The capacity of a screen is dependent upon screen as well as material type
- The screen available area
- Aperture (size and type)
- Slope of screen
- Vibration amplitude
- Number of decks
- Size and shape of material
- Moisture content in the material
- Bed depth of material
- Dry or wet screening
The basic feed rate to a screen is calculated by the formula
FA = Basic feed rate (ton/hour/meter of screen width),
LA = Aperture in mm (square type),
pB= Bulk density ton/m3,
C= Percentage of material in the critical size in the feed
[Critical size (C) of the material is defined as the size of the material in the input material which are very close to the screen opening (aperture) i.e. between 0.75 to 1.5 times of the aperture opening.
Example:
For a 16mm screen opening the critical size of the material are the size fraction between (16 x 0.75) = 12mm to (16 x 1.5) =22 mm
Actual capacity of the screen can be defined as
R = Efficiency factor,
W= Screen width in Meters
C is an Efficiency factor which depends upon so many factors,
C = C1 x C2 x C3 x C4 x C5 xC6 xC7 x C8 x C9 x C10 x C11
The details of the value of C are given as under
- Mass factor: (C1)
It is the factor if the material under screening has different Bulk density then 1.602 ton/m3 (which is standard), If a material has BD of 1.5 t/m3 then C1 becomes,
- Screen Open area factor: (C2)
It is the ratio of %age of Open area of the screen to the base open area. This factor is generally taken as 1, if standard mesh is used.
- Correction factors for Under size :(C4)
The standard undersize of material in the feed related to aperture size is 40%, if it is below or higher a factor is to be taken as per the table For a multiple deck screen the under size to specific deck is calculated as the example given, If in a double deck screen, if the screen feed contains 35% passing half the lower deck aperture size and 70% passing upper deck aperture size then the percentage of the under size to lower deck is 35/70 = 50%.
- Correction factor for Oversize: (C3)
The standard oversize in the feed is mainly taken as 25%, If the feed material has lower or higher than 25% then a factor is taken as per table.
- Screen Efficiency factor: (C5)
The correction factor for different efficiency has been given as
Deck Factor (C6):
The number of deck in the screen will have a factor.
- Correction due to screen slope:(C7)
Due to different screen slopes the factor changes as the material travelling rates is changed. The factors are
- Correction for aperture slot type:(C8)
For square type use 1, for rectangular use 1.6, for circular use 0.8 etc.
- Correction for Particle size (C9):
If the size of the particles have different types of shape than a factor is to be taken for correction.
- Correction for type of screening (C10):
Wet screening is beneficial for efficiency, but it also depends upon the aperture size, if the aperture size is higher than wet screening is less effective, as the aperture decreases correction factor is increased till 2-3 mm, below this again the efficiency decreases. The factor related to aperture is as below
(The water requirement for screening (wet)is 25 liters/min ( i.e 1.5KL) per 1 m3 of feed if the gangue material is low, if the gangue material is high and sticky in nature then increase the water quantity in some proportion depending upon the experience. If water quantity is very high then wet screening becomes less effective due to agglomeration of sticky particles.)
- Correction factor due to moisture content (C11)
This factor is applicable for dry grinding only, for wet grinding it does not have an effect, but the efficiency is increased.
Example:
Find out the TPH of a single deck vibrating screen inclined at an angle of 20 degree and mesh size (16 x 16)mm square. The sieve analysis of the input material is as fallow. +22 mm=30%, +16 mm=20%, +12 mm=10% , +5 mm=10%, -5 mm=30%,
Bulk Density =1.63 t/m3, The screening is done by wet method, size of the screen = 1639 x 5300mm
The critical size for 16mm screen is 0.75×16=12mm and 1.5×16=24mm, from the sieve analysis of the material the %age of material in between 12mm to 24 mm is 60%.
Let us find out the basic feed rate of the screen.
Let us calculate the factors for tph of the screen,
C1= (Mass factor) =
C2 = (Open area factor) = 1
C3= (Correction factor for oversize) = 1.25 (from table)
C4 = (Correction for under size) = 1.5 (from table)
C5 = (Screen Efficiency factor) =at 80% efficiency of the screen it is 1.1(from table)
C6 = (Deck factor) = 1 (for single deck)
C7 = (Correction due to screen slope) = 0.9 (from table)
C8 = (Correction due to aperature type)= 1
C9 = (Correction due to Particle size) = 1
C10 = (Correction due to type of screening) = 1.2 (from table)
C11 = (Correction due to Moisture) =1
Factor C = C1 x C2 x C3 x C4 x C5 x C6 x C7 x C8 x C9 x C9 x C10 x C11
Screen TPH is calculated,
