化工原理课件CHAP8-2

Measurement of flowing fluidsIt is important to measure the rate at which a fluid flowing through a pipe or other channel. Many different types of meters are used industrially Most meters operate on all the fluid in the pipe or channel and are known as full-bore meter. Others, called insertion meters, measure the flow rate, or more commonly the fluid velocity, at one point only 华南理工大学 化工学院 伍钦ceqwu@http://www.77cn.com.cn 020-87114377

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FullFull-bore metersThe most common types of full-bore meters are venturi and orifice meters and variablearea meters such as Rotameters. Other full-bore measuring devices include Velement, magnetic, vortex-shedding, turbine, and positive-displacement meters; ultrasonic meters. shedp182华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377

Venturi meterVenturi meter is constructed from a short flanged inlet section A , consisting of a short cylindrical portion and a truncated cone; a flanged throat section B; and a flanged outlet section C , consisting of a long truncated cone.

E

H

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A

B华南理工大学 化工学院

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C

At the junction of the cylindrical and conical portion, an annular chamber is provided, and a number of small holes drilled from the inside of the tube to the annular chamber. The annular chamber and the small holes has the function of averaging inpidual pressure, it is called piezometer ring.

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The average pressure is transmitted through the upstream pressure connection. A second piezometer ring is formed in the throat section. A manometer or other means for measuring pressure difference is connected between two piezometer ring.p182

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In the venturi meter , the velocity is increased , and the pressure decreased, in the upstream cone. The pressure drop in the upstream cone is used to measure the rate of flow through the instrument.

Since separation does not occur in the contracting cross section, the upstream cone can be made shorter than the downstream cone with but little friction, and space and material are thereby conserved.华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377

The basic equation for venturi meter To write the Berboulli equation for incompressible fluids between the two pressure station .

u u pa pb = + hf 2 2 ρ2 b 2 a

Friction is neglected

u u =2 b 2 a

2( pa pb )华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377

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ρ

(8-31)

The continuity relation can be written, since the density is constant, as

da u a = ub = β 2 ub d b

2

(8-32)

ua is estimated from Eq(8-31) and Eq(8-32)ub =p183

1 1 β4

2( pa pb )

ρ

(8-33)

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Venturi coefficient Eq(8-33) applies strictly to the frictionless flow of non-c

ompressible fluids. To account for the small friction loss between locations a and b, Eq(8-33) is corrected by introducing an empirical factor Cv and writing

ub =p183

Cv 1 β4

2( pa pb )

ρ

(8-34)

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When db is less than da/4, the approach velocity and the term β can be neglected, since the error is less than 0.2 percent. For a well-designed venturi, the constant Cv is about 0.98 for pipe diameters of 2 to 8 in. and about 0.99 for large size.

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Flow rate The flow rate of practical interest is the mass and volumetric flow rates through the meter.

q = ub S b =

S b Cv 1 β4

2( pa pb )

ρ

(8-36)

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Pressure recovery Friction cannot be completely eliminated, and a permanent loss in pressure and a corresponding loss in power do occur. Because of the small angle of pergence in the recovery cone , the permanent pressure loss from venturi meter is relatively small.华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377

Orifice meterThe venturi meter has certain practical disadvantages for ordinary plant practice. It is expensive, it occupies considerable space, and its ratio of throat diameter and pipe diameter cannot be changed.p184华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377

For a given meter and definite manometer system, the maximum measurable flow rate is fixed, so if the flow range is changed, the throat diameter may be to large to give an accurate reading or too small to accommodate the next maximum flow rate.p184华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377

The orifice meter meets these objections to the venturi but at the price of a larger power consumption.

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The principle of the orifice meter is identical with that of the venturi. The reduction of the cross section of the flowing stream in passing through the orifice increasing the velocity head at the expense of the pressure head, and reduction in pressure between the taps is measured by the manometer.p184华南理工大学 化工学院 伍钦 ceqwu@http://www.77cn.com.cn 020-87114377