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.

The Trouble With Watts – Efficiency

June 2nd, 2015

One of the problems with using the number of watts consumed to produce a particular output of another form of energy (light, motion, heat, etc) is that there are always losses when one form of energy is converted to another.  Although the law of conservation of energy always applies, energy lost in conversion to other forms of energy is not always obvious.  Let’s consider devices that convert electrical energy into light as an example.

There is no better example of the trouble with watts than we see in the lighting industry today.  When Thomas Edison invented the first practical incandescent light bulb, he rated his bulbs in candlepower.  In his wisdom, he knew that the “deliverable” was the amount of light produced.  Also in his wisdom, however, he knew that the customer would use a certain amount of energy to produce that light – - and guess who provided the energy.  He quoted new lighting installations based on so many light fixtures each providing the equivalent light of so many candles as it was the amount of light produced that was important to the potential buyer.  As time passed, the candlepower rating gave way to watts as a way to rate light bulbs.  Although I have not been able to find a clear explanation of why this happened, I suspect that the cost of electricity had something to do with it.  Knowing the rate of energy consumption of each bulb provided a way to determine how much energy would be used overall.  This determination, of course, would allow for the time that each bulb would be illuminated and could be converted directly into cost.

Despite his best efforts, Edison’s first light bulbs were not very efficient.  It takes a lot of energy to heat a piece of carbonized bamboo to incandescence with a lot of the energy ending up as heat.  Over the years, there were continued efforts to produce more light using less energy.  Over time, the efficiency of the light bulb did improve but, having become a convention, watts remained the way of expressing the light output capacity of a light bulb.  In simple terms, a 60 watt light bulb, for example, became a bit brighter with each improvement.  The changes in efficiency were not huge so people were happy to continue to use watts as a way to, indirectly, define the brightness of a light bulb.

In more recent years, starting with the invention of the fluorescent bulb, improvements in efficiency became significant enough to begin the downfall of the watt as a means to express the brightness of a light source.  At first, the unique design of fluorescent lamps (tubes instead of bulbs) provided enough distinction that the inequality of watts vs. brightness wasn’t an issue.  However, as technology advanced even further to light emitting diode light sources and fluorescent lamps took on a form emulating that of the incandescent bulb, the inequity in the relationship between watts and brightness became so significant that a new way had to be found to express the brightness of a light source that was not linked to its energy consumption.  As a result, light bulbs today are rated in lumens with watts being used as an indication of efficiency.  One lumen is defined the amount of light that falls on a one square foot area at a distance of one foot from a burning candle (right back to Edison).  Now, instead of shopping for a 60 watt light bulb (for example) we shop for a light source that produces a certain number (or range) of lumens.  The following chart shows some approximate ranges for comparison.

Light Bulb Watt Equivalents Revised

Watts have given way to lumens as a way of specifying light sources.  But this is just one problem with watts.  Others will be discussed in upcoming blogs.

-  FJF  -

Ultrasonic Machining – A New Use for Ultrasonics?

May 28th, 2015

Over the years, there have been several anecdotal references to otherwise unexplained changes in the properties of surfaces exposed to ultrasonic energy in a liquid.  In some cases, it would make sense that the change was due to increased cleanliness.  In others, however, the benefit of cleanliness alone would seem questionable.  One incident in particular sticks in my mind.

Many years ago, a printing company reported that ink rollers (not analox rolls but just smooth rolls) carried ink much more efficiently and uniformly after they were ultrasonically “cleaned.”  In a series of (not so) controlled experiments, it was shown that the use of a chemical cleaner played virtually no measurable role in whatever was happening.  Rolls exposed to ultrasonics using tap water showed the same change in ink carrying properties as those cleaned ultrasonically using a variety of chemicals.  It was finally deduced that ultrasonic exposure had “roughened” the surface of the steel rolls just enough to allow the ink to adhere more effectively.  At that time (30 or so years ago) we did not have the means to measure what exactly had changed when the rolls were exposed to ultrasonics.  No difference could be seen using a microscope - the result was strictly empirical but significant.

Contemplating the above over the years, I have come to the conclusion that there are two possible explanations for the change seen in the ultrasonically cleaned ink rolls – -

  • The implosions of cavitation bubbles may have created small “craters” in the surface which improved the “tooth” of the surface.
    or
  • The surface had been “machined” sufficiently to remove a thin skin of oxide or other barrier which otherwise prevented adhesion of ink.

Either of the above is logically possible based on our knowledge of ultrasonic cavitation and implosion.  But, the important thing at that time was not what happened but the fact that something happened.

Since that time there have been instances in which, although not quite as dramatic, I have seen evidence of changes in surface characteristics that are not consistent with cleaning alone.  But the purpose here is not to teach but to learn.

Ultrasonics is still a growing technology.  The more I know (after nearly 50 years) the more I find that I don’t know.  I see this potential use of ultrasonics under controlled conditions and with known parameters a rich opportunity to enhance coating technology and to improve bonding technology.  On a somewhat larger scale, we already commonly use sand blasting, as an example, as a way to increase bond strength when two surfaces are to be held together using an adhesive.  Maybe ultrasonic “machining” would provide a means to improve bond strength on a much smaller scale – maybe in optical coatings, semiconductor and other applications.  In fact, this effect may already be benefiting in instances where the operative term should not be cleaning but, rather, “machining.”

With the capability we now have to utilize a variety of ultrasonic (and megasonic) frequencies, power levels and waveforms, I believe it would be worthwhile to research this, as yet, poorly understood potential use for ultrasonics.

-  FJF  -

 

 

Ultrasonics – Revisiting Watts Per Gallon

May 21st, 2015

Watts per gallon as a measure of ultrasonic cleaning tank capability is under growing scrutiny.  In a series of blogs over the next several weeks, I hope to explore the origin of watts per gallon and the changes and ambiguities which have brought its continued use as a measure of ultrasonic capability into question.  Also, if watts per gallon is no longer a meaningful “yardstick” what might replace it?  Is there a more meaningful and appropriate way to measure power in an ultrasonic tank and, if so, what is it?  It seems there are lots of questions but no obvious answers regarding the use of watts per gallon as a unit of measure for ultrasonic tank capability.

The war of watts in ultrasonic cleaning has been raging since the very beginning of ultrasonic cleaning history in the 1950′s.  It was during this period of time, I suspect, that the first battle of the war began with each of a handful of ultrasonic manufacturers touting the power of their generators as an indication of the effectiveness of their ultrasonic cleaning tanks.  It was a fairly level battleground as most ultrasonic cleaning tanks were only a few gallons in size.  Watts per gallon became the accepted way to “rate” ultrasonic cleaning tanks.  In the beginning, the limiting factor was the amount of ultrasonic power that could be produced by ultrasonic generators which at that time used tube technology.  Semiconductors had not yet come on the scene.  Tubes, in general, were challenged to provide huge amounts of power at ultrasonic frequencies.  Magnetostrictive transducers of that era, however, were quite inefficient and forgiving and could accommodate much more power input than could be provided by tube type generators.  So, the battle of watts in the beginning, I believe, was all about generators.  Powerful generators meant powerful ultrasonic activity and superior cleaning results.

Todays ultrasonic cleaning systems are far advanced from those of the 1950′s.  Now, nearly all ultrasonic generators utilize semiconductor technology which is capable of delivering huge amounts of power at ultrasonic frequencies without much difficulty.  Most ultrasonic transducers are of the piezoelectric type which, although highly efficient, are, typically, rated for an input power of around 40 watts per transducer element.  This has led to the ultrasonic transducers being the first limiting link in the ultrasonic chain.  Meanwhile, many users of ultrasonic systems still view watts per gallon as the ultimate measure of ultrasonic cleaning tank effectiveness.  This results in an interesting conundrum.

In a effort to satisfy customer hunger for watts per gallon, there continues to be a seemingly irresistible temptation on the part of ultrasonic manufacturers to add more and more transducers to their cleaning tanks to accommodate more and more ultrasonic power.  However, there are certain laws of physics that, if ignored, can lead to diminishing results as transducer populations become more dense.  Maybe more power is better, but, remember, only the power that is effectively delivered in to the cleaning tank as ultrasound that creates cavitation bubbles that implode  has any bearing on cleaning.  Therein lies the root of the second and perhaps more difficult to overcome limiting link in the quest of more ultrasonic power intensity.

In short, placement of ultrasonic transducers must be done in such a way that each transducer works in harmony with neighboring transducers.  This dictates specific spacing between transducers for optimum results.  Without proper transducer spacing, the result is much akin to a four wheel drive vehicle with one wheel operating in the wrong direction.  Although more power (watts) is consumed, the resulting travel distance (ultrasonic cleaning) is diminished as the one wheel operating in opposition to the others consumes power with negative results.  Also, as excess power is delivered, an effect analogous to “spinning the wheels” occurs as the transducer looses traction with the load, so to speak.  At large transducer amplitudes, connection between the transducer (tire) and liquid in the cleaning tank (road) is lost which can result in a reduction or total loss of ultrasonic energy being delivered to the cleaning tank.

So, it may no longer be a matter of watts per gallon.  Stay tuned as we discuss the above and other issues as they relate to the evaluation of ultrasonic cleaning system effectiveness.

-  FJF  -

Capillary Flow

May 18th, 2015

In the process of parts cleaning, capillary flow can be our friend or our enemy – sometimes both at the same time.  So let’s take a minute or two to understand this interesting phenomenon.

Capillary flow is a property that is exhibited when liquids spontaneously penetrate narrow passageways.  The classic example of a capillary (narrow passageway) is a small internal diameter tube.  The following illustration demonstrates how three different liquids might respond in a capillary space.

Liquids climb to different heights in the same diameter capillary space depending on their properties.  The size and contact angle of the meniscus where the liquid meets the capillary wall also changes based on the properties of the liquid.

Liquids climb to different heights in the same diameter tube depending on their properties. The size and contact angle of the meniscus where the liquid meets the capillary wall will also change based on the properties of the liquid.

The flow of liquid into a capillary space is spontaneous and occurs without pressure, vacuum or any other outside influence.  The height (or depth) of penetration can, of course, be affected by pressure or vacuum but without them, the degree of penetration in similar capillary spaces is related most directly to the surface tension and specific gravity of the liquid.  The meniscus formed at the point where the liquid contacts the surface of the capillary space is analogous to the contact angle measurement for surface tension.   Contact angle was discussed in previous blogs as both an indicator of cleanliness (if the liquid is held constant) or as a measure of surface tension of a liquid (if the contact surface is maintained constant).

Although we often think of capillaries as tubes, the same phenomenon will occur wherever there are closely spaced surfaces of any geometry including flat plates or more complex shapes like screw threads in contact with a threaded fastener or tapped hole.

Effect of varying capillary spacing

Varying the capillary spacing using the same liquid and surface conditions will produce different results in capillary flow. Closer spacing results in deeper penetration.

The degree of capillary penetration is also directly related to the dimension of the capillary space or gap.  A larger capillary gap results in less penetration.  Once a specific spacing is exceeded (depending on the surface activity of the capillary walls and the surface tension of the liquid), there will be no noticeable spontaneous penetration into a capillary space although the meniscus effect will still be evident where the liquid and surfaces forming the capillary space meet.

Note – Capillary flow is also commonly referred to as “wicking” as it is this effect which causes liquids to rise in the wick of an oil lamp or candle.  Liquid may also “wick” in porous solids.

In cleaning, capillary flow results in difficulties in both rinsing and drying.  Once liquid has entered a capillary space (by overcoming gravity) the only way to remove it is evaporation or use of mechanical means such as high pressure air or centrifugal force.  The liquid will not just “drain” out.

The purpose of this blog is not to suggest ways to overcome the effects of capillary flow but, rather, to make the reader aware that the effect exists any time small passageways are encountered.  The result may be inconsequential in some instances but in others is a means for real concern.

-  FJF  -

Servos

February 17th, 2015

We have all heard of servos (short for servomechanism) but exactly what characterizes a “servo” is often a cloudy topic.

Steam engine governor developed by James Watt.

Steam engine governor developed by James Watt.

Most sources are in agreement that the centrifugal ball governor invented by James Watt and used to control the speed of steam engines was the first application of a powered servo mechanism.  This eloquently simple device utilized centrifugal force to raise or lower a set of balls that were attached to a shaft driven by the engine being controlled.  As the speed of the engine increased, the balls would move higher and higher as centrifugal force increased.  The height if the balls controlled a valve which delivered more or less steam to the engine as required to maintain the desired ball height, hence the speed of the engine.

In the cleaning industry, servos are often used to control temperature and motion.  So exactly what is a “servo?”  The characteristic of a servo which differentiates it from other control schemes is that it is interactive.  Consider the following illustrations.  In both cases, the task is to move an item (in this case a box, for example) from position “A” to position “B.”  This is to be accomplished with a conveyor belt with a means to detect position.  In a conventional control system, limit switches are used to detect when the conveyor belt is in position “A” or position “B.”  The limit switches control a fixed speed motor by turning the motor on or off.  A flange attached to the conveyor belt activates the limit switches.

In a conventional control system, position is sensed with limits are met. Between limits, there is no feedback on position.

In a conventional control system, position is sensed with limits are met. Between limits, there is no feedback on position.

Although the speed of the motor may be variable, the speed of the belt does does not change during the transfer from position “A” to position “B” except for starting and stopping.  Considerations in a conventional control system are

  • Acceleration must be limited to a speed that will not damage or cause slipping of the load on the conveyor belt.
  • The speed must be limited to prevent “overshoot” when the desired position is achieved.
  • Conditions are totally unknown between the start point and the end point.

Now the servo system.

In the case of the servo, a position sensor detects the position of the conveyor belt throughout its travel. An amplifier controls the speed of the motor based on comparing the feedback from the position sensor with the desired target positions

In the case of the servo, a position sensor detects the position of the conveyor belt throughout its travel. An amplifier controls the speed of the motor based on comparing the feedback from the position sensor with the desired target positions

A servo system includes a position feedback device which reports the position of the conveyor constantly.  This feedback device can be in the form or a potentiometer, an optical counter or any of a number of suitable devices.  The information from this indicator is received by a control interface (often called an “amplifier” by convention) which compares it to the desired value at any given time.  The control interface interprets the feedback and applies an appropriate corrective signal to drive the controlled device to the target.  The control interface can be customized to react in specialized ways.  For example, if the feedback indicates that the target is some distance away, it might increase the speed of travel and then ramp down as the target is approached.  As a result, the time required to move from “A” to “B” in our example above can be greatly reduced as shown in the graphs above each illustration.

Servo systems can also be used to help control temperature as described in the blog on temperature controls.

Servo controls, although usually more costly than more conventional controls have the benefit of being able to provide precise control in a programmable way.

-  FJF  -

Is it Viscosity or is it Surface Tension??

February 13th, 2015

Viscosity and surface tension are properties that are often intuitively linked to one another.  Because these properties are of primary importance in cleaning it will be worth the while to understand them a bit better.

Surface tension is the property of a liquid that exists at the interface of the liquid with another media (usually air) to hold the surface intact much like the “skin” of a balloon.  Surface tension is created as molecules or atoms at the surface of the liquid have a stronger attraction for one another than they do for the particles or atoms of the adjoining media.  The higher the surface tension, the more stable the barrier between the two that prevents their mixing.  As you have probably heard, a blob of liquid in an environment with no gravity will form a perfectly round ball as it seeks the lowest energy state.  “Lowest energy” meaning the least stretching of the “skin” formed by surface tension.  The surface tension of a liquid nearly always decreases as the temperature of the liquid is increased.  Soaps and other chemical additives also reduce surface tension.  In fact, surface tension is a good measure of the effectiveness of many cleaning agents.  One means of measuring surface tension is by determining the contact angle where a drop of a liquid meets a surface.  If the activity of the surface is known, the contact angle measures surface tension. If the surface tension of the liquid is known, the contact angle indicates the activity of the surface.

Viscosity is a measure of a liquid’s resistance to motion.  Viscosity is weird, taking on one of five general characteristics.  Some materials increase in viscosity as motion is increased while others decrease in viscosity as motion is increased.  These are known as non-Newtonian liquids and include things like quicksand and a mixture of corn starch and water.  It is sufficient to know for our purpose here that viscosity is the resistance to motion of a liquid.  The viscosity of tooth paste is greater than the viscosity of motor oil while the viscosity of motor oil is greater that that of water.  In most cases except for a few “engineered” fluids like particular kinds of motor oil, viscosity decreases as temperature is increased – - sort of the “molasses in January” thing.

So why the confusion?  In general, we think of very viscous liquids as having high surface tension and intuitively link the two.  In fact, a very viscous liquid like wood glue has very low surface tension to be able to penetrate and bond to the irregular wood surface.  Liquid mercury, on the other hand has relatively low viscosity and extremely high surface tension.  The table below shows the viscosity and surface tension of some familiar liquids.

Viscosity and Surface Tension Table

The important “take away” here is that one can not assume that just because a liquid has low viscosity that it also has low surface tension or vice versa.  The two properties are different and not related in the way we would intuitively think.  Liquids with high surface tension and/or high viscosity can be difficult to remove from a surface.  Liquids with low surface tension and low viscosity are good cleaners.  Increasing temperature moves both properties in the direction in favor of successful cleaning.

Things to ponder - Is the viscosity of “silly putty” high or low?  What about its surface tension?  How about mayonnaise? (answers are available on the internet)

-  FJF  -

Considerations for Water Makeup

February 10th, 2015

Adding water to cleaning and rinsing tanks is inevitable in a cleaning process.  Most modern machines include a means to sense liquid level and add more water or other appropriate liquid as necessary.  It would seem simple enough, but there are a lot of factors to consider in managing liquid makeup.

The first thing to determine is where the cleaning or rinsing solution is going.  In general, water is lost in one of two ways, either through evaporation or by being carried out on the parts being cleaned or rinsed.  In the case of evaporation, the loss is primarily water as it evaporates leaving chemistry or contaminants behind.  In the case of carry-out from a cleaning tank, both water and chemistry are depleted from the tank over time.  Of course, in the real world, losses from a cleaning or rinsing tank are usually due to a combination of the above.

Liquid lost to evaporation requires only water makeup.  Liquid lost to carry-out contains chemistry which must be replaced along with makeup water.

Liquid lost to evaporation requires only water makeup. Liquid lost to carry-out contains chemistry which must be replaced along with makeup water.

To replace water lost to evaporation, it is only necessary to replace water.  A level sensor senses low liquid level and opens a valve allowing fresh water to enter the tank until the proper operating level is achieved.  In the case of chemical and water loss due to carry-out, both the water and the chemical component need to be replaced.  This can be accomplished by monitoring the chemical concentration using one of the means we have discussed in previous blogs and adding chemistry automatically or manually as needed.  Another option is to replenish using a mixture of water and chemistry.  This can be accomplished using chemical dispensing devices.  The important thing to understand is that the chemical concentration of the replenishing liquid usually doesn’t need to be as high as the target concentration in the tank.  This is because part of the losses are probably due to water evaporation.  When setting a system of this type, one should consider monitoring chemical concentration over time to determine the proper makeup concentration to maintain the desired chemical concentration in the tank.

If water or water/chemical is added in large quantity there may be other process ramifications.  For example, adding a large quantity of cold water may significantly reduce the temperature of the bath.  This, of course, could have a detrimental impact on cleaning if the process is temperature dependent.  One solution would be to use hot water for makeup.  A second consideration in ultrasonic tanks is that the addition of fresh solution that has not been degassed will add entrained gas into the system which may have a detrimental effect on ultrasonic cavitation.  For these reasons, level controls should be adjusted so that any one addition of makeup liquid is as small as practical and doesn’t exceed 2 to 5% of the overall tank capacity.  In a case where a large amount of makeup solution is necessary, such as when a tank is drained and re-filled, production should be halted until the solution temperature recovers and, in the case of an ultrasonic tank, the solution has been fully degassed.

Finally, in the case of overflow rinses where water is being added continuously, flows should be limited to the minimum that will give the desired result.  Heaters should be sized to assure that liquid temperature is maintained at the highest anticipated flow rate.

-  FJF  -

 

“Oil” is oil. Right?

February 5th, 2015

Wrong!

I don’t think most of us would consider using motor oil to moisten our dry skin.  By the same token, we wouldn’t use body oil in our car’s engine.  The notion seems to persist in the cleaning industry, however, that oil is oil.  We often neglect to differentiate between oils when it comes to developing or recommending a cleaning process or chemistry.  In fact, there are probably thousands of different oils each with its own set of characteristics which can have a great impact on the process and chemistry required to remove them.

Oils can be distillates of crude oil, man-made or synthetic oil, animal oil, or vegetable oil or mixtures of any of these.  Additives including wetting agents, stabilizers, emulsifiers and a host of others are added to give the oil the properties required for a particular application.  As an example, let’s look at a two classifications of oil, their general characteristics and what impact those characteristics might have on removing them from a substrate.

Lubricants -

Lubricants, in general, are oils designed to penetrate into capillary spaces and remain in place for a long period of time.  They are not water soluble and have relatively low surface tension to allow them to penetrate and wet the surfaces being lubricated.  However, their surface tension needs to be high enough to prevent their continued migration too far beyond the location requiring lubrication.  They also have low volatility to slow evaporation so that they will continue to lubricate over the long term.   In some cases they must be able to resist high temperatures.  Viscosity is tailored depending on the application.  Because of their relatively low surface tension, lubricating oils are best removed by a chemistry that will emulsify them into solution.  Finding a chemistry that will preferentially wet the substrate (“splitting” the oil) is not generally an option.

Coolants -

Coolants and some types of cutting oils are usually emulsions of oil and water.  A surfactant is used to assist in the emulsification process.  The primary purpose of a coolant is to cool the cutting tools and substrates during a machining process.  In general, the lubricating function, although important, is a secondary characteristic.  Coolants are usually delivered in large quantity to completely flood the area where machining is taking place.  The oil component of a coolant usually leaves a thin coating of lubricant on the surface while the water component cools the surfaces.  Because of their relatively low surface tension, coolants are often best removed using a chemistry that “splits” the oil from the substrate.  The benefit to using a “splitting” chemistry is that the chemistry is not consumed or depleted during the cleaning process.  In some cases, the recovered oil can even be re-used.

The list of industrial oil classifications goes on to include penetrating and/or water-displacing oil, hydraulic oil, rust inhibitors and many others such as the oil used for dampening in shock absorbers.  Add to this the classifications of oil used in non-industrial applications such as skin care and pharmaceuticals and the list becomes almost limitless.

The “take away” here is that removing oil from parts is far beyond a case of “no brainer” solutions.  Just because a process is effective in one oil removal application does not mean that is going to work in any other even if the two appear to be similar.  Knowing the type of oil being removed and its inherent characteristics is critical to success.

-  FJF  -

More for Less with Accumulators

January 13th, 2015

As a prolog to the following let me just say, as I may have said before in this blog, that being an engineer is both a blessing and a curse.  A blessing because you intuitively understand how things work (or at least think you do) that other people see as magic, and a curse because it is not possible to turn off being an engineer, which can be frustrating.  Also, interesting engineering is sometimes found in the least expected places.  This one, which I feel has a potential use in cleaning system applications, I first discovered at Epcot Center in Disney World.  Should I be surprised?

In the center of the Innovations area of Epcot Center, there is a huge fountain which performs in synchronization with music.  Multiple jets of water spurt up into the air at least 150 feet with microsecond timing and cascade back down over the edge of the fountain.  Looking at the fountain (as an engineer) I was at first impressed with the capacity of pumps that must be required to pump all that water.  During a later trip when the fountain was drained for maintenance and the workings were revealed, it became clear to me that the pumping requirement wasn’t as huge as I had at first imagined.  The “trick” is that each nozzle is fed by an “accumulator” consisting of a pressure canister.  During the time that the nozzles are not functioning, the accumulators are filled with water which pressurizes the air within them.  Then, at the proper moment, the accumulated water, pressurized by the trapped air, is released to the nozzle.  In short, the pumping capacity required is only a fraction of what I had first thought since it is water under pressure from the accumulators that actually propels the jets and not a pump.  Since each jet is on for only a short period of time, there is time for refilling and pressurizing the accumulators between bursts.  In fact, this is not unlike a toilet which accumulates water in a tank which is released during flushing in a large volume to achieve the desired result.

Subsequently, it has occurred to me that this same concept might be used in cleaning systems in cases where a relatively large volume of water or water under high pressure (as in a deluge rinse or spraying application) is required for only a short period of time.  Instead of having a pump that can, for example, deliver 10 gallons per minute at a pressure of 100 pounds per square inch, why not have a pump that can deliver 1 gallon per minute at 100 pounds per square inch (at lower cost) and accumulate it for use at the appropriate time propelled by the pressure of air from an accumulator?

Accumulator System

In the lower of the two illustrations above, a short burst of high pressure liquid can be supplied using a lower capacity pump thereby reducing the pump capacity and short term energy requirement as pressure is built up in the accumulator(s) during the time that the nozzle is not in use.

The pragmatist might counter that as the air in the accumulator expands, the available pressure decreases.  Although true, many applications will tolerate reducing pressure.  If not, there are a couple of options.

  • Size the accumulator(s) so that the pressure will be adequate even despite the pressure drop at the end of the anticipated requirement.  This can be accomplished by increasing the size or number of devices.
  • Pressurize above the required pressure and use a pressure reducing valve to control a constant pressure to the nozzle.

Again, I believe this arrangement has potential use in cleaning system applications where significant amounts of liquid are required for only a short period of time.

-  FJF  -

De-Ionized Water – Things I Did(n’t) Know – Part 2

January 8th, 2015

Since DI water quality is critical to many cleaning processes, it is important to understand its behavior.  In too many cases, the quest for numbers results in high costs in the use of DI water.  In short, delivering super-high resistivity water does not always have a significant impact on the resistivity of de-ionized water in the ultimate application.

DI water systems can supply water with resistance ranging from 1 megohm to greater than 18 megohms depending on the configuration of the system.  Systems producing higher resistivity water are more costly to both purchase and maintain.  The rating of a system is based on the water quality that will be delivered directly at the outlet of the system.  The road from there is a bumpy one as the pure DI water is looking to gobble up ions wherever they can be found – and they can be found everywhere.  In most cases, the quality of water delivered to the process is much more dependent on what happens as the water navigates itself to the point of use than on its quality at the outlet of the de-ionizing system.  The following links to provide some insight into the actual use conditions for DI water and what water quality can be maintained in some typical applications.

http://www.circuitnet.com/experts/86458.shtml

http://www.circuitnet.com/experts/54704.shtml

Although extremely illuminating, the above are a bit lengthy so let me just summarize them quickly.

  1. High quality DI water (over 15 megohm resistivity) seldom exists other than within the confines of the plumbing as it exits the de-ionizing equipment.  Once the water is exposed to the open environment (including external plumbing), the resistivity immediately starts to decrease.
  2. Maintaining anything above 1 megohm resistivity in a tank or any container open  to the atmosphere is near impossible no matter what quality of water is used at the inlet.  Increasing the inlet water from a purity of 1 megohm to 18 megohms will not necessarily result in higher resistivity in the process tank, at least over the long term (longer than 15 minutes).
  3. Testing for DI water quality other than directly at the site of use is very difficult.  Once the water is collected and transported, the resistivity will likely have dropped significantly (usually by an order of magnitude).  Any verification testing should be done directly at the point of use.  Most DI water systems incorporate a sensor that reads resistivity within the plumbing at the outlet of the system but one can not assume that the quality of water will be maintained into the process tank.
  4. In most cases, the quality of DI water used in an application is considerably lower than it is measured at the source.  In most rinsing applications, use of water with a resistivity above 500k ohms does not contribute to its effectiveness.  A de-ionized water feed of over 2 megohm resistivity only ads cost to the process.
  5. De-ionized water should not be sprayed in air.  Spraying only increases the surface area available for contaminants to enter the liquid.

In summary, it is important that the process engineer understand the behavior of DI water in designing a cleaning process.  Similarly, the equipment supplier needs to understand what it takes to achieve and maintain high quality DI water under the intended conditions of use.  Certain measures may be taken to minimize absorption of ions.  They include – -

  1. Minimizing the length of plumbing between the DI water source and the point of use.  This plumbing should also be selected specifically for DI water use.
  2. The surface area of the water that comes into contact with air should be minimized.  Using narrow, deep tanks is better than using wide shallow tanks.
  3. DI water inlets should be located below the liquid level of tanks to prevent splashing as the DI water enters the tank.

 -  FJF  -