There is more than one way to dry the air

Use these hints and tips for evaluating equipment that wrings the moisture out of your air system.

By Bob Roseman

The four untruths of compressed air are that it is free, it is efficient, it is clean and it is safe These misstatements have lulled many a plant engineer into costly mistakes that affect the competitiveness of the company. After years of being the "back room utility", compressed air is now recognized as a costly but essential power source for the modern plant.

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Along with this new awareness of cost, drying and cleaning the compressed air is no longer an option for the modern plant engineer. The four commercial methods of removing water from compressed air are refrigerated (condensing), deliquescent (absorbing), desiccant (adsorbing) and membrane (molecular separation). Understanding how each method removes moisture from compressed air is an important part of selecting the correct air treatment equipment for your plant's back room utility.

Start with the basics

The appropriate selection of a compressed air dryer, however, begins with your needs, not the capabilities of the equipment. Whatever equipment you use or purchase must, of course, meet your particular needs but the competitive marketplace for your company's products must drive you to reach your objective at the lowest possible cost. If your production costs are higher than your global competitor, you will ultimately lose. Be aware of the difference in operating costs of the methods of drying air. Striving for a lower dew point or a greater degree of drying increases the cost significantly. Avoid this pointless approach unless the application requires such heroic measures.

If the application requires that level of drying, then you must provide it; however, don't increasing your costs without making a significant study of the trade-offs. A move from refrigerated dryers to desiccant dryers increases operating costs by a factor of six to eight, depending on the chosen method of desiccant regeneration.

Cold is one way to go

Each method has unique features to benefit the customer. Refrigerated dryers represent the largest segment of installed equipment and currently provide the greatest amount of drying capacity in the United States. It is the least expensive of the four commercial methods of drying air. Refrigerated dryers in their simplest incarnation are large refrigerators containing compressed air tubing. The function of the refrigeration system is to reduce the temperature of the air and condense the water vapor into liquid water. This liquid water is then separated from the compressed air stream and drained.

The compressed air circuit begins in an energy recovery air-to-air heat exchanger referred to as a precooler/reheater. This part of the equipment reduces the temperature of the compressed air by transferring the heat of compression in the inlet air to the exiting air. The net effect is to reduce the load imposed on the dryer's refrigeration system.

After the air is precooled, it is routed to the main evaporator/chiller portion of the dryer. There the air stream is cooled to the lowest temperature possible but still above the freezing point of the condensate. The condensate collects in the separator and an automatic drain ejects it from the dryer. The cool dry air is then piped back to the energy recovery exchanger where it is reheated by the warm, moist incoming air.

Adsorption on a surface

Desiccant dryers operate on a simple principle — pass the moist compressed air over a material of very low moisture content. Doing so causes the water vapor to move from the relatively "wet" air to the relatively "dry" desiccant. This raises the question of what to do when the so-called dry material is now used up and saturated with captured water.

To allow for more than a single use of the desiccant material, most driers of this type feature dual towers that permit the regeneration of the desiccant in one tower while the drying process continues in the other. There are a few key design parameters that must be met for this form of dryer to work. See the sidebar for the basic design guidelines manufacturers follow regarding bed velocity, contact time and dynamic loading in making cost and performance tradeoffs.

Absorbtion into a volume

Deliquescent dryers are single tower dryers that consume the deliquescent desiccant--usually a salt--during operation. An example is the small silica gel packets that are packed with new cameras and some electronic gear. Deliquescent dryers have advantages: they are simple, they have a low initial cost, and they require no electrical power. When properly applied, these dryers reduce or suppress the dew point of the compressed air by 20 to 40 degrees Fahrenheit . This type is not common in general plant compressed air systems. They find use in specific applications in which the design advantages outweigh the limited dew point performance and high operating costs.

Passage through a surface

Air, compressed or otherwise, is a mixture of oxygen, nitrogen, water vapor, and other trace elements. Membrane dryers deal with the separate gaseous components of compressed air and permit the water vapor and some oxygen molecules to diffuse across a membrane. The driving force across the membrane is the differential between the pressure and relative humidity of the compressed air and that of the atmospheric air surrounding the membrane fibers. The amount of air lost in this process ranges from 25 to 40% of the inlet air flow. Other than a few pioneering applications, use of this approach to air drying is currently limited to air flows less than 50 standard cubic feet per minute for specific point of use applications.

Common sense advice

A common error in dryer selection is assuming standard operating conditions and basing the selection only on flow (usually measured in standard cubic feet per minute). One of the most significant factors affecting the sizing of every dryer types is inlet temperature. As can be seen from a psychrometric chart, the amount of moisture in a physical cubic foot of compressed air doubles for every increase in temperature of 20 degree Fahrenheit.

Sizing the dryer on the basis of compressor pressure setting is another error that can result in undersized treatment equipment. The pressure declines from the discharge of the compressor as it flows through the separator, aftercooler, filters and piping on its way to the dryer. As the pressure in the air system drops, the volume of the air increases and its velocity increases. Faster flow means shorter a residence time in the dryer. This results in lower separator efficiencies in refrigerated dryers and desiccant dusting in that type of dryer. Pay proper consideration to the lowest operating pressure when selecting the equipment.

Picking a winner

There are general guidelines for selection of dryer types. The practical plant engineer realizes that "dry" compressed air actually means no liquid water at the point of use. For most manufacturing applications in which the compressed air equipment and piping distribution system is inside a heated space, refrigerated dryers are the proper selection.

In some applications, any moisture, even in vapor form, may be harmful to the process or process equipment. In these situations, as well as those in which the compressed air distribution system is exposed to temperatures below freezing, desiccant dryers may be required. Don't select a desiccant dryer without first making a detailed examination of actual need because this type of dryer has a high operating cost relative to that of a refrigerated dryer. Proper selection is the first step in effective operation of clean up equipment for compressed air systems.

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