2024-03-11
What is an air dryer?
In pneumatic systems, the compressor sucks in ambient air that always contains a certain amount of water vapor. The process of compressing the air to about 100 psi will increase the temperature of the air, but it will also increase the dew point and water retention capacity. Any subsequent cooling downstream may cause some water vapor to condense. Liquid water in pneumatic systems can cause problems such as corrosion and bacterial growth, which can lead to slow control or component failure. Therefore, air dryers are often required to alleviate the water problem.
In a typical system, air flows from the compressor to an aftercooler, which lowers the temperature, causing condensation and removing most of the water. Coalescing filters are sometimes installed to remove excess water. However, if the circuit requires further processing, it must be equipped with an air dryer. They remove most or all of the water from the compressed air until it reaches the critical component or process. The following are the main types.
Refrigerated air dryers remove water by cooling the compressed air temperature and condensation. The internal moisture separator collects liquid water and sends it to the drain. Freeze dryers typically produce air with a pressure dew point between 35 and 40 °F. They are commonly used in general plant operations. They may not be suitable for more critical processes that require extremely dry air, and they are not suitable for circuits with temperatures below freezing.
Freeze dryers are considered very economical to purchase and operate. They are divided into two categories, bicycles and non-bicycles. As the name implies, one type runs intermittently and the other runs continuously. Users should consider using a circulating dryer, which can only be powered to meet demand, thereby reducing power consumption and energy costs.
Desiccant air dryers work on a different principle: they absorb moisture from the air stream in a reversible manner and adsorb onto the desiccant. They produce low dew points and are therefore a good choice in sub-freezing conditions or when the process requires extremely dry air. There are two types: unheated and heated.
The unheated desiccant dryer is filled with desiccant in two adjacent tanks (called the drying tower and the regeneration tower). The moisture-laden compressed air flows into the drying tower, where it passes and combines with the porous desiccant. The extremely dry air leaves the dryer with a pressure dew point of -40 to -100 °F.
This adsorption process also generates heat, which typically raises the air temperature by as much as 20 °F. In order to remove water from the desiccant, about 15% of the dry air of the first column is introduced into the second regeneration column. Here, the dry compressed air expands to atmospheric pressure when passing through the desiccant, and water is sucked out of the desiccant, which is caused by the increase in the air temperature due to the heat of adsorption. The moist air is then expelled from the system. The dryer cycles between drying and regeneration operations at regular intervals, so that a tower always dries the incoming air.
As the name suggests, heated desiccant dryers have a heater in the loop. Just like the unheated version, a desiccant in the tower can remove moisture from the air. The resulting pressure dew point range is -40 to -100 °F. The second column regenerates the used desiccant. The valve diverts approximately 8% of the air leaving the drying tower and passes it through the heater. This dry hot air then passes over the desiccant in the regeneration tower, releasing the previously captured moisture. The moist air is then vented to the outside, usually through a muffler.
When weighing the advantages of an unheated dryer and a dry air dryer, keep in mind the economics. The cost of producing compressed air can be substantial, and heated dryers use about 50% less compressed air for regeneration. On the other hand, the heater may require a large amount of power. Therefore, users should review the details of the application to determine which type is less costly.
A related type of desiccant dryer is a compression heat dryer. These are special versions of desiccant dryers that reuse the heat generated by oil-free air compressors and are highly energy efficient. In an adsorption dryer, high temperature, high pressure air (usually above 300 °F) from the compressor first enters the regeneration tower and releases moisture from the desiccant. The air stream then exits and passes through an aftercooler, reducing the air temperature to about 100 °F and removing some of the water. The cold air then flows through the drying tower and exits at a dew point of approximately -40 °F.
The adsorption dryer is combined with an oil-free compressor to produce high-quality compressed air without water and oil, suitable for the food, beverage and pharmaceutical industries. Compared with refrigerated and conventional desiccant dryers, adsorption dryers have higher upfront costs. However, because they are powered by the waste heat of the compressor, the operating cost of adsorption dryers is extremely low.
Membrane air dryers rely on the selective permeability of specially designed membrane materials and the pressure differential inside the dryer. Small water molecules in the air can pass through the tiny holes in the membrane; larger nitrogen and oxygen molecules cannot.
The dryer consists of a cylindrical container containing a bundle of hollow membrane tubes. In fact, untreated compressed air enters the dryer and flows through the duct. Also, the volume outside the tube but inside the vessel itself is at atmospheric pressure, creating a pressure differential on the tube wall. Intake air passes through the duct, and the pressure difference allows only water molecules to pass through the membrane, while dry air leaves the device.
A certain percentage of dry "purge" air (usually 10% to 20%) is recirculated outside the tube and carries away the water vapor. Depending on the design, the pressure dew point ranges from 40 ° to as low as -40 °F.
Membrane air dryers are compact, lightweight, require no power and no moving parts, so they are inexpensive to operate and do not require routine maintenance. These units are usually recommended for point-of-use applications, close to electrical or explosion hazards, and remote areas.
For any type of dryer, experts recommend installing standard filters and coalescing filters upstream. This prevents particles, oil and liquid water from entering the dryer, maintaining high efficiency and helping to ensure a long life.
Another suggestion is not to over-specify the dryer. It is rarely necessary to run every part of the compressed air system at the lowest dew point, which is expensive and almost always wasteful. On the other hand, do not leak water during drying and specify that the dew point is too high. Water damage in the system is equally expensive. Experts say that users should only provide the dryness required for each application, especially when the dryness varies depending on the process or machine. And, as with any system, upfront and operating costs, flow and performance should be considered when specifying a dryer.
What, dryer, air, desiccant, drying, generating, compressed air, them, dew point, system
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