MaterialandEnergyBalances.ppt

Material and Energy Balances 2.Expressions of Water(moisture) content of solids (1)w=mass of water per unit mass of moist solid [water content on the wet basis] 3.Material balances [Refer to Figure 5-6(countercurrent dryers)，results also apply to cocurrent dryers.] (1)W(mass rate of water vaporization [kg water/s]) (2)L (mass rate of dry air required, kg dry air/s) 4.Heat balances [Refer to Figure5-7]  ?Question: For certain W(water vaporized from moist solids) and certain H2(air humidity at the dryer exit), when selecting blower for the drying system, are the atmospheric conditions determined by summer or winter? (2)Supplementary Heat transfer rate QD ?Approximate method of heat balances (p.257) Substituting into Eq.(5-32) and rearrangement give 5.Thermal efficiency of drying process ? ?Methods for increasing ?: ?Methods for increasing ?: 6.Air states when passing through the drying system (1)Through the preheater (2)Through the dryer From Eqs.(5-30) and (5-31), 1)Drying process under constant enthalpy等焓干燥过程 2)Non-constant enthalpy drying process 2)Non-constant enthalpy drying process (3)Determining the air conditions at the dryer exit. ?Analytical method: Determining H2 and I2 by using equations of material and heat balances for drying process. ?Graphical method: Using Humidity Chart of Air-Water System (H-I Diagram). Firstly, find operating line equation from given conditions and using equations of material and heat balances, then draw operating line and determine H2 and I2 . [Examples 5-5, 5-6, 5-7] Assignments：5-3，5-5 Hints of [Example 5-5]: Hints of [Example 5-5]: Hint of [例5-6]: Hints of [例5-7]: ?For solution of complex problem, using the following equations: (3)Equation for enthalpy calculation Another solution of [例5-7] : Using the approximate equation: Problem 5-3. The moist air with a dry-bulb temperature of 20oC and a humidity of 0.009 kg water/kg dry air is heated to 50 oC in the preheater and then sent to a dryer under atmosph