Medical Air Compressor Technologies

     In this section, we list the typical air compressor technologies found on a medical air compressor system

Piston:

      There are (2) types, or styles, of piston air compressors that are used for medical air compressor systems. (ISO standards do allow another type of piston compressor, but the USA follows NFPA-99 recommendations not ISO) The first is called "oil-free" and the second is "oil-less." The names sound similar but they are indeed very different from each other. Both styles use rings that are teflon or other materials that do not require oil for lubrication. Remember, the ultimate goal is to provide the patient with clean, dry air.

    Oil-Free

      An oil-free compressor utilizes a piston which, via a drive motor, gets driven up and down within a cylinder. Just as you would expect, the piston has various rings to keep the seal tight between it and the cylinder wall. The main bearing connecting the piston rod to the crank shaft, is lubricated via an oil bath. Yes, an "oil-free" compressor has oil in the compressors crankcase. The trick here is to keep oil from the crankcase "creeping" up into the compressor chamber. How is this done? Simple...the cylinder housing is elongated so that part of the piston rod is exposed to the atmosphere. Per NFPA-99 this "visual distance piece" must be 1 1/2 times the diameter of the shaft (NFPA-99 5.1.3.5.4). There are seals on each side of the shaft which keep the oil in the crankcase, where it belongs, and the other is to seal off the dry compression chamber from contaminants.


Oil-Less

      An oil-less compressor - just like the oil free type - utilizes a piston which, via a drive motor, gets pushed up and down within a cylinder. Again, the piston has various rings to keep the seal tight between it and the cylinder wall. The main bearing connecting the piston rod to the crank shaft is of the permanently sealed type and does not need lubrication. An oil-less compressor has zero oil in it...the crankcase, as well as the compression chamber, is void of any oil.

   
Scroll:

      This type of compressor has a very unique design. There are (2) scrolls that look like loosely rolled up pieces of paper - one rolled inside the other. The orbiting scroll rotates inside of the stationary scroll. Air is tramped in the pockets between each scroll. As the orbiting scroll "rotates," it compresses the air toward the exhaust port located in the center of the scrolls. The intake is at the outer edges of the scroll...intake near the outer edges, exhaust in the center.

Liquid Ring:     

       As the name implies, liquid ring compressors require a liquid to create a seal. For medical applications, liquid ring compressors are always sealed with water. Never seal a medical liquid ring compressor with oil! The operating principle is as such: an impeller, which is offset so the impeller is not in the center of the pump housing, rotates and traps pockets of air in the space between the impeller fins and the compressor housing. The impeller is typically made of brass. As the impeller turns, there is a pocket of air that is trapped in the space between each of the fins. The trapped air is compressed between the impeller and the pump housing, sealed with the water ring. As the air is compressed, it's then pushed out of the pumps discharge. Since the compressed air is ultimately being used by a patient, care must be taken to ensure the compressor does not introduce contaminants. This is done by ensuring the compressor is always getting a supply of fresh sealing water. This can be accomplished (2) ways*:

          1.) a "once thru re-circulating" system: sealing water is introduced into the compressor, discharged, and sent directly down the drain. The sealing water is only used once.

           2.) a "partial re-circulating" system: sealing water is introduced into the compressor and discharged. Some of the discharged water is re-circulated and used again to seal the compressor. With this type of system, since some of the water is re-used, it does not require as much water as a "once thru" to operate.

           * liquid ring compressors should never be operated as a "total re-circulating" design. Not constantly introducing a fresh water supply may produce contaminants.

Screw:

      A screw compressor utilizes (2) shafts that are cut similar to a screw you'd use to hold a piece of wood down. The grooves of each shaft overlap and they get closer and closer to each other near the discharge or compressed air end. Pressure is created by pulling air into the spinning grooved sections. The air is compressed into a smaller and smaller space until it reaches the designed pressure.

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