Decanter Solidbowl Centrifuge

The solidbowl centrifuge is commomonly used to seperate, clarify, or classify solid particles from liquid

Solidbowl centrifuge
  • Overview
  • Design
  • Machine Variables

The Decanter solidbowl centrifuge is commomonly used to seperate, clarify, or classify solid particles from liquid. The solidbowl centrifuge is often used in processing chemical, food, mineral, rendering, and both industrial and municipal wastewater industries.

 

ScreenbowlCentrifugeDiagram

 

  1. Feed pipe
  2. Thrust bearings
  3. Driven sheave
  4. Main bearings
  5. Vibration isolators
  6. Feed compartmnet
  7. Screw conveyor
  8. Planetary gearbox
  9. Backdrive

Approximate capacities of Decanter solidbowl centrifuges

 

 Solidbowl Machine Size  

Capacity Per Unit 

 
  150 Mesh x 0 20-40% Feed Solids   Motor HP  GPM Cap
 36" x 72"  12-14 TPH  100  275
 40" x 96" high capacity  18 TPH  500  380
 44" x 132"  27-28 TPH  500  450

 

NOTE: We recommend that the total feed slurrry flow to the centrifuges NOT exceed the GPM capacity. All machines are sized specifically for material feed size and GPM of slurry.

 

Machine variables and their effecto on process


Machine Variables
Speed, Feed Flow Rate, Pool Depth, Gearbox Ratio, Differential Speed, Conveyor Speed, and Conveyance Speed.

Effects of machine variables on process:

Speed (RPM)
The effect of speed (operating RPM of the Bowl) on the process follows:

  1. Increases or decreases the G level.
  2. Increases of decreases the differential speed and resonance time of the settled solids (how fast the settled solids are removed by conveyor).
  3. Alters the particle size distribution in the cake (rejects) as follows: 

a. Higher speed settles more solids inside diameter of bowl and fewer solids in the effluent (product). In classification this makes a finer cut (more G's to settle solids, higher G seconds).
b. Lower speed settles lesses solids to bowl and more solids in effluent. In classification this coarsens the cut and reduces the amount of G seconds.

    4. Increases or decreases cake moisture because of more G's.

Speed can be increased or decreased while the machine is operating if the motor is a variable speed drive. If not speed is fixed by the design of the motor RPM and sheave sizes.

Feed Flow Rate
The effect of flow rate (GPM) on the process follows: 

    1. Increase of the flow reduces the time the slurry is in the bowl (residence time).

 

        This will usually:

        a. Increase the amount of solids in the effluent phase.

        b. Sometimes it will effect the cake moisture.

        c. In classification it will decrease the separation efficiency of the machine and coarsen the cut 

    2. Decrease in the flow increases the time the slurry is in the bowl (residence time).

 

        This will usually:

        a. Decrease the amount of solids in the effluent phase.

        b. In classification it will increase the separation efficiency of the machine and make the cut finer.

The flow rate is adjustable while the machine is in operation. Flow rate should not vary more than 2-3% or it will effect the particle size distribution in classification of particles.

Increase in flow rate also will increase the pool depth due to the effect of the head which overflows the weir plates.

Pool Depth
The weir plates on the liquid end of the machine control the pool depth. By increasing the pool depth (i.e. going from 1" to 2" we increase the pool volume (have a deeper pool). The same occurs when you reduce the pool level you reduce the pool volume.

Weir Plate Adjustment
The effects of pool depth follows:
Decrease: (lowering the pool depth, i.e. going from 3" to 2"') in pool depth reduces the time the slurry is in the bowl (residence time). 

    This will usually: 
    1. Increase the amount of fines (solids) in the effluent phase. 
    2. In classification it will decrease the efficiency of the machine and coarsen the cut. 
    3. Reduces G seconds. 
    4. Increases the length of beach (conical end of bowl) not covered by the pool. Typically this will decrease the moisture of the discharged cake solids. 
    5. Can increase gearbox torque levels.

Increase in pool depth increases the time the slurry is in the bowl (residence time). 

    This will usually: 
    1. Decrease the amount of solids in the effluent phase. 
    2. In classification it will increase the efficiency of the machine and make the cut finer. 
    3. Deeper pool will decrease the length of beach (conical end of bowl) not covered by the pool. Typically this will increase the moisture of the discharged cake solids.

Pool depth can not be adjusted while the machine is in operation.

Gearbox Ratio
Typically the gearbox ratio is fixed. It is a function of the design of the gearing and usually comes in 20, 40, 116, 130, and 140:1 ratios. The higher the gearbox ratio (i.e.: 130 versus 40:1) the lower the differential speed.

The gearbox ratio affects the differential speed and controls how fast or slow the settled solids are inventoried inside the machine. In other words how quickly the solids are conveyed out of the machine.

The higher the differential speed the faster the settled solids (rejects) are removed.

In classification it can affect the particle size distribution.

Differential Speed
The differential speed is the difference in speed between the bowl (driven by the motor) and the conveyor (driven by the gearbox). The operating speed determines the differential speed on a fixed ratio gearbox. 

    Typical effects on process as follows: 

    1. High Differential Speed 
        a. Usually have wetter cake. 
        b. Tends to decrease solids in effluent due to lower residence time of solids in bowl. 
        c. In classification it can increase the solids in the effluent due to turbulence of conveyance (a stirring effect) 

    2. Low Differential Speed 
        a. Usually a drier cake (longer residence time on beach). 
        b. Tends to increase solids in effluent due to longer residence time of solids in bowl. 
        c. In classification it can decrease the solids in the effluent due to less turbulence of conveyance (less stirring effect).

Note: In classification some products are not effected by the change in differential speed.
Conveyor Speed
Conveyor Speed is the rotating speed (RPM) of the conveyor. This speed is determined by subtracting the differential speed (DS) from the bowl speed (BS).

To find the speed of the conveyor use the following formula:     
    BS - DS = Conveyor Speed 
    Where BS = Bowl Speed and DS = Differential Speed
Conveyance Speed
The linear rate at which the solids are moved along the inside diameter of the bowl is called conveyance speed. This rate is a function of the differential speed and conveyor lead (pitch).

It can be calculated as follows:

    Formula: = inches/sec. Conveyance Speed

    Where: DS = Differential Speed CL = Conveyor Lead 

    Example: DS = 23 RPM, CL = 12" = 4.6"/sec

Machine Variables Summary (Especially Applicable to Solidbowl Systems)
The following is a summary of machine variables and how they effect process.

 Variables Adjustable (during operation)  Fixed by Design  Effects on Process 
 Speed  Yes***  Yes***  Increases or decreases G level. Higher speed makes finer particle size cut and lower speed makes coarser cut.
 Feed flow rate  Yes    Increases or decreases residence time of slurry in bowl. Lower flow rate makes finer cuta nd higher flow rate makes a coarse cut.
 Pool depth    Yes**

 Increases or decreases residence time of slurry in bowl. Deeper pool makes cut finer and shallow pool makes cut coarse.

 Gearbox ratio    Yes*  Controls how fast the settled solids are removed from bowl. (see differentrial)
 Differential    Yes*  Controls how fast the settled solids are removed from bowl. Lower differential can make the cut finer and high differential makes the cut coarser. However, sometimes it has no effect at all on classifying particles.

***Normally is either fixed or utilizes a variable speed motor

**Normally is non-adjustable when the machine is in operation

*Some machiens have a back-drive device to control or vary the speed of the normally stationary first stage pinion. This will make these items variable

Note: When making change to a machine change one variable at a time to determine the effect on process