As we head in to the colder months here in Ohio, I will soon be getting my humidifier out of the basement and set up in my bedroom. The dry air that accompanies the onset of winter chaps my lips, cracks the skin on my knuckles, affects my nasal passages, and oftentimes makes me wake up with a sore throat…something I definitely don’t want to happen in the middle of a pandemic! So I put some water vapor in my home’s air, on purpose, to take care of all of that.
Moisture in an industrial compressed air system, however, isn’t good for anything. It’ll corrode your pipes, get rust in your pneumatic tools, motors, and cylinders, and spit out of your blow off devices, all over whatever you’re using your air to blow off. Depending on the type of compressor, where, and how, it’s used, there are different types of dryers. Today, dear reader, we’re taking a look at one of the most basic moisture removal systems: the deliquescent dryer. The principle of operation is as follows:
Deliquescent dryer: how it works (1)
Incoming compressed air enters near the base, where a form of mechanical separation occurs…the air flows back & forth, around trays of desiccant. The simple act of changing direction causes a certain amount of free liquid to just fall out and collect in the bottom.
The air then flows upwards through the desiccant bed. The desiccant in a deliquescent dryer absorbs moisture (as opposed to the adsorption that occurs in a regenerative desiccant dryer) until they get so wet, they dissolve.
The desiccant level has to be monitored (commonly via a sight glass) so it can be replaced as it’s consumed.
After the desiccant does its job, moisture free air flows out the top, and gets on with it’s work.
Deliquescent dryers, owing to their simplicity, are the least expensive air dryers. They have no moving parts and no electricity, so the only maintenance involved is replacing the desiccant media as it’s consumed. This makes them especially popular in mobile/on-site applications involving portable or tow-behind, engine driven compressors, since they don’t need power to run.
There are several disadvantages, also owing to their simplicity:
The deliquescent media has to be periodically replenished. If you don’t stay on top of it, you can find yourself shut down while you go back to the shop to get a big bag of salt. That’s time your boss can’t charge your customer for. Also, the cost of the new media is a continual operating cost of the dryer…something you don’t have to account for with the regenerative desiccant models.
Disposal of the waste media can be a concern…you definitely want to check your local environmental regulations before dumping it in the garbage. Your boss won’t like talking to the EPA about THAT either.
They have to be equipped with a particulate filter on the discharge to keep the deliquescent media (which, being a salt, is corrosive in nature) from entering your system. That would be even worse than water moisture…which this is there to prevent in the first place.
They don’t get near as low of a dewpoint as other dryers – the best you can hope for is 20°F to 30°F. Which is fine, given the above mentioned nature of applications where these are commonly used. You just wouldn’t want to use them to supply a product like an EXAIR Vortex Tube…which can turn that in to -40°F cold air, causing the water vapor to turn to liquid, and then to ice. In a hurry.
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Most people are familiar with desiccant from the small packets we find enclosed with a new pair of shoes, in a bag of beef jerky, or in some medication bottles. These packets almost always say “Do Not Eat,” and I get that for the ones in the beef jerky or the pill bottles, but I just don’t understand why they put it on the desiccant packets bound for a shoe box…
Anyway, desiccant (in MUCH larger volumes than the household examples above) are also used to get water vapor out of compressed air. Desiccant dryers are popular because they’re effective and reliable. The most common design consists of two vertical tanks, or towers, filled with desiccant media – usually activated alumina or silica gel.
These materials are prone to adsorption (similar to absorption, only it’s a physical process instead of a chemical one) which means they’re good at trapping, and holding, water. In operation, one of these towers has air coming in it straight from the compressor (after it’s become pressurized, remember, it still has just as much water vapor in it as it did when it was drawn in…up to 5% of the total gas volume.)
When that tower’s desiccant has adsorbed water vapor for long enough (it’s usually controlled by a timer,) the dryer controls will port the air through the other tower, and commence a restoration cycle on the first tower. So, one is always working, and the other is always getting ready for work.
There are three methods by which the desiccant media can be restored:
Regenerative Desiccant Dryers send a purge flow of dry air (fresh from the operating tower’s discharge) through the off-line tower’s desiccant bed. This dry air flow reverses the adsorption process, and carries the water away as it’s exhausted from the dryer. This is simple and effective, but it DOES use a certain amount of your compressed air.
Heat Of Compression Desiccant Dryers use the heat from pressurized air straight from the compressor(s). This hot air is directed through one tower, where it removes moisture from the desiccant. It then flows through a heat exchanger where it’s cooled, condensing the moisture, before it flows through the other tower to remove any remaining moisture. This method doesn’t add to your compressed air usage, but it only works with oil-free compressors.
The third method uses a hot air blower to flow heated air through the off-line desiccant bed. It’s similar to the Regenerative type, but it doesn’t use compressed air. However, they DO require a certain amount of wattage for the heater…remember, electricity isn’t cheap either.
As an EXAIR Application Engineer, it’s my job to help you get the most out of our products, and your compressed air system. If you have questions about compressed air, call me.
Russ Bowman
Application Engineer
EXAIR Corporation
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