Industrial Centrifuge are being used to separate solids from liquid under different conditions according to the output requirement. The centrifuges are being classified according to the process type, positioning and mechanism used. According to the particle size separation size also the centrifuges can be classified as follows;
From separation of fine calcium carbonate particles of less than 10 μm to coarse coal of 0.013 m. For the design of the centrifuge, the input conditions take according to the previous equipment output conditions. The conditions of the input are, solids concentration 0.36, solid mass flow rate 1024 kg/hr, particle size 180 um.
In many industries, for the purpose of separating solids, centrifuges are being used frequently and centrifuges covers a broad range of applications, in this design a filtration centrifuge is selected since the input has a high solid particle diameter
Filtering centrifuges are broadly categorized as continuous operating and batch operating. Both continuous and batch filtering centrifuges use some type of filtration media fitted against the basket (bowl) wall. As the solid-liquid mixture is introduced, the liquid filters through the solids, through the filter media, and typically through perforations in the basket shell. Typical solids retention times range from 5 to 45 s for continuous operating filtering centrifuges and 5 to 180 min for batch operating filtering centrifuges . For the selection of the centrifuge the following design table uses;
According to the Criteria’s a centrifuge can be selected. Industrial centrifuges are unique with compare to lab-size centrifuges where it has a complex mechanism for the centrifugation. In industry the range of capacity is from about 1 ton/h for small (250mm diameter) units up to about 120 tons/h or more in the largest units is available according to the industry requirement (1250-mm-diameter).
Equipment sizing of Industrial centrifuge
The design of the chemical equipment consists of 3 major steps namely, Chemical design, Mechanical design & Civil design. Each of this section have a vital role in the equipment sizing where all the criteria should interconnect in order to have a proper equipment
2.1 Chemical design of Industrial Centrifuge
The chemical design is where all the separation process principles applies as well as the filtration theories. The following assumptions take for the equipment sizing ;
- If the particles go to the furthest point with the given centrifugal force it will separate the rest of the particles in any particular place
- Temperature does not change during the centrifugation process
8 Design criteria’s
- Basket length
- Basket outside diameter
- G-value and RPM
- Particle settling time
- Particle settling criteria
- Cake thickness dynamics
- Stoke length and time
1. Basket length
The basket length of a pusher centrifuge varies. Considering the design purpose the basket length can selected as required initially but when the rest of the criteria are also need to check whether it suits with it.
2. Basket outside diameter
The basket outside diameter is a very critical criterion where it has a much signification on the centrifugation forces and separation layers. As the design guides given the basket diameter ranges from 0.25 -0.75 m . initially a value of can select but when the rest of the criteria met the basket outside diameter needs to change in order to get the correct centrifugation.
3. G-Value and RPM
The G-value considers to be the force which gives the particle to moves towards the basket outer shell, the G-value can be determined by using the following equation for the industrial centrifuge;
The ɷ value for the pusher centrifuge gives from the design guide can take the centrifuge G value for the centrifuge takes from the table given below for pusher centrifuge;
4. Particle settling time
For the finding of the particle settling time, the liquid layer generating at the given rpm should be found.
5. Particle settling criteria
The Stokes settling velocity of a spherical particle under centrifugal field gives by the following equation
Useful relationships have established on continuous sedimentation by studying the kinematics of settling of a spherical particle of diameter d in an annular rotating pool. Equating the time rate of change in a radial position to the settling velocity, and the rate of change in an axial position to bulk-flow velocity.
The following factors should consider for the design;
- If the particle to go the bowl distance the following criteria should meet;
6.Cake Thickness Dynamics in Industrial Centrifuge
For a pusher centrifuge the cake filtration is the step where it removes the water from the slurry.
Accordingly, for a pusher centrifuge the minimum Intrinsic permeability(k’) ≥ 2× (10^ −9) and specific cake resistance(α) to be ≥5 × 10^8), by using this criterion with ε for the respective product, the Thickness can be estimated but Darcy’s law and the pressure on the cake
7. Stroke Characteristics
In order to remove the separated solids from centrifuge a shaft mechanism with a slider crank uses for this purpose. For the shaft movements a slider crank mechanism must use.
Mechanical Design of Industrial Centrifuge
Mechanical Design of Industrial Centrifuge
The Mechanical design is a very critical operation where it involves the mechanical aspects which requires to operate the equipment smoothly. Moreover, the mechanical design of the Pusher centrifuge consists of the following 7 subsections;
- Strength (theory of failure)
- Material buckling
- Main motor selection
- Shaft design
- The strokes of the crank
- Crank motor selection
The fluid in a rotating centrifuge exerts pressure on the basket. The minimum thickens required to counter this load can determine by using the following equation.
From the literature it has found that the alloy type 304 stainless steel commonly uses for the purpose of making baskets for centrifuges.
In order to generate values due to the self-pressure and for the calculation;
purpose the bowl thickness can be assumed a value, if the value gives a lesser value for the wall thickness from the calculation then the bowl wall thickness assumed value can accept.
For the filter cake, a Polyester material uses which can easily be easily with stand the pressure exerted due to its higher (393 MPa)
Since the assumed value is higher than the calculation is correct, as a convection the minimum thickness of an industrial stainless steel is 5mm.
Because of this reason the making of the outer thickness of the basket the 5mm uses even the calculations has given a different value. from the design guides the minimum corrosion allowance is given as 2mm
Strength (theory of failure)
In below info, the material properties needed to check if the material uses for the purpose of making the Industrial centrifuge.
The principal stresses are the maximum values of the normal stresses at the point; which act on planes on which the shear stress is zero. In a two-dimensional stress system, the principal stress at any point are related to the normal stresses, the following equations are being used for substitution ;
By substituting the calculated values from values from the where is not significant and assumed to be 0 .
The third principal stress, that in the radial direction , will usually be negligible for thin-walled vessels
Allowable Stress intensity
The maximum intensity of stress allowed will depend on the particular theory of failure. Using this criterion, the maximum stress intensity at any point is taken for design purposes as the numerically greatest value of the following: if the allowable stress does not increase beyond the material maximum allowable design stress at the given temperature then the criterion is fulfilled ;
Hence, under conditions for the centrifuge where the resultant axial stress z due to the combined loading is comprehensive. The vessel may fail by elastic instability (buckling).
Local buckling will normally occur at a stress lower than that required to buckle the complete vessel.
Main motor selection
For the given RPM (1231.48), the motor can select as from (references) 1GG6, 1GH6, 1HS6, Size 180, Efficiency 86 %
Bearings selects to mount according to SKF standard YSP 210 SB-2F bearings can select for the industrial centrifuge.