Drying compressed air is similar to removing the humidity in the air when using an air conditioning system.
From a functional standpoint, what does this really mean? What will take place in the compressed air system if the air is not dried and the moisture is allowed to remain?
The answer is in the simple fact that moisture is damaging. Rust, increased wear of moving parts, discoloration, process failure due to clogging, frozen control lines in cold weather, false readings from instruments and controls – ALL of these can happen due to moisture in the compressed air. It stands to reason, then, that if we want long-term operation of our compressed air products, having dry air is a must.
A heat of compression type dryer is a regenerative desiccant dryer which uses the heat generated by the compression of the ambient air to regenerate the moisture removing capability of the desiccant used to dry the compressed air.
When using one of these dryers, the air is pulled directly from the outlet of the compressor with no cooling or treatment to the air and is fed through a desiccant bed in “Tank 1” where it regenerates the moisture removing capabilities of the desiccant inside the tank. The compressed air is then fed through a regeneration cooler, a separator, and finally another desiccant bed, this time in “Tank 2”, where the moisture is removed. The output of “Tank 2” is supplied to the facilities as clean, dry compressed air. After enough time, “tank 1” and “tank 2” switch, allowing the hot output of the compressor to regenerate the desiccant in “tank 2” while utilizing the moisture removing capabilities of the desiccant in “tank 1”.
If you have questions about your compressed air system and how the end use devices are operating, contact an EXAIR Application Engineer. We’ll be happy to discuss your system and ways to optimize your current setup.
A critical component on the supply side of your compressor system is the dryer. Atmospheric air contained within a compressed air system contains water vapor. The higher the temperature of the air, the more volume of moisture that air is capable of holding. As air is cooled, this water vapor can no longer be contained and this water falls out in the form of condensation. The temperature where this water will drop out is referred to as the dew point.
At a temperature of 75°F and 75% relative humidity, approximately 20 gallons of water will enter a 25HP compressor during a 24-hour period. As air is compressed, this water becomes concentrated. Since it’s heated during the compression process, this water stays in a vapor form. When this air cools further downstream, this vapor condenses into droplet form.
Moisture within the compressed air system can result in rust forming on the inside of the distribution piping, process failure due to clogged frozen lines in colder weather, false readings from instruments and controls, as well as issues with the point of use products installed within the system.
In a membrane dryer, compressed air is forced through a specially designed membrane that permits water vapor to pass through faster than the air. The water vapor is then purged along with a small amount of air while the rest of the compressed air passes through downstream. Generally, the dew point after the membrane dryer is reduced to about 40°F with even lower dew points also possible down to as low as -40°F!
With such low dew points possible, it makes a membrane dryer an optimal choice in outdoor applications that are susceptible to frost in colder climates. Membrane dryers also are able to be used in medical and dental applications where consistent reliability is critical.
A membrane dryer does not require a source of electricity in order to operate. The compact size makes it simple to install without requiring a lot of downtime and floor space. Since they have no moving parts, maintenance needed is minimal. Most often, this maintenance takes the form of checking/replacing filter elements just upstream of the membrane dryer. The membrane itself does need to be periodically replaced, an indicator on the membrane dryer will display when it needs to be changed. If particular instruments or processes in your facility are sensitive to moisture, a membrane dryer might be the best option.
However, there are some drawbacks to these types of dryers. They’re limited to low capacity installations, with models ranging from less than 1 SCFM up to 200 SCFM. This makes them more applicable for point-of-use installations than for an entire compressed air system. The nature in which the membrane dryer works necessitates some of the air to be purged out of the system along with the moisture. To achieve dew points as low as -40°F, this can equate to as much as 20% of the total airflow. When proper filtration isn’t installed upstream, oils and lubricants can ruin the dryer membrane and require premature replacement.
Make sure and ask plenty of questions of your compressor supplier during installation and maintenance of your system so you’re aware of the options out there. You’ll of course want to make sure that you’re using this air efficiently. For that, EXAIR’swide range of engineered Intelligent Compressed Air Products fit the bill. With a variety of products available for same-day shipment from stock, we’ve got you covered.
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 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.
No matter what your use of compressed air entails, moisture is very likely an issue. Air compressors pressurize air that they pull in straight from the environment and most of the time, there’s at least a little humidity involved. Now, if you have an industrial air compressor, it’s also very likely that it was supplied with a dryer, for this very reason.
For practical purposes, “dryness” of compressed air is really its dew point. That’s the temperature at which water vapor in the air will condense into liquid water…which is when it becomes the aforementioned issue in your compressed air applications. This can cause rust in air cylinders, motors, tools, etc. It can be detrimental to blow offs – anything in your compressed air flow is going to get on the surface of whatever you’re blowing onto. It can lead to freezing in Vortex Tube applications when a low enough cold air temperature is produced.
Some very stringent applications (food & pharma folks, I’m looking at you) call for VERY low dew points…ISO 8673.1 (food and pharma folks, you know what I’m talking about) calls for a dew point of -40°F (-40°C) as well as very fine particulate filtration specs. As a consumer who likes high levels of sanitary practice for the foods and medicines I put in my body, I’m EXTREMELY appreciative of this. The dryer systems that are capable of low dew points like this operate as physical filtration (membrane types) or effect a chemical reaction to absorb or adsorb water (desiccant or deliquescent types.) These are all on the higher ends of purchase price, operating costs, and maintenance levels.
For many industrial and commercial applications, though, you really just need a dew point that’s below the lowest expected ambient temperature in which you’ll be operating your compressed air products & devices. Refrigerant type air dryers are ideal for this. They tend to be on the less expensive side for purchase, operating, and maintenance costs. They typically produce air with a dew point of 35-40°F (~2-5°C) but if that’s all you need, they let you avoid the expense of the ones that produce those much lower dew points. Here’s how they work:
Red-to-orange arrows: hot air straight from the compressor gets cooled by some really cold air (more on that in a moment.)
Orange-to-blue arrows: the air is now cooled further by refrigerant…this causes a good amount of the water vapor in it to condense, where it leaves the system through the trap & drain (black arrow.)
Blue-to-purple arrows: Remember when the hot air straight from the compressor got cooled by really cold air? This is it. Now it flows into the compressed air header, with a sufficiently low dew point, for use in the plant.
Non-cycling refrigerant dryers are good for systems that operate with a continuous air demand. They have minimal dew point swings, but, because they run all the time, they’re not always ideal when your compressed air is not in continuous use. For those situations, cycling refrigerant dryers will conserve energy…also called mass thermal dryers, they use the refrigerant to cool a solution (usually glycol) to cool the incoming air. Once the glycol reaches a certain temperature, the system turns on and runs until the solution (thermal mass) is cooled, then it turns off. Because of this, a cycling system’s operating time (and cost) closely follows the compressor’s load – so if your compressor runs 70% of the time, a cycling dryer will cost 30% less to operate than a non-cycling one.
EXAIR Corporation wants you to get the most out of your compressed air system. If you have questions, I’d love to hear from you.
Visit us on the Web
Follow me on Twitter
Like us on Facebook
In this blog, I would like to turn your attention to our accessories that support our Intelligent Compressed Air® Products; the Filter Separator and the Oil Removing Filter. EXAIR products use compressed air to coat, conserve, cool, convey and clean. So, to keep our products running properly and efficiently, we need to supply them with clean, pressurized air. If we look at the two types of filters that we offer, we can see how they can play an important part in your compressed air system.
Filter separators are used to remove bulk liquid and contamination from the compressed air stream. They have a 5-micron filter and work very well if you get a slug of liquid in your compressed air system. They use mechanical separation to remove the large particles of dirt and water from the air stream.
Most facilities use some type of compressed air dryer in their system to dry and condition the compressed air. But, if a system failure occurs, then water, oil, and dirt can be pushed into the compressed air lines and perhaps into your EXAIR products.
Even if you have good quality air, it is still important to keep your products protected. I would consider the Filter Separator as a minimum level of protection that should be used.
The Oil Removal Filters are used to keep the compressed air even cleaner yet. They work great at removing very small particles of dirt and oil. The 0.03-micron media of the Oil Removal Filter is designed to “coalesce” the fine liquid particles into large droplets.
Thus, allowing gravity to remove it from the compressed air stream. Some common issues allow for dirt and oil particles to collect in “dead” zones within the air lines. As it piles up and grows, portions can break off and get into the air stream affecting pneumatic devices.
The Oil Removal Filter will be able to help eliminate this long-term problem in your compressed air system. As a note, Oil Removal Filters are not great for bulk separation. If you have a system with lots of water, you should use a Filter Separator in front of the Oil Removal Filter to optimize the filtration.
Now that we went through each type, how do we use them together to get the best supply of compressed air? We always want them to be installed upstream of a Regulator. This is because the velocity is lower at higher pressures.
Lower velocities mean lower pressure drops which is great for supplying the proper amount of compressed air to EXAIR products. If you are using a combination of both filters, the Filter Separator will be upstream of the Oil Removal Filter. The Filter Separator will knock down the large particles and liquid slugs allowing the Oil Removal Filter to remove the smaller droplets and particles.
EXAIR offers a range of sizes to help support our products. They range from ¼” NPT ports up to 1 ½” NPT ports. The size of the ports determines the flow rating for each unit. EXAIR also has Mounting Brackets to mount the filters to walls or frames.
To support each type of filter, we have replacement elements and bowl kits. Since the function of the filter is to remove debris, we recommend to change the filter element once a year or when it reaches 10 PSID pressure drop; whichever comes first.
If we can analyze the compressed air systems, I would like to categorize it into a good and premium quality. For the good quality of compressed air, you can have the compressed air run through the Filter Separator. For the premium quality of compressed air, you can have your compressed air run through the Filter Separator and then through the Oil Removal Filter.
With clean quality air, your EXAIR products will provide you with effective, long-lasting performance without maintenance downtime.
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.
Visit us on the Web
Follow me on Twitter
Like us on Facebook
All atmospheric air contains some amount of water vapor. When air is then cooled to saturation point, the vapor will begin to condense into liquid water. The saturation point is the condition where the the air can hold no more water vapor. The temperature at which this occurs is knows as the dew point.
When ambient air is compressed, heat is generated and the air becomes warmer. In industrial compressed air systems, the air is then routed to an aftercooler, and condensation begins to take place. To remove the condensation, the air then goes into separator which traps the liquid water. The air leaving the aftercooler is typically saturated at the temperature of the discharge, and any additional cooling that occurs as the air is piped further downstream will cause more liquid to condense out of the air. To address this condensation, compressed air dryers are used.
It is important to dry the air and prevent condensation in the air. Many usages of the compressed air are impacted by liquid water being present. Rust and corrosion can occur in the compressed air piping, leading to scale and contamination at point -of -use processes. Processes such as drying operations and painting would see lower quality if water was deposited onto the parts.
There are many types of dryers – (see recent blogs for more information)
Refrigerant Dryer – most commonly used type, air is cooled in an air-to-refrigerant heat exchanger.
Regenerative-Desiccant Type – use a porous desiccant that adsorbs (adsorb means the moisture adheres to the desiccant, the desiccant does not change, and the moisture can then be driven off during a regeneration process).
Deliquescent Type – use a hygroscopic desiccant medium that absorbs (as opposed to adsorbs) moisture. The desiccant is dissolved into the liquid that is drawn out. Desiccant is used up, and needs to be replaced periodically.
Heat of Compression Type – are regenerative desiccant dryers that use the heat generated during compression to accomplish the desiccant regeneration.
Membrane Type– use special membranes that allow the water vapor to pass through faster than the dry air, reducing the amount water vapor in air stream.
The air should not be dried any more than is needed for the most stringent application, to reduce the costs associated with the drying process. A pressure dew point of 35°F to 38°F (1.7°C to 3.3°C) often is adequate for many industrial applications. Lower dew points result in higher operating costs.
If you have questions about compressed air systems and dryers or any of the 15 different EXAIR Intelligent Compressed Air® Product lines, feel free to contact EXAIR and myself or any of our Application Engineers can help you determine the best solution.