Conduction seems to be the “go to” when it comes to drying systems.  Heat from a heat source such as an electric heater is used to heat air by conduction.  The heat moves from the heater to the air by physical contact between the two.  The heated air is then delivered to the drying site using a blower or fan.  The heat in the air is then used to facilitate the drying process which involves evaporating any water (or other liquid) that remains on the part after being cleaned and rinsed.  Let’s think through this scenario.

In earlier blogs we learned that air is not only a poor conductor of heat but also has a relatively low capacity to store heat.  Once the heat in the air is delivered to the drying site, its motion over the wet part creates evaporative cooling of the remaining liquid as the top layer of the liquid evaporates.  Since the liquid is a better conductor than air, the cooled liquid pulls heat from the part beneath it thereby cooling it as well.  Quite simply, the heat it the part is much more available and abundant than the heat in the air blowing over the part.  In many cases, the temperature of the part being dried is actually reduced only to increase after all of the water is evaporated.  Those of us who have conducted drying trials with hot air dryers know how difficult it can be to dry parts that hold significant amounts of water.

One option to a hot air dryer is one using radiant heat.  Radiant heat is transmitted by electromagnetic waves rather than by contact between two surfaces.  Radiant heat from the sun easily travels 93 million miles through a total vacuum.   Radiant heat from the sun even passes easily through normal window glass to heat the spot on the rug where your pet likes to nap without significantly heat the window glass itself.  Radiant heat only heats opaque and/or non reflective objects which are in its path.

Now let’s consider how radiant heat might be used to dry parts.  First off, the drying of glass and other transparent or reflective objects is off the table!  That’s not going to work.  However, there are lot of parts that are good absorbers of radiant heat.  Heat from a radiant heater will pass through air and water remaining on a part and heat the part itself directly.  The part, then, will efficiently deliver that heat to the water on the part by conduction (assuming that the part is a good conductor).  The heat delivered to the water from the part will facilitate its evaporation while more heat is added from the hot part below to overcome evaporative cooling much more effectively than air will in the conduction example given above.  A low flow of dry (not necessarily hot) air over the surface being dried will accelerate evaporation (which is its only purpose).

In the next blog, we’ll look at the comparison between hot air and radiant heat as a means of drying parts in somewhat more detail and consider how an efficient and fast radiant heat dryer might be designed.

The more I think about it, the more I wonder why we don’t utilize radiant heat more frequently as an efficient and fast way to dry parts.

-  FJF  -

Heat Capacity -

Different materials require that different amounts of heat be added or absorbed per unit of volume to achieve a given change in temperature.  Water, for example, requires a great deal more heat to achieve a given temperature change than does something like an equal volume of air.  Likewise more heat can be absorbed from water than from air.  Water, then, can “store” much more heat than air.  Consider this simple example -  – If you have two towels, one dry and the other wet, and you put them in an oven at a temperature of 200 degrees Fahrenheit, which towel will remain hot longer once you remove them from the oven?  Anyone who has been offered one of those hot towels on a long airplane trip knows that the wet towel will retain its temperature much longer than a dry towel.  For much the same reason, you fill a hot water bottle with hot water, not hot air to keep it warm for a long period of time.  Based on weight, water has about 4 times the heat capacity of air (and remember the air requires a much greater volume to produce the same weight).  Steel has about 1/2 the heat capacity of air (per weight) while aluminum has a heat capacity about double that of steel but still not equal to that of air.  So, air has a reasonable ability to hold heat but it requires a lot more air than water to hold a specific amount of heat.

Thermal Conductivity -

Thermal conductivity is a measure of the capability for heat to move through a material.  Again, a simple example – - Consider an oven set to a temperature of 350 degrees Fahrenheit.  This is a common temperature used for baking in your oven at home.  Even though the temperature of the air in the oven is 350F, most of us wouldn’t hesitate to stick our hand in there and move it around for a few seconds.  HOWEVER, if there is a pie baking in that oven and it has been in there long enough for the pie dish to have achieved a temperature of 350F, I think we would all pass on touching that pie dish and use a pot holder instead.  The reason we would do that is because the pot holder is made of materials that have low thermal conductivity.  In relative terms, water conducts heat about 25 times better than air.  Steel conducts heat about 4 times better than water while aluminum and copper conduct heat 3.5 and 7.5 better than steel, respectively.  So when it comes to thermal conductivity, water is a far superior conductor of heat than is air.

In drying, we are looking to move heat from place to place to change temperatures to achieve particular goals.  As we continue on our discussion of drying in upcoming blogs, Heat Capacity and Thermal Conductivity are concepts that will be used to explain the mechanics of the drying process in more detail.

-  FJF  -

Note - The therodynamic rigor for the following discussion is extremely complex. For those wishing to “crunch the numbers” there are a plenty of sources that will provide the tools to do that.  For our purpose here, it is the concept that is important – I’ll save you the detail for now.

Any time matter changes from one phase to another (solid to liquid, liquid to gas) there is heat either released or absorbed.  By heat, we mean BTUs, not temperature.  A BTU (British Thermal Unit) is defined as the amount of heat that is required to raise the temperature of one pound of liquid water one degree Fahrenheit at standard atmospheric pressure.  BTUs are what change temperature.  Add BTUs of heat and temperature rises.  Take BTUs away and temperature lowers.

When BTUs of heat are added to water, the temperature increases. At two temperatures, when the water melts and when it turns to steam, the temperature stands still although BTUs are still being added. At these two temperatures, the water is said to be “changing phase.”

Although we commonly only think of phase changes at the melting and boiling points, the same rule applies whenever there is a phase change.  When water evaporates, it changes phase from a liquid to a gas.  As a consequence, BTUs are absorbed.  We are all familiar with evaporative cooling.  Our bodies employ evaporative cooling to maintain a constant body temperature.  Moisture is produced and evaporates from the surface of the skin to lower our body temperature on a hot day.  On and extremely hot and HUMID day, the body produces more and more moisture in attempt to produce the required cooling effect but, because of the high relative humidity, not all of the moisture is able to evaporate.  The excessive moisture is what we commonly refer to as “sweat.”

Although we commonly think of water changing from liquid to vapor only at the boiling point, phase change by evaporation occurs at any temperature, not just at the boiling temperature.

Now it gets interesting (and maddening) – - In earlier blogs we said that temperature (we now know that this means more BTUs) is good for drying because it decreases relative humidity of the air surrounding the parts and increases the vapor pressure of the water being dried from parts.  But as always happens in nature, there is a balance.  As it turns out, decreased relative humidity and increased vapor pressure both accelerate the rate of evaporation.  When the rate of evaporation is increased, the expenditure of BTUs is increased thereby creating a cooling effect.  So now we’re in a position where we must not only provide BTUs to decrease relative humidity and increase vapor pressure but also enough additional BTUs to overcome the effect of evaporative cooling resulting from the otherwise beneficial factors in drying process.

In the drying process, there are many factors in addition to relative humidity and vapor pressure that impact the amount of heat lost to evaporation.  In an upcoming blog, I’ll discuss some of these and how the effects of evaporative cooling might be minimized in the drying process to make it faster and more efficient.

-  FJF  -

Vapor Pressure -

Vapor pressure, in simple terms, is the equilibrium pressure over a liquid in an enclosed space.  A thorough explanation of the whole thing about vapor pressure is a little complex and won’t contribute to our discussion here so let’s just cut to the graph.

As temperature is increased the vapor pressure of a liquid is increased. In the above example of water, the vapor pressure equals atmospheric pressure at a temperature of 100 degrees Celsius. At this point, the water is said to boil.

Liquids become more volatile as their temperature is increased.  More volatile means that evaporation will occur at a faster rate thereby speeding the drying process.  When the vapor pressure of a liquid reaches that of atmospheric pressure, the liquid boils.  It is important to point out that this all depends on the temperature of the liquid and NOT the temperature of the air surrounding it as was the case with relative humidity.  And this does make a difference.

Note – Before proceeding let me say that the astute reader may already have deduced that vapor pressure is what vacuum drying is all about.  Yes, that is correct. But, for now, I’ll just talk about it in terms of more conventional drying and leave the vacuum drying discussion for a later blog.  OK?  – Thanks

Liquid on a part is usually in a relatively thin layer compared to the geometry of the part being dried.  Because of this, the temperature of the liquid on a part is usually at or near the temperature of the part rather than to the temperature of the surrounding air.  It’s all about conduction and thermal mass – air simply does not conduct heat as readily as do most solids and liquids.  In order to benefit from the increase in vapor pressure due to increased temperature, a way must be found to increase the temperature of the liquid and, usually, this means raising the temperature of the part.  Unfortunately, using the hot air in a dryer is not a very effective way to increase the temperature of the part for this and a couple of other reasons which we will discuss later.  Introducing the part to the dryer at an elevated temperature or using radiant heating are much more effective ways to accomplish this goal.

OK, so we all agree that heat is good for the overall drying process.  But there are some downsides as well.  Many parts will not tolerate high temperatures.  Plastics, for example, may become deformed if heated above what would be considered a “moderate” drying temperature of 150F.  Many metals will become discolored or experience rapid oxidation at high temperatures.  So, basically, the sky may not be the limit when it comes to increasing temperature to enhance drying.  Drying, in fact, is an art.  There are a surprisingly large number of variables involved.  Upcoming blogs will discuss these variables and how to achieve an effective drying process in more detail.

-  FJF  -

Drying simply means removing liquid remaining on the parts as a result of the cleaning and rinsing process.  This is accomplished in one of two ways.  One is physical removal.  Physical removal of liquids may be as simple as placing the part in an orientation that will allow liquid to drain due to gravity.  Or, it may involve using a blast of air or some other means such as centrifugal force or vibration to cause removal of liquid from the part being dried.  The other (and probably more common) method of drying is evaporation.  Evaporation of liquid is usually enhanced through the use of heat and the movement of air over the parts.

At first, drying by evaporation would seem very simple.  The evaporation of liquids, after all, is nothing spectacular.  It’s a process we see every day.  It rains, the sidewalk gets wet.  The rain stops and the sun comes out and the water on the sidewalk evaporates and is gone.  Voila!  A deeper look, however, reveals that there is more to evaporation than one might think.  The rate of evaporation depends on temperature doesn’t it?  The higher the temperature, the faster evaporation takes place?  Well, actually, yes but in fact no!  The rate of evaporation is actually driven by the relative humidity to a greater degree than by temperature.  But, in fact, the two are inter-related.  As the temperature of air is increased, it can absorb more liquid and, therefore, the relative humidity is decreased.  Lower relative humidity promotes faster drying.  The following chart and graph which both show essentially the same data are very interesting.

As temperature is increased, the amount of water required to saturate a specific volume of air increases.

This graph shows that as the temperature of air increases, the amount of water required to saturate it increases exponentially. A few degrees of increase in temperature has a an increasingly large effect on the saturation point.

It is a common mis-belief that air can “hold” more water as the temperature is increased.  In fact, air no more “holds” water than does a sponge.  If a sponge is submerged in water and squeezed several times, the water displaces air from the cells of the sponge and, eventually, water occupies all of the internal spaces of the sponge.  The sponge is “saturated” with water.  When the sponge is removed from the water, a large percentage of the water will drain out as there is nothing really “holding” it in the sponge.  Air at 100% humidity is saturated with water.  If a volume of air saturated with water is heated, the level of saturation is decreased and the air requires additional moisture to again become saturated (or less unsaturated if you’re left-handed).  Air that is saturated with water is at a relative humidity of 100%.  Air that contains only 50% of the water required to be fully saturated is at a relative humidity of 50%.  Similarly, if the temperature of a volume of air that is saturated is reduced, water comes out of the air as a fog or water droplets.  The “dew point” is the temperature at which air becomes fully saturated.  In weather terms, this is when it rains.

Relative humidity in percent is the total water required for a volume of air divided by the amount of water that would be required to be totally saturate that volume of air.  In drying, it is important to understand the role of both temperature and humidity and how they are related.  I will explore this more in the next blog.

-  FJF  -

Wetting -

Wetting refers to the ability of a liquid to be attracted to or to adhere to a surface.  In general, ingredients that facilitate wetting reduce the surface tension of a liquid.  In addition, they may make the cleaning formulation have an affinity to be be highly attracted to a particular surface like, for example, aluminum.  Wetting agents do two things – first, their ability to reduce surface tension allows cleaning solution to penetrate into areas where surface tension might otherwise impede access.  This facilitates cleaning surface features with finely detailed geometry.  Low surface tension also allows liquids to surround and penetrate between small particles and substrates which facilitates separation of the particles from the substrate.  This is especially true in the case of ultrasonic cleaning.  Once the particles are free of the substrate, they can be rinsed away.  Secondly, wetting agents may exhibit what are called surface active properties.  In simple terms, the wetting agents are formulated to be highly attracted to a surface.  If the cleaning solution is more highly attracted to the substrate than the contaminant (oil, for example), the contaminant looses the ”tug-of-war” and is displaced by the cleaning solution to be rinsed away.  The wetting properties of a cleaning solution benefit the removal of particles as well as other contaminants.

Emulsification -

Emulsifying agents allow suspension contaminants that would otherwise not mix with water.  In essence, emulsifiers contain complex molecules called “micelles.”  One end of each micelle is hydrophilic (water loving) while the other end is hydrophobic (resisting bonding with water but able to bond with hydrocarbon or other specific hydrophobic liquids).  Acting as intermediaries, micelles allow water and oil to co-exist as a mixture (as opposed to a solution as described in an earlier blog).  By allowing oil (or other contaminants) to mix with water, emulsifiers promote the removal of otherwise water-resistant contaminants.

As an extension of the use of emulsifiers, they can also be used to make cleaning formulations that exhibit the cleaning properties of two discreet cleaning agents.  For example, mixtures of water based chemistry and a solvent such as kerosene can be used as an emulsion to remove a mix of contaminants that couldn’t be removed by either component alone.  Emulsifiers allow the co-existence of the water based component and the solvent based component in a uniform mixture.

Note – Most coolants used in machining operations are emulsions of water and oil in one form or another combined with emulsifying agents.  Coolants are frequently used effectively for in-process cleaning of parts during the manufacturing process.

As stated at the beginning of this series of blogs, their intent is not to be an exhaustive study of cleaning chemistry but, rather, a general overall view of some of the basic concepts involved.  The formulation of cleaning chemistries is a science of its own which is constantly evolving to meet new requirements and hardware capabilities.

-  FJF  -

In heavy-duty applications, caustics like sodium hydroxide and potassium hydroxide are diluted with water in concentrations that may exceed a 1 to 1 mix.  Such concentrations, along with heat and mechanical energy are effective at removing oil, grease, and even oxides in some cases.  High concentrations of caustic chemicals are generally safe to use on substrates which include steel, cast iron and stainless steel.  Such strong concentrations, however, are not suitable for use on copper, brass and other relatively “soft” alloys.  Aluminum, for example, is readily dissolved by concentrated caustics.  The benefits of using caustics, however, is undeniable.  This fact has led to the development of caustic chemistries that are compatible with almost any substrate.

One way to improve the compatibility of caustic chemistry with a wider variety of substrates is to reduce the concentration.  Many “caustic” cleaners contain the actual caustic ingredients in concentrations far less than 10%.  Another way to reduce the aggressive qualities of caustics is to use “inhibitors” as part of the formulation.  Inhibitors, in essence, alter the characteristics of caustic ingredients rendering them chemically non-aggressive against specific substrates such as aluminum as well as brass and copper and their alloys.

Since caustic chemistries do not naturally possess the low surface tension properties required to make them effective at penetrating capillary spaces, most caustic cleaning formulations include wetting agents and other ingredients to improve their ability to penetrate.  Most caustic cleaning chemistry also includes any of a long list of other ingredients to make them suitable for a specific use.  These ingredients include emulsifiers, saponifiers, chelating agents (to “soften” water), and the list goes on.  Most formulations are proprietary and are not disclosed other than an occasional mention of an ingredient that may be potentially toxic or otherwise environmentally unfriendly.

Although the formulation of caustic chemistries at first may seem relatively straight forward, it is really a very complex science and, in some ways, an “art.”  Interactions between components and totally unexpected results (good or bad) are not uncommon.

Finally, a word of caution!  If you find a chemistry that works in your application, stick with it!  There will always be something out there that is “the same or better” than what you are presently using.  Before making any move, first make sure that it produces the same result!

As you might imagine, this has been only a “brush over” of caustic cleaning chemistries.  There are, it seems, whole libraries devoted to the subject.  Future blogs will go into more detail on these and other chemistries.

-  FJF  -

Three types of water-based cleaning chemistries are alkaline, acid and a third variety (which really isn’t a solution) called an “emulsion.”  Nearly all cleaning chemistries also include ingredients in addition to the main constituent which may enhance or in some other way tailor its overall performance.  Let’s start off by looking at acid type cleaners.

Acid Based Cleaning Chemistry

Acidic cleaning chemistries are probably the least frequently used of the three characteristic groups mentioned above when it comes to parts cleaning.  Acid cleaners often clean by initiating a chemical reaction with the contaminant that is being removed.  A good example of an acidic cleaner is a formulation designed to remove rust from a steel part.  The acid reacts with the iron oxide (rust) to remove it from the steel part.  “Cleaning” is a borderline term for such uses.  At the initiation of this blog nearly two years ago, we defined “cleaning” as removing a contaminant from a surface without changing the characteristics of the surface.  In most applications using an acidic cleaner, there is a little more going on than just removing a contaminant.  Acidic cleaners, for example, are used extensively in the plating industry to “pretreat” surfaces prior to plating.  Although the acid cleaners often contain ingredients which also remove such things as oil and some soluble contaminants, the real purpose in using an acidic chemistry is to remove surface oxides from the substrate to provide an “active” and sometimes “etched” surface to facilitate the plating process.  This effect falls, technically, outside the realm of “cleaning.”  If the only goal was to remove the contaminants (and not condition the surface for plating), non-acidic chemistry would probably be equally effective.

Acids are also used in applications where the effect of an acid results in other desired effects.  One of these applications is in the “passivation” of medical prosthesis such as knees, hips, pacemaker housings and the like.  Although commonly called “passivation,” one of the major goals in this application is to kill and remove residues of any biologically active contaminants or “pathogens.”  Again, it may be a bit of a stretch to call this a “cleaning” application.

The more prevalent main constituents of acidic chemistries are sulfuric, hydrochloric, acetic, citric and phosphoric acids.  These acids are usually used in relatively low concentrations in formulations that also contain wetting agents and other ingredients to provide a desired effect.  Since acids are generally aggressive against many substrates, it is not uncommon to use “inhibitors.”  “Inhibitors” are ingredients which limit the activity of the acid to prevent excessive etching of the parts being “cleaned” or to extend the life of the vessels used to contain the acidic formulation.

Finally, the effectiveness of acidic chemistries, like any others, is dependent on a number of variables which include time, temperature and concentration.  Any chemistry should be used according to the manufacturer’s recommendations.

-  FJF  -

The “classic” orifice as we have defined it earlier is a passageway through an otherwise solid piece of material which has both a length and a width and is open on both ends.  Cleaning using spray washing, for example, can be quite effective at removing liquid contaminants.  Once the bulk of the contaminating liquid is removed, continued flushing driven by the pressure of the spray completes the cleaning process.  Removing a solid contaminant, however, presents a greater challenge.  The smaller the orifice is, compared to its length, the greater the challenge.  This is because mechanical energy must be delivered and/or an exchange of liquids must occur at the location of the contaminant.

The orifice on the left above is more easily cleaned than the one on the right because ultrasonic energy and/or liquid exchange can reach the contaminant. If the length of the orifice on the left were to be increased to 5L (the same ratio of W to L as the one on the right) cleaning would be more difficult but not as difficult as the one on the right.

If the width of the orifice is large, delivering mechanical energy and an exchange of liquid are relatively easy as shown in the illustration above.  As the ratio of width to length decreases, the difficulty of cleaning increases accordingly.

Ultrasonic energy produces two mechanisms that can be effective in removing contaminants from an orifice.  The first is ultrasonic cavitation.  If the width of the orifice is large enough to accommodate a cavitation bubble (about .001″ at a frequency of 40kHz) then cavitation can occur within the orifice to enhance cleaning.  The depth of penetration of cavitation events will vary depending on the size of the orifice and the frequency and power of the ultrasonic energy.  As we’ve discussed in previous blogs, higher ultrasonic frequencies produce smaller cavitation bubbles.  The second mechanism is the effect of pressure waves that can penetrate much greater distances than actual cavitation within a long orifice to enhance cleaning.  These pressure waves can also penetrate orifices that are curved or angled.  As the depth of the orifice increases the effect of cavitation and pressure waves decreases as energy decays over distance and is absorbed by the surrounding solid material.  The ratio of length to width of an orifice that can be effectively cleaned using ultrasonics varies with ultrasonic power, frequency and the nature of the material surrounding the orifice.

Although the length of an orifice that can be cleaned using ultrasonics is limited in the classic case, there are special cases where orifices of nearly unlimited length can be cleaned ultrasonically.  In these cases, the orifice, rather than penetrating a solid with some thickness, is surrounded by only a thin wall of material that will conduct ultrasonic energy.  These special cases include things like hypodermic needles, glass tubing and thin-walled metallic or ceramic tubing.  In these cases, ultrasonic energy can be delivered through the wall surrounding the orifice to produce mechanical effects including ultrasonic cavitation within the orifice independent of its length.  The tube is simply immersed in an ultrasonically activated liquid.  A flow of liquid through the thin-walled orifice is, of course, required to clear the loosened contaminants from the interior of the orifice.

In summary, one must consider both the Width to Length ratio of an orifice and its overall size to determine cleanability.  Cleanability increases as the Width to Length ratio increases.  Larger orifices are more easily cleaned than smaller ones due to better accessibility within the orifice.  The cleaning of orifices with thin walls that conduct ultrasonic energy is often possible in cases where an orifice with the same Width to Length ratio through a solid material would not be possible.  This is due to the ability to introduce ultrasonic energy through the conductive wall to deliver mechanical energy to its interior.

-  FJF  -

Liquids -

Liquids are common contaminants in orifices.  Liquid contamination can result from manufacturing operations but also may be the result of use as in the orifice of a nozzle, for example.  Liquids are usually the simplest contaminants to remove from orifices.  This is because even if the orifice is completely obstructed by the contaminating liquid, the gross amount of the contamination can be removed by simply applying pressure to one end of the orifice thereby pushing it out the other end.  This applies for virtually any width or length orifice.  Once the contaminant has been sufficiently removed to allow cleaning liquid to penetrate the length of the orifice, cleaning of any residual liquid contaminant is dependent on mechanisms that will be discussed in more detail in ongoing blogs.

Solids -

As you might imagine, solid contaminants present a significantly different challenge than liquids when it comes to removing them from an orifice.  Solids may either partially or completely block an orifice.  Solid material in an orifice might be either soluble or insoluble.  Solids may be held in place by a number of different mechanisms ranging from adhesion (a hardened glue, for example) or something as weak as cohesive forces (fine particles from a grinding or broaching process, for example).  Let’s look at these things and some of the interactions between them one at a time.

If an orifice is partially blocked by a solid there is a chance that a cleaning chemistry can be used to penetrate the length of the orifice and either dissolve or, in some cases, deliver sufficient mechanical energy to break the solid contaminant free so that it can be flushed out.  If the orifice is completely blocked, the effectiveness of either of the above mechanisms will depend on being able to introduce liquid into the length of the orifice leading to the blockage and provide physical contact with it.

Soluble solid contaminants are best removed using a chemistry that will dissolve them.  Dissolution may be achieved using a simple flush.  The use of other mechanisms depends on the geometry of the orifice.  Non-soluble solid contaminants require some means of delivering sufficient mechanical energy to the attachment site to physically displace them.

In cases where insoluble solids are adhered along the length of an orifice the removal mechanism nearly always involves the delivery of mechanical energy.  Sometimes a simple flushing with liquid is adequate to do the job but in many cases more energy than can be delivered by moving liquid is required.

Note – An especially difficult (almost nearly impossible) to remove contaminant is a solid particle that is wedged into an orifice along its length.  Curled chips which are made “springy” by the cutting process, for example, often defy removal by normal batch cleaning processes.

It is in such cases that probes, ultrasonics and other mechanisms are required.  The effectiveness of these mechanical means depends on the geometry of the orifice and, to some degree, the placement of the orifice within its surrounding mass.

Subsequent blogs will discuss means of delivering mechanical energy within an orifice and how geometry may affect the ability to do this.

-  FJF  -