There are many types of compressed air dryers. Different types of air dryers suit different applications and operating conditions. Below are some basic distinctions between different types of dryers.
A deliquescent air dryer is basically a pressure vessel filled with desiccant tablets, such as Van Air Systems Dry-O-Lite. Deliquescent tablets absorb moisture from wet inlet compressed air. The desiccant deliquesces, or more simply it melts as it absorbs moisture and dries the compressed air. The desiccant is consumed during the drying process. This dryer type is well suited for harsh operating environments, outdoor installations, and intermittent point-of-use applications.
Typical applications are: sawmills, ready mix plants, foundries, landfill gas and biogas, hazardous areas, paint booths, and outdoor mobile sandblasting and surface preparation.
Deliquescent dryers have a low capital cost relative to other types of compressed air dryers.
Operating costs are determined by the rate of desiccant consumption. Electric power is not required and the pressure drop is typically 1 PSI or less
Maintenance is simple. Add desiccant as needed and drain the condensate daily. The dew point is 20⁰F to 63⁰F lower than the temperature of the compressed air.
A refrigerated dryer uses a refrigerant compressor, condensing unit and heat exchanger to chill compressed air. Chilling causes water vapor to condense. Compressed air is reheated before leaving the dryer, so that the compressed air dew point is lower than its temperature.
Refrigerated dryers are ideal for continuous demand, indoor applications, where the ambient air quality is relatively clean and their air lines do not pass through an area subject to freezing temperatures. Some uses include food processing (do not, however, run air lines through a walk-in freezer when relying on a refrigerated air dryer), machine shops, and general manufacturing environments.
Refrigerated dryers typically cost slightly more than deliquescent dryers, depending on the brand and options.
The cost of operating a refrigerated air dryer is the electrical power to run the refrigerant compressor and condenser fan motor(s). Energy saving or cycling dryers are available, although their initial purchase price is more than a non-cycling dryer.
The pressure loss through a refrigerated dryer is normally 5 PSI, depending on filtration options.
Maintenance tasks includes keeping the condenser and automatic drain clean. Repairs to the refrigerant circuit require special skills.
Regenerative Desiccant Dryers
Desiccant dryers are sometimes called twin tower dryers, adsorption dryers, or regens. At Van Air Systems we usually call them regens. Regen dryers contain a reusable desiccant, typically activated alumina or molecular sieve, which adsorbs the moisture vapor from the wet inlet compressed air stream. The dryer has two pressure vessels each filled with the desiccant. Each vessel alternates between drying and regenerating. Regeneration is accomplished by using a portion of the dried outlet air or a combination of dry air and heat from an electric or steam heater. A regenerative dryer produces a -40F outlet pressure dew point.
Typically a regenerative dryer is specified by a customer who needs virtually all of the moisture removed from the compressed air stream. Some applications include refineries, petrochemical, pharmaceutical, power plants, and most process industries. Regenerative dryers are also used when needing to protect air lines from freezing due to low ambient temperatures.
Regenerative dryers are the most expensive type of air dryer, from both an initial and an operating cost perspective. Therefore this class of dryer should only be used when the application absolutely needs -40F or -100F dew point compressed air.
Certain regenerative desiccant dryers use up to 18% of the dry process air to regenerate the off stream tower. Other types use heat plus dry process air. In either case there’s a heavy utility load needed to run this type of dryer.
Maintenance tasks include changing of filters, replacements of valve seals, monitoring of upstream drains on separators and filters, and changing of the adsorbent desiccant every 3-5 years.
The main advantage of a regenerative dryer is it virtually removes all of the moisture vapor in the compressed air stream. All processes and outdoor piping are completely protected.
A membrane air dryer uses bundles of hollow membrane fibers. The compressed air passes thru the center of the fibers and the moisture vapor permeates thru the fiber walls. A portion of the dried compressed air is used to regenerate the membrane by sweeping away the moisture vapor. Typically 18%-25% of the dried compressed air is used for “sweep air”
This type of dryer is typically used for low flow applications up to 100 scfm or less. They are used for intermittent point of use applications such as a laboratory air, instrument cabinets, and laser purging.
Membrane dryers require little to no direct maintenance. However the membranes are high susceptible to oil and liquid contamination and pre-filters and drains must be consistently serviced. The membrane must be protected from liquid water and oil contamination so you must install a coarse coalescing & fine coalescing prefilter in series to protect the dryer.
The membrane dryer is relatively easy to install and reasonably priced for low flow applications that require a low dew point.
It’s not always easy to know what type of compressed air dryer is best for a given application. If you’re not sure what air dryer type best meets your needs, give us a call at 814-774-2631 and ask for technical sales.