuring instrument is within ±0.5% of the average static pressure, and the testing system is determined to be in a stable state. If the flow rate at the measurement point is not less than 90% of the maximum flow rate, based on the upper and lower peak values ​​of the collected flow-pressure difference ΔP fluctuations, the fluctuation range of the flow-pressure difference ΔP is calculated to be within ±0.5% of the average pressure difference, and it is considered to be in a steady state.

[0017] Furthermore, in step s5, the test system returns to an unstable state, that is... Figure 1 When the pressure fluctuation reaches a point between the upper and lower pressure fluctuation limits 1, in step s6, the test system determines that the adjusted percentage flow measurement point has returned to an unstable state, i.e. Figure 1 The pressure fluctuation point is located between the upper limit 2 and the lower limit 2 of the pressure fluctuation.

[0018] Furthermore, in step s8, if the value of the non-steady-state critical point A or the value of the non-steady-state critical point B does not meet the requirements, the test system determines that the tested fan has a defect.

[0019] Furthermore, in step s7, if the auxiliary fan has been adjusted to zero speed and the flow rate of the opened nozzle is less than 10 m / s during the valve adjustment process, the test system first reduces the number of opened nozzles, then controls the valve opening degree, and finally controls the speed of the auxiliary fan to adjust the flow rate of the opened nozzle to the range of 10-35 m / s.

[0020] The advantages of this invention over the prior art are as follows:

[0021] This invention only requires upgrading the existing ventilation fan air performance testing system with corresponding test programs, controlling the three variables of the test system resistance adjustment, and adding automatic steady-state determination and peak detection to the acquisition program. It automatically determines the critical steady state based on the fluctuation of the upper and lower limit peak values, thereby improving testing efficiency. In the process of ventilation fan air performance testing, this invention can effectively identify the non-steady-state critical state of airflow during ventilation fan testing, improve the need for product design improvement, avoid selecting corresponding working conditions for engineering sites, and prevent the after-sales maintenance costs caused by abnormal phenomena at the engineering site. It solves the difficulty of critical non-steady-state determination in the industry's fan testing process. Attached Figure Description

[0022] Figure 1 This is a static pressure-flow curve of the fan under test.

[0023] Figure 2 This is a schematic diagram of the air performance testing system for the ventilation fan involved.

[0024] Figure 3 This is a schematic diagram for identifying the unsteady-state critical point of the tested fan under operating conditions at ±3% of its design speed. Detailed Implementation

[0025] like Figure 1 , Figure 2 As shown in the figure, an automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system according to an embodiment of the present invention includes the following steps:

[0026] s1. After the tested fan is connected to the test system, the test system runs the tested fan and the auxiliary fan, controls the valve to open 100% and keeps a set number of nozzles on the nozzle wall in the open state. By adjusting the speed of the auxiliary fan, the flow velocity at the open nozzles is kept within the range of 30-35m / s, so that the static pressure value P measured by the static pressure measuring instrument is 0Pa.

[0027] s2. Under static pressure of 0Pa, the test system collects and records the flow rate pressure difference ΔP fluctuation data measured by the differential pressure gauge within three minutes, and calculates the maximum flow rate under static pressure of 0Pa based on the value of the flow rate pressure difference ΔP.

[0028] s3. Divide the maximum flow rate into intervals of 10% to determine the flow rate measurement points of 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, and 0%.

[0029] s4. Adjust the flow rate by first controlling the auxiliary fan speed and then controlling the valve opening degree, so that the flow rate is adjusted from the highest to the above percentage flow measurement points in sequence, until the test system determines that the adjusted percentage flow measurement point has reached a stable state.

[0030] s5. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed, until the test system returns to the unstable state. The test system identifies and records the unsteady critical point A. If the test system cannot return to the unstable state, the test system determines the unsteady critical point A to be 100% of the maximum flow rate.

[0031] s6. Continue the test of the next percentage flow point in the order of step s3 until the test system determines that the adjusted percentage flow measurement point has returned to an unstable state;

[0032] s7. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed, until the test system returns to a stable state. The test system identifies and records the unsteady critical point B.

[0033] s8. The value of the unsteady critical point A should be greater than 70% of the maximum flow rate, while the value of the unsteady critical point B should be less than 50% of the maximum flow rate.

[0034] s9. Determine the actual operating area of ​​the ventilation fan product between the unsteady critical point B and the unsteady critical point A.

[0035] In step s1, the tested fan operates at its design speed, and the static pressure 0Pa is the arithmetic mean of multiple test data from the static pressure measuring instrument within three minutes after the valve opening degree and the auxiliary fan speed have stabilized.

[0036] In steps s4 to s7, the testing system determines whether a certain measurement point has reached a stable state. The flow rate at the measurement point is processed separately according to whether it is less than 90% of the maximum flow rate. If the flow rate at the measurement point is less than 90% of the maximum flow rate, the fluctuation range of the static pressure value P measured by the static pressure measuring instrument is within ±0.5% of the average static pressure, and the testing system is determined to be in a stable state. If the flow rate at the measurement point is not less than 90% of the maximum flow rate, the fluctuation range of the flow-pressure difference ΔP is calculated to be within ±0.5% of the average pressure difference after collecting the upper and lower peak values ​​of the flow-pressure difference ΔP, and it is considered to be in a steady state.

[0037] In step s5, the test system returns to an unstable state, that is... Figure 1 When the pressure fluctuation reaches a point between the upper and lower pressure fluctuation limits 1, in step s6, the test system determines that the adjusted percentage flow measurement point has returned to an unstable state, i.e. Figure 1 The pressure fluctuation point is located between the upper limit 2 and the lower limit 2 of the pressure fluctuation.

[0038] In step s8, if the value of the non-steady-state critical point A or the value of the non-steady-state critical point B does not meet the requirements, the test system determines that the tested fan has a defect.

[0039] In step s7, if the auxiliary fan has been adjusted to zero speed and the flow velocity of the opened nozzle is less than 10 m / s during the valve adjustment process, the test system first reduces the number of opened nozzles, then controls the valve opening degree, and finally controls the speed of the auxiliary fan to adjust the flow velocity of the opened nozzle to the range of 10-35 m / s.

[0040] This invention only requires upgrading the existing ventilation fan air performance testing system with corresponding test programs, controlling the three variables of the test system resistance adjustment, and adding automatic steady-state determination and peak detection to the acquisition program. It automatically determines the critical steady state based on the fluctuation of the upper and lower limit peak values, thereby improving testing efficiency. In the process of ventilation fan air performance testing, this invention can effectively identify the non-steady-state critical state of airflow during ventilation fan testing, improve the need for product design improvement, avoid selecting corresponding working conditions for engineering sites, and prevent the after-sales maintenance costs caused by abnormal phenomena at the engineering site. It solves the difficulty of critical non-steady-state determination in the industry's fan testing process.

[0041] In practical use, the invention will be described in conjunction with the accompanying drawings for ease of understanding;

[0042] Case 1: Assume that the unsteady-state critical point A of a certain type of ventilator is 75% of the maximum flow rate, and the unsteady-state critical point B is 25% of the maximum flow rate. The process of identifying the unsteady critical points by the air performance testing system is as follows:

[0043] s1. After the tested fan is connected to the test system, the test system runs the tested fan and the auxiliary fan, controls the valve to open 100% and keeps a set number of nozzles on the nozzle wall in the open state. By adjusting the speed of the auxiliary fan, the flow velocity at the open nozzles is kept within the range of 30-35m / s, so that the static pressure value P measured by the static pressure measuring instrument is 0Pa.

[0044] s2. Under static pressure of 0Pa, the test system collects and records the flow rate pressure difference ΔP fluctuation data measured by the differential pressure gauge within three minutes, and calculates the maximum flow rate under static pressure of 0Pa based on the value of the flow rate pressure difference ΔP.

[0045] s3. The flow rate is adjusted by first controlling the auxiliary fan speed and then controlling the valve opening degree. At the measurement points of 90% and 80% of the maximum flow rate, the test system determines that the above measurement points are both in an unstable state.

[0046] s4. The flow rate is adjusted by first controlling the auxiliary fan speed and then controlling the valve opening degree. When the measurement point is 70% of the maximum flow rate, the test system determines that the measurement point is in a stable state.

[0047] s5. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed. The test system determines that the measurement points at 71%, 72%, 73%, 74%, and 75% of the maximum flow rate are all in a stable state, and identifies the measurement point at 76% of the maximum flow rate as an unstable state. Based on this, the value of the unsteady critical point A is determined to be 75% of the maximum flow rate.

[0048] s6. The flow rate is adjusted by first controlling the auxiliary fan speed and then controlling the valve opening degree. At the measurement points of 60%, 50%, and 40% of the maximum flow rate, the test system determines that the above measurement points are all in a stable state. At the measurement point of 30% of the maximum flow rate, the test system determines that the measurement point is in an unstable state.

[0049] s7. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed. The test system determines that the measurement points at the maximum flow rate of 31%, 32%, 33%, and 34% are all unstable states, and identifies the measurement point at the maximum flow rate of 35% as a stable state. Thus, the value of the unsteady critical point B is determined to be the maximum flow rate of 35%.

[0050] s8. The value of the unsteady critical point A should be greater than 70% of the maximum flow rate, while the value of the unsteady critical point B should be less than 50% of the maximum flow rate.

[0051] s9. The actual area of ​​use for this ventilation fan product should be between 35% and 75% of the maximum flow rate.

[0052] In addition, the tested fan may operate at ±3% of its design speed during actual use. This fan air performance testing system can also identify the critical point of ±3% of the design speed, as detailed below:

[0053] Operating at +3% of design speed

[0054] Based on the unsteady critical point A measured by s5, the unsteady critical point A is reproduced through programmed constant flow control. Keeping the overall system resistance K constant, the tested fan is increased by 3%, and the point is re-evaluated to determine if it is the pressure critical steady-state point: If the pressure fluctuation remains within ±0.5%, the valve opening is further increased by fine-tuning the valve to control a 1% change in maximum flow, obtaining the pressure critical steady-state point A+ with a 3% increase in flow; if the pressure fluctuation exceeds ±0.5%, the valve opening is decreased by fine-tuning the valve to control a 1% change in maximum flow, obtaining the pressure critical steady-state point A+ with a 3% increase in flow. See [link to relevant documentation]. Figure 3 .

[0055] Based on the unsteady critical point B measured by S7, the unsteady critical point B is reproduced through programmed constant flow control. Keeping the overall system resistance K constant, the tested ventilator is increased by 3%, and the point is re-evaluated to determine if it is the pressure critical steady-state point: If the pressure fluctuation remains within ±0.5%, the valve opening is further reduced by fine-tuning to control the maximum flow rate change by 1%, obtaining the pressure critical steady-state point B+ with a 3% increase in flow rate; if the pressure fluctuation exceeds ±0.5%, the valve opening is increased by fine-tuning to control the maximum flow rate change by 1%, obtaining the pressure critical steady-state point B+ with a 3% increase in flow rate. See [link to relevant documentation]. Figure 3 .

[0056] Operating at -3% of design speed

[0057] Based on the unsteady critical point A measured by s5, the unsteady critical point A is reproduced through programmed constant flow control. Keeping the overall system resistance K constant, the speed of the tested fan is reduced by 3%, and the point is re-evaluated to determine if it is the pressure critical steady-state point: If the pressure fluctuation remains within ±0.5%, the valve opening is increased by fine-tuning the valve to control a 1% change in maximum flow, obtaining the pressure critical stable point A- with a 3% speed reduction; if the pressure fluctuation exceeds ±0.5%, the valve opening is decreased by fine-tuning the valve to control a 1% change in maximum flow, obtaining the pressure critical stable point A- with a 3% speed reduction. See [link to relevant documentation]. Figure 3 .

[0058] Based on the unsteady critical point B measured by S7, the unsteady critical point B is reproduced through programmed constant flow control. Keeping the overall system resistance K constant, the speed of the tested fan is reduced by 3%, and the point is re-evaluated to determine if it is the pressure critical steady-state point: If the pressure fluctuation remains within ±0.5%, the valve opening is further reduced by fine-tuning to control a 1% change in maximum flow, obtaining the pressure critical stable point B- with a 3% speed reduction; if the pressure fluctuation exceeds ±0.5%, the valve opening is increased by fine-tuning to control a 1% change in maximum flow, obtaining the pressure critical stable point B- with a 3% speed reduction. See [link to relevant documentation]. Figure 3 .

[0059] Case 2: Now assume that the unsteady-state critical point A of a certain type of fan is 100% of the maximum flow rate (i.e., there is no unsteady-state critical point A), and the unsteady-state critical point B is 25% of the maximum flow rate. The process of identifying the unsteady critical points by the air performance testing system is as follows:

[0060] s1. After the tested fan is connected to the test system, the test system runs the tested fan and the auxiliary fan, controls the valve to open 100% and keeps a set number of nozzles on the nozzle wall in the open state. By adjusting the speed of the auxiliary fan, the flow velocity at the open nozzles is kept within the range of 30-35m / s, so that the static pressure value P measured by the static pressure measuring instrument is 0Pa.

[0061] s2. Under static pressure of 0Pa, the test system collects and records the flow rate pressure difference ΔP fluctuation data measured by the differential pressure gauge within three minutes, and calculates the maximum flow rate under static pressure of 0Pa based on the value of the flow rate pressure difference ΔP.

[0062] s3. Divide the maximum flow rate into intervals of 10% to determine the flow rate measurement points of 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, and 0%.

[0063] s4. The flow rate is adjusted by first controlling the auxiliary fan speed and then controlling the valve opening. When the measurement point is 90% of the maximum flow rate, the test system determines that the above measurement point is a stable state.

[0064] s5. The flow rate was gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed. The test system determined that the measurement points of 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, and 100% of the maximum flow rate were all in a stable state. Since the test system could not return to the unstable state, the test system determined that the unsteady critical point A was 100% of the maximum flow rate.

[0065] s6. The flow rate is adjusted by first controlling the auxiliary fan speed and then controlling the valve opening degree. At the measurement points of 80%, 70%, 60%, 50%, and 40% of the maximum flow rate, the test system determines that the above measurement points are all in a stable state. At the measurement point of 30% of the maximum flow rate, the test system determines that the measurement point is in an unstable state.

[0066] s7. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed. The test system determines that the measurement points at the maximum flow rate of 31%, 32%, 33%, and 34% are all unstable states, and identifies the measurement point at the maximum flow rate of 35% as a stable state. Thus, the value of the unsteady critical point B is determined to be the maximum flow rate of 35%.

[0067] s8. The value of the unsteady critical point A should be greater than 70% of the maximum flow rate, while the value of the unsteady critical point B should be less than 50% of the maximum flow rate.

[0068] s9. The actual area of ​​use for this ventilation fan product should be between 35% and 100% of the maximum flow rate.

[0069] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for automatically identifying unsteady-state critical points in a ventilation fan air performance testing system, characterized in that, Includes the following steps: s1. After the tested fan is connected to the test system, the test system runs the tested fan and the auxiliary fan, controls the valve to open 100% and keeps a set number of nozzles on the nozzle wall in the open state. By adjusting the speed of the auxiliary fan, the flow velocity at the open nozzles is kept within the range of 30-35m / s, so that the static pressure value P measured by the static pressure measuring instrument is 0Pa. s2. Under static pressure of 0Pa, the test system collects and records the flow rate pressure difference ΔP fluctuation data measured by the differential pressure gauge within three minutes, and calculates the maximum flow rate under static pressure of 0Pa based on the value of the flow rate pressure difference ΔP. s3. Divide the maximum flow rate into intervals of 10% to determine the flow rate measurement points of 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, and 0%. s4. Adjust the flow rate by first controlling the auxiliary fan speed and then controlling the valve opening degree, so that the flow rate is adjusted from the highest to the above percentage flow measurement points in sequence, until the test system determines that the adjusted percentage flow measurement point has reached a stable state. s5. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed, until the test system returns to the unstable state. The test system identifies and records the unsteady critical point A. If the test system cannot return to the unstable state, the test system determines the unsteady critical point A to be 100% of the maximum flow rate. s6. Continue the test of the next percentage flow point in the order of step s3 until the test system determines that the adjusted percentage flow measurement point has returned to an unstable state; s7. The flow rate is gradually increased by 1% of the maximum flow rate, first by controlling the valve opening degree and then by controlling the auxiliary fan speed, until the test system returns to a stable state. The test system identifies and records the unsteady critical point B. s8. The value of the unsteady critical point A should be greater than 70% of the maximum flow rate, while the value of the unsteady critical point B should be less than 50% of the maximum flow rate. s9. Determine the actual operating area of ​​the ventilation fan product between the unsteady critical point B and the unsteady critical point A.

2. The automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system according to claim 1, characterized in that: In step s1, the tested fan operates at its design speed, and the static pressure 0Pa is the arithmetic mean of multiple test data from the static pressure measuring instrument within three minutes after the valve opening degree and the auxiliary fan speed have stabilized.

3. The automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system according to claim 2, characterized in that: In steps s4 to s7, the testing system determines whether a certain measurement point has reached a stable state. The flow rate at the measurement point is processed separately according to whether it is less than 90% of the maximum flow rate. If the flow rate at the measurement point is less than 90% of the maximum flow rate, the fluctuation range of the static pressure value P measured by the static pressure measuring instrument is within ±0.5% of the average static pressure, and the testing system is determined to be in a stable state. If the flow rate at the measurement point is not less than 90% of the maximum flow rate, the fluctuation range of the flow-pressure difference ΔP is calculated to be within ±0.5% of the average pressure difference after collecting the upper and lower peak values ​​of the flow-pressure difference ΔP, and it is considered to be in a steady state.

4. The automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system according to claim 3, characterized in that: In step s5, the test system returns to an unstable state, that is, it reaches the pressure fluctuation point between the upper limit 1 and the lower limit 1 of pressure fluctuation in Figure 1. In step s6, the test system determines that the adjusted percentage flow measurement point has returned to an unstable state, that is, it reaches the pressure fluctuation point between the upper limit 2 and the lower limit 2 of pressure fluctuation in Figure 1.

5. The automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system according to claim 4, characterized in that: In step s8, if the value of the non-steady-state critical point A or the value of the non-steady-state critical point B does not meet the requirements, the test system determines that the tested fan has a defect.

6. The automatic identification method for non-steady-state critical points in a ventilation fan air performance testing system according to claim 5, characterized in that: In step s7, if the auxiliary fan has been adjusted to zero speed and the flow velocity of the opened nozzle is less than 10 m / s during the valve adjustment process, the test system first reduces the number of opened nozzles, then controls the valve opening degree, and finally controls the speed of the auxiliary fan to adjust the flow velocity of the opened nozzle to the range of 10-35 m / s.

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