Air Pollution Continuous Monitoring Technology in Japan
An overview of the principles of automatic monitoring devices used in air pollution continuous monitoring

2. Measuring instruments for stationary source

2.8. Flue gas flow meter

1.Measuring method of flue gas flow with Pitou tube
Pitou tube was developed by Pitot in 1732. It is atube with only one end being open, and placed parallel to the stream with the opening end to the upper stream, so that the opening be a stagnate point, and total pressure p0 of the stream is same with the one in the tube. (refer to Fig.4.2.17) Such a device to measure the flow is called Pitot tube or total-pressure tube. Another device which consists of a dual tube with a hole on a side of the outer tube to measure static pressure p1, and another hole at the end to the upper stream of the inner tube just same with the one described above to measure the total pressure p0 which is,

p0 = p1 + dv**2/2

where d is density and v is velocity of the stream. With the difference between p0 and p1, stream velocity can be estimated, and the device is called Pitot static tube or simply Pitot tube. (refer to Fig.4.2.18)

Fig.4.2.17

Fig.4.2.18

Pitot tube used in an industrial area for on-line measurement is illustrated at Fig.4.219, which has a double tube vertical to the stream pipe with one tube at upstream side having several holes, and another tube at downstream side also having several holes. Each tube detects the total-pressure and the static pressure respectively. The difference of both pressure values is measured with a differential pressure transmitter, and volume flow rate can be estimated through square root operation.

Fig.4.2.19

Pitot tube is one of differential pressure type flow meter, and other ones are orifice, Venturi tube, and flow nozzle. These flow meters including the Pitot tube measure the flow velocity and the flow rate with Bernoulli's law,

v**2/2 + gh + p/d = constant

where v is flow velocity, g is gravity, h is height, and d is flow density. Differential pressure type flow meter has advantages of simple structure, inexpensive, lots of experiences at the fields, many studies of the devices, and standardization. On the other hand, it has disadvantages of relatively large error, permanent pressure loss (energy loss), and block with slurry or particles. Pitot tube type has less pressure loss with only the resistance of the vertical tube than other types. To measure flue gas flow rate in negative pressure and less differential pressure, Pitot tube type flow meter is suitable, with taking care of particles which might cause choke of the holes.