- Contents :
- Rotary Drum Filter
- Rotary Drum Vacuum Filter
- Rotary Drum Filter Diagram
- Rotary Drum Filter Construction
- Rotary Drum Filter Working
- Advantages of Rotary Drum Filter
- Disadvantages of Rotary Drum Filter
- Rotary Drum Filter Design
Rotary Drum Filter
• A rotary drum filter is the most common type of continuous vacuum filter. In this filter filtration, washing, partial drying and discharge of cake all take place automatically.
Construction :
• A rotary drum filter is shown in Fig. 5.9 (B). It consists essentially of a cylindrical sheet metal drum mounted horizontally. It may be from 50 to 400 cm in diameter and 50 to 800 cm long.
• The outer surface of the drum is formed of perforated plate. A filter medium such as canvas covers the outer surface of the drum which turns at 0.1 to 2 r/min in an agitated slurry trough. Inside the outer drum, there is a smaller drum with a solid surface.
• The annular space between the two drums is divided into number of compartments/sectors by radial partitions and separate connection is made between the compartments and a special type of rotary valve. As the drum rotates, vacuum and air are alternately applied to each compartment.
• Apart from cast iron, the other materials of construction of this filter include stainless steel, titanium, plastics such as PVC, etc. These materials give much improved corrosion resistance for many slurries.
Working :
• The drum is immersed to the desired depth in the slurry which is mildly agitated to prevent the settling of the solids. Vacuum is then applied to the portion of drum which is submerged in the slurry through the rotary valve. Because of this, the liquid (filtrate) is sucked into the compartment and solids get deposited on the cloth to form a cake of the desired thickness which can be regulated by adjusting the speed of the drum. With higher speeds, thinner cake will be formed and consequently, high rate of filtration will be achieved. The filtrate from the compartment then goes to a filtrate collecting tank through the internal pipe and rotary valve.
• As the portion of the drum on which the cake is formed comes out of the slurry, the cake is washed by spraying wash liquid. The wash liquid leaves the filter through the rotary valve and is collected separately in a separate tank. After washing, the cake enters into a drying zone as the drum rotates where the cake is partially dried by sucking air through the cake of solids. After the cake of solids has been sucked as dry as possible, vacuum is cut off and the cake is removed by scrapping it off using a adjustable doctor's knife. A little air is blown in under the cloth to aid the removal of the cake. Once the cake is removed from the drum sector, it re-enters the slurry and the cycle is repeated.
• Usually, one-third of the cycle is used for filtration, one-half for washing and air drying and one-sixth for cake removal.
Advantages of Rotary drum filter :
1. It is entirely automatic in action and thus the man-power requirement is very low.
2. With cake consisting of coarse solids, it is possible to remove most of the liquid from the cake before discharging.
3. It has a large capacity for its size. Therefore, it is widely used for the filtration of large quantities of free filtering material.
4. By changing the speed, it is possible to built up cakes of varying thickness. With fine solids, the thickness of cake is small and is large with coarse solids.
Disadvantages of Rotary drum filter :
1. The maximum available pressure difference is limited to less than one atmosphere.
2. As it being a vacuum filter, a difficulty is encountered in the filtration of hot liquids due to their tendency to boil.
3. It cannot be employed for materials forming relatively impermeable cakes or cakes that cannot be easily removed from cloth.
4. Initial cost of the filter and vacuum equipment is high.
Also Read : Cyclone Separator