Centrifuges

The Home brewer has a number of challenges from a filtration perspective. These include contaminants that seem liquid or semi – liquid in form which either simply flow through barrier filtration or quickly block it. One of the biggest challenges is the removal of what is commonly referred to as HMPEs – these are not always visible in fuel and will not always settle out quickly by gravity settling alone. For WVO users there is a requirement to filter to sub-micron levels, but this too can be time consuming and messy. Centrifuges have been successfully used for accomplishing fine filtration within an hour or two and can be done without mess and without the need for consumables (such as filter elements).

Centrifuges come in many guises, but they are basically a device for separating suspended contaminants that have a different specific gravity (SG). By spinning at high speed the massive amount of G-force generated, acts on solids with higher SG, which are flung to the wall of the centrifuge at the same time the oil exits through it's ports or drain. Water being of a higher specific gravity can be separated too, but in the pressured oil type of centrifuge, whatever water does not stick to the contaminant cake, drains with the oil (see section below)

For the home bio brewer there are two main types: Motor driven & Pressure driven. Centrifuges are also used for hydraulic oils, waste motor oil, swarf oil etc. Other types not discussed include water separators – these are very expensive and tend to be used for large commercial operations.

Motor Driven The motor driven types are generally produced from a single milled piece of aluminium with a channel and port for the oil and a channel and port for the water. The oil drains in one direction and the water drains in the other. The motor is mounted vertically, so manufacturers modify a standard motor with a special seal that protects the motors bearings from water. Advantages of this design is the ability to remove solids of far greater size and the ability to separate water. The disadvantages are that it can be a messy process and not unattended and it's a flower flow rate. They are also pretty pricey at £1400. The best known design is WVO designs. They can be homemade, but given the speed that the bowl needs to be spun at to work, extreme care should be taken to ensure that the bowl is well balance and that the bearing that locates it to the motor shaft is perfectly secure.

Pressure driven Originally developed by Mann & Hummel for fitting to the lube oil system on large diesel engines, as a way of removing carbon and wear material not caught by the engine's oil filter – this has enabled longer oil service intervals and greater engine life. When being used for either WVO filtration, as final polish for biodiesel or as a way or removing high melting point elements (often referred to as HMPE's), a gear pump is required plus fittings which include tees, lever ball valve, a hose assembly, a pressure relief valve and a pressure gauge. The centrifuge has tangentially opposed jets at the bottom of it's rotor, through which oil is forced through causing high speed rotation. The rotor has a bowl / cap that acts as a wall for the contaminant cake. This design has an inlet pot (often 1/4” BSP or 3/8 BSP) and a drain hole at the bottom of the unit. Differences between different types of unit are viton vs. butyl rubber seals and some are dynamically balanced for reduced noise.

The centrifuge needs to be driven with oil pressure of between 5 – 7 bar and a relatively low flow (not greater than one and a half times the throughput of the centrifuge at 7 bar).

The advantage of oil pressured centrifuges are: An unattended process with no mess, cleaning down to sub-micron levels at a about 1/3 the price of motor driven devices. Disadvantages: despite the claims of some, these are not an effective water removing device and requires pre-straining (250 µ) and heating oil (probably 50 – 60°C) in the case of WVO.

Many recognise the benefits of using a centrifuge to clean WVO to sub-micron levels or to finely polish biodiesel and remove high melting point elements that cause waxing. But questions are often asked about what pumps are suitable:

Gear pumps Oil pressured centrifuges need 5 - 7 bar pressure to work well with our application (when used on an engine's lube circuit, it's not so critical).

The gear pump's flow is important too! Flow should be rated a just a bit higher than the throughput from he centrifuge at 7 bar (coz pressure and flow will change slightly due to the oils viscosity - bio will have a bit of slip through the gears for example resulting in slightly lower pressure / flow).

The range of flow from the different models used by bio / WVO types are between 4.6 and 8 LPM. So pumps should be < 9LPM. This needs to be compared with the gear pumps that are actually available. For example 3cc per rev driven by a motor at 1400rpm or 2645rpm or 2755 rpm. 5cc per rev pumps are commonly available too.

Power steering pumps have been successfully used. If driven by a motor via a belt, using different gears would allow you to get a decent match. It would seem that a power steering pump from a small car would suffice.

The pump (and centrifuge) should be protected from a 'dead head of pressure' using a pressure relief valve. RickDaTech in his SVO Tutorial wesite has a good page about this. Any PRV should be configured to relief the pump if a jet blocks and the centrifuge if the pump pushes >7 bar pressure.

A Tam pump will not work, neither will any centrifugal or peripheral pump - The flow would be to high and they're not good for anywhere near the sort of pressure required to drive a centrifuge. Gear pumps are very different to other pumps that we use, are very rugged and have a much longer life. Gear pumps can be purchased from online auction sites and can be pretty cheap in China, these usually need mating to a 1/2 HP or 3/4 HP motor via a coupler with a shroud covering the coupler.

In terms of port size a 1" isn't great as you need to reduce to 1/2" before the lever ball valve which acts as a pressure / flow regulator (a 1" ball valve is difficult to use and be precise). If using a pump with a 1/2" discharge couple to a 1/2" tee with the ball valve on the branch of the tee and hose to the centrifuge on the run of the tee. If using a pump with a 1/4" discharge use a 1/4" tee with a 1/4" ball valve on the Tee's branch.

If you are using the pump with WVO you should pre-strain at 250µ and heat the oil, as you have to heat it any way, why not do it as part of dewatering, but heating to >70°C centrifuging and then leaving to settle and running any water and creamy stuff off the bottom. If you plan to do this check the temperature rating of your pump. When centrifuging Biodiesel, the fuel should be settled for as long as possible at the coldest temperature range that it is like to be used at. This will ensure that any high melting point elements that form will be removed, before they can settle out into the fuel tank.

If using the pump with bio check for Viton seals. You can also use the pump to drive the oil through hardwood shavings or a polish pot if you're reducing water ppms on dispense. Gear pumps are not self priming and therefore be configured with flooded suction. The most effective way to do this is by using a conical tank and pulling from the bottom with the pump cited below the level of the tank.