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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Russ Bowman Application Engineer EXAIR Corporation Visit us on the Web Follow me on Twitter Like us on Facebook
If you operate an air compressor, you’re drawing water vapor into your compressed air system. Factors like climate control (or lack thereof,) and humidity will dictate how much. If (or more to the point, when) it condenses, it becomes an issue that must be addressed. There are several types of dryer systems to choose from, usually when you buy your compressor…we’ve covered those in a number of blogs. Some of these can leave a little more water vapor than others, but remain popular and effective, when considering the cost, and cost of operation, of the different types.
So, how do you handle the condensate that the dryer doesn’t remove?
Receivers, or storage tanks (like EXAIR Model 9500-60, shown to the right,) are commonly used for several reasons:
By providing an intermediate storage of compressed air close to the point of use, fluctuations across the system won’t adversely affect an application that needs a constant flow and pressure.
This also can keep the air compressor from cycling rapidly, which leads to wear & tear, and additional maintenance headaches.
When fitted with a condensate drain (more on those in a minute,) they can serve as a wet receiver. Condensate collects in the bottom and is manually, or automatically emptied.
Condensate drains, while popularly installed on receivers, are oftentimes found throughout larger systems where the vapor is prone to condense (intercoolers, aftercoolers, filters and dryers) and where the condensation can be particularly problematic (drip legs or adjacent to points of use.) There are a couple of options to choose from, each with their own pros & cons:
Manual drains are self explanatory: they’re ball valves; cycled periodically by operators. Pros: cheap & simple. Cons: easy to blow down too often or for too long, which wastes compressed air. It’s also just as easy to blow down not often enough, or not long enough, which doesn’t solve the condensate problem.
Timer drains are self explanatory too: they cycle when the timer tells them to. Pros: still fairly cheap, and no attention is required. Cons: they’re going to open periodically (per the timer setting) whether there’s condensate or not.
Demand, or “zero loss” drains collect condensate until their reservoir is full, then they discharge the water. Pros: “zero loss” means just that…they only actuate when condensate is present, and they stop before any compressed air gets out. Cons: higher purchase price, more moving parts equals potential maintenance concerns.
The “last line of defense” (literally) is point-of-use condensate removal. This is done with products like EXAIR Automatic Drain Filter Separators. They’re installed close to compressed air operated devices & products, oftentimes just upstream of the pressure regulator and/or flow controls…the particulate filter protects against debris in these devices, and the centrifugal element “spins” any last remaining moisture from the compressed air flow before it gets used.
Good engineering practice calls for point of use filtration and moisture removal, such as that provided by EXAIR Filter Separators.
Efficient and safe use of your compressed air includes maintaining the quality of your compressed air. If you’d like to find out more about how EXAIR Corporation can help you get the most out of your compressed air system, give me a call.
Russ Bowman
Application Engineer
EXAIR Corporation
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The supply side of a compressed air system has many critical parts that factor in to how well the system operates and how easily it can be maintained. Dryers for the compressed air play a key role within the supply side are available in many form factors and fitments. Today we will discuss heat of compression-type dryers.
Heat of compression-type dryer- Twin Tower Version
Heat of compression-type dryers are a regenerative desiccant dryer that take the heat from the act of compression to regenerate the desiccant. By using this cycle they are grouped as a heat reactivated dryer rather than membrane technology, deliquescent type, or refrigerant type dryers. They are also manufactured into two separate types.
The single vessel-type heat of compression-type dryer offers a no cycling action in order to provide continuous drying of throughput air. The drying process is performed within a single pressure vessel with a rotating desiccant drum. The vessel is divided into two air streams, one is a portion of air taken straight off the hot air exhaust from the air compressor which is used to provide the heat to dry the desiccant. The second air stream is the remainder of the air compressor output after it has been processed through the after-cooler. This same air stream passes through the drying section within the rotating desiccant drum where the air is then dried. The hot air stream that was used for regeneration passes through a cooler just before it gets reintroduced to the main air stream all before entering the desiccant bed. The air exits from the desiccant bed and is passed on to the next point in the supply side before distribution to the demand side of the system.
The twin tower heat of compression-type dryer operates on the same theory and has a slightly different process. This system divides the air process into two separate towers. There is a saturated tower (vessel) that holds all of the desiccant. This desiccant is regenerated by all of the hot air leaving the compressor discharge. The total flow of compressed air then flows through an after-cooler before entering the second tower (vessel) which dries the air and then passes the air flow to the next stage within the supply side to then be distributed to the demand side of the system.
The heat of compression-type dryers do require a large amount of heat and escalated temperatures in order to successfully perform the regeneration of the desiccant. Due to this they are mainly observed being used on systems which are based on a lubricant-free rotary screw compressor or a centrifugal compressor.
No matter the type of dryer your system has in place, EXAIR still recommends to place a redundant point of use filter on the demand side of the system. This helps to reduce contamination from piping, collection during dryer down time, and acts as a fail safe to protect your process. If you would like to discuss supply side or demand side factors of your compressed air system please contact us.
