Removing water from factory air and lowering the dew point protects automation equipment.
One of the foremost causes of costly downtime and emergency maintenance in industrial automation systems is not oil or contaminants, which are easily removed with proper filtration, but water vapor and the resulting condensed water that is trapped in factory compressed air.
Water in facility airlines can cause corrosion, rust and pipe scale which can break loose to block or adhere to air passageways that can lead to increased pressure drop and loss in machine performance.
These machine components will suffer premature wear and parts deterioration from water:
- pneumatic cylinders will have their pre-lubricants washed away, corrode and respond slower
- solenoid valves’ rubber seals will stiffen, become more susceptible to rupturing and leaking
- instrumentation can malfunction even with the presence of a small amount of moisture/water
- air powered tools will likely stick, jam and perform less efficiently
End products are also at risk of suffering from quality deterioration because of water:
- spray painting will be adversely affected by change in color, adherence and finish quality
- industrial ink-jet printers will be adversely affected by change in adherence and finish quality
- blow molding plastic bottles’ viscosity and material consistency could be adversely affected
- gluing/taping adhesiveness of cardboard boxes could be adversely affected
- pharmaceuticals compound mixing and integrity could be adversely affected
- food processing may be contaminated because of micro-organism growth
Recognize water entry points
All atmospheric air contains various contaminants and water vapor which is concentrated during the compression process. The heat generated during compression increases the capacity of the compressed air to hold moisture, thus avoiding condensation inside the compressor. Lubricants used to improve the efficiency and life of the compressor also become part of the contaminant load as do wear particles.
A compressor with a 100-hp capacity, operating at 86° F ambient temperature, a relative humidity of 80%, compressing air to 100 psi, can produce approximately 30 gallons of condensate during an eight hour shift.
Removing liquefied water from compressed air
Aftercoolers — The compressed air exiting the compressor can reach temperatures as high as 300° F, which is unusable for industrial applications. This hot, saturated air will release 70-80% of its excess water vapor if simply cooled to near room temperatures. To achieve this type of cooling and bulk moisture removal, air or water cooled aftercoolers are often employed. Condensed liquid is removed from the aftercooler via automatic condensate drains and disposed.
The air exiting the aftercooler will still be saturated but at a much more manageable temperature. Because the air is at a dew point of 100° F or more, it is still vulnerable to further condensation into liquid water should it be exposed to temperatures lower than this dew point. The piping in most industrial facilities would provide such temperatures and opportunities for condensation as the air moves throughout the plant.
Drip legs — The next line of defense for removing the water vapor saturated compressed air is a drip leg. A drip leg is a vertical pipe plumbed at the air drop line (below the horizontal header pipe) to allow for water to be easily and efficiently drained away using the principle of rapid air expansion, or adiabatic expansion, to condensate water vapor into liquefied water.
To broadly summarize adiabatic expansion, temperature is the average heat or kinetic energy of all the particles divided by a given volume of air; if the volume of air is increased through expansion, the heat is divided by a larger volume number, therefore decreasing the air temperature. If the drop in air temperature falls below its dew point, then a condensate will form.
The condensate is then removed by a drain at the bottom of the drip leg which can be automatically or manually drained to avoid overflow of contaminants.
Water separators — Water separators will use mechanical separation techniques to remove condensed water in bulk from factory air either by directing inlet air into a spiral and using centrifugal force to separate the water out from the compressed air or by passing the inlet air through a special resin filter element with large meshes to trap water particles that will drop down to a collection bowl, allowing the compressed air to pass through.
SMC’s AMG series water separator is capable of removing water droplets up to a 99% water removal rate, using a special resin filter to trap water droplets. The AMG is easy to install and requires no electrical power and can be either a standalone unit or integrated in to a modular air prep system.
However, the AMG water separator is not designed to remove water vapor or lower dew point which will require a refrigerated air dryer or desiccant dryer.
Lowering dew point
Failure to remove water vapor from factory air can quickly become a costly maintenance headache.
Aftercoolers, drip legs and water separators are used to remove water condensate from factory compressed air. However, this air is still at 100% relative humidity and is still at risk of condensing into water should the surrounding temperatures drop to its dew point.
In order to increase protection of expensive automation equipment, factory compressed air must remove as much water vapor as possible to avoid any condensation further downstream. This is done by lowering its dew point.
Drying compressed air at the highest pressure consistent with the facility’s demands will result in the most economical dryer operation. For most industrial applications, the rule is to first set the pressure dew point to meet general requirements, then adjust it 20° F lower than the facility’s lowest ambient temperature. Hence, factory air dryness or dew point is relative to the application’s specific requirements.
Refrigerated dryers are the most common measure to lower dew point. A refrigerated dryer will further cool the compressed air by removing heat at its inlet side and lowering its temperature dew point down to 37° F, then expelling the condensate through an automatic condensate drain. The dryer will then reheat the dried compressed air back to ambient temperature by recycling the previously removed heat using a heat exchange process. This reheating of the compressed air to ambient temperature will eliminate “sweating” cold pipes when working in humid factory conditions.
It is recommended that a coalescing filter be installed upstream from the refrigerated dryer to remove any compressor oil and other contaminants that may still be trapped in the compressed air to ensure the dryer’s proper functioning. Oil coating the cooling surfaces decreases efficiency while coalescing filters saturated with liquid water will reduce its drying capacity.
n circumstances where factory piping is exposed to ambient temperatures lower than the dew point achievable by refrigerated drying, alternate methods of drying must be considered.
Membrane dryers use hollow fibers composed of a macro molecular membrane through which water vapor passes easily, but is difficult for air (oxygen and nitrogen) to pass through. When humid, compressed air is supplied to the inside of the hollow fibers, only the water vapor permeates the membrane and is drawn to the outside due to the pressure differential between the moisture inside and outside the hollow fibers. The compressed air becomes dry air and continues to flow unimpeded out of the membrane dryer.
A portion of the dry air from the outlet side is passed through a very small orifice to reduce the pressure and purge the outside of the hollow fibers. The moisture that permeated to the outside of the hollow fibers is discharged to atmosphere by the purge air which in turn creates a low partial pressure allowing the dehumidification process to continuously perform.
By altering the air flow rate and membrane configurations, pressure dew points from 55° to –44° F can be achieved. Membrane air dryers are a cost effective solution for point-of-use applications for pharmaceutical manufacturing, packaging, laboratory environments and other applications.
Desiccant dryers pass air through beds of desiccant, an absorbent material such as silica gel or activated alumina, which adsorb water vapor to its surface to effectively lower dew points to temperatures well below that which a refrigerated dryer can achieve. Heatless regenerative models use a pair of desiccant beds which alternate in service while the off-line bed is regenerated via a pressure swing adsorption process.
Pressure dew points from 16° to –40° F and beyond can be achieved with a desiccant dryer.
Both membrane and desiccant dryers are adversely affected by the presence of oils or liquid water and must be protected with a quality coalescing filter.
What is the appropriate dew point?
Over specifying an application’s or a facility’s dew point can be very costly due to exorbitant energy bills just as the maintenance costs for water vapor damage to product lines can be for an under specified dew point.
Drying the entire factory compressed air supply to –10° F dew point is unnecessary and extremely wasteful.
It is a sensible practice to dry the compressed air to a dew point 20° F lower than the factory’s lowest ambient temperature then subdivide each compressed air supply by application using zone or point-of-use membrane or desiccant dryers to provide the appropriate level of dryness.
The costs of energy, downtime, replacing production components, end product defects or even loss of brand value are just a few factors to consider when determining an appropriate dew point.
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