Y CENTRIFUGAL SEPARATORS

by

Donald C . Roman Pres i dent

Roman Associates Sci tuate Harbor, Massachusetts

Presented a t S M E ' s

"Me ta I work i ng Cool ants" Program May 6-8, 1986

Schaumburg (Chicago), I 1 1 i no i s

THE APPLICATION OF CENTRIFUGAL SEPARATORS TO FACTORY COOLANTS

By: Don Roman ROMAN ASSOCIATES 99 Beaver Dam Rd. Scituate Harbor Ma. 02066

The theme of this S.M.E. sponsored program is "Metalworking Coolants". The more obvious benefits of clean coolant are improved precision and finish, increased tool life, and a better work enviroment. There are numerous systems, and a variety of equipment available for coolant purification and treatment.

This portion of the discussion pertains to the application of centrifuges in cleaning the various liquids classified as coolants. W e will be discussing the removal of tramp oil, insoluable solids and bacteria.

The intent and goal of this presentation is to simplifiy for you, an often confusing equipment technology. I am attempting to share with you, a condensation of 2 1 years of "hands on" experience, gathered from hundreds of case histories. During the time alloted, we will be discussing some of the basic theories of sedimentation, along with the design, application and misapplication of centrifuges. Hopefully, by the end of our discussion, some of you will have gained more confidence and understanding for your future exposure to centrifugal separators. There are seemingly endless successful centrifuge installations, in both food and industrial applications. If I had to pick one segment of industry where centrifuges have a less than favorable image and history, it would be the metalworking industry. The reasons for this are typically, misapplication and lack of understanding. W e intend to cover this problem in detail. later i n this disucssion.

In the time alloted today we will try to confine our discussions t o the treatment of metalworking coolants. Listed below are some additional factory applications for centrifuges that fall outside the coolant catagory:

Grinding coolant clarification VPaint booth water clarification Hydraulic oil purification Wire drawing lubricant clarification Quench oil clarification Cutting oil clarification Lubricating oil purification Fuel oil purification Press lube oil purification Waste treatment and sludge dewatering

- 2 -

Today environmental concerns and regulations have given increased importance to the word "separation". Within a short period of time, the disposal costs for some factory wastes has risen 500%. In addition, increasing quantities of necessary factory-process fluids are being classified as hazardous materials. A simple cost benefit analysis should make it obvious to all concerned, the economic importance of keeping these fluids "in plant" and "in use", for the longest possible period.

After making the decision to remedy the problem, the next step is often selection of the process system and equipment. For both water soluable coolant and mineral oil purification, the following process requirement data will be required by potential equipment suppliers:

The results and problems of your existing system. Flow rate requirement and desired results. Feed slurry composition ( X liquid/liquid/solids x vol.). pH, toxicity, flashpoint, temperature and viscosity. Is foaming o r areation a potantial problem? Rate of product separation. (Test tube work) Rate of filteration (if applicable) Nature of insoluable solids (erosive, packing, etc.) Dryness requirement of recovered solids Manpower availability

A f t e r identifying the above parameters and requirements, s decision can be made regarding a Sedimentation or filteration approach to the problem.

For the purpose of this exercise, we are going to eliminate the filteration concept. Let's assume, if you will, that t h e solids are fine and slimey, and that the separation involves three phases. Excessive filter aid, and costly additions1 equipment, would be required with the filteration approach t o this problem,

N o w that we have decided upon the sedimentation method of product separation, we will follow some logical steps towards the simplest and most economical solution. Equipment is often selected on s basis of flow rate vs capital equipment costs. If we follow this concept in our selection process, we will typically be considering simple, static-settling tank designs first, and high-speed disc bowl centrifuges as a last resort.

- 3 -

In a few moments, we are going to begin an indepth study and discussion of the design and application of both static and centrifugal separators. In order to relate coolant purification during this part of the program, It's important now to identify the chemical and physical changes in the used water soluable coolant; Specifically, those changes that we wish to correct. (Mineral oil coolant will play only a minor part in our discussion, mainly because past experience and records make this application relatively predictable).

Many soluable oil coolants are emulsions. Water soluable coolants form true chemical solutions through a reaction similar to soap chemistry. Tramp oils contamination of coolants is from various mineral oils used in the manufacturing process. Typically, these tramp oils can be hydraulic oils from machine tool actuators, way lubricant, o r lubricating oils. When tramp oil first enters the coolant it is usually in a free floating state and easily removed. The longe.r-..it is allowed . . t~g_-r.ea.ain in t he._salu.t i on the n o re d i f f i cu 1 t C he se~~a~r.a~~.o.n_~~become 9-. ~~

Con t nu e d pumping andd -sAiKar ~~'pr-o-vla e;i~ by machining opera t ions reduce the oil droplet size and help form stable emulsions. This tramp oil is present in the coolant in several forms, ranging from free floating tramp oil, to emulsified oil and coolant, to .true suspensions of sub-micron droplets. rhis later condition is s i - - u s - t he mechanical effect of homo Cnizing milk-and butter fat The fat Darticls a2-e &<>-remd to iess tba n 2 microns in size. by high s h e a r gtreaa, s i n g a high pressure pump. Referring to "Stokes Law", we can appreciate the effect of the particle size in the equation-given below.

Other problems present in coolant purification involve the particle size, erosiveness and packing nature of the insoluable solids. Some of the fine particles become oil laden, and often form a forth phase, that must be handled in a three-phase separator.

The free floating tramp o i l is easily removed by floation or belt skimmers. My experience with the problems of centrifugally removing bound and emulsified tramp oil from water soluable coolants, is that centrifugal forces near 4000 x G are required, that is, if one is to maintain reasonable high flow rates.

"Stokes Law" describes settling velocity as a function of particle size. particle density, and the viscosity of the liquid medium. Centrifugal separation is a faster and more efficient method than gravity settling, but the basic principals of "Stokes Law" applied to both concepts. It is important to understand the value and effect of each factor in the equation listed below:

- 4 - 2

..

r i . *

? , .

V - Settling (or separation rate) in distancefamount of time

r = Radius of the heavier partiacle

s - Specific gravity of the heavier particle

s t - Specific gravity of type lighter particle 7

e - Viscosity coeflcient'of the lighter particles 4 -

A quick glance at the simple formula illustrates the importance of the particle size. Note its squared function in the equation. Also note the important effect of (e). A viscosity with a higher (e) number, reduces the value of the settling rate (v).

For all practical purposes, gravity is constant in nature. By the use of centrifugal force, the G forces can be multiplied many thousands of times.

These basic lawaof settling remain unchanged when evaluation a settling tank compared to a centrifuge. Many refinements to the basic principals are involved in centrifuges design, but certainly the most important would be the t h i n , STRATA distribution created with the disc stack. If we increase the gravity effect by 4 , 0 0 0 through centrifugal force, then the addition of the disc stack can multiply the total effect a further 100 times.

T h e other important points to remember, are that larger particles settle faster than smaller ones, and that they naturally settle at a faster rate in liquids of lower viscosity.

A s we consider the separation of insoluable solids from liquids, or several imiscable liquid phases from each other, the use of earth's gravity is often the first, most logical, and most inexpensive consideration. The energy is free, and the maintenance relatively low. The design faults are, (compared to centrifugation), that gravity separation I s relatively slow, and that solids compaction is minimal.

- 5 -

The advantages of centrifugal force were known to ancient people. Honey was extracted from the comb in this fashion. A vessel spun about one's head on a 20' rope, at 60 revolutions per minute, produces a centrifugal force o f 25 times gravity.

Continuous gravity settlers (separators) have been in successful use for centuries, and will continue to maintain a place in industry. There are many manufacturers, and the better designs take full advantage of "Stokes Law". In order to decrease the settling time, many of these static separators are equipped with dividing plates, installed to stratify the process liquid into thin layers. This has the effect of decreasing the time by decreasing the distance. Another common approach i n continuous gravity settler design, is the use of helicies, o r porous media, to Coalese the lighter liquid phase droplets into a larger size. Typical applications for this type of separator are:

Bilge water separators (oily waste water) Water soluable coolant purification

W e would now like to review some of the basic principals of centrifugal separation. Using overhead color projections as an aid, w e observe first the simple continous gravity settler depicted in Fig. #I. Figure 1 2 is a design improvement, in that, the incoming feed slurry is directed towards the bottom of the settler by a baffle. Hopefully, now that we have forced the insoluable solids to the bottom, they will remain there under the force of (1) "G". Fig. 1 3 depicts further design improvements, in that the same volume of liquid is being processed in a vessel of one-half the height of Fig. W2. This means that the solids should settle to the bottom in one-half of the time required in Fig. 1 2 .

is FIG.*I I

n FIG.%

I 0 1 FIG. *3

us t rates separation

ard into particles.

et, centrifuge ranging from type of x C up to in t h e 2000 x

A lot of our initial study and discussion, regarding centrifugal theory, will begin with this most simple design. Different products require more or less "G" force to achieve the desired separation. Internal design improvements can improve the efficiency and flow rate of centrifuge but the fact remains that some product separations require m o r e "G" forces than ochers.

Commerical centrifuges advertise the "G" force ratings, at the maximum outside diameter of the rotor. Other than to serve as a guide i n comparing centrifuges, this method of evaluation is misleading and useless. The important "C," force calculation is taken at a diameter within the centrifuge where the work is being done. The following formula for calculating "G" forces w i l l be helpful in our evaluation work w h e n using test tube centrifuges.

2 G = RPM X dia in (inches)

70,800

Calculating the advertised "G" forces at the 0"D" of the rotor in Fig. # 5 , we find the answer to be 1,193 X G. This number is useless information since the liquid will never be exposed to these forces. I n the design configuration of Fig. # 5 , the liquid enters the rotor through the feed tube. There is some slippage and turbulance i n the liquid-filled rotor, since the feed material must be accellerated from zero up to the rotor speed. There are no baffles in this design to force the incoming feed to any greater diameter than the innermost pool area, depicted as small diameter (d). The feed slurry will follow a path upward, in this innermost pool area until i t exi t s from the bowl or rotor. The area outside of this inner pool surface now becomes little more than holding space for the insoluable solids to be collected. When attempting to conduct a test tube centrifuge evaluation, and select the correct commercial centrifuge, (of the Fig. #5 configuration) we should calculate the "G" forces at small diameter (d). This calculates to 565 x G. The test-tube centrifuge should be operated at this range for evaluation of separation performance. W e must also consider tha i,nner pool ares' offers limited reside design. The length 5f T F m e r - ~- excessive. The spin test for solids compaction can be conducted at a higher "G" force, and for longer periods.

- 8 -

The centrifuge rotor design depicted in figure 8 6 is arranged with a feed baffle (distributor). That forces the incoming slurry to enter the bowl at a larger diameter. This is a standard configuration in high speed centrifuge design. ~ 0 t h this design and that depicted in Fig. 115 should be equipped with verticle accellerating vanes in order to keep the liquid up to speed and prevent turbulance.

I f w e consider the centrifuge bowl design i n Fig. #6 a s a typical high speed centrifuge, we can add the following dimensions and values for general calculation.

Bowl RPM - 6,000 RPM Bowl O . D . = 15" = ( 7 , 6 2 5 x G) Feed Baffle O . D . - 9" - ( 4 , 5 7 5 x G) Liquid Pool I . D . - 4' - (2,030 x G) T h e important number here is the 4 , 5 7 5 X G, arrived at

from the calculations of the "G" forces, at the 9" O . D . feed baffle. T h e feed material is subjected to this high force, then travels upward and inward towards the discharge point. We have theorectically created a separating effeciency that is 4 , 5 7 5 times greater than that achieved in the tank depicted i n Fig. 112 .

F l G . . ' 6

- 9 -

Fig. 117 illustrates a centrifuge bowl of the same dimensions and speed as that in 1\13 above. Dividing plates or discs have been installed to stratify the liquid to be treated into thin layers. This effect was discussed earlier when covering the design principals of Fig. 1 4 .

In a typical high-speed disc bowl centrifuge, the spacing between the discs is .020" (or 0.5 mm). In some special applications, particularly with high viscosity liquids, the spacing between the discs must be increased. The angle of the discs is extremely important, and a steeper angle will add efficiency to the separator. The liquid flow pattern through these discs is similar to the flow through a pipe or a river. The current, or flow, is strongest through the middle and is progressively slower, or less, near the pipe walls or the river bank. The force vector acting upon the insoluable solid particles is outward in a horizontal motion. These particles strike the underside of the discs, where the liquid flow is minimal and slide outward to the solid's holding space.

If we look at the chart on the next page, we can obtain a relative efficiency comparison of the methods we have discussed thus far.

F I k ' 7 q '. :

. .

. ._. . ~~ ...

- IO-

h

v 2

T min

7. min

36000 I/H

- 1 1 -

LiquidILiquid Separation

So far our discussion has dealt with the clarification of solid particles from a liquid. principals of Liquid Liquid separation. Fig. 18 shows a vessel filled with two imiscable liquids and a trace amount of insoluable solids. Fig. # 9 shows the condition of this mixture after some period of settling. liquild mixture (interphase) that has formed between the two phases. soluable coolants, and w e w i l l discuss this in detail later. Fig. d10 shows an ideal separation that has occured after sufficient settling time.

We will now review the

Please note the unseparated

This is a common occurance when working with water

,I

. .I . , P.'.

! !

- 12 -

In Fig. #11 we have a Three-phase separator, quite similar in construction to the liquid/solid clarifier shown previously in Fig. # 6 . The design difference is that we have installed a baffle (or top disc) in the upper portion of the centrifuge. It's purpose is to trap the lighter liquid phase and provide separate passages for the two separated phases to leave the centrifuge bowl. This is also illustrated in Fig. 1/12 which shows the three-phase separator with the discs installed. In order for this three-phase design to function properly, the discs and feed-distributor baffle must be provided with distribution holes. The placement of these distribution holes is critical and is different for a mineral oil purification than you will find in a milk application, or water-soluable coolant separator. When designing a separator for a specific application such as mineral oil, the distribution holes are placed where the natural interface will occur with the liquids to be treated. (This is not the case with coolant purifier designs). Density and temperature variations require that some method of adjustment be available to position this interface within the area of the subject distribution holes. In a standard centrifuge, this is either done with a weir (ring dam or gravity disc), or by applying back pressure to one of the liquid phase outlets.

- 13 -

This important interphase adjustment is shown more easily in the two settling-tank illustrations, Fig. 0 1 3 and Fig. 114. This clearly shows the movement of the interface with changes in the diameter of the gravity disc.

FIG."13

U

/

L- G R A V I T Y D I S C

A quick study of this interphase is warranted here, because in coolant purification, this is a problem area. A s you examine Fig. 1\13 or Fig. 6 1 4 you w i l l notice that there are three liquid phases, and outlets for o n l y two phases. In a continuous separator this interphase will continue to grow until a portion of it begins to exit w i t h one or the other phases. The natural effect is that the interphase column will grow towards the outside diameter of the vessel and therefore presents itself as a contaminant in the coolant. T o prevent this unwanted coolant contamination and also fouling of the disc stack, certain centrifuge design adjustments must be made for coolant processing.

- 14 -

Figure. 1115 and Fig. b 1 6 shows a half section of an oil purifier bowl and a coolant concentrator bowl ( 1 1 1 6 ) . The distribution holes in the standard mineral oil purifier ( 1 ! 1 5 ) are near the outside diameter of the disc stack, and the light phase or oil has to travel the greater distance through the disc. The interphase is actually being positioned just outside of the disc stack.

Please note that in the coolant concentrator design (Fig. %16), that the distribution holes have been moved toward the inside diameter of the discs and that the often troublesome coolant interphase is being held or positioned inward of the distribution holes. This helps to prevent plugging of the disc stack when troublesome emulsions are encountered. Often these emulsions contain fine, oil laden, metallic particles that can eventually contribute to plugging of the disc passages, if the interphase is not positioned properly. The tramp oil phase and any unbroken emulsion are being concentrated and usually discharged as an unclean light phase. In this design, the coolant, or heavy phase is being forced to travel the greater distance through the disc stack, all the while being subjected to higher " G " forces. The top disc in this design is usually cut off to a smaller diameter. This helps to reduce the back pressure restriction or the heavier phase, allowing for higher flow rates in this area. It also provides some additional solid holding space.

... . . . .

..

- 1 5 -

Metallurgy

The high-speed disc bowl, centrifuges that we have been discussing thus far are constructed primarily of 300 series stainless steel. In the designs intended for straight mineral o i l purfication, some of the internal bowl parts are constructed of bronze. The centrifuges to be used in water-based coolant purification should have all product contact parts constructed of stainless steel.

Solids Holding Capacity and Cleaning

The subject contrifuge designs have rather limited solids holding space. It can range from one liter in .the smaller, manual clean, designs up to ten liters in the larger automatic solids ejecting types. This i s a meaningfull limitation to the user and often some pre-settling of the solids is required. This primary separation is frequently accomplished in settling tanks equipped with drag-out conveyors, filter screens or with inexpensive, automatic discharge, basket-type centrifuges.

A simple material balance of the feed slurry ( X solids by V o l . and flow rate) will quickly determine if the manual clean design is feasable for your application. Most of the coolant separators i n use today are of the automatic discharge design (solids ejecting. self-cleaning). This bowl design make use of a hydraulically actuated sliding-bowl bottom or sliding-piston arrangement. This mechanism is actuated and i n fractions of a second, discharges either all of the bowl's contents, or only a predetermined portion. Again, it is important to pre-separate the larger solids before using th% type o f centrifuge. lecting (and packing) at the rim of the c;ntrifuge bowl at the higher 'GG" force area (typically 7,000 X G ) for relatively long periods of time. The internal hydraulic pressure at the rim of this bowl is 1,000 P . S I I . G . and when the bowl opens the discharge is quite violent. At the above pressures abrasive particles will quickly erode t&i-

' centrifuge-sea s u s Some of th; designs being offered have a rathe&discharge mechanism. the discharge i n these designs presents a problem of the solids bridging in the bowl. W h e n this happens, potentially dangerous imbalance can occur. It is important with these slower designs, to fully discharge the entire contents of the bowl each time.

+Depending upon the frequency of the discharge cycle, some additonal coolant will be lost as a result and the sludge discharged will be quite wet.

Investment Cost

The lack of violence to

T h e price range f o r these high speed disc bowl centrifuges can range from $10,000 fo r a smaller (manual clean) solid bow+ unit capable of precessing 180 gallons per hour up to $160,000 f o r a larger automatic unit capable of 3,000 gallons per hour,