A high speed centrifuge is a type of separator which is used on ship to remove contamination from liquids such as fuel and lube oils. It is imperative to carry out this treatment in order to remove solid impurities and water before they are supplied to the marine engine. Thus, the task of centrifuge is to remove solid contamination from liquid and to remove undesirable liquid (water) from useful liquids (fuel).
Principle of Working
The separation principle of high speed centrifuge depends on the difference in the specific gravity of two different liquids. To understand, let’s take a settling tank where fuel is stored and because of the difference in the gravity of water and fuel (water is heavier) the water gets collected at the bottom part of the due to the effect of gravity. Mathematically this process can be represented by:
Fs = ∏/6x D3 (ρw-ρo) g
Where Fs is the separating force, ρw is density of water, ρo is density of oil and “g” is gravitational force.
Now if we convert the tank into a conical rotating object, then the gravitational factor g will be replaced by the centrifugal force ω2 r, where ω2 is angular velocity of rotation and r is effective radius.
Fs = ∏/6x D3 (ρw-ρo) ω2 r.
Now the separating force will be much higher in centrifuge as compare to settling tank.
Construction of High Speed Centrifuge:
Basic components of the centrifuge are as follows:
The exterior frame work is normally made up of caste iron which supports the internal bowl and disk parts and carries water line, feed line and outlet line connections.
Bowl and disk:
There are bowls inside the frame, which can be a solid assembly operating non continuous and have space enough to retain the separated sludge. There can also be an arrangement in which the upper and lower parts are separate for discharging the accumulated sludge by a continuous operation. These parts are normally made up of high tension stainless steel.
The Vertical shaft is used to transform the electrical motor output into rotational motion for rotating the bowl in high speed through spur gear and horizontal shaft or belt. The material used for vertical shaft construction is an alloy of steel.
Horizontal shaft or belt drive:
The electrical motor drives the horizontal shaft through clutch pads and is used for transmitting the rotational motion to bowl assembly. A special belt having elastic character is used in some models in place of horizontal shaft, thus removing the use of the gear assembly. The horizontal shaft material is a special alloy of steel.
A spur gear is placed between the horizontal and vertical shafts for the transfer of rotational motion. These gears are manufactured by special aluminum bronze material.
Clutch or friction pads:
An electric motor will get overloaded if it is connected directly to the bowl assembly for the rotation of the same as the complete assembly is very heavier. To avoid this, clutch or friction pads and drum assembly are installed on the horizontal shaft. Normally the number of pads varies from 2 to 4 depending upon the frequency supply to the motor.
As the motor starts, the pads inside the drum moves out gradually due to centrifugal force and cause friction in the internal wall of the drum resulting in rotation of the shaft and the bowl gradually without overloading and damaging the motor and gears.
Attached Gear pump:
A general construction of centrifuge consists of a horizontal shaft driven attached supply or discharge gear pump. In some system an external supply pump may be installed in place of the attached pump.
Types of Centrifuge:
There are normally two types based on the application:
1) Purifier: When a centrifuge is arranged for separating two liquids of different densities, for e.g. water from oil, it is known as a purifier. The main component of purifier is correct size gravity disc or dam ring which is responsible to create interface between the oil and water.
2) Clarifier: When a centrifugal is arranged to remove only impurities and small amount of water, it is called as clarifier. Since it is used mainly for that fluid where mostly solid impurities are to be removed, gravity disc is not used in clarifier; instead a sealing ring is used to keep the impurities intact unless desludged.
The basic operations of clarifier and purifier are:
– It contains stack of disk numbering up to 150 and are separated from each other by very small gap. A series of holes are aligned in each disk near the outside edge which permits the entry of dirty oil.
– Due to difference in gravity and centrifugal force, the heavier impure liquid (water) and particles moves outside and lighter clean oil flows inwards and get separated.
– The collected sludge and impurity can be discharged continuously or at a time intervals, depending upon the construction, automation and system incorporated.
You may also like to read Oily Water Separator-Construction & Working.
Introduction to Marine Engineering by D.A Tayler
Marine Aux. Machinery by H.D McGeorge