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Desiccants are substances that attract and hold water very well. When you purchase a product that could be damaged by the presence of water you often find a little envelope full of what looks like small beads packed inside the box. This is desiccant. Two types are commonly used: activated alumina and silica gel. Unlike the salt tablets used in the deliquescent dryer, these desiccants don’t dissolve when they absorb water. They fill up with as much water as they can hold and maintain their shape. Once they’re full they need to be replaced or be regenerated (have the water removed). Some desiccants are treated with chemical indicators to change color when regeneration or replacement is needed.
Another name commonly used for desiccant dryers is “Twin Tower” dryers. This is because each dryer consists of two towers filled with desiccant, with each tower being capable of handling the rated flow capacity of the dryer. While one tower is drying compressed air, the other is being regenerated.
Desiccant dryers are used primarily where very low dew points are required. The most common ratings are at pressure dew points of –40°F and -100°F. Depending on size, their initial purchase cost can be less than or more than a refrigerated dryer. Operating cost is a more serious consideration with desiccant dryers. You don’t want to use them unless you really need the low dew point.
Operating cost of these dryers varies with the method used to remove water from (regenerate) the desiccant. This process is referred to as “purging” and the three most common methods used are (1) Heatless purge, (2) Heated purge and (3) Blower purge. What all three methods have in common is that air is used to dry out the desiccant and carry the moisture to atmosphere.
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Heatless regenerative desiccant comporessed air dryers take a portion (about 15%) of the dry compressed air leaving the dryer and pass it through the desiccant of the
tower that is being regenerated to abosrb the moisture from the desiccant and
carry it out of the dryer.
A dryer of this type that is rated for 500 CFM will use about 100 CFM for purge.
100 CFM is the output of a 25 hp air compressor. A comparably sized refrigerated air dryer would use a total of about 5 hp. There are optional controls
available to make these dryers more energy-efficient.
Dew Point: up to -100°F
CFM Range: 3-5400
Heated purge regenerative dryers take advantage of the fact that hot air
can hold more water than cold air. These dryers take about 5% of the dry
compressed air leaving the dryer and pass it through an electric heater before
sending it through the wet desiccant bed of the tower that is being regenerated.
A 500 CFM heated purge dryer will use 25 CFM (7½ hp worth) of compressed air and a 4½ kw (6 hp worth) heater. The cost of this dryer is offset by
energy consumption of a little more than half (13½ hp) of that used by the heatless dryer. There are also optional controls avaliable to make this dryer operate even more efficiently.
Dew Point: up to -100°F
CFM Range: 150-5000
Blower purge regenerative dryers use no compressed air for regeneration. A blower mounted on the dryer sends atmospheric air through an electric heater which and
then through the wet desiccant bed that is being regenerated. A 500 CFM blower purge dryer uses a 12 kw (16 hp worth) heater and a 3 kw (4 hp worth)
blower.
In this size the blower purge dryer is still more efficient than the heatless
dryer but not as efficient as the heated purge dryer. In larger sizes this may
not be the case. Blower purge dryers are also useful in situations where there
is not enough excess compressor capacity to feed production and dryer purge
at the same time.
Dew Point: up to -100°F
CFM Range: 150-5000
Manually regenerated desiccant air dryers are most commonly used for intermittent low-flow applications. They are basically a filter that has been filled with desiccant instead of a filter element. The life of the desiccant is expressed in how many cubic feet of compressed air it can process before requiring regeneration.
Dew points as low as –40°F are obtainable, but the actual dew point rises as the desiccant adsorbs more and more moisture. When the desiccant has removed a fixed quantity of moisture from the compressed air passing through it, the desiccant must be either replaced or regenerated by baking off the adsorbed moisture in an oven. Many of the desiccants used in manually regenerated dryers are treated a color change chemical that will provide a visual indication as to when the desiccant must be changed or regenerated.
