GATE 2021 AG – Question 64
A 1 tonne/h two-screen air cleaner takes grain with 8.5% impurities. Clean-grain loss in blower, first-screen overflow, second-screen underflow is .3%,1.2%,.8% respectively. Product has .6% impurities. Find cleaning efficiency in percent, to one decimal.
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Correct answer: 93.0 to 94.0
Explanation
Use the source’s concentration-based cleaning efficiency $E=(x_f-x_p)/[x_f(1-x_p)]$,where impurity fractions are x_f=.085 and x_p=.006. Then $E=(.085-.006)/[.085(.994)]=.934999$,or **93.5%**. The separate clean-grain losses total2.3% of feed and describe recovery;they are not part of this purity-based efficiency definition. A combined recovery/separation effectiveness would be a different metric.