大学课件-化工原理-干燥english.ppt

1、大学课件-化工原理-干燥english The Property Of Wet-Air(Air-Water)wet-air:dry air+humidity-ideal gasHumidification termsdry air is const over drying process unit mass of dry air is as basisHumidity HMass ratio of water vapor to dry airaVaVaaVVnnnnnMnMH622.02918kg water/kg dry airFor ideal gas：VVpPpH622.0nv,n a-

2、kmolSaturated Humidity Hs Partial pressure of humidity in wet air is equal to vapor pressure ps at fixed temperatureSSSPPPH622.0Percentage relative humidity )PP(%100Pp)PP(%100PpSVSSV reflects unsaturation of wet air,and drying power，drying power=1，no drying powert，pS，drying powerp=101.3kN/m2273t273)

3、H244.1773.0(H273t273184.22273t273294.22HH Humid volume H(m3/kg dry air)Humid heat cHHHcccVaH88.101.1（kJ/kg dry air C）basis:at 0 C water in liquid phase dry airH2490t)H88.101.1(H)tcr(tcIV0a r0:latent heat of water steam at 0C，2490 kJ/kg Enthalpy I(kJ/kg dry air)tas,tw,tt:dry-bulk temperaturetw:wet-bu

4、lk temperaturetas:adiabatic saturation temperature Adiabatic saturation temperature tasadiabatic saturation process:mass and heat transfer to equilibriumair,tas,Has air,t,Htaswaterastt asHH)HH(crttasHasas )H,t(ftasIentry=Iexport)HH(rkttWWHW tw is not state functiontw=f(t,H)t fixed,H，tW，unsaturated d

5、egreeWet-bulb temperature tw Dew point tdH is fixed，t saturation t=tdddpPPH622.0Pd：vapor pressure at td，=partial pressure of air at initial state pvHHPpd622.0PHfpd,unsaturated air：t tas=tw tdsaturated air：t=tas=tw=td t fixed，tas、tw unsaturated degreeRelation among tas,tw,t Air-water system湿度湿度Humidi

6、ty chart Chart to express variation of wet air 1Heating and cooling process2 Adiabatic saturation process3 Non-adiabatic wetting process4.Gas mixing processFlow Sheet of DryingblowerdryerHeat exchangerExpression of moisture in wet materialwet basis:wkg/kgmaterial wetof massmaterial wetin moisture of

7、 massw dry basis:Xkg/kgmaterialdry of massmaterial wetin moisture of massX wwX1XXw1Moisture evaporated21mmmWqqq221121)(wqwqXXqqmmmcmw1212221111wwwqwwwqqmmmW121HHqqlmWmL kg干空气/kg水 amount of dry airkg/s12HHqqmWmLHeat balance1.preheater)tt)(H.(L)II(LQooP101881011H=const,t 2.dryer Ideal process21II P-X*

8、curve (or P*-X curve)-X curveEquilibrium moisture Free moisture Bound moisture Non-bound moisture moisture presented in two formsdrying curve X *drying rate curve R XMechanism Of Moisture Transferred In Wet Material diffusion theory of drying capillary theory of drying Wet materialporosity material

9、non-porosity materialmechanism of moisture transferred in wet material Drying Time For Constant Drying Process 1.constant rate period )(10111CCXXCXXARGRdXAGdC 2.falling rate period=1+2 3.total drying time Operating and design parameters（1）Humidity and temperature of inlet airH t（3）outlet temperature

10、 of wet materialexperimental data:)2015(max2（2）outlet temperature of airduring co-current operating,difference of outlet temperature between air and solid material is about 1020 during counter-current operating,100 normally be initial design parameter Dryer Equipment 1.tunnel dryers.箱式干燥器（盘架式干燥器）2.rotary dryers 转筒式（回转式干燥器）3.drum dryers气流式4.pneumatic dryers流化床（沸腾床）5.spray dryers 喷雾