cleaning times the second cleaning process the oil skimmer part iii.pdf
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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.
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""""""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
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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
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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 [email protected].
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.
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