Mechanical Operation: Screen Efficiency/Effectiveness

Screens are used to separate particles based on their sizes. Separation is important for various processes in chemical industries. The efficiency of a screen is defined as the sharp separation of particles. An ideal screen means all desired particles (specified particle size) are recovered 100% from the feed and no undesired particles come with the desired one. 

the efficiency of screen depends on

1. When the screens are used for many times, the screen gets damaged. The wires displace from their original position which leads to changes in mesh opening.

2. The time of operation and capacity (i.e. feed rate) also affect the effectiveness of screen. 

3. Moisture content also effect the effectiveness of screen.

4. orientation of particle

The efficiency of screen is defined as : recovery × rejection

Recovery: the fraction of desired particle recovered from feed:P(yP)/F(yF) 

Rejection: the fraction of undesired particle rejected:R(1-yR)/F(1-yF)

F, P, R are the feed rate, product (desired size) and reject flow rate.

yF, yP, yR are the fraction of desired product in feed, product and reject.

ideally yP should be 1 and yR should be 0 and the recovery and rejection both should be 1

Now the question rises how to find yF, yP, yR?

suppose we have a mixture of 3 different sizes material say 4, 5 and 6 cm & 5 cm is the desired size particle, now, how can we calculate the mass fraction of 5 cm size particle?

In industrial screens there will be some overlapping of particle sizes, i.e. there will be presence of oversize material and undersized  particle in streams. for that we will use new screens on a sample of 3 streams which has an efficiency of 1 (new screen, less capacity, standard moisture content and proper orientation). this fraction calculated will be used to find the efficiency of the industrial screen.