| 摘要 |
PROBLEM TO BE SOLVED: To accurately detect blocking without delay by storing the correlation curve between a flow rate and pressure, and by judging the blocking of a filter when the correlation curve exceeds a preset setting value. SOLUTION: In ultrasonic sensors 43a and 43b, the average flow velocity of an outdoor-air intake path 23 is measured and is multiplied by the sectional area of the intake path 23 to obtain a flow rate. The pressure of the intake path 23 is obtained by a pressure sensor 41. When a filter 21 is not blocked, a first correlation curve is shown by a solid line (a). In the case of blocking, the pressure is decreased according to the degree of the blocking as shown by dashed line (b) or (c), which corresponds to a second correlation curve. The relationship between the width of the fluctuation and the degree of the blocking is experimentally verified in advance. The difference between a flow-rate value from the correlation curve of the solid line (a) corresponding to the pressure value being detected by the pressure sensor 41 and that being detected by supersonic sensor 43a or the like at the same time is obtained for calculating the fluctuation width of the flow rate. When the fluctuation width exceeds a setting value, it is judged that the blocking bas occurred, and a warning is issued.
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