干燥速率曲线.doc

一、实验目的 掌握干燥曲线和干燥速率曲线的测定方法。 学习物料含水量的测定方法。 加深对物料临界含水量Xc的概念及其影响因素的理解。 学习恒速干燥阶段物料与空气之间对流传热系数的测定方法。 学习用误差分析方法对实验结果进行误差估算。二、实验内容 每组在某固定的空气流量和某固定的空气温度下测量一种物料干燥曲线、干燥速率曲线和临界含水量。 测定恒速干燥阶段物料与空气之间对流传热系数。三、实验原理1. 干燥速率的测定U=dWSdWS (7-1)式中：U干燥速率，kg /（m2h）；S干燥面积，m2；时间间隔，h；W时间间隔内干燥气化的水分量，kg。2. 物料干基含水量X =G-GCGc （7-2）式中：X物料干基含水量，kg水/ kg绝干物料；G固体湿物料的量，kg；Gc绝干物料量，kg。3. 恒速干燥阶段，物料表面与空气之间对流传热系数的测定Uc = dWSd = dQrtwSd = (t-tw)rtw （7-3）=Ucrtwt-tw （7-4）式中：恒速干燥阶段物料表面与空气之间的对流传热系数，W/（m2）；Uc恒速干燥阶段的干燥速率，kg/（m2s）；tw干燥器内空气的湿球温度，；t干燥器内空气的干球温度，；rtwtw下水的气化热，J/ kg。 4. 干燥器内空气实际体积流量的计算由节流式流量计的流量公式和理想气体的状态方程式可推导出：Vt=Vto273+t273+t0 （7-5）式中：Vt干燥器内空气实际流量，m3/ s；t0流量计处空气的温度，；Vt0常压下t0时空气的流量，m3/ s；t干燥器内空气的温度，。VT0 =C0A02P （7-6）A0 =4d02 （7-7）式中：C0流量计流量系数，C0=0.67A0节流孔开孔面积，m2； d0节流孔开孔直径， d0=0.050 m；P节流孔上下游两侧压力差，Pa；孔板流量计处t0时空气的密度，kg/m3。四、实验装置图1干燥速率曲线测定实验装置图1离心风机； 2-孔板流量计； 3-孔板流量计显示仪； 4，11重量传感器显示仪； 5干燥物料； 6电加热器； 7干球温度计； 8，14湿球温度计显示仪； 9洞道干燥室； 10废气排出阀； 11废气循环阀； 12新鲜空气进气阀； 13-电加热控制； 15温度显示仪； 16孔板流量计压差变送器和显示仪。五、 操作流程1、将干燥物料放入水中浸湿。2、调节送风机吸入口的蝶阀12到全开的位置后启动风机。3、 用废气排出阀10和废气循环阀11调节到指定的流量后，开启加热电源。在智能仪表中设定干球温度，仪表自动调节到指定的温度。4、在空气温度、流量稳定的条件下，用重量传感器测定支架的重量并记录下来。5、 把充分浸湿的干燥物料5固定在重量传感器4上并与气流平行放置。6、在稳定的条件下，记录干燥时间每隔2分钟干燥物料减轻的重量。直至干燥物料的重量不再明显减轻为止。7、变空气流量或温度，重复上述实验。8、 关闭加热电源，待干球温度降至常温后关闭风机电源和总电源。9、实验完毕，一切复原。六、实验数据记录表表 1 操作条件与物料性质记录表备注：表2 干燥速率曲线测定原始数据记录表序号时刻/min总质量/g序号时刻/min总质量/g1262273284295306317328339341035113612371338143915401641174218431944204521462247234824492550七、数据处理1、操作条件数据计算干燥面积（帆布面积）S=2ab=2*0.157*0.08=0.02512m2 ；绝干物料重量 27.5g； 支架重量（80.5+81.3）/2=80.9g ；进口温度（35.5+44.5）/2=40.0 ；此时空气密度空气=1.128kg/m3；湿球温度（27.0+28.2）/2 =27.6； 干球温度（60.0+60.0）/2=60.0；空气流量计压差读数：0.89 kPa；孔板流量计孔径d0=0.0450m， A0 =4d02 = 4*0.04502 = 1.59010-3m2；VT0 =C0A02P = 0.671.59010-320.891031.128 = 0.0423m3/s；Vt=Vto273+t273+t0 = 0.0423273+60.0273+40.0 = 0.0450 m3/s；u =VtA =Vtbh =0.04500.125*0.180=2.000m/s。2、实验数据整理计算表3 干燥速率曲线测定数据整理表序号时刻/min总质量/g失重/gU/（kg/(m2h）G/gX (kg水/ kg绝干物料)X (kg水/ kg绝干物料)12177.096.102.49524175.91.11.31495.002.4552.47536174.71.21.43393.802.4112.43348173.51.21.43392.602.3672.389510172.31.21.43391.402.3242.345612171.31.01.19490.402.2872.305714170.11.21.43389.202.2442.265816169.01.11.31488.102.2042.224918168.01.01.19487.102.1672.1851020166.91.11.31486.002.1272.1471122165.81.11.31484.902.0872.1071224164.71.11.31483.802.0472.0671326163.71.01.19482.802.0112.0291428162.61.11.31481.701.9711.9911530161.51.11.31480.601.9311.9511632160.51.01.19479.601.8951.9131734159.31.21.43378.401.8511.8731836158.21.11.31477.301.8111.8311938157.30.91.07576.401.7781.7952040156.11.21.43375.201.7351.7562142155.20.91.07574.301.7021.7182244154.11.11.31473.201.6621.6822346153.01.11.31472.101.6221.6422448152.01.01.19471.101.5851.6042550150.91.11.31470.001.5451.5652652149.81.11.31468.901.5051.5252754148.81.01.19467.901.4691.4872856147.71.11.31466.801.4291.4492958146.71.01.19465.801.3931.4113060145.61.11.31464.701.3531.3733162144.61.01.19463.701.3161.3353264143.51.11.31462.601.2761.2963366142.51.01.19461.601.2401.2583468141.41.11.31460.501.2001.2203570140.41.01.19459.501.1641.1823672139.31.11.31458.401.1241.1443774138.40.91.07557.501.0911.1073876137.31.11.31456.401.0511.0713978136.50.80.95555.601.0221.0364080135.41.11.31454.500.9821.0024182134.41.01.19453.500.9450.9644284133.31.11.31452.400.9050.9254386132.40.91.07551.500.8730.8894488131.41.01.19450.500.8360.8554590130.31.11.31449.400.7960.8164692129.21.11.31448.300.7560.7764794128.40.80.95547.500.7270.7424896127.41.01.19446.500.6910.7094998126.31.11.31445.400.6510.67150100125.31.01.19444.400.6150.63351102124.40.91.07543.500.5820.59852104123.31.11.31442.400.5420.56253106122.40.91.07541.500.5090.52554108121.50.91.07540.600.4760.49355110120.51.01.19439.600.4400.45856112119.70.80.95538.800.4110.42557114118.80.91.07537.900.3780.39558116118.10.70.83637.200.3530.36559118117.40.70.83636.500.3270.34060120116.80.60.71735.900.3050.31661122116.30.50.59735.400.2870.29662124115.80.50.59734.900.2690.27863126115.50.30.35834.600.2580.26464128115.10.40.47834.200.2440.25165130114.70.40.47833.800.2290.23666132114.40.30.35833.500.2180.22467134114.10.30.35833.200.2070.21368136113.80.30.35832.900.1960.20269138113.50.30.35832.600.1850.19170140113.20.30.35832.300.175080.40.47831.900.1600.16772144112.50.30.35831.600.149030.20.23931.400.142070.60.71730.800.120010.60.71730.200.0980.10976152110.80.30.35829.900.0870.09377154110.60.20.23929.700.0800.08478156110.30.30.35829.400.0690.07579158110.00.30.35829.100.0580.06480160109.80.20.23928.900.0510.05581162109.50.30.35828.600.0400.04582164109.40.10.11928.500.0360.03883166109.20.20.23928.300.0290.03384168109.00.20.23928.100.0220.02585170109.00.00.00028.100.0220.02286172108.90.10.11928.000.0180.020计算举例：以第二组为例，失重W=Wn-Wn+1 = W1-W2= 177.0=175.9=1.1g；干燥速率U = dWSd WS = 1.110-30.02512(260) = 1.314kg/(m2h)；湿物料重量G=W-支架重 = 175.9 - 80.9 = 95.0g；干基含水量X =G-GCGc = 95.0-27.527.5=2.495 kg水/ kg绝干物料；平均含水量X=（X1+X2）/2=（2.495+2.455）/2=2.475 kg水/ kg绝干物料。3、根据以上数据画出图形如下：4、恒速干燥段，物料表面与空气间对流传热系数计算tw =27.6时，水的汽化热rtw=2437.8kJ/kgUC =1.314看过/（m2h），t=t干 =60.0 =Ucrtwt-tw = 1.3142437.810360-27.613600=27.46287W/(m2)八、误差分析尺寸相对误差D(a）=D(b)=0.5mm；Er(a)=D(a)|a| = 0.5157=3.18510-3；Er(b)=D(b)|b| = 0.580=6.25010-3D(S)=D(2ab)= 2bD(a)2+2aD(b)2=(20.080.0005)2+(20.1570.0005)2 =0.176210-3Er(S)=D(S)|S|=0.176210-30.02512=0.701410-3时间相对误差D()=0.5s, =2min=120s,D()=2D()2=20.52=0.7071s,Er()=D()|=0.7071120=5.892610-3失重相对误差W=W1-W2,由仪器参数，D（W）=(200-0)0.2%=0.4g,D(W)= 2D(W)2=20.42=0.5656gW=UcS=1.3140.02512260103=1.100g Er（W）=D(W)|W| =0.56561.100=0.51418干湿球温差相对误差D(t)=D(tw)=1500.5%=0.75, D(t-tw)= D(t)2+D(tw)2=20.752=1.0607Er(t-tw)=D(t-tw)|t-tw|=1.060760-27.6=0.03274rtw是常数，误差为0Er()=Er2S+Er2+Er2W+Er2(t-tw) =(0.701410-3)2+(5.892610-3)2+0.514182+0.032742=0.51526则D()=Er()|=0.51526*27.46=14.148915 W/(m2)故=3101 15 W/(m2)各因素在其中占的比例：Er2(S)Er2()=(0.701410-3)20.515262 =0.0001854%； Er