Dryer technology can be classified into two main types: mechanical and chemical. Mechanical air dryers use refrigeration to remove moisture from the air. They cool the air to a temperature where the water vapor condenses out. When dew point requirements are above 32°F, refrigerated air dryers are the best choice.

Chemical dryers, on the other hand, use materials that attract water from the air. They do this with two different types of materials - absorbents and adsorbents.

Absorbent chemicals used in air dryers are known as "deliquescent desiccants". They attract moisture by the chemical reaction known as absorption. Absorption involves the desiccants combining with the water vapor from the air to form a liquid that is then drained off.

Adsorbents remove the moisture from the air by attracting and holding the water to the surface of their particles. These desiccants are not used up in the process, but they need to be regenerated periodically with heat or dry air, or a combination of the two. Generally, these types of air dryers are called "desiccant dryers" or "regenerative dryers". These dryers come in two basic designs - heatless and heat reactivated.

The best desiccants are solid materials which combine very large surface areas with strong surface forces. The most common materials that possess this combination are silica gels, activated alumina, activated charcoal, and molecular sieves. Adsorbents come in many shapes, such as round balls, pellets (extrusions), and irregular granules, which are used for dynamic systems. Sizes will vary from less than 1/16" to 1/2". Preferred Physical Characteristics are

  • Chemically Inert
  • Physically Durable
  • Easily Regenerated
  • Thermally Stable
  • Larger Surface Area for Adsorption

There are three basic types: alumina, silica gel and molecular sieves.

Silica gel is slightly acidic in nature and is resistant to all organic acids, except HF, and is attacked by alkaline materials. It, therefore, cannot be used in a basic environment. Checking the pH of the condensate prior to the bed may be used to determine if alkaline attack is a possibility.

Molecular sieves are alkaline in nature and are attacked by mineral acids in contrast to silica gel. However, strong basic environments can attack the binder materials and eventually weaken them physically.

Activated alumina is alkaline in nature and subject to attack by mineral acids, as are the molecular sieves. Operation in an acidic environment will lead to formation of extremely fine dust due to the attack of the binder that holds the desiccant together.

Desiccant dryers operate on the principle of adsorption. Adsorption is an operation in which a gas, vapor, or liquid brought into contact with a solid is concentrated on the solid's surfaces by the utilization of surface forces. Adsorption is a purely physical phenomenon, generally reversible, in which neither the adsorbent nor the adsorbate undergo any change in structure. Absorption is a process in which a gas, vapor, or liquid penetrates a solid structure to produce a solid solution, resulting in either a temporary or a more or less permanent chemical reaction or phase change.

Air temperature determines water loading. Desiccant dryers look at air temperature in terms of saturation to determine the required desiccant quantity. Bed temperature is also affected by the inlet saturation temperature, and this will further reduce bed capacities and adsorption efficiencies.

Inlet air pressure affects dryer tower velocity. Higher pressures typically allow for smaller dryer size for a given loading. Conversely, lower-pressure operation increases tower velocity requiring larger dryer size.

Inlet air pressure and temperature work together in determining the actual dryer size. Low inlet temperatures in conjunction with high inlet pressure will use a smaller dryer size for a given flow rate. Conversely, an inlet temperature with a low inlet pressure will require a larger dryer size for a given flow.

Dryers are specified with a flow rate in terms of SCFM, which is the flow in cubic feet per minute corrected to standard conditions. However, dryers are sized based on ACFM, or cubic feet per minute at actual conditions. This is where the inlet pressure and temperature can affect the dryer size when the SCFM is corrected to ACFM.

Dryers are specified with a flow rate in SCFM at 100 PSIG and 100°F. However, it is recommended that the actual operating conditions be determined to correctly size a dryer. It is rare that a given application will actually be running at the typical specified conditions. If the actual pressure is below the specified pressure and/or the actual temperature is above the specified condition, the dryer will most likely be undersized for the given flow rate and will most likely not produce the desired effluent dew point.

Ambient temperatures and the relative humidity do not directly affect the dryer design calculation. However, the temperature and relative humidity will affect compressor output, which can affect the SCFM flow rate at the dryer inlet between winter and summer conditions. Typically, this difference will not significantly affect the size of a desiccant dryer on a flow-rate basis. However, the ambient conditions can affect the heat exchanger performance due to changes in cooling water temperatures and, especially, where an air-to-air heat exchanger is used. These changes can have a significant impact on the dryer size because of the changes in the inlet temperatures to the dryer. This is an area that should be taken into consideration when selecting the type of heat exchanger to be used and the type of reactivation system used.

Dynamic adsorption, as practiced commercially, is a process which generally may be reversed as a function of temperature. In other words, adsorbed moisture may be freed (reactivation) from the adsorbent by the application of heat and/or change in pressure. The adsorbent is then cooled and is fit for re-use as a desiccant.

Typically, there are two basic types of reactivation systems - heatless (pressure swing) and heat reactivated. Heat reactivated systems can be broken down into two basic types - externally heat reactivated and internally heat reactivated. Externally heat reactivated can be broken down into dry gas purged, closed loop convection and open loop convection.

A pressure swing dryer, also known as a Heatless Dryer, is a desiccant dryer that uses changes in pressure to reactivate the bed. No heat source is used for the reactivation process. These dryers are typically short cycle dryers (minutes).

Convection dryers are heat reactivated dryers that use an external blower, heater and cooler combination to reactivate the bed. They can either use atmospheric air or a closed loop reactivation system. The common denominator is that they do not use any process air for reactivation, although there are variations that use some process air for cooling.

These dryers are a derivative of the heatless dryer. By heating the dry purge air with a heater, they can extend the cycle time from minutes to hours and are classified as a heat reactivated dryer. They use about 6-8 percent of the process air for purge. They are a form of convection, although the reactivation heater is only really sized to heat the purge air to increase purge air efficiency.

These dryers use heaters installed in the desiccant bed to reactivate the desiccant. The heaters can be multiple elements installed in heater wells or, in some cases, installed in a single heater well in the center. The multiple heater element arrangements are typically more efficient. These dryers also use the least amount of purge (typically 3% of the process air) of any dryer because of the high efficiency of the multiple element arrangements. Where other heat reactivated dryers use purge air calculations directly linked to inlet air flow rates and loadings, this dryer purge flow is independent of the inlet conditions and is a function of dryer size only.