I see from looking at the space shuttle cabin air supply system that it provides O2 and N2 through a common manifold into a 14.7 psi regulator. What is not clear to me is where the cabin air goes, and if there's much of a flow through that regulator, or if its mostly idle. The spec suggests it can manage 20lb/hr. My question is when and how much it operated... Did they continuously or periodically vent cabin air only to have it top up from this system? Is the system only supplying fresh air when the cabin pressure falls, as a result of leaks or waste dumping? Any info which helps me understand the way in which air was supplied through this regulator, especially when and why, would be great.
Here's the normal orbit configuration for the Orbiter's Pressure Control System (PCS).
~850 psi O2 (yellow) flows in from the cryogenic tanks (also used to feed the fuel cells) in the Orbiter midbody, over a heat exchanger with the Freon loop which warms the O2, through a flow restrictor and filter into a 100 psi regulator.
~3000 psi N2 (purple) flows in from dedicated tanks in the midbody through a 200 psi regulator, then a filter, into the cabin plumbing.
One of the two PCS systems is configured to flow either O2 or N2 through the cabin regulator. The diagram shows PCS System 1 configured to flow. Nothing flows unless the cabin pressure drops below 14.7. When the pressure does drop below 14.7, the regulator cracks and start flowing gas. Which gas it flows is controlled by the O2/N2 Control Valve, which in turn is controlled by a circuit that senses the partial pressure of O2 (PPO2) in the cabin.
If the PPO2 is less than 2.95 psi, the system will flow O2. If it is greater than 3.45, it will flow N2. For intermediate values of PPO2, the valve remains in its last position. Again, nothing flows unless the cabin pressure drops below 14.7
Nothing major was intentionally vented but O2 is consumed by crew breathing, N2 is consumed by (minor but always present) leakage overboard (there is a nominal 3 lb/hr vent through the vacuum vent system all the time), toilet ops, etc.
The diagram says "1st half" because midway through the mission, the systems were configured to flow from System 2, just to even out wear and tear.
As far as flow amounts, you can see the flow restrictors on the oxygen lines which limit the O2 flow to ~25 lb/hr, but the cabin reg can flow up to 75 lb/hr or more if it kicks into high flow - note there are no flow restrictors on the nitrogen lines.
Control switches for the electrically operated valves are found on Panel L2, to the left of the commander's seat. Here's a picture I took of the panel in Endeavour. You can see how the schematic drawn on the panel matches the drawing above pretty well. Valves that don't have switches on this panel are manually operated valves located in the middeck.
Reference: used to teach the system for a living. The picture comes from training material. Put everything in past tense. The writeup in the Shuttle Crew Operations Manual is pretty good, page 2.9-4