7/29/2019 Cleaning times The second cleaning process The oil skimmer Part III.pdf

http://slidepdf.com/reader/full/cleaning-times-the-second-cleaning-process-the-oil-skimmer-part-iiipdf 1/3

cleaningtimes

Figure 2:Plotted inthis figureare data fromTable1,withthe time to riseforoildroplets of80micronsin hydraulic size.Atdroplet sizesof 80 micronsand above,one canexpect rise times ofaround10minutes ina tank of fluid about 1 ft high.Thisshouldallowadequate space for separation of sub-surfaceoil fromcleanedparts.

~\ ~

..

-, -.......

........-...

""""""III'

The Second Cleaning Process:

The Oil Skimmer-Part III

32080400060

Table1: Calculated TimeNeededfor Rise ofOilDroplets(p= 0.85glee)

Coalescers can function effectively

in both size ranges.

THE CUTOFF POINT

The cutoffpoint between larger and

smaller droplets is different in the

minds of everyone who studies this

industry or manufactures a product

for sale within it. Units of this cut

off point are the hydraulic dimen

sions, usually in microns, of the oilparticles that do rise rapidly

enough to reach the surface, and

those that do not.My cutoff point is around 80

~ D r o ~ l e t Size. Imicrons

----------'%min 300

1 Vz min 150

8 Vz min 60

35 min 30

2 % hrs 15

20 hrs 5

1200

Particle Size, Microns~ Stokes' Law - Time to Rise fo r 80Microns (minutes)

• Oil that reaches

the surface is in

the form of larger

droplets and is

best removed by

one of theenhanced-gravity

separators.• Oil that does not

reach the surface(smaller droplets)

is best removed

by centrifugal

separators.

40

10000

III

1000::lC

1:Ill" 10041J :u

.5N 10

...

41III

ii:o..41

Ei= 0.10

o

THE BASIC CONSTRAINT

The basic constraint iswhether or no t

the oil reaches the fluid surface of thecleaning tank or rinsingvessel.

• enhanced-gravity separation (ofwhich there are at least two types);

• centrifugal separation; and

• coalescers.

Collected Oil Skim

~il-Water

EntryPoint

Figure 1:Larger droplets risefaster inoilthan dosmallerones,whichmaynot riseat aiL

THE BASIC CAPABILITIES

There are three types ofoil skimmers

useful for removal of oil from aque

ous cleaning baths so that parts areno t reinfected with oil:

THE BASIC SITUATION

Shown in Figure 1, oil-based soils

are:

• removed from parts in anaqueous cleaning bath,

• broken into small pieces duringinternal fluid processing,

• reassembled due to gravitational

forces into a surface skim at th e

top of aqueous baths, therefore

• resoUing parts within cleaning orrinsing baths, or on removal

through their fluid surface.

In the June column, I covered the

problem oil skimmers are intended

to solveby keeping cleaned parts from

being infected with tiny oil droplets

previously removed from parts.

In the July/August column, I cov

ered three types of oil skimmers. In

this space I will propose criteria forselection of the right one for differ

ent applications.

www.metalfinishing.com September2008 Imetalfinishing I 59

7/29/2019 Cleaning times The second cleaning process The oil skimmer Part III.pdf

http://slidepdf.com/reader/full/cleaning-times-the-second-cleaning-process-the-oil-skimmer-part-iiipdf 2/3

cleaningtimesl _ ~ m Feature i Poor II Good il Better

IBest

IAbility to recover oil Gravity Coalescer EnhancedGravity Hydrocyclone

droplets smaller than 80Fig.2, july/Aug. 2008 issue Fig. 6, july/Aug. 2008 issue Fig. 4, july/Aug. 2008 issue Fig. 5. july/Aug. 2008 issuemicrons at a high level of

efficiency -$200 to $500 -$5000

Mater ial treated by Fluid rising to the Any fluid contact ing Fluid on t he surface Any flu id fed to deviceseparator surface of the tank device of a tank

Quality of oil-rich stream Fairly free of wate r Can be fairly free of water Very free of water No t free of water

Qual ity of water -rich Somewhat free of oil Not well freed of oil Can be fairly free of oil Very free of oi l

stream

TabLe 2:ComparisonofOiL-Water SeparationSystems

microns. See Table 1, and the valuesalso calculated from Stokes' Law

and graphed in Figure 2, for the rea

son behind this choice. This situa

tion is shown in Figure 3.

COMPARISON OF TYPES

Table 2 shows a comparison of all

three skimming systems. Droplet

size of immiscible oils in effluents

from cleaning baths is no t a factor

generally controllable by the operato r or designer of the cleaning sys

tem. Managers of cleaning systems

are pleased to remove all oil (non

water-soluble soils) f rom par ts and

do no t normally care what physical

size the oil takes in the waste water.

Therefore, managers select separa

tion systems based upon the nature

of oil distributions they f ind within

and around aqueous cleaning

machines.

Oily water separation efficiency

for all three separator types is high

est with large oi l droplets. Very

small droplets are more difficult toseparate.There is a reason to prefer one

type of oil-water separator over

another: the ability of systems to

recover smaller-sized droplets of oil

from a "slurry" of oil particles in

COMPLETE FINISHING SYSTEMS

Ovens • Washers • Conveyors

Engineered Systems

Superior f inishing SlJscems.

estoblished in 1950. offe rs

a full turn-keu approach

blJ providing all the

elements essenool co

lJour coacing needs

• Pre-creacmenc Washers

• Ovens

• SpralJ [)oochs & touiprnenc

• Conveqor Lines

• Dorch SlJscems

• Dip Tonks

• Recrofics. Porcs& Serv ice

• Silicate-free

• Heavy-duty

• Soak and Ultrasonic

• Amine-free

• Biodegradable

• Free rinsing

• Proven performance on steel ,

stainless steel, and copper alloys

tryRD·531Aqueous Cleaner

Struggling with Surface Prep

prior to Coating or Bonding?,....

Test samples available

Tel.716.763.4343www.miraclean.com

@SUPERIOR FINISHING SYSTEMS, LLC

Phone : 336 956 20 0 0 • fa x: 336 956 5635

E-ma il: superiorsys@cr iadb iz.rr.com

www.superiorfinishingsystems.com

Circle055 on reader information card or goto www.metalfinishing.com/advertisers Circle049 on reader information card or goto www.metalfinishing.com/advertisers

September2008 Imetalfinishing I 60 www.metalfinishing.com

7/29/2019 Cleaning times The second cleaning process The oil skimmer Part III.pdf

http://slidepdf.com/reader/full/cleaning-times-the-second-cleaning-process-the-oil-skimmer-part-iiipdf 3/3

cleaningtimes TYPICAL SEPARATOR PERFORMANCEOil-Wa te r Systems

Figure 3: Performance datafor all three typesof oiL separation devices areshown.Datacourtesyof

ULtraspin (www.uLtraspin.com.au!TutoriaL-4.htm). which manufactures hydrocycLoinic oiL-water separa-

tors.Gravity-based systemsworkwell with Larger oiL dropLets, andthe usefuL rangefor hydrocycLone

machines iswherethe oiL dropLets havebeenhighLy sheared into tiny ones. FortunateLy, this situation is

rarein metaL cleaning. Note: thesedata suggest that gravity-based systemsareusefuL above-80

micronsin hydrauLic particlediameter.

1 V - /I /' JV

V /'

I /' V) ~ ~

Oil-Droplet Size, Microns

1604020

-0 - Hydrocyclone

1000

If any reader has available dataabout recovery of materials and its

financial impact on their operation,

I would be pleased to collect it, and if

suitable, convert it into a subsequent

column.

810

John Durkee is the author of the book

Management of Industrial Cleaning

Technology and Processes, published

by Elsevier (ISBN 0-0804-48887). He is

an independent consultant specializing inmetaland critical cleaning. You can con

tacthimat POBox847, Hunt,TX 78024

or122Ridge RoadWest, Hunt, TX 78024;

(830) 238-7610; Fax (612) 677-3170; orat jdurkee@precisioncleaning.com.

6000

100

80

60

40

20

o

o

~ Gravity system ........ Coalescer

manufacturer of the cleaning

machine, and, therefore, no t manu

factured by them.

Alternatively, as readers of the

advertising in this magazine know,

separation devices may be pur

chased as stand-alone equipment

and integrated into existing equip

ment by their owner. That is often

the bes t way to go about a system

upgrade. These devices are small

enough that one can see them

demonstrated at a trade show.

SUMMARY

This three-part series has covered

the dominating factor controlling

selection and use of an oil skimmer

(droplet size), the three types ofcommercially available skimmers

(gravity, centrifugal, or coalescer),

and in what circumstances each

should be used (when droplet size is

matched to capability).

PURCHASE OF AN OIL SKIMMER

These separation devices are pur

chased as add-on systems by the

water. This is shown in Figure 3,

where the performance of gravity

based, hydrocyclonic, and coalescing

units are compared on a consistent

basis by particle size.

RECOVERY OF MATERIALS

Material recovery has become an

important aspect of performance

quality with regard to aqueous

cleaning machines.

When the primitive skimming

device shown as Figure 7 in my June

column was chosen by users, all they

wanted was for most of the oil to dis

appear and for cleaned parts no t tobe reinfected with this oil.

Today, standards are higher. Both

the recovered oil and water haveacknowledged financial value to an

enterprise. Oftentimes, some of that

value is avoidance ofan environmen

tal-related cost.

But of even greater value is that of

the specially formulated aqueous

cleaning fluid dissolved in the water.

Costing $10 to $50 per gallon as a

concentrate, because it cleans at a

near-neutral pH and separates clean

ly at a desirable temperature range,

recovery of these fluids is essentialto the financial health of the clean

ing system.

Today, skimmers are more often

justified based on the value of recov

ered materials, and not just on the

cosmetic or sanitary value of remov

mg a mess.

THOSE IN THE KNOW READ MFW ...

The Metal Finishing Weekly newsletter keeps readers informed in between

publication of the monthly magazine.

The best part is it's delivered directly to your inbox-and it's all free! But you

must opt in!

(Visit metalfinishing.com to register.)

www.metalfinishing.com September 20081 metalfinishing 161