The wet feed material to a continuous dryer contains 50 wt% water on a wet basis

Question

The wet feed material to a continuous dryer contains 50 wt% water on a wet basis and is dried to 27 wt% by countercurrent air flow. The dried product leaves at the rate of 907.2kg/h. Fresh air to the system is at 25.6 degrees C and has a humidity of H=.007kg H2O/kg dry air. The moist air leaves the dryer at 37.8 degrees C and H=.02 and part of it is recirculated and mixed with the fresh air before entering a heater. The heated mixed air enters the dryer at 65.6 degrees C and H=.01. The solid enters at 26.7 degrees C and leaves at 26.7 degrees C. Calculate the fresh- air flow, the percent air leaving the dryer that is recycled, the heat added in the heater, and the heat loss from the dryer.

Explanation / Answer

H1=.007 H2=.02 H4=.01 Ls=907.2 X1=1=.5x2 X2=.54=27 x2 use G1H1+G6H2=(G1+G6)H4 plugging in the known H's we can solve and find that G6=23.1% is recycled air (G1+G6)H4+LsX1=(G1+G6)H2+LsX2 use these to find G1 the amount of fresh Air flow plug in known numbers and use linear substitution to solve G1=32100 Heat added in heater= Cs(Gt)(T1-T2) plug in known numbers to get 439kW Heat loss in dryer is none because all of the heat is transferred from the air to the water which is evaporated back to the air. There is no temp change in the solid so no heat is transfered.

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