Deliquescent Dryer Fundamentals: An Engineer’s Perspective

Van Air Systems is one of the few companies producing deliquescent desiccant and deliquescent dryers. It’s our specialty in a modest niche market. Because deliquescent dryers are specialized, they’re often a source of confusion on the part of both equipment distributors and end-users.  I receive a lot of questions about the basic operation of a deliquescent dryer.  People want to know what’s in the desiccant. Customers often ask how we size deliquescent dryers.  So I thought it’d be helpful to write up an overview that addresses some of these questions.

But before getting into sizing principles, let’s review what deliquescent desiccants are.

Deliquescent desiccants are special drying tablets that we form from hygroscopic materials. Hygroscopic means water attracting. This page has some pictures of deliquescent desiccant tablets.

These materials typically absorb 3.5 to 4 times their own weight of water vapor. The absorption causes deliquescence, or the dissolving of the tablet into a liquid.  We produced this video last year to help illustrate deliquescence.

The best deliquescent materials are salts with a strong affinity for moisture. Common deliquescent salts include calcium chloride, magnesium chloride, potassium chloride, and lithium chloride.  We blend these materials in proprietary ratios, along with binders, to produce a tablet that performs optimally under typical air system pressures and temperatures. Not all deliquescent desiccants are equal. The final formulation and properties of the desiccant can have significant impact on the design of a dryer tank.

Van Air Systems’ deliquescent desiccants are formulated and manufactured to be very hard.  A hard tablet retains its basic shape as it dissolves into brine within the dryer vessel. This property is critical to proper operation of the dryer and prevents desiccant caking, fusing, crushing, and the associated problems of breakthrough due to channeling.

The tablet sizes within a dryer tank range from the smallest at the contact zone at the bottom of the desiccant bed, to the largest at the top of the bed’s active zone.  Beyond the active zone no drying takes place; the tablets here are simply reserve capacity.

Van Air Systems has tested over 900 desiccant formulae to arrive at the two best deliquescent desiccants for drying compressed air.  Dry-O-Lite establishes within a compressed air system a relative humidity of 55%, or a dew point suppression up to 20ºF.  10BF establishes 10-13% relative humidity, or a dew point suppression up to 65ºF.  Just subtract the suppression temperature from the dryer’s inlet air temperature to predict the outlet dew point of the dry compressed air. (Note: We also produce a third deliquescent desiccant called Special Purpose (SP), but it’s almost exclusively use in natural gas applications, because of temperature constraints.)

Most compressed air dryers are designed based on saturated (100% RH) inlet conditions at a pressure of 100 PSIG.   We also use these design conditions for deliquescent dryers.  The inlet air temperature, however, is not assumed to be 100F, as with a refrigerated or regenerative dryer.  Rather, we advise that the temperature be as close as possible to the ambient temperature, whatever that may be.  Cooler is always better.   This is because compressed air holds considerably larger amounts of water at increased temperatures.  So cooler inlet temperatures to a deliquescent dryer mean less desiccant consumption and a better outlet dew point.

Under most conditions, deliquescent desiccants will absorb between 3.5 and 4 pounds of water per pound of desiccant.  10BF will be consumed faster than the Dry-O-Lite under identical conditions because more water must be absorbed to obtain its higher dew point suppression.

A deliquescent dryer also acts as a high efficiency liquid knock out and mist eliminator, which is required downstream of a cooler.  This is why you’ll often see a deliquescent dryer installed without an upstream, standalone moisture separator or coalescing filter. However, if a deliquescent dryer is being installed downstream of a piston-type compressor – particularly an one that coughs up lots of lubricant– a coalescing filter is a good idea.

A primary consideration when we design our deliquescent dryers is bed diameter.  We calculate this based on maximum or minimum gas velocities and desirable storage volume.

In most cases, it is more attractive to design a bed with the largest possible diameter. This allows the bed to hold a large storage volume of desiccant without become excessively tall, thereby increasing the time between maintenance, desiccant refilling, and keeping a reasonable height for the fill hatch.

Maximum diameters are closely related to desirable actual minimum bed velocities.  The actual minimum velocity of compressed air flow through a bed of deliquescent desiccant can be low if the air is evenly diffused into the bed via mechanical means.  A full area bed support will also help evenly distribute the compressed air to the full bed.

Very low velocities can be used if the deliquescent desiccant is formulated properly to remain hard while deliquescing. Not meeting this requirement was the downfall of a few defunct competitors.  It’s very difficult to form a desiccant that doesn’t soften as it deliquesces.  Dry-O-Lite and 10BF meet this critical objective.

With the proper desiccant, channeling is minimized with low velocities because the desiccant in contact with wet gas will begin to deliquesce, shrinking in size and will cause the gas to dynamically diffuse across the desiccant drying zone. The velocity is best kept in a range to permit storage desiccant to replace consumed desiccant as the deliquescing desiccant falls in on itself at the point of consumption. Extremely low velocities can cause isolated channeling which require routine maintenance to correct.

Van Air Systems designs a standard line of compressed air deliquescent dryers with recommended inlet flows having velocities in the range of 15 to 18 actual feet per min when using high quality Dry-O-Lite or 10BF desiccants.   These tanks also permit the largest storage capacity at the lowest practical height for fixed plant air applications.

Sometimes it is desirable to design the dryer for a small footprint. This is the case in portable applications, especially for the mobile abrasive blasting and coating industry, where contractors often put a dryer on the flat bed of a truck, where there’s limited space.  In this  market flow rates are usually intermittent, where more frequent maintenance can be tolerated, and large desiccant storage volumes are not a requirement.

Higher flows through a desiccant bed require limiting the compressed air’s actual velocity to avoid bed fluidization and in extreme cases entrainment of the brine with the compressed air leaving the bed.

Since a bed must have sufficient desiccant to maintain > 3 seconds of drying zone contact time prior to adding desiccant,  a bed that is used with high velocity flows near fluidization limits will have short drying periods between refills or it  will be very tall to permit some desiccant storage.

Compressed air dryers designed to be used in portable applications are normally designed for actual velocities ≤ 30 feet per minute which will permit a small footprint, reasonable storage at a low height and stay well below fluidization velocities.

In practical terms this means that our portable deliquescent dryers (Blast Pak Series) have approximately 40% of the desiccant storage capacity of a standard stationary dryer (D Series).

As always, if you have any questions email me or give me a call.