An oxygen concentrator uses molecular sieves (or sieve filters / sieve bed filters / sieve tank) that bind nitrogen and let oxygen pass through. Since this technology only works under high pressure, a compressor is needed. Behind the sieve filter the oxygen pressure is again reduced, the flow controlled and then delivered to the patient.
Apparently the filters get saturated with nitrogen after a few seconds, so that the filters need to be flushed. This is done in reverse direction with a part of the just filtered oxygen. Therefore the inlet valve switches over to an exhaust opening. Because it takes time for building up pressure, flushing, switching over and releasing the nitrogen, the oxygen flow to the patient would be interrupted for some time. That is why oxygen concentrators always have two sieve filters which work alternately, one filters the oxygen while the other gets flushed. The slightly fluctuating oxygen pressure is then led into a buffer storage tank before it gets to the patient.
Here the functional process in detail:
Ambient air gets into the concentrator via a foam dust filter. One part of the air enters the compressor trough another fine filter, the other part is needed to cool the compressor unit. Behind the compressor a cooling coil with a fan cools down the warm compressed air before it gets into the sieve tank system.
1. Inlet valve V2 is open. Pressurized air from the compressor of 1.4-2 bar (20-30 psi) enters sieve tank 1.
2. Oxygen passes through filter 1 but nitrogen accumulates at the filter intake.
3. The oxygen gets through a restrictor into the reservoir tank. Through another restrictor a part of the oxygen enters sieve tank 2 through the outlet.
4. The trapped nitrogen of sieve tank 2 is flushed through the intake and gets through the open exhaust valve V3 and the muffler out of the concentrator.
5. After 7-8 sec V1 and V3 close and the equalization valve V5 opens for 1 sec. The different pressures of the sieve tanks get balanced.
6. Now V4 and V1 open. Air gets into sieve tank 2, oxygen from sieve tank 2 fills the reservoir tank and sieve tank 1 gets flushed.
The reservoir tank is connected to a pressure regulator where the system pressure is reduced to patient pressure of 0.5 bar (8 psi). The flow can be adjusted with a flow meter within a range of 1 to 5 litres per minute. The value is shown by a little ball floating in a glass tube.
Between pressure reducer and flow meter another bacterial filter is inserted. This filter does not only protect the patient but also the concentrator of humidity from the outlet when the concentrator is not in use.
For safety reasons an internal measurement device for the oxygen concentration is often connected between the reservoir tank and the flow meter. When the oxygen concentration drops due to a fault, an alarm is given. The alarm will also sound when the system pressure is too high or too low.