The bed is partially pressurized by the previously charged bed and from the pressurized reserve tank of air. When one bed receives fresh gas, the other's valves isolates it from the fresh gas and vents its tank to the atmosphere. The PSA system uses a three-way Versa valve. My system uses a normally-closed valve, so it needs power to open it to allow the fresh gas through to the bed. The model number for the 3-way, 1/4 brass, normally closed, 120vac Versa valve is VSG-3321-120. There may be an additional letter at the end.
Fresh gas enters from the top and flows into the bed when there is 120vac on the valve. When the power is off, the valve closes to the fresh gasflow and opens to the atmosphere. Exhaust gas flows from the bed out the hole on top to the left of the inflow port. Since there are two beds there are two valves. A controller handles powering and de-powering the valves.
The enriched gas leaves the bed at the bottom of the tank. Remembering that only one tank is under pressure, we need a means to prevent the pressurized, enriched gas from entering the other tank from the bottom. The system uses a check-valve that only lets gas flow out.
I got the check valves for a few dollars on Ebay. There is one under each bed.