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A Closer Look
Heat-Les Regenerative Dryers...How Do They Work?
The most popular regenerative compressed air dryer design in use today is the “Heat-Les” Dryer. Most users of the heat-les dryers are familiar with the basic operation
of alternating pressure vessels back and forth through a series of switching valves. Some users know that keeping all liquids out of the dryer is of paramount
importance to achieving low dewpoint air. Some even know that the purge rate requirement is 15% of the inlet compressed air flow when operating at 100 PSIG.
But very few know how water vapor is adsorbed out of the air stream and onto the surface of the activated alumina desiccant. Furthermore, very few know how the
water vapor is desorbed from the desiccant and exhausted during regeneration.
The focus of this Closer Look is to analyze how water is adsorbed and desorbed from the desiccant. At Fluid Energy, we believe that understanding this basic
concept is important when analyzing your compressed air system and troubleshooting your heat-les dryer(s).
Before analyzing this subject specifically, it is important to understand the basic operation of heat-les air dryers.
Figure 1 graphically illustrates operation of the most prominent heat-les air dryer design in use today. Compressed air enters through inlet valve V1, passes
through a dry bed of activated alumina desiccant, and exits through check valve CV1. While the on-line bed is drying the air at full line pressure, the off-line
chamber is being regenerated at atmospheric pressure or 0 PSIG. Regeneration air is expanded to atmospheric pressure through a flow meter assembly, which usually
consists of an adjustable valve, a pressure gauge, and an orifice. By controlling air pressure upstream of an orifice the compressed air flow is known.
When operating at 100 PSIG, the purge air requirements are 15% of the inlet flow.
As the compressed air passes across the activated alumina desiccant bed, a natural process of equilibrium occurs. Because the compressed air is 100% saturated,
it is holding all of the water vapor it can. Conversely, the activated alumina desiccant is very dry and it naturally wants to come into equilibrium with the
saturated inlet air. In other words, the desiccant wants to hold the same proportionate amount of water vapor as the inlet air so that both the air and the desiccant
are in equilibrium.
The same phenomenon occurs naturally everyday in real life. An example would be spilled water on a table eventually disappearing because of evaporation.
The dry ambient air wants to hold as much water vapor as it can and the liquid water eventually disappears.
Referring to Figure 1, consider a dryer designed to provide a –40 pressure dewpoint for a capacity of 1000 SCFM at inlet conditions of 100 PSIG, 70ºF, and
100% relative humidity. As discussed in a previous edition of the Closer Look, the actual volume flow under these conditions is about 128 actual cubic feet
per minute or 128 ACFM.
As stated earlier, the purge air requirement when operating at 100 PSIG is 15% of the inlet airflow. 15% of 128 ACFM is 19.2 ACFM at 100 PSIG. Based on the
ideal gas laws, the amount of purge air expanded to atmospheric pressure is:
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114.7 PSIA
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19.2 ACFM x
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14.7 PSIA
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= 150 ACFM at Atmosphere
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Not surprisingly, this is 15% of the dryer design rating of 1000 SCFM.
During the drying cycle, only the upper 1/3 of each desiccant bed is used. The other 2/3 is used for storage in the event of overload and also for
insulation to keep the heat of adsorption in the bed. This “heat of adsorption” also aids in the regeneration process by adding heat to the purge air.
The golden rule in the dehydration of air and gases is that “the amount of water vapor that air and gases can hold is a function of temperature only.”
In the continuing example of a 1000 SCFM dryer, the actual volume flow being dried is 128 ACFM at 100 PSIG, 0 PSIG, or 200 PSIG, the amount of water
vapor it can hold is still .15 pounds per minute.
Referring back to the example, 128 ACFM of saturated air is being stripped of water vapor in the on-line chamber. Meanwhile, 150 ACFM of dry air is
flowing countercurrently in the off-line chamber to desorb the water vapor from the desiccant. In a perfect world, the regeneration air would need
only to be 128 ACFM. But because it is very difficult to achieve 100% relative humidity, regeneration air needs to be supplied at a 15% rate or 150 ACFM for this example.
All of this explanation is provided to explain one important fact. That is, the volume of regeneration air needs to be slightly greater than the
volume of air being dried. If anything occurs to upset this from happening, high dewpoints can be expected. Some of these potential upset conditions are:
Excessive inlet air flow
Low inlet air pressure
Leaking check valve or inlet valve
Malfunctioning exhaust valve causing back pressure
Plugged exhaust muffler causing back pressure
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