Our GIEBEL aeration dryers are mainly filled with silica gels because their absorption capacity is about 40%, and thanks to the addition of color indicators they make it visible by a color change when an adsorber should be replaced. In addition, silica gel can be easily regenerated and reused lt. Silica gel filters dry the air to an average of 10% RH.
Molecular sieves are ideal for selective separation of gases – in other words, they are used when other substances besides moisture are to be adsorbed. They are also used when a high degree of drying is desired, such as in the ventilation of optical equipment.
Activated carbon adsorbs non-polar substances that are immiscible with water and is therefore suitable for adsorbing odors, oil mist, vapors and other pollutants – we therefore also like to call our activated carbon filters oil mist separators and use them where the environment is to be protected from oil mist and other pollutants.
Valves are used in respiratory dryers when air flows through the adsorber only infrequently during its service life – i.e., the system is only occasionally in operation. In this case, valves prevent moisture from diffusing through the holes in the bottom of the adsorber and gradually loading the respiratory dryer with moisture from the ambient air. For systems that are in regular or continuous operation, the effect of valves is negligible. Here it is more profitable to use the cheaper adsorbers without valves and to accept a minimal loading by the ambient air via the adsorber bottom.
If a new, unsaturated adsorber is installed, it achieves a residual air humidity of approx. 0% – this means the dew point is approx. -40°C. When fully loaded, i.e. at a saturation of approx. 33%, the residual air humidity is approx. 40% RH and thus has a dew point of approx. 6°C (depending on the temperature).
In general, the sooner an adsorber is replaced and fresh desiccant is used, the lower the dew point and the lower the water entry into a system. For use on hydraulic units, gearboxes, storage tanks, drums and IBC containers, it is sufficient to replace an adsorber when it is fully discolored, i.e. at 33% load. Up to this point, the amount of water entering as well as the dew point have no significant negative influence on the system.
As a rule of thumb: amount of silica gel x max. absorption capacity = water absorption capacity. The maximum absorption capacity of silica gel is calculated at 40%. Accordingly, an adsorber with 4.0 kg of silica gel can absorb a maximum of 1.6 liters of water.
Factors such as relative humidity and temperature influence the water absorption capacity. For example, silica gel can absorb up to 40% water at 100% RH, and only about 30% of its own weight at around 60% RH. High temperatures also have a negative effect on water absorption. Since the water molecules begin to oscillate with increasing temperature, they are less easily adsorbed. At about 80°C, the molecular movement is so strong that the use of silica gel can no longer be considered useful.
Hydrophobic membranes are very microporous and highly hydrophobic. They retain water, but at the same time allow water vapor and air to pass through. Water molecules from the air are therefore not separated. This makes the membranes unsuitable for ventilation drying. However, they are used in numerous other areas, for example in waterproof textiles, microfiltration, packaging materials or pressure equalization elements.
Nitrogen blanketing of containers is preferable if the substance is to be protected from oxygen to prevent oxidation. However, if the main purpose is to protect the substance from moisture, an adsorber is a much better choice. It is easier to handle and cheaper to use.
The use of a tank air dryer in combination with the blanketing of an inert gas, such as nitrogen, is not recommended. The blanketing creates a continuous positive pressure, while an adsorber requires negative pressure to draw in air and dry it. Thus, the two processes work in opposite directions. The expensive nitrogen would continuously escape via the adsorber.
After mounting an adsorber, moisture from the ambient air should normally not enter the system. However, if there is water in the system after a certain time, check for other air openings, such as cylinder seals or damage.
If hygroscopic substances are present in an IBC container, it makes sense to use an adsorber. It prevents the contents from coming into contact with atmospheric moisture and guarantees consistently high product quality. To keep the maintenance intervals as long as possible, normally adsorbers with valves are used. They prevent the adsorber from being loaded by the humid ambient air.
However, if an IBC with a valve adsorber is to be emptied at high speed, i.e. within a few minutes, a high negative pressure is created and the plastic walls of the IBC are drawn inwards. The effect becomes visible when about 2/3 of the contents have been removed. If you intend to empty an IBC at a high rate, you must use an adsorber with the largest possible cross-section to minimize the pressure build-up. In addition, the aeration dryer should in no case have valves!
If water is already dissolved in a hydraulic oil, it can no longer be absorbed by the adsorber. In this case, active drying is recommended. We have developed a so-called Tank Dryer for this purpose. This is a desiccant bag that is lowered to the bottom of a tank on a long cord and absorbs the water there. For more information, please visit www.giebel-desiccants.com.
If a plant is operated at permanently low humidity, the use of a breath dryer is not necessary. Adsorbers are used when plants contain hygroscopic liquids, air humidities from 70% rH and ambient temperatures from 25°C prevail or plants are exposed to temperature fluctuations of +/- 10°C.
To determine the optimum and maximum air flow through an adsorber, the physical properties of the desiccant, the geometry of an adsorber and the ambient conditions (temperature, humidity) play a role. To determine the performance of an aeration dryer and quantify the efficiency, the adsorber index can be used. This is determined by the influencing factors mentioned above. An optimum adsorber index of 1.0 (100%) is achieved with long loading times and low residual moisture. Under poor conditions, i.e. low loading time and high residual moisture, the adsorber index is low and reaches a limit value. In practice, an adsorber index of 0.7 has proved successful. With this value, a maintenance interval of approx. 1 year is aimed for (depending on the application cycles) as well as a residual air humidity after drying of 10% rH on average and a dew point of approx. -12.5°C.
Adsorbers are usually mounted vertically, with the connection at the bottom. This type of mounting ensures that the desiccant is evenly distributed and the air inlet openings are protected from the weather, which increases the IP protection.
It is also possible to mount an adsorber in a horizontal position. However, some factors must be observed during installation to ensure the function of the adsorber and to prevent damage to the system. We only recommend this type of installation for series 1 and 2. When mounted horizontally, adsorbers of series 3 and above, can create an air pocket that affects drying efficiency. In addition, a greater leverage force acts on the connection here, and damage cannot be ruled out.
So from series 3 on (diameter 110mm), a different type of mounting (For example, by means of an angle adapter or a wall bracket) is recommended.
Yes – but if an adsorber is mounted from top to bottom, it is important to prevent the ingress of dirt or water through the openings. If the adsorber is installed outdoors, a specially manufactured adsorber of the TB or TM series should be used.
When mounting an adsorber without valves, the openings are closed with protective plugs. Thus, the adsorber can be stored for a longer period of time without loading. During commissioning, the plugs must be removed. For plants with slow air flow it is sufficient to remove 1-2 plugs, for plants with high air flow all 4 plugs should be removed.
The maintenance interval of an adsorber depends on the plant to be ventilated and the environmental conditions. If the plant draws in a high volume of air that must be dried by the adsorber, the maintenance interval is shortened. The same applies to high humidity levels, which introduce a high quantity of water molecules into the adsorber. And if there is already moisture in the system, it will additionally load the adsorber. This is quickly visible by a discoloration of the silica gel from above.
Of course, the maintenance interval also depends on which adsorber is used for a system. We will be happy to advise you on this.
If an adsorber does not change color over a longer period of time, this is a good sign. The valves protect the desiccant and extend the service life. Nevertheless, it may be useful to check the function. To do this, the desiccant can be weighed. The difference to the dry mass indicates the water absorption.
If the adsorber discolors from above, this indicates moisture escaping from the system. This is the case when a ventilation dryer is not installed right from the beginning, but after a few months (or years) of operation. Then the air in the tank and possibly also the stored liquid already contain a certain amount of water. This moisture is absorbed by the adsorber and is shown by the coloration from above.
If a ventilation dryer becomes saturated very quickly, this can have several causes. The most common reason is a high volume flow to be dried and an adsorber selected too small. If the system already contains a high amount of moisture, the adsorber also absorbs it and loads from both sides.
When orange gel comes into contact with water, it turns green as it becomes more saturated. However, some substances, such as amines and AdBlue, can cause a violet coloration if they steam out of the tank and enter the adsorber. This silica gel is no longer able to absorb water and must be replaced.
When using an adsorber, the silica gel may discolor differently depending on environmental conditions and operating parameters. Adsorbers without valves have air inlet openings in the bottom and allow air (and thus moisture) to flow continuously to the desiccant. If the granules are loaded slowly and constantly (mounted on a system with very weak or no air movement), the load will discolor uniformly over the entire body. This is because the silica gel wants to reach equilibrium and therefore distributes the water. The orange hue will change to a dark green hue with water, but only when the silica gel is about 15 to 20% charged. In the stage before that, the orange becomes darker and darker and appears slightly brownish.
A brownish to black color, on the other hand, is caused by oil aerosols or other vapors. This means that when the upper activated carbon disc (black layer) can no longer absorb the escaping particles, they slowly settle on the silica gel. If oil particles emerge from a system, the use of an upstream oil droplet separator or an oil mist separator from our adsorber VG-D series is recommended.
In principle, it is possible to install an adsorber to dry the supply air on a compressor. However, it is absolutely not recommended. Due to the large amount of air that a compressor draws in, an average-sized adsorber with 3kg of silica gel would be loaded within a very short time and would require maintenance. This enormous maintenance effort is out of proportion to the benefit. It is more advantageous to insert an adsorber in the compressed air line after the compressor.
KC Drying Beads® and Sorbead Chameleon® from BASF are also silica gels with comparable properties. A slight difference comes from their content of approx. 3% Al2O3, which improves heat transport. This factor does not come into play in aeration dryers with less than 10 kg adsorbent. Due to their very early color change at approx. 6% loading, BASF products are also rather unsuitable makes.
However, they are ideally suited for adsorption plants that adsorb and regenerate cyclically. Here, their higher half-value diffusion coefficients and the resulting higher number of regeneration cycles offer clear advantages.
Silica gels adsorb moisture from the air, or in the form of mist. However, when liquid water is adsorbed, so much energy is released that the silica gel grains break. For the adsorption of liquid water, therefore, a special silica gel – silica gel water resistant – is used.
Oil significantly interferes with the water absorption capacity of silica gel. This means that where oil mist or oil droplets occur and silica gel is used for drying, further measures are required to prevent silica gel and oil from coming into contact with each other.
Color indicators are added to silica gels to indicate the state of loading with water by means of a color change. They are divided into ph indicators and metal salts. pH indicators change color because their originally acidic or basic environment develops toward pH 7 (neutral) with increasing water absorption. Methyl violet, which is used in our silica gel orange-green, is such a pH indicator. Metal salts form complexes when they absorb water – either through the incorporation of water of crystallization or through a reaction of the metal salts with water and chloride ions. Iron(III) chloride, which is used in Silica Gel Orange Colorless, is such a metal salt.
Another color indicator that has gained notoriety in connection with silica gel under the name blue gel is the heavy metal cobalt(II) chloride. However, this substance is considered to be carcinogenic as well as mutagenic – its use is therefore expressly not recommended!
Silica gels and molecular sieves are both desiccants with high water absorption capacities. Molecular sieves are mainly used as an alternative to silica gel when low relative humidities of < 10%RH prevail and strong drying is desired.
In addition, molecular sieves can adsorb other substances besides water, such as gases. You can find out more about this on our desiccant website www.giebel-desiccants.com.
Pure silica gel does not require labeling according to the EC Directive (67/548/EEC or 1999/45/EC) and is not hazardous to health or the environment. In addition, silica gel is non-flammable and chemically stable. If handled and used properly, no ecological problems are to be expected.
If a color indicator is added to the silica gel, its labeling requirement depends on the color indicator used and its concentration. At GIEBEL, sustainability as well as products that are compatible with health and the environment are very important to us, which is why we only sell silica gels that do not require labeling. Safety data sheets provided by the manufacturer in accordance with Regulation (EC) No. 1907/2006 always give you information about the use and compatibility of chemicals – take a look!
Adsorbents can be regenerated by raising the temperature, lowering the partial pressure and passing a dry liquid through them. However, the regeneration temperatures of molecular sieves and silica gels are very different. While molecular sieves require a temperature of 300 °C to release water completely, 140 °C is optimal for colorless silica gels and 120 °C for orange gels. It is possible to dehumidify silica gels in a commercial oven without any problems. To do this, lay it flat on a baking tray if possible. Regeneration in a microwave oven is not suitable – the resulting hotspots explosively dissolve the water molecules and damage the silica gel grains. For the regeneration of molecular sieves, ovens must be used that generate correspondingly high temperatures.
If oil-laden silica gel is regenerated, the oil reacts at approx. 110 °C on the silica gel surface and the color indicator is destroyed. If the dry beads are moistened again, they absorb significantly less water. In short: Oil-laden silica gel should be replaced.
All original GIEBEL Adsorbers® are made of harmless and reusable materials. These include: Acrylic glass, aluminum, galvanized steel, polyamide (PA), polyoxymethylene (POM), NBR, FKM, EPDM, activated carbon, silica gel orange.
At the end of their service life, GIEBEL adsorbers are disposed of in accordance to the relevant legal regulations. Metal and plastic parts should be sorted and disposed of separately. The desiccant silica gel, which is loaded with water, can be disposed of with household waste. Components loaded with oil must be disposed of in accordance with the specifications of the oil used – this also applies to the silica gel if it has been loaded with oil.