Improving Air System Efficiency

by: R. Scot Foss, Plant Air Technology

Part 10: There are a number of ways to deal with omnipresent water vapor in compressed-air systems to avoid the problems of its evil companion, liquid water.

In compressed-air systems, water is an insidious contaminant.  It enters a system invisibly.  It causes problems – corrosion, clogging, washing away lubricants – internally, while still unseen.  When water finally shows up at an exhaust port, its damage already may be done.

Water can be damaging to pneumatic production equipment on its own, but water also is a carrier.  Many other contaminants depend on water to distribute them around the system or require liquid water to initiate a destructive reaction.  One of the most problematic is acid gas, a common airborne constituent in most industrial atmospheres.  When mixed with water in the cooling stages after compression, this gas can form hydrochloric and sulfuric acids.  Without water, acid gas and many other contaminants could not do their dirty work.

Almost all water enters a compressed-air system in the form of vapor as part of the ambient air drawn in by the compressor.  Water vapor causes no problems.  Only when it condenses does it become a contaminant.

Terminology:

Special terminology has evolved to explain the interrelationship of water vapor, temperature, pressure, and liquid water.  Misunderstanding and misuse of this terminology has led to a great deal of confusion about the behavior of water vapor in compressed-air systems and how to avoid the damage liquid water can cause.

There is a well-defined, maximum amount of water vapor that any gas can hold at any given temperature and pressure.  Compressed air under these conditions is said to be saturated.  If its temperature is increased, the air can hold more water vapor.  If the pressure of saturated air is increased or the temperature decreased, some of the water vapor will condense.  If compressed air containing an arbitrary amount of water vapor is cooled at constant pressure, some water eventually condenses.  The temperature at which condensation begins is called the saturation temperature.

Any statement about saturation temperature must always include the system’s pressure.  For the special case where the air pressure is equal to atmospheric pressure, the saturation temperature is called the dew point.  However, some people use the terms dew point and saturation temperature synonymously.  Hence, it always is good practice to include a statement of pressure when mentioning dew point temperatures as well.  This precision avoids confusion. (When a pressure does not accompany dew point information, atmospheric dew point is assumed.)

In systems that operate below 0^o F, the user may measure dew point from an air sample that expanded from system pressure to atmospheric pressure.  That resulting dew point is significantly lower than that which would have been measured at pressure.  The user then thinks that he has achieved an adequate dew point, when the dryer on line may be operating at 30^o to 40^o F above the rated performance.

For engineering calculations, humidity usually is written as the weight of water per unit weight of moisture-free compressed air (e.g., 0.001 pounds of water per pound of dry air).  Frequently, these numbers are very small decimal fractions, so it is more convenient to express them as parts of water vapor per million parts of dry air.  The conversion to ppm by weight merely involves moving the decimal point six places.  But again, care must be exercised.  Most constituents of an air sample are analyzed on a volumetric basis; hence, chemists often report water and contaminants as ppm by volume.

The percentage relative humidity (%RH) is 100 x the ratio of partial pressure of water vapor to the vapor pressure of water at the stated temperature.  Many people confuse percentage relative humidity with percentage saturation (which sometimes is called percentage humidity).  Percentage saturation is 100 x the ratio of the existing weight of water vapor per unit weight of dry air to the weight of water vapor that would exist per unit weight of dry air if the air were saturated at the existing temperature and pressure.  Another way to express water or water vapor content is as weight (grains or pounds) of water per unit volume (standard cubic foot) of compressed air.

Removing Water Vapor:

There are many methods that will remove water and water vapor from compressed air.  Mechanical methods, such as centrifugal separation or porous-media filtration, usually will take out liquid water, mist, and droplets only; vapor remains.  Note that these methods are very velocity sensitive.  Too high or too low an air steam velocity will degrade performance.

Refrigeration drying is limited to the saturation moisture content of the air at the refrigerant temperature.  All of the moisture will condense down to the airside heat-exchanger temperature in the refrigerant-to-air heat exchanger.  The balance of moisture present will exist saturated at the discharge temperature from the dryer.  You must also provide a mechanical separator to remove the liquid condensate, which results from this process.  The efficiency of the dryer is dependent on the efficiency of the separator.  Many separators in dryers are 80 to 95% efficient in moisture removal.  The liquid water not removed in the separator will be absorbed into a vapor state in the air stream in the reheat cycle of the refrigerant dryer.  This will result in higher than intended pressure dew points.  The efficiency of the separator is largely dependent on its ability to isolate the separated liquid from the air stream so that it cannot be re-entrained.  This is why most 35^oF dew point refrigerated dryers perform much better than those rated for 40^o to 42^oF.

Chemical Drying:

Deliquescent drying involves chemicals – usually in tablet form – made up primarily of salt and desiccated urea.  The compressed-air stream is usually directed through a bed of deliquescent tablets, which adsorbs the water vapor.  A chemical phase conversion produces a liquid brine solution that runs down into a holding tank  (A timer or solenoid-operated drain valve arrangement typically removes the brine.)  This type of dryer can reduce pressure dew points by 20^o to 40^o F.

One of the potential problems associated with deliquescent drying is the highly corrosive nature of the tablets and the brine.  Another is the bed geometry.  Optimum dew-point reduction depends on the bed geometry as well as saturation temperature and mass flow of the compressed air.  As the bed dissolves during its normal functioning, its geometry changes and the dew-point reduction performance can degrade.

Obviously, maintaining the bed geometry or desiccant level is important to the operation of the dryer.  Maintenance involves topping off the bed with fresh tablets.  While this sounds simple, remember that this dryer is a pressure vessel.  To add desiccant, the dryer first must be isolated from the system and depressurized.  If a bypass valve is installed to provide isolation, gross water may drop out downstream when the valve closes.  If there is no valve, the whole air system has to be shut down – if possible.  What happens in the real world is that the bed is seldom serviced until the moisture content of the plant air gets so high that there are complaints form the production people.

Another factor in desiccant-dryer operation is that the chemicals will absorb lubricants and liquid water from the air stream.  Both of these liquids interfere with the tablets’ performance.  So for desiccant dryers to function at their best, it is very important to provide pre-filtration that removes all liquids and aerosols prior to drying.

Adsorption Drying:

In the adsorption process, vapors are attracted to and condense on the interior surfaces of certain solid materials or tablets, which contain literally millions of submicroscopic pores and cavities.  There is no chemical reaction, so adsorption is generally superior to absorption because it is not corrosive an does not require the frequent material refills.  When the chemical bed becomes saturated, it can be regenerated by driving out the collected moisture with heat of flowing dry air through it.

The adsorption process can easily produce consistent pressure dew points of –20^oF and lower.  (When necessary, a properly engineered and applied adsorption dryer can produce pressure dew points below –100^oF.)

Adsorption dryers typically are constructed as twin towers.  While one active tower is drying air, the other tower is regenerating.  Once the first tower’s desiccant bed is saturated, a valve shifts to divert the raw air stream to the second dry tower.  If the regeneration is performed by dry air – which reabsorbs the moisture from the bed – the technique is identified as heatless.  This heatless technique uses a portion of the dry air exiting from the drying tower to regenerate the wet tower.

In a typical heatless dryer rated at –20^o to –40^oF Pressure dew point, the quantity of air consumed for regeneration is about 15% of the rated flow capacity of the dryer at pressure.  If the pressure is higher, the purge flow will be higher.  If the pressure is lower, the dryer may be unable to produce the desired pressure dew point.  Dew points lower than –40^oF will require higher flows, sometime approaching 25 to 30% of the dryer’s rated performance.  The drying cycle for tower changes usually is about five minutes.

The other approach towards regeneration is with heat.  The heat source can be internal or external.  Calrod heaters can be installed right in the bed, or electric heater elements can be mounted on the towers’ exteriors.  (An alternate internal heat source is steam, if available.)  Note that drying temperatures can approach 450^oF.  This can be a problem if a lubricated compressor supplies the system; most compressor lubricants have flash points well below 450^oF.

The initial cost of externally heated dryers is significantly more than internally heater or heatless dryers.  The operating cost of heated dryers doesn’t vary much from type to type, but they do require so purge air in order to function.  Generally to purge flow consumes about 6% of the dryer capacity.  It is important to understand that when we say a percentage of dryer capacity, we do not refer to the mass being processed.  If a dryer is rated at 1,000 scfm, it will use the same amount of purge flow whether it is operating at 10% or 100% of capacity.

Another type of dryer uses the heat of compression to regenerate the bed.  The hot air flow exiting the compressor is channeled directly to the tower to be regenerated, then through the aftercooler where some moisture is condensed, and finally to the active tower for more drying.  This arrangement is by far the most economical of all forms of regeneration.  Its drawback is that maintaining the proper bed temperatures may be difficult.  If the airflow is too high or the temperature is too low, the dryer will not work correctly.  Too many heat-of-compression dryers have not performed properly because flow and temperature were not considered.

Although there are other methods for drying compressed air – such as microwaves and membranes – the systems just described represent the large majority of those used commercially in industry for water and vapor removal.

Temperature Considerations:

Like everything in life, good ideas poorly applied do not work.  One drawback of all heat-regenerated dryers is the high bed temperature.  The temperature of air discharging into the system immediately following tower switching can be 200^oF or higher.  The air temperature will reduce to 100^oF in time for the towers to switch again.  It is possible that this cyclical temperature swing will adversely affect some downstream production processes.  Remember: the weight flow of the air at 200^oF and 100 psig is 0.470 lb/cf, while the same cubic foot of air weighs 0.551 lb/cf at 100^oF and 100 psig.  The 15% lower density at the elevated temperature may be a problem.

Other Water-Ingression Points:

In addition to the well-documented point that most water vapor comes into a system with the air in the compression inlet, there are other less-commonly understood ways for water to enter.  Leaks – even pinhole types – can be a source of water contamination.  The assumption that nothing can get into a pressurized system because pressure inside is greater than pressure on the outside is not always valid.  There are a least three effects that allow this intrusion: jet pump, shortened diffusion path, and molecular flow.

The jet pump effect siphons stagnant external contaminants into a system when a high-velocity pressurized gas steam passes over a leak point. The entry opening can be as small as 0.00001 in.  It only needs to be larger than the molecule of moisture or contaminant that is drawn into the system in what is called a direct hit.

The shortened diffusion path effect occurs when a leak or leaks allow outflow from a pressurized system.  The hole in the pipe acts as an orifice, and the cross-sectional area of the jet formed by leaking compressed air may decrease over a finite distance downstream from the inner edge of the hole where the leak originates.  Eddy currents then form near the hole.  The refrigeration effect from the expansion of the leaking air may be sufficient to cause water vapor to condense both in the vicinity of and around the edges and sides of the hole.  The actual path length a water molecule must travel to enter the system may not be the metal thickness but rather a shorter distance near the leak’s inner edge.

Molecular flow can occur if the partial pressure of the vapor inside the system is sufficiently lower than the partial pressure of the vapor in the ambient air.  Water vapor will diffuse through a solid pipe wall and enter the system.  This phenomenon occurs where compressed air systems are operating at pressure dew points of –40^oF or less and the outside-saturated temperature is high.

The proper selection of the piping material can significantly reduce the potential for contaminant ingression via this effect.  Systems that have pressure dew points of -75^oF to -100^oF or lower should use copper or pickled-and-lined pipe to prevent this diffusion path problem from downstream contaminants.  An extreme application error would be to use very dry compressed air for open blowing where the atmosphere is quite humid.

In fact, the most ludicrous method of contaminating a system with water is to drive the dew point of the air down to the point where it is hydroscopic, and then expose the gas to a liquid downstream of the drying process.  Vapor seeks the lowest vapor pressure.  A condensation problem can occur after regeneratively drying compressed air at the source to a low dew point and then using the air for one of these applications downstream:

• Sparging or agitating any liquid with compressed air,
   

• Dripping or atomizing oil via point-of-use lubricators into the air steam,
   

• Agitating sludge or chemicals with air,
   

• Aspirating oil or other liquids with compressed air,
   

• Transporting slurries or powdered solids through a pipeline using compressed air, and
   

• Spilling high-pressure, wet compressed air into a drier compressed-air source.

In most systems, this effect can occur when the air is extremely dry and the atmosphere is very wet, or when very dry air comes in contact with liquid.  The greater the dryness extremes between the air and the contaminant, the greater the problem.   When molecular diffusion brings water into a system where water contamination is unacceptable to production, the uneducated solution may be to make the air even dryer.  This, of course, exacerbates the problem rather than solving it.

Conclusion:

Compressed air needs only to be dry enough to suit its application.  Excessive drying may fix one problem, but it creates new ones in the process – and more drying costs more.
 

Reprinted with permission from R. Scot Foss, president of Plant Air Technology, Charlotte, N.C., a company specializing in system auditing and design.  This article is based on his book, "Compressed Air System Solution."  A portion of the proceeds from sales of the book is donated to children’s charities.  To order a copy of the book, please contact Southern Corporation.