有机无机化学化工热力学教学课件BlowersandCompressor

1、1Exercise 1 Water( = 1000 kg/m3) is transported from a low tank to a high tank. When the valve at C is open,the following data are detected:flow rate q, head H, pressure at inlet of pump p1, pressure at outlet of pump p2 and shaft power PB. Now one of the following variation is conducted, determine

2、the change in q,H,p1,p2 and PB.1 open the valve C wider;2 transport another liquid ( =1260kg/m3)28.5 Measurement of flowing fluids 1. Type of flowmeters2. Venturi meter（文丘里流量计文丘里流量计）3. Orifice meter（孔板流量计孔板流量计）4. Rotameter（转子流量计转子流量计）5. Pitot tube（测速管测速管）31. Type of flowmetersAccording the sensing e

3、lement ,the meters can be classed as :Sensing elementPressure dropArea differentThermalMagnetic 4ClassificationPositive-displacement metersThermal meter 热丝电式仪表Full-Bore metersInsertion metersMeasure the average velocityMeasure velocity at one point.Venturi meterOrifice meterRotametersV-elementPitot

4、tube5 Selection the flowmeter Applicability of the instrument to the specific problem Rangeability Accuracy Cost of install and operation6throat section is called文氏喉2.Venturi meter 文丘里流量计(head meter压差流量计)short conical inlet sectiona long discharge coneEnough straight pipe7ab8baaabbppVV222bbabaVVDDV2

5、2Writing the Bernoulli equation for incompressible fluids across station a and station b. the continuity relation (assume the density is constant): )(214baabbppV9bavbppCV214Cv , : V e n t u r i coefficient babvbbppSCSVq214volumetric flow rate mass flow rate babvbbppSCSVqm214(8.37) (8.38) The basic e

6、quation of venturi meter10For a well-designed venturi, the constant , is about 0.98 for pipe diameters of 2 to 8 in. and about 0.99 for larger sizes. C113. Orifice meter 孔板流量计(head meter)Orifice 1-11drilled plate with the hole concentric with the pipe flangesPressure tapsvena contracta tapstable 8.2

7、 Enough straight pipe12ab13baooppCu214baooooppSCSuq214uo = velocity through orifice.Co = Orifice coefficients, velocity approach not included.So = cross-section area of the orifice.(8.39)Volumetric flow rate14 C0 is determined experimentally. It varies considerably with changes in and with Reynolds

8、number at the orifice Reo.ooooDmuD4ReC0, is almost 0.61 and independent of provided Reo is greater than about 30,000. 15Pressure recovery 16FIGURE 8.19 Overall pressure loss in orifice meters. = ratio of orifice diameter to pipe diameter 17 EXAMPLE 8.4 184. Rotameter (Area meter ) tapered glass tube

9、Float: the indicating elementstainless-steel,lead, aluminum, glass, plasticRotameter 1-1219ab20 Write Bernoulli equation between upsteam a and downstream b . a represents the position right below the float, b is the up-surface of the float, Continuity equationabbaAAVVWhere: Ab is the annular area be

10、tween the float and the tube wall. Aa is the cross section area of the tube right below the float)(222aabbbaVVgZpp21By force balance gVAppfffba)(Where Vf: volume of float Af: maximum cross section area of float f: density of float : density of fluid.22And continuity equationabbaAAVV)(222aabbbaVVgZpp

11、22()()2abfffbbaappAZAgAVV gVAppfffba)(Using 23 volumetric flow rate Area meter:The area is related to the flow rate. Pressure drop is constant in a rotameter.Calibration curvefffaabbbAgVAAV)(212fffaabbbbbAgVAAAVAq)(22245. Pitot tubeInsertion metersMeasure the local velocity at one point Pitot 1-1025

12、26 Impact tube (冲压管）冲压管） a : parallel to the direction of the flow. psStatic pressure + kinetic energy = stagnation pressure (驻点压力) psStatic tube（静压管）b: perpendicular to the flow direction. Static pressure po27002ppusF o r i n c o m p r e s s i b l e fluids,the velocity is given by the equation below: ps= mesured by the impact tube a, is the stagnation pressure of the fluid p0= the static pressure, measured by b.(8.46)28Actual pitot tube, a correction factor should be applied.)(200ppCusF