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.
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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.
For properly designed compressed air systems, air compressors will use primary storage tanks, or receivers. They are necessary to accommodate for fluctuations in airflow demand and to help prevent rapid cycling of the air compressor. (Reference: Advanced Management of Compressed Air – Storage and Capacitance) There are two types of primary receivers, a wet receiver tank and a dry receiver tank. The wet receiver is located between the air compressor and the compressed air dryer where humid air and water will be stored. The dry receiver is located after the compressed air dryer. In this blog, I will be reviewing the wet receivers and their requirements as a storage tank.
Air compressors discharge hot humid air created by the internal compression. A byproduct of this compression is water. By placing a wet receiver on the discharge side of the air compressor, this will create a low velocity area to allow the excess water to fall out. It will also give the hot air time to cool, allowing the compressed air dryers to be more effective. With wet receivers, it will reduce cycle rates of your air compressors for less wear and store compressed air to accommodate for flow fluctuations in your pneumatic system.
But, there are some disadvantages with a wet receiver. For compressed air dryers, it is possible to exceed the specified flow ratings. If the demand side draws a large volume of air from the supply side, the efficiency of the compressed air dryers will be sacrificed, allowing moisture to go downstream. Another issue with the wet receiver is the amount of water that the air compressor is pumping into it. As an example, a 60 HP air compressor can produce as much as 17 gallons of water per day. As you can see, it would not take long to fill a wet receiver. So, a condensate drain is required to get rid of the excess water.
Condensate drains come in different types and styles. They are connected to a port at the bottom of the wet receiver where the water will collect. I will cover the most common condensate drains and explain the pros and cons of each one.
Manual Drain – A ball valve or twist drain are the least efficient and the least expensive of all the condensate drains. The idea of having personnel draining the receiver tanks periodically is not the most reliable. In some cases, people will “crack” the valve open to continuously drain the tank. This is very inefficient and costly as compressed air is being wasted.
Timer Drain Valves – These valves have an electric timer on a solenoid to open and close a two-way valve or a ball valve. The issue comes in trying to set the correct time for the open and close intervals. During seasonal changes, the amount of water going into the wet receiver will change. If the timer is not set frequent enough, water can build up inside the receiver. If too frequent, then compressed air is wasted. Compared to the manual valve, they are more reliable and efficient; but there is still potential for compressed air waste.
No-waste Drains – Just like the name, these drains are the most efficient. They are designed with a float inside to open and close a drain vent. What is unique about the float mechanism is that the drain vent is always under water. So, when the no-waste drain is operating, no compressed air is being lost or wasted; only water is being drained. The most common problem comes with rust, sludge, and debris that can plug the drain vent.
All wet receivers require a condensate drain to remove liquid water. But, the importance for removing water without wasting compressed air is significant for saving money and compressed air. EXAIR also has a line of Intelligent Compressed Air® products that can reduce your compressed air waste and save you money. You can contact an Application Engineer for more details.
Recently we have blogged about Compressed Air Dryers and the different types of systems. We have reviewed the Desiccant and Refrigerant types of dryers, and today I will discuss the basics of the Membrane type of dryers.
All atmospheric air that a compressed air system takes in contains water vapor, which is naturally present in the air. At 75°F and 75% relative humidity, 20 gallons of water will enter a typical 25 hp compressor in a 24 hour period of operation. When the the air is compressed, the water becomes concentrated and because the air is heated due to the compression, the water remains in vapor form. Warmer air is able to hold more water vapor, and generally an increase in temperature of 20°F results in a doubling of amount of moisture the air can hold. The problem is that further downstream in the system, the air cools, and the vapor begins to condense into water droplets. To avoid this issue, a dryer is used.
Membrane Dryers are the newest type of compressed air dryer. Membranes are commonly used to separate gases, such as removing nitrogen from air. The membrane consists of a group of hollow fiber tubes. The tubes are designed so that water vapor will permeate and pass through the membrane walls faster than the air. The dry air continues on through the tubes and discharges into the downstream air system. A small amount of ‘sweep’ air is taken from the dry air to purge and remove the water vapor from inside the dryer that has passed through the membrane tubes.
Resultant dew points of 40°F are typical, and dew points down to -40°F are possible but require the use of more purge air, resulting in less final dry compressed air discharging to the system.
The typical advantages of Membrane Dryers are-
Low installation and operating costs
Can be installed outdoors
Can be used in hazardous locations
No moving parts
There are a few disadvantages to consider-
Limited to low capacity systems
High purge air losses (as high as 15-20% to achieve lowest pressure dew points
Membrane can be fouled by lubricants and other contaminants, a coalescing type filter is required before the membrane dryer.
If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.