Shopping for a Centrifuge

Q. We’re shopping for a centrifuge for processing our vibratory finishing solution with the intention of reusing it. Options range from manually removing accumulated sludge to semi- and fully automated methods. Price differences are great. We’re willing to spend more for a fully automatic system, but do you think the expense is justified? We’d also like your comments on how to best use such a system, and what to expect of its performance. S.B.

A. Let’s begin with some basic terms. Particles that are removed get packed around the centrifugal bowl and are referred to as sludge, cake, sediment or precipitate. Processed water is supernatant or centrate. The style of centrifuge used for this application is a decanter centrifuge, which can be thought of as a very fast-settling tank. The performance difference is that the centrifuge will settle out smaller particles depending on the added force it generates.

To justify the cost difference of different cake removal systems, first estimate how often this operation should be performed. Most manual systems have a small capacity for cake, usually only 30–50 lb. Most of the cake comes from worn ceramic media, so you can expect to clean the centrifuge once for every 50-lb box of media you use.

Most manual systems require 15 min for a clean-out. If your operator has the time, you have the answer. Just don’t expect this to be his or her favorite job.
Semi-automatic cleaning operations are far less messy and have more sludge capacity, so they need to be cleaned less frequently. They are usually 50–100% higher priced than manual systems—not too hard to justify if you consider operator convenience.

A fully automatic system requires an expensive control system to direct mechanical operations, including starting and stopping the centrifuge. About a third of the cost of one of these systems is in the control panel, and maintenance can be quite costly. A warranty based on a preventive maintenance contract is worth the price if you want to minimize downtime over many years.

In any system, if you don’t clean the sludge as often as necessary the smallest particle sizes removed become larger and larger because the cake reduces the effective radius of the bowl. Particle size separation is directly proportional to the G-forces generated in the fluid, and G-forces are directly proportional to radius. If a bowl with a radius of 10 inches has two inches of cake pasted against its sides, it now has an effective 8-inch radius, with G-forces reduced by 20%.

How many Gs do you need? I’ve experimented with 1,000 to 4,000 Gs. The water was pretty cloudy at 1,000 G; acceptable for most finishing operations at 2,000 G; and very good at 3,000 G. Crystal clear water was attained somewhere between 3,000–4,000 Gs. When selecting a centrifuge, get as close to 2,000 Gs as is reasonable. Unfortunately, most commercially available centrifuges for mass finishing produce 1500 Gs or less and may need an additional filtration system.

Other factors that determine particle size removed include: particle mass, electrical charge and absorption characteristics; time in the system; and fluid viscosity. I said before that gravitational force in a centrifuge is directly proportional to radius; it’s also proportional to the square of the rpm. For any given radius, if you double the rpm you increase the G-force four times. It follows, then, that you want the highest-rpm centrifuge that is practical.
The design of the complete system is also of importance. Industrial centrifuges are designed to give the fluid one pass through the spinning chamber, severely restricting the time during which the fluid will be exposed. Particle separation depends on G-force exposure time.

The practical way to increase time in the centrifuge is to constantly recirculate the centrate along with dirty solution returned from the finishing operation. To minimize re-mixing of clean with dirty solution, a conical bottom tank is used. It has a 10–12-inch diameter tube fixtured in the center, extending to two-thirds the depth of the tank. The centrate is returned to the top of this cylindrical divider. This is also where premixed fresh solution should be introduced. With 20 gpm coming into this tube from the centrifuge, the downpour velocity in the tube is about 20 ft/min, ensuring the cleanest solution will be nearest to the top of the tube. Fresh make-up solution is added to this tube and the intake for the pump sending clean solution to the vibrators is located in the tube, below the lowest operating level for the tank.

Effluent from the vibrators is pumped in at the top of the conical tank at an angle aiming it along the top of the tank and flowing in a counter-clockwise direction. This creates a swirling action within the cone, encouraging heavy particles to work their way quickly to the bottom of the cone where the fluid and particles are sucked into the intake pipe for the centrifuge. Since the centrifuge is removing several times as much fluid as is pumped in from the vibrators, there is a constant downward, spiral motion of the fluid. The cleanest solution is always at the top of the interior tube, ready to be reused.
Tank capacity should be at least twice the hourly flow of compound going to the vibrators. A medium-size centrifuge may be rated to process 20 gal/min. You continue processing the holding tank water at 20 gal/min, while sending about 4 gal/min of the clean solution back to the mass finishing operation. On average, the fluid will pass through the centrifuge five times to remove the particles.

Particle size removal is dependent on G-force. However, each time the fluid is processed, it will remove about 75% of the particles of sizes within its effective sorting range. After five passes, less than 0.1% of the original available particles will remain. Therein lies the secret to efficient use of a centrifuge. The final centrate will still have about 5% of the original particles you wanted separated because some of the fluid is being processed for the first time.

Some caveats: 1) Limit maximum capacity of the pump feeding the centrifuge to no more than the maximum capacity of the centrifuge; 2) Prevent leakage of air into the centrifuge; 3) Minimize need for flow restriction valves anywhere in the system; 4) Use a large enough conical bottom tank to prevent the vortex from reaching the bottom; 5)Prevent pulsing of the flow to the centrifuge; 6)Do not feed the centrifuge any more fluid than its maximum rated capacity