COMPRESSED AIR DRYERS WHITEPAPER HOW TO SELECT THE RIGHT DRYER
The most basic requirement of a dryer is to keep up with the air production of the compressor to meet the respective process requirements. Fortunately, there are calculation tools available that help users figure out which dryer is best. These tools should definitely be used to be sure that the sizing of the machine is properly done. Guessing the dryer size, or simply replacing an old dryer by a similar sized dryer, will not always lead to the expected outcome.
To determine which dryer is the best fit, there are six major criteria that should be calculated and taken into account:
1. Maximum air flow rate in m³/min 2. Desired pressure dew point 3. Air inlet pressure 4. Air inlet temperature 5. Ambient air temperature (and water temperature if the condenser is water-cooled) 6. The installation environment of the dryer
Adsorption dryers can provide an ultra-low dew point, generally around -40°C. A reliable and predictable dew point can be important for demanding applications in sectors such as pharmaceuticals and food processing. Refrigerant dryers typically achieve a dew point of about 3°C, but they are less expensive to purchase, operate and maintain than adsorption dryers. They are mostly used for protecting the installation, rather than necessary for specific applications.
Watch the wiki video for more information on how to choose the correct air dryer for your compressor:
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Refrigerant dryers are the most commonly used type of compressed air dryers. They consist of an air-to-air heat exchanger and an air-to-freon heat exchanger. Here is how they work: The compressed air from the compressor first passes through the air-to-air heat exchanger. In this phase, the incoming air is pre-cooled by the outgoing, cooler air and the outgoing air is heated. In the second phase, the compressed air passes through the air-to-freon heat exchanger, where it is further cooled down to a temperature of approximately 3°C. At this temperature, the present water vapor is partly being condensed, collected and removed. At this stage the relative humidity of the compressed air is still 100%. In the next step, the compressed air is heated by the incoming air, and, thanks to the resulting increase in temperature, the relative humidity of the outgoing air drops below 50%. Refrigerant dryers, which are available in water-cooled and air-cooled versions, are used in compressed air systems to avoid water and corrosion in the system. A relative humidity below 50% is sufficient to accomplish this goal.
A refrigerant dryer is equipped with a compressor that compresses the refrigerant gas into a liquid. Conventional refrigerant air dryers typically use a reciprocating (piston) compressor that operates at a fixed speed. Essentially, it means that this compressor is either “off” or “on.” When it is on, it runs at full speed, which is not very energy efficient.
Conversely, a VSD refrigerant dryer uses a screw compressor with a screw element that orbits a second fixed element. It is compact and quiet, and the motor speed can be adjusted to meet a specific demand, which is much more energy efficient.
In addition to its great efficiency, another key benefit of a VSD refrigerant dryer is its reliable supply of high-quality air at a stable and low dew point, regardless of the ambient air temperature. This is a major advantage over thermal mass dryers, which offer some energy savings. However, in that case, a thermal mass is cooled and then dries the air as the mass slowly warms up. The result is a dew point that rises and falls with the temperature of the thermal mass, which means that the quality of the compressed air may vary by up to two purity classes. This fluctuation can harm the operation’s production quality and reliability. In contrast, a VSD refrigerant dryer continuously supplies dry air at a stable dew point.
Adsorption dryers are used when the compressed air application requires a dew point lower than 0°C. In most cases, the dryers consist of two pressure vessels placed next to each other. These two containers are filled with hygroscopic beads, which are a type of desiccant.
Compressed air passes through one container as the moisture in the air is adsorbed by the beads. Once they have captured a certain amount of moisture, the beads are saturated. At this point, the air is guided to the second container. While the compressed air passes through the second container, the beads of the first container are regenerated, which means the captured moisture is released again and disposed of. When the beads in the second container are saturated, the air is once again guided through the first, regenerated container and the process begins anew.
There are different types of adsorption air dryers on the market, each with a specific method for regenerating the desiccant:
The moist compressed air flows from bottom to top through the desiccant, which adsorbs the moisture. Dry air from the outlet of the drying column is expanded to atmospheric pressure and then sent through the saturated desiccant, thus removing its moisture. Following the desorption process, the decompression valve is closed and the tank is repressurized.
Using an electric air heater, these dryers heat the expanded purge air, which limits the required purge flow to approximately 8%. This type of dryer uses 25% less energy than dryers without heated purge.
The ambient air, expelled by the blower passes through the external heater. The air is then directed to the saturated desiccant removing its moisture.
HOC dryers regenerate the desiccant by using the available heat from the compressor. Instead of removing heat from the compressed air in a final cooler, the hot air is used to regenerate the desiccant. This type of dryer can provide a typical dew point of -20 °C without any additional energy input. A lower dew point can also be achieved by adding additional heaters.
Rotary drum adsorption dryers consist of a single vessel that contains a drum instead of desiccant beads. This drum features a honeycomb structure on which the adsorption material is impregnated. The drum rotates at a very low speed (just a few revolutions per hour). One part (3/4th) of the drum is used to dry the compressed air, the other part (1/4th) is being regenerated.
Hot compressed air from the compressor is used for regeneration. If the compression process provides insufficient heat, it may be necessary to increase the heat to reach the required pressure dew point. The advantage of rotary drum dryers is that they have no switching valves. The electric motor and the rotary drum are the only moving components in this type of dryer. It is not necessary to filter the compressed air upstream of the dryer and, as the active material is bound to the drum, there is no need to filter the air at the outlet.
Rotary drum dryers do not require a separate cooling cycle. Their benefits also include a very low pressure drop and a very low energy consumption.
Cerades™ is a solid desiccant designed by Atlas Copco that revolutionizes the design, efficiency and performance of desiccant dryers. In dryers equipped with the structured desiccant, the compressed air is routed directly through the solid desiccant instead of making its way through the many tiny desiccant beads.
This has many benefits:
Membrane dryers use the process of selective permeation of gas components in the air. The dryer consists of a cylinder that houses thousands of narrow hollow polymer fibers with an inner coating. Through selective permeation, these fibers are able to remove water vapor.
The performance of membrane dryers is defined by the temperature and humidity of the incoming air. They do not provide a fixed dew point at the outlet but deliver a dew point suppression instead. Their design is very simple and reliable. There are no moving components, which means that they are 100% maintenance free. However, the average purge consumption of these types of dryers is around 25%.
