Dryer Technologies - Medical Gas Info dot com

Dryer Technologies

In this section, we will present the common dryer technologies found on a medical or instrument air system...each has their pros and cons. We make no judgment as to which technology is best, but in order for the end user / engineer to make the most educated decision as to the type of dryer design they wish to use, we have chosen to present the "good and the bad" associated with each dryer technology along with other pertinent information.

Refrigerated Dryers:

            This type of design uses a refrigerant to cool the compressed medical air that goes into the dryer. As the air is cooled, moisture is condensed, trapped, and expelled from the dryer. The compressed air coming from the air compressor passes through cold, copper coils. As the compressed air passes the cold coils, an exchange of heat takes place. As the warm air from the compressors is cooled, moisture is "wrung" out of the compressed air. (This is the same principle that causes your commode to "sweat." As the warm, moist air in your bathroom comes in contact with the cool sides of the tank, the moisture in the warm air condenses on the sides of the cold water tank!) The refrigerant is compressed and used over and over again and the condensed moisture is expelled via the dryers drain. There are (3) types of refrigerated dryers:

                 1.) non-cycling: this type of dryer is always "on" the dryer stays on 24 / 7. This is the most widely used.
                 2.) cycling: this type of dryer will turn off when no flow is present. Not used in the medical market.
                 3.) heatsink: the cooling coils are submerged in a cold gel. The cold gel ensures the incoming air - even low air flows - will pass by cold coils drying the air to acceptable levels.

            So, is this saying really true?:

"you can't dry the medical air below 32° F or the dryer will freeze. What the hospital is left to contend with is a dryer that can     only dry the air to approximately 35° F. The dew point will alarm above 39° F. That only leaves about 4° F for the medical air system to operate under."

            The answer to this statement is yes and no. Yes, a refrigerated dryer will only dry the air to a dew point level of about 35° F at 100 psi; however, NFPA-99 tells us that the dew point level shall be measured after the final line regulator...at 50-55 psi. At this reduced pressure level, a refrigerated dryer can dry the air to a dew point level well below 32° F. This is due to the fact that as the air is reduced in pressure (from 100 psi to 50 psi) the air expands. As the air expands, the dew point level will always drop. The key is two fold: #1 ensure that the dryer has close to its designed air flow feeding into it. This holds especially true with non-cycling dryers. (i.e a 50 CFM dryer is designed to have 50 CFM of air flowing through it) #2 ensure that the dryers drain expels all of the water (an electronic drain is a good idea). If there is water sitting in the dryer's float bowl, this water can get "re-picked up" by the air....increasing the dew point level! .

                  Here is a 100 psi vs. 50 psi dew point table illustrating this concept:

(click on file to launch)

Desiccant Dryers:

            This type of design uses desiccant (either in loose "bead" form or a compressed "core" form) to dry the medical air. (Understand that there are various types of desiccant. The term desiccant refers to any substance that has a high affinity for water that is used as a drying agent. Manufacturers of dryers use various substances as desiccant: silica gel, activated alumina, etc. It also should be noted that there are "heatless" and "heated" desiccant dryers. This refers to the way the moisture that is captured is expelled from the dryer. A heatless dryer use compressed air to blow off the moisture from the desiccant while the heated style uses warm air created to dry off the desiccant. Most, if not all medical gas compressor system manufacturers use the heatless style) The principle of operation of a desiccant heatless desiccant dryer is simple: 1st, there are (2) separate drying chambers (one is used to dry the air while the other is "purging" or blowing the moisture off of the desiccant.) As moist air passes into chamber "A", the desiccant catches the moisture from the passing air. When the air exits chamber "A" it's typically dry to a dew point level of -10° F to +20° F. After a preset time (or when the dew point reaches a predetermined level) a switchover valve (or shuttle valve) will move so that chamber "B" is now the side which is drying. Since chamber "B" is now drying the compressed air, chamber "A" can rid itself of the accumulated moisture. The purge port to chamber "A" opens and compressed air is used to blow off the moisture which the desiccant has trapped. A desiccant dryer typically uses 15% of the systems useable air flow to rid itself of moisture. This is the downside to a desiccant dryer....the fact that it uses some of the medical air for purging or "regenerating." After a preset time (or preset moisture level) chamber "B" will purge and chamber "A" will once again dry the compressed air.

            We've touched upon the main downside of a desiccant dryer system - the fact that it uses approx 15% of the compressors air to purge itself of the trapped moisture. This leads to the question of: "what can I do to control the amount of purge air used?" There are (2) ways to accomplish this. Both ways use moisture level of the exiting air as the basis for purging rather than time:

                  1.) most desiccant dryer manufacturers offer some time of purge saving system that can be purchased with the dryer. This involves monitoring the moisture level of the air exiting the dryer and will suppress the purge until the dew point reaches a certain level.

                  2.) medical compressor system manufacturers will tie the desiccant dryer into the facilities dew point monitor. The dryers will only purge when the dew point level has risen to a predetermined point. It must be noted that if the dew point monitor were to fail in the "dew point level is ok" mode, the desiccant dryers would not purge, possibly allowing medical air with high moisture levels to be sent throughout the facility.

Membrane Dryers:

Membrane dryers reduce moisture levels by trapping trapping water molecules as the molecules pass through a permeable membrane wall. The membrane typically is made of synthetic materials that have an affinity for moisture. The design is very simple - similar to a filter canister. Membrane technology is typically not used in the medical market due to the amount of air needed for purging and the fact that controlling the purge rate is very difficult to do. Please contact us if you would like more information on membrane dryers.