Sharp JX-9400 Technické informace stáhnout pdf (Strana 38)

 is the reduction ratio, which is the ratio of the smallest diameter to the

diameter of the pipe.

The ﬂow may be restricted with an oriﬁce plate, a nozzle or a Venturi tube. The

most sophisticated and expensive is the Venturi tube in which the discharge

coeﬃcient is nearly 1 and constant for Re > 2  10

and higher than 0.94 if

Re > 50,000. Moreover, this device does not induce a large pressure drop in

the ﬂow. At the other end of the spectrum is the simple and cheap oriﬁce

plate, which induces a large pressure drop and shows discharge coeﬃcients

that may be as low as 0.6, depending on the Reynolds number. The charac-

teristics of the nozzle lie in between. The device that is most sensitive to

perturbation is the oriﬁce, then the nozzle. The Venturi tube is the least

sensitive.

All these ﬂowmeters should be mounted between two straight pipes, the

upstream pipe being up to 30 pipe diameters long, depending on the type of

perturbation upstream, and the downstream part at least 3 diameters long. If

a straightening vane 2 diameters long is installed upstream, the distance

between this vane and the ﬂowmeter may be reduced down to 10 diameters.

The literature (for example, ASHRAE, 2001) provides detailed drawings of

such devices.

Velocity traverse

If the velocity of the air, v, is measured at enough points in the duct, the volume

airﬂow rate can be deduced by integrat ion over the whole area, A, of the

cross-section as shown:

Q ¼ 

vdA ð2:2Þ

where  is the density of the air.

Equipment to measure air speed

Provided the direction of ﬂow is parallel to the duct, which may normally be

assumed, then velocity can be determined by the measurement of air speed

alone.

The air speed measurement devices should be small enough to enable them

to be easily inserted through small holes in duct walls. The most common

examples are hot wire or NTC anemometers, helix anemometers and Pitot

tubes.

The hot wire anemometer and NTC anemometers measure the temperature

drop of a heated wire or a heated resistor (with a negative temperature coeﬃ-

cient), which, in each case, is dire ctly related to the temperature and speed of

the air ﬂowing over it. The sensors are heated by an electric current and

measurements are made of the voltage drop, which depends on the tem-

perature. The temperature of still air is taken into account by the use of a

reference sensor shielded from the ﬂow. Such devices can measure speeds

Airﬂow Rates in Air Handling Units 17

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 210 211

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