Air dryers are extremely important to the health of your air system. They remove excess moisture which eases the load on filtration and allows for better removal of other contaminants. Moisture is the enemy of your air system and the dryer is responsible for removing it.
Due to the importance of moisture removal, ensuring you have the best dryer for your application is essential. There is little difference between the air dryers in many instances, and your decision may come down to upfront cost and energy consumption. It is crucial to understand your system before adding anything new to it.
Refrigerant air dryers are the most commonly used air dryers, and this is due to their price and ease of use across multiple applications. By cooling the air down using refrigeration, these dryers cause condensation to be formed and directly removed from the air stream. These systems do not get below 38°F to avoid the risk of potential freezing in the system. Even with this inherent risk, these dryers remain tried and true in the compressed air industry.
Desiccant air dryers, often called adsorbing dryers, operate differently than refrigerant air dryers. Rather than cooling the air down, the air passes through desiccant beads with a high affinity for water. As the air passes through, moisture is collected by the beads and removed from the air stream. This technique allows the air to reach extremely low dewpoints. With low dewpoints comes a decreased likelihood of condensation build-up at the end of the line.
Membrane air dryers are the least commonly used variation of compressed air dryers. These dryers use water migration by passing it over a membrane with a high affinity to water. This membrane builds up the water vapor until it passes through to the low-pressure side. This side has to have air passed through it to move the water vapor elsewhere to be collected and removed from the system. The downside to these dryers is the requirement of precious compressed air to be used away from the point of use.
So as air exists around us, it exists at atmospheric pressure, about 14.6 PSI at sea level. So if you have 1 square foot of air at atmospheric pressure, and then you increase that pressure to 150 PSI, you are reducing the space that air exists into a tenth of its size. So all the water molecules floating around in that space have been forced into a much smaller area.
Desiccant air dryers are known for reducing the air’s dewpoint to extremely low temperatures. A standard dew point that a regenerative dryer expects is −40 °F; when the air leaves, there is as much water in the air as if the air had been “cooled” to −40 °F. These air dryers typically come in two variations: single canister and twin tower units. As the names suggest, the difference between the two is the number of chambers filled with desiccant. Typically, these models are split between heatless and heated.
Refrigerated dryers are the most common type of compressed air dryer. They remove water from the air stream by cooling the air to approximately 3 °C (38 °F) and effectively condensing out the moisture in a controlled environment. 3 °C (38 °F) is the realistic lower limit for a refrigerated dryer because a lower temperature runs the risk of freezing the separated water. They are typically specified as primary dryers and generally produce air quality that is appropriate for approximately 95% of all compressed air applications.
Desiccant is an exceptionally adsorbent material due to its high affinity to water. In addition, they pull moisture from the air due to their hygroscopic properties. In other words, it can keep the area in immediate contact with the air dry by removing all the moisture into the center of the bead and containing it to allow more water to be adsorbed. This process continues until the beads become entirely saturated and need to be switched out. These dryers work year round with no problem due to the dew point can be made by a desiccant. These dryers have the capability to filter moisture in air that is as low as -100°F.
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