Blog Purpose

The CTG Technical Blog is intended as a source of information on subjects related to industrial and precision cleaning technology. The writer of the blog, John Fuchs, has 40+ years of experience covering the entire gamut of cleaning. Mr. Fuchs has extensive knowledge of ultrasonic cleaning technology having been employed by Blackstone-Ney ultrasonics and its predecessors since 1968. The blog is also intended to serve as a forum for discussion of subjects related to cleaning technology. Questions directed to the blog will be responded to either in the blog (if the topic has general interest) or directly by email. Emails with questions about the current blog should be entered in the comments section below. Off-topic questions related to cleaning may be sent to jfuchs@ctgclean.com.

Automation – Load and Unload

June 9th, 2014

This series of blogs about automation will address automation as an “add-on” or accessory to an existing or planned cleaning system.  In many cleaning systems, automation, or at least partial automation, is an integral part of the system as it is required for effective cleaning process.  This is especially true of spray washers which often rely on part rotation to allow spray access to all surfaces of the part(s) being cleaned.  But even with systems which already have automation, adjuncts like conveyors and lift tables can be added to facilitate or simplify material handling.

Load and Unload Locations

One of the basic considerations for an automation system is where the parts will be picked up and where they will be delivered.  Depending on circumstances, load and unload stations can be arranged to facilitate the flow of parts through the manufacturing process – as long as the parts are being moved anyway, why not utilize the move to your advantage?  For example, in some instances, it may be best to have the load and unload stations immediately adjacent to one another to eliminate the need for a single operator to walk from one end of the machine to the other to load and unload parts.  In the case where there are two operators, however, delivery to a downstream location may be a desired feature.  One example of this might be when cleaned parts, once cleaned, are delivered directly into a clean room or packaging area.

To assure uninterrupted process flow, it is often beneficial to provide a conveyor at the load and/or unload positions capable of queuing 20 to 30 minutes of production ready for cleaning or transfer to the next step.  This is usually accomplished using either a simple gravity conveyor or a powered conveyor with sequencing capability.  When space is limited, load and unload conveyors can be positioned perpendicular to the flow of parts through the system to reduce the length of the overall machine.

Other considerations for the load and unload stations might be a lift table or hoist in the case of large or heavy parts or a demagnetizer for parts which might hold residual magnetism from the manufacturing process.

One thing that is often overlooked in the design of automation is that parts baskets or fixtures used to process parts through the cleaning process need to be returned to a location prior to cleaning for reloading once emptied.  In a high volume cleaning operation, a secondary conveyor might be used to return baskets from the unload end of the machine back to the load station or another point prior to cleaning.

Although the above considerations may seem trivial, they provide the starting point in the design of an automated cleaning system.  Just as important (but more obvious) are knowing the exact cleaning process as well as the required system throughput in terms of the number of baskets or fixtures of parts to be processed.  In the next several blogs I will describe a number of automation options along with their benefits and shortcomings.

-  FJF  -

Automation – Introduction

June 3rd, 2014

Many cleaning systems are automated.  There are a number of benefits that can be realized through automation -

  • Reduced labor cost
  • Increased throughput
  • Improved process consistency

Reduced Labor Cost

The cleaning process is inherently labor intensive.  Parts to be cleaned must be prepared and fixtured or put into suitable carriers.  They must then be moved through the steps of the cleaning process following a prescribed program of times and temperatures.  Features including such things as ultrasonics, sprays, rinses, etc. must be activated as required.  Finally, parts must be removed from baskets or fixtures and moved on to the next process step.  Granted, it is difficult to remove all labor from the cleaning process but automation can reduce the labor requirement significantly.  Automation, for example, can move baskets or fixtures through the cleaning process leaving the operator free to perform other tasks.  Automation can also be utilized to initiate process functions as necessary to assure proper processing.

Increased Throughput

Although automation can not increase the capacity of a cleaning machine already running at full capacity, it can assure that the ultimate capacity is met on a consistent basis.  A manually operated machine, even with automatically timed process steps, is often not attended full time.  Operators are distracted with other things, take breaks, etc. resulting in usable machine time being wasted.  Parts, for example, staying in a tank for a time beyond that required by the process will detract from overall process efficiency.  Automation can help minimize this inefficiency especially if provisions are made to queue parts prior to and after processing so that processing can continue even if the operator is absent or distracted for short periods of time.

Improved Process Consistency

Although often not considered as a major motivation for automation, when process consistency is important, automation is almost a necessity.  Properly designed automation can assure that each part processed gets the proper treatment.  This is especially true in cases where different parts processed through the same system require different treatment.  In most cases, process times migrate toward the maximum of all process steps (time, temperature, etc.) in the interest of assuring that all parts receive sufficient treatment.  In cases where over-treatment is a possibility, all parameters will migrate toward their minimums in a manually operated system to avoid the possible consequences of over-treatment.

Where to Start?

The exercise of defining an automation system can be a daunting one indeed.  The decision making process is seldom well-defined nor is it intuitive.  A good starting point is a full understanding of the process, the geometry and number of parts being processed and, finally, the “culture” of the manufacturing operation surrounding the cleaning system.  By “culture” I mean things like what is the process flow, what is the lot size, can part lots be mixed or must they be kept separate, are the parts to be processed handled in baskets or individually, is cleaning continuous or intermittent, etc.  The type and layout of an automation system will, in part, be a result of these and (often) other factors as well.  In upcoming blogs I will attempt to better define various automation options and how one goes about the decision making process for automation.

-  FJF  -

 

What is a “Closed Loop” cleaning system?

May 23rd, 2014

The ultimate “closed-loop” cleaning system would produce no effluents requiring disposal. In many manufacturing facilities, disposing of waste liquid containing chemicals and other contaminants removed from parts being cleaned is either very costly or just not possible using municipal sewer or other available facilities.  Even in the best cases, most cleaning system effluents require pre-treatment prior to disposal.  Municipalities frequently monitor waste coming from manufacturing locations on a connection by connection basis and, believe me, you want to be in compliance or you will pay!  In many instances, the only solution is to have collected waste hauled away for treatment – an expensive option unless your effluent stream happens to include precious metals such as gold or silver!

A “closed loop” cleaning system may reduce the cost of disposal of used process liquids significantly or, in some instances, completely.  The basic concept is that water is used in progressively dirtier process steps and, in the end, is disbursed by evaporation as water vapor.  In order to successfully accomplish this goal, water usage must be minimized.  The article Ten Minutes to better Rinsing demonstrates how water usage for rinsing can be significantly reduced by using a reverse cascade system.  If the required amount of water required to refresh the cascaded rinse tanks can be reduced sufficiently, then it may be possible to use the overflow from the first (final in the sequence of flow) rinse tank to makeup the cleaning tank.  If the evaporation rate from the cleaning tank is sufficient to vaporize all of the liquid received from the rinse tanks then a “closed loop” system has been achieved.  There are no effluents.

The problem with this scheme, of course, is that eventually all contaminants removed from the parts being cleaned will end up in the cleaning tank.  Although solid contaminants (particles) may be removed from the cleaning tank by filtration, water soluble or emulsified contaminants will continue to build up in the tank until cleaning is no loner effective.  Therefore, even in a well-engineered “closed loop” cleaning system removing contaminants that can mix with water, the solution in the cleaning tank will occasionally need to be dumped and replaced.  Disposal of the dumped, contaminant-laden liquid can be achieved by evaporating the liquid in an evaporator, by neutralizing it and sending it to drain, or by hauling it away for disposal.  In any event, there is a distinct benefit to reverse cascade flow even if a totally “closed-loop” system is not achieved.

Water is an expensive commodity.  Disposing if it once it’s used is often even more expensive.  Any means that can be put to use to reduce water consumption, therefore, reduces the cost of cleaning.  Although not applicable in all situations, consideration of a “closed-loop” system may be well worth the effort.  In most cases, it will mean that the initial cost of the equipment is higher, but the higher cost is repaid multi-fold by the savings in water and waste disposal.

-  FJF  -

 

Ultrasonics – Frequency vs. Exposure Time

May 13th, 2014

The blog about maximizing the effect of multiple frequency ultrasonics explained in some detail the mechanics of cleaning using multiple ultrasonic frequencies and the benefits of sequencing through a series of frequencies multiple times to achieve maximum cleaning effect.  This blog will further explore these benefits especially in cases where extended exposure of a part to certain frequencies and power intensities may cause damage to the part.

It is well known that lower ultrasonic frequencies have a higher likelihood of damaging soft substrates including aluminum, brass and others.  This is mainly because lower frequencies produce larger cavitation bubbles which implode releasing more energy than the smaller cavitation bubbles produced at higher frequencies.  It is also well known that if there is a tendency for a particular frequency to damage a substrate, the potential for damage is increased with longer exposure times.  Most substrates survive short exposures to lower frequency ultrasonics without damaging effects.  Yet, the value of using lower frequencies can not be denied.  Using multiple discreet ultrasonic frequencies allows utilization of lower frequencies while minimizing the potential for part damage by limiting the exposure time and/or intensity at lower frequencies.

For example, consider a part that is contaminated with particles of varying sizes which requires the use of multiple ultrasonic frequencies to effectively clean.  Also, let us assume that this part is susceptible to damage due to cavitation erosion if exposed to high power low frequency ultrasonic energy for an extended period of time.  A microprocessor controlled multiple frequency ultrasonic cleaning system offers several options.

1) A frequency sequence that limits the exposure time at lower ultrasonic frequencies.  This provides the benefit of lower ultrasonic frequency to remove larger-sized particles while staying below the time limit threshold for part damage.

Multi-Frequency sequency with varying time

In this example sequence, the time at lower frequencies is reduced to prevent cavitation damage while still benefiting from the full range of frequencies available.

2) A frequency sequence limiting the exposure intensity at lower frequencies that may cause part damage.

Multi-Frequency sequence with varying power

In this example sequence, the power at lower frequencies is reduced to prevent cavitation damage while still benefiting from the full range of frequencies available.

3) There may also be instances where both time and power as well as the sequence order itself is beneficial.  The following illustrates a sequence utilizing all of the variables.

Multi-Frequence sequence with all variables utilized

In this example, sequence, time, power and sequence order are all varied.

Realize that the above only illustrates the use of a three frequency system.  Today’s ultrasonic equipment offers as many as 7 frequencies depending on the manufacturer and model.

The next question, of course, is which combination of variables works best for what application.  Unfortunately, this is, as yet, uncharted territory.  There are “indicators” which suggest that particular sequences are more effective than others but the proof, as always, is in the result.  My personal guess is that frequency sequences, time and power will, in time, become as important as today’s time, temperature and chemistry options.  As cleaning requirements become more and more challenging it is good to at least know that the options are available even if not fully explored.  I am sure that diligent researchers will utilize these options to achieve the desired result.

-  FJF  -

 

Ultrasonics – Applying Multiple Frequencies for Maximum Benefit

April 9th, 2014

As described in a preceding blog, changing ultrasonic frequency has a demonstrated effect on the size of particles removed from a contaminated surface depending on frequency. Low frequencies produce large cavitation bubbles which implode with high energy.  High energy implosions are able to dislodge relatively large particles from a surface. Higher frequencies produce smaller cavitation bubbles which, although they implode releasing less energy, are able to remove smaller particles as they are able to penetrate the “barrier layer” found near the surface being cleaned.  This knowledge and the relationship of particle size vs. the ultrasonic frequency can be very useful when contaminants consist of consistently sized particles.  In the real world, however, a particle population is likely to consist of particles of varying sizes in which case the frequency/particle size relationship becomes a bit more complex.

Being aware of the benefits of using multiple frequencies for cleaning, many users employ ultrasonic equipment which is capable of delivering ultrasonic energy at a number of discreet frequencies.  The general notion is that one first subjects a part to cleaning at one frequency and then at another to derive the benefits of multiple frequencies.  A model can be put forth, however, that tells us that a better way would be to sequence repeatedly through a number of frequencies.  Let me explain – -

First, we must assume that the contaminant involved consists of a collection of particles of varying size distributed on a surface.  Further, let us assume that the particles have a natural attraction to both the substrate (surface being cleaned) and to each other.  The illustration below shows such a situation.

Illustration showing a surface contaminated with particles of varying size

The substrate (surface being cleaned) is contaminated with particles varying in size. The smaller particles in this illustration would, typically, be 1 micron or less in size.

The effect of first applying a lower frequency will be to dislodge and remove the largest particles on the surface as well as some of the smaller particles near the surface.  The remainder of the smaller particles on the surface are not affected as a result of the barrier layer which is created by the remaining larger particles below as shown in this illustration.

Showing the effect of the first application of low frequency ultrasounc

First applying a low ultrasonic frequency results in the removal of large particles on or near the surface of the contaminant layer.

Applying a higher frequency will remove some of the smaller particles remaining on the surface.  However, due to their lack of intensity, the larger particles will remain as shown below. 

After the application of low and high frequency ultrasonics

A higher frequency will remove smaller particles from the surface of the contaminant layer but leave more larger particles exposed.

The exposed layer of larger particles is removed by a subsequent application of lower frequency ultrasonic energy.  However, smaller particles thus exposed are not removed due to the barrier layer effect.

Second application of low frequency ultrasonics

A subsequent application of low frequency ultrasonic energy removes more large particles exposing more small particles below.


 Remaining small particles are removed by a subsequent application of high frequency ultrasonic energy.

Removal of smaller particles using a second application of high frequency ultrasonic energy.

Remaining small particles are removed by again applying high frequency ultrasonic energy.

The message of the above is that simply applying one, two, or more ultrasonic frequencies in succession may (likely) not produce the required cleaning effect in many situations especially if the particle population is made up of particles of differing size.  Repeated sequential application of frequencies may produce a better cleaning result.  The appropriate “recipe” of frequencies and exposure times can be developed by conducting cleaning trials and comparing the results using various combinations of frequencies.

-  FJF  -

 

Ultrasonics – The What and Why of Sweeping Frequency

March 24th, 2014

Note – The following blog is adapted from a paper recently written by Timothy Piazza, President of Blackstone-NEY ultrasonics and is re-printed here with his permission.  This would probably be a good time to mention that guest blogs or suggestions for blog topics are always welcome.  My email is jfuchs@ctgclean.com.  FJF

Blackstone~NEY Ultrasonics – Sweep Frequency
By Timothy Piazza Ph.D.

By way of introduction, and to put the subsequent discussion into context, an operational review of the equipment is required.  All modern ultrasonic units are characterized by the center frequency at which the unit operates (effectively the average frequency of operation) as well as what is called the sweep bandwidth, and sweep frequency. In ultrasonic cleaning, sweep is defined as a purposeful deviation from the center frequency during operation as a function of time.  Typically, this deviation occurs symmetrically about the center frequency. The driving frequency is continuously varied within some window, about the center frequency.  The following provides a graphical example of the frequency modulation commonly referred to as sweep.

Illustration showing frequency sweep with a regular period and frequency excursion.

Frequency sweep with a fixed frequency deviation and sweep rate.

In this example the center frequency is 104 kHz with a sweep bandwidth of 4 kHz (f =104 kHz ± 2 kHz).  The rate at which the frequency changes is known as the sweep rate.  In Figure 1 the sweep rate is 500 Hz.  Additional information including an audio clip of the sound of sweep is available in a preceding blog.

Sweeping the ultrasonic frequency has been demonstrated to greatly enhance cavitation activity, and thus the ability of a given system to effectively clean.  This results primarily for two reasons.  The first is that ultrasonic transducers are electro-mechanical devices with manufacturing tolerances.  As a geometrically driven quantity, resonant frequency is strongly dependent upon the size and shape of a transducer.  Manufacturing tolerances create a natural variation in the center frequencies of each individual member of a multi-transducer ultrasonic array.  Thus there is no single “center” frequency, but instead a distribution of center frequencies characterizing an array.

Illustration showing that use of a single frequency results in unequal transducer output

Sweeping frequency promotes uniform intensity from an array of transducers with slightly varying frequencies.

Use of non-sweeping ultrasonics can only maximally excite one or a few of the transducers in an array, i.e. those whose center frequency happen to match the driving frequency.  The other transducers in the array will be acoustically dim, with lower power output, in comparison.  This leads to uneven sonication and cleaning within a tank.  Constantly changing the frequency via sweep ensures that all transducers are excited at their center frequency during the sweep cycle.  Also, if the sweep rate is sufficiently high, compared to the lifetime of a sound wave in a tank, the result is a uniform acoustic field and cleaning.  The second reason for sweep is that the introduction of a “band” of frequencies excites a significantly larger bubble population, resulting in shorter degas times, as well as enhanced cleaning.

Historical Note – The effect of non-uniform excitation of an array of transducers operated at a fixed frequency was recognized decades ago.  In an effort to correct the problem, some ultrasonic manufacturers sorted assembled transducers by resonant frequency prior to bonding to minimize variations in frequency.  Subsequently, it was discovered that the resonant frequency of a transducer changed in an unpredictable way once it was bonded making this effort futile.  The simple and elegant solution of sweeping frequency to provide uniform transducer output was invented by William Puskas in 1988.

The above is intended to demonstrate that there is no single frequency that can be used to characterize an ultrasonic cleaning tank.  Indeed, in a correctly operating system, there are a range of frequencies present in a tank at any given time.

 

Chemical Concentration – Economic and Process Considerations (cont.)

March 10th, 2014

My previous blog addressed the chemical cost of using too much (or perhaps too little) “soap” in a cleaning process based on chemical cost.  Today we look at process issues.

You might be saying, “What the heck?  So I use too much chemical.  Soap is cheap and I look at it as “insurance”.”  Well, that “insurance” might not be as insuring as you think!  Excess chemistry can lead to a number of related issues including but not limited to the following – -

Physical Effects -

Most cleaning chemistries are soluble (or miscible) in water only up to a given concentration.  This is true of both liquids and powders.  Excessive chemical concentration may result in the separation or stratification of liquids or, in the case of powders, failure of the chemical to completely dissolve thereby leaving sludge in the cleaning tank or reservoir which may interfere with the proper operation of pumps, nozzles and filters as described in more detail below.  Excessive concentration of liquid chemistries may result in a “slick” of surfactant on the liquid surface which may add to part contamination.

Equipment -

Most cleaning equipment is designed for use with a particular level of chemistry concentration.  Things like pumps, valves, pressure and level sensors and filters, are selected with the assumption that they will operate in a “friendly” environment (unless it is known in advance that other conditions can be expected).  Pumps, as one example, require different materials of construction and seals for use with a 50% solution of sodium hydroxide than they do for use with a 5% solution of the same chemistry.  Improperly specified optical, capacitive or conductivity-based level sensors may be “blinded” by excess chemical buildup.  I the case of the more conventional mechanical type level sensor, the mechanism may get “gummed up” with residue from excess chemicals over time.

Spraying systems and  overflow weirs are, likewise, designed for a particular concentration of chemistry.  Excessive use of chemistry and the resulting physical changes in the cleaning solution may cause plugging of spray nozzles or excessive foaming in pumping systems as well as overflow weir reservoirs.  Yes, these problems are typically a result of radical chemical overuse but there are others that can result from relatively minor changes in chemical concentration.

Rinsing -

The more soap used in cleaning, the more difficult it is to rinse cleaned parts to remove the soap residue remaining from the cleaning process.  A relatively small change in chemical concentration can have a significant impact on the effectiveness of a rinsing process.  And even if the rinsing is adequate from a equipment standpoint, one must consider the increased cost of makeup water to maintain rinsing effectiveness and, finally, the cost of waste disposal (since all of that soap has to go someplace).

The proper use of cleaning chemicals is an important part of a successful cleaning process.  The best “early warning” of excess chemical concentration is usually found in the rinse.  If parts are not rinsing properly (streaks, haze, fogging) there is a good chance it is due to an excess of cleaning chemistry.  You could, of course, “fix” the rinse by increasing the makeup flow, time, or any one of several other parameters but, as I have stated frequently, the root cause of the problem may be found upstream from the point of its manifestation.  Don’t overlook the possibility of excess chemical concentration in the cleaning stage.

-  FJF  -

Chemical Concentration – Economic and Process Considerations

March 7th, 2014

I have talked before on the blog about the subject of chemical concentration and its relationship to cleaning.  A couple of recent incidents prompt me to re-address the subject of chemical concentration but from a little different angle.

It’s a “no-brainer” that cleaning chemicals are expensive and, with the possible exception of heat and labor, probably top the list of the ongoing costs of industrial cleaning.  It makes sense, then, to minimize the consumption of cleaning chemistry when possible.  We have all heard the stories about over-use of cleaning and other chemicals in the home.  For example, I have seen reports that indicate that, on average, we use at least 4 times more laundry soap and dish detergent than necessary to get our clothes and dishes clean, and at least twice the amount of shampoo and hair conditioner needed to keep our hair shiny and fresh-looking.  In fact, there is a growing belief that toothpaste isn’t required at all for dental health!!  This over-use of chemicals has been compounded by the fact that there is substantial evidence that householders have not reduced consumption to compensate for the new “extra strength” formulations. It should come as no surprise that the situation is not substantially different in the world of industrial cleaning with the prevailing belief that “more is better.”

Most technical data sheets for chemistry suggest appropriate ranges of temperature and chemical concentration for best results in typical applications.  Although well-intended, these recommendations can be misleading especially in cases involving a range of contaminants or if there is a change in the cleaning chemistry.  Many of the cleaning chemistries today are multi-purpose meaning that they can be used on a variety of substrates to remove a variety of contaminants.  The recommended use temperature and concentration provided in the technical data sheet, in most cases, brackets the entire range of uses often not supplying specific recommendations for any of them individually.  In most cases, concentrations on the lower end of the spectrum are adequate to get the job done.  An important step in reducing cleaning costs is to determine what concentration is required to get the parts clean and to put a procedure in place to assure that the requirement  for chemistry is met but not exceeded.  Instructions like “add two scoops of soap” or (and yes I’ve heard this) “add three glugs from the gallon jug” do not promote consistent and economic use of chemistry.  Also, the practice of adding chemistry to “refresh” the cleaning solution may or may not be appropriate depending on the cleaning mechanism of the chemistry being used.  If chemistry is being consumed in the cleaning process, yes, but if the addition is to compensate for the fact that the cleaning solution has become contaminated with soils removed by the cleaning process then the answer is no.  In this case, the cleaning solution should be discarded and replaced.  Otherwise it is akin to taking a bath in previously used bathwater – Yuck.

Despite the above, the final message here is not that everyone should use less cleaning chemistry but, rather, that cleaning chemistry should be used appropriately and in a consistent manner.  Experiment with reducing or increasing the concentration of chemistry in your cleaning bath.  If you can achieve the same result with less chemistry, why waste money!  In the next blog I’ll discuss some reasons other than $$ to know and understand your cleaning chemistry and use it appropriately.

-  FJF  -

 

Cleanliness Testing – White Glove and Swab Tests

March 4th, 2014

I have spent considerable time on the blog disclosing and discussing a variety of cleanliness testing methods.  A couple of tests that escaped earlier discussion, however, are the “white glove” test and the closely-related “swab” test.  These tests are conducted by rubbing or wiping a surface using a white (usually cotton) glove or a cotton swab (like a Q-tip).  After wiping, the glove or swab is observed for any evidence of residue having been left on the surface being tested.

Back in the 1950′s, these tests were quite popular and were, indeed, considered the “acid tests” for cleanliness.  In fact, they were, and in some cases still are, pretty good indicators of cleanliness in a number of situations but there were, and still are, limitations using this type of test technique.

The first of such problems is access to the contamination.  The white glove is useful on flat surfaces and the swab is useful both on flat surfaces and in bores (ID’s, tapped holes, etc).  Neither, however, is able to reach into a confined recess (for example) to extract a sample of contaminant.  Cleanliness testing using these techniques is limited to surfaces which can be directly accessed.

Another problem is that, for the test to be meaningful, the contaminant must be of a color that will contrast with the white surface of the glove or the swab.  A clear or white contaminant may confound the test completely indicating that a surface is free of contaminant when, in fact, it is not!

The test is severely subjective depending on the acuity of the eyesight of the person evaluating the test and, to some degree, on the observing conditions during evaluation.  Contaminants, for example, are not as easily seen in the relatively dark confines of a factory environment as they might be in a well-lit laboratory.  Even more powerful lighting (sunlight) may reveal discolorations that may not be seen otherwise.

The above reveal instances in which these tests may provide a false indication of cleanliness.  There are, however, cases where these tests may falsely indicate a contaminated surface. Some surfaces, notably aluminum, lead and graphite or carbon, will discolor a glove or swab even if they are “completely clean.”  These surfaces, when abraded, release particles of the base substrate which, depending on the definition of “clean” may or may not constitute contamination.

These tests are also very sensitive to the pressure and aggressiveness of the person doing the test.  Repeated abrasion (even in the same location) or excessive pressure may cause discoloration of the glove or swab while a more gentle approach will not.  These things are very difficult to control.

Despite all of the above, the white glove and swab tests are amazingly popular even to this day.  As I stated earlier, they are appropriate in specific cases but their limitations must be recognized.  As a “quick check” for cleanliness in relatively non-critical applications these simple and inexpensive tests may be all that is required.  In critical applications, however, it would be best to employ techniques that are more reliable and accurate providing a quantitative result that can be documented.

-  FJF  -

Other Oil Removal Options

February 28th, 2014

In some cases, previously described oil removal technologies based on the gravity separation of oil from the cleaning solution are unjustifiably cumbersome and expensive.  Fortunately, smaller scale solutions are available for use in such applications.

Oil Skimmers -

Oil skimmers utilize a material which preferentially attracts oil (hydrophobic) to skim floating oil from a surface leaving the majority of any aqueous component behind.

Illustration showing the principle of an oil skimmer

Oil adheres to a hydrophobic surface as it is withdrawn through a layer of oil on the surface of an aqueous based liquid.

The oil, which coats the surface of the attractant is then removed from the surface and collected for disposal or re-use.  The oil-attracting material is usually configured as a belt or disc which is partially submerged in the liquid being skimmed.  A squeegee or scraper is used to continually remove oil for disposal or re-use.  A typical example is shown below.

Illustration of a belt-type oil skimmer

An endless belt driven by a motor (not shown) collects oil from the liquid surface. A scraper or other device removes the oil from the traveling belt.

Devices like that shown above work well in many relatively non-critical applications to remove oil.

Oil Absorbing “Filters” -

In more critical applications additional means such as oil absorbing filters are required for complete oil removal.  Such “filters” are similar in construction to those used to filter out particles and, in fact, in some cases may remove both particles and oil from a recirculated solution.  They contain materials which absorb oil and reject aqueous liquids.  Limitations include the fact that most of these devices have only a limited capacity to remove oil.  Once the absorbant has become saturated, the canister (filter) must be discarded and replaced. This, of course, is not a practical solution if there is a large amount of oil present.

Ultrafiltration -

Under specific and controlled conditions, ultrafiltration may be utilized to remove oil from a processing bath even if it is in the form of an emulsion.  Basically, the cleaning chemistry must be designed to have a molecular size smaller than that of the oil being removed.  Oil molecules are typically rather large compared to those found in many cleaning chemistries.  To filter out the oil molecules typically requires the use of a filter made of porous stainless steel (or other material) which is coated with a material which provides the very small passages required to allow water and cleaning chemistry to pass while retaining the oil molecules.  Problems with this method of oil removal include the possibility of clogging of the filter media.  I will provide a more thorough discussion of ultrafiltration as a means of oil removal in a later blog.

In summary, oil removal is a major consideration in any cleaning process where oil is present as a contaminant.  Selection and implementation of the proper oil removal means will determine success or failure in many applications and should be considered early in the design phase of any cleaning equipment.

-  FJF  -