Hi there Pequod, and Welcome to the forum!
Glad you found us.
In your initial example a ductless system provides 200CFM and 80CFM of fresh air is required. Does that mean if the same system were ducted, you would still need 200CFM but 80CFM from that would be injected into the system from outside?
Exactly. The 200 CFM figure is what needs to circulate through the room, regardless of where it comes from and where it goes. The 80 CFM figure is how much fresh air you need in order to keep the people alive and healthy and happy, because that's how much stale air (CO2 and other nasty stuff) you need to remove, and replace with the same amount of fresh air (O2). These are two different concepts, but interconnected.
If so, would that mean the air handler specs only need to be least 120CFM & therefore run at a lower speed than it would otherwise? Or does the fresh air just feed into the air handler which still pumps out 200CFM?
The AHU still needs to move 200 CFM in this hypothetical room, regardless of whether it is ductless or ducted. That's how much air needs to be circulated through the room, in order to provide the cooling, dehumidification, and general air circulation. That's what you need to keep the conditions evenly spread out through the room, and moving around nicely. How much
fresh air is in that, isn't really relevant to the amount of air that must be moved through the room. (It is only relevant to keeping people alive / healthy / happy.) In some cases (open loop systems), all of the incoming air is fresh, and all of the outgoing air is stale: there is no return path in such systems. That's the ideal, but very expensive to run in extreme climates. In that case, too, the AHU would still need to move 200 CFM, because that's how much flow the room needs to keep things moving smoothly.
I've also seen it suggested that an ERV or other fresh air delivery system could just dump air right over the inlet of the ductless Air Handler.
Right! That is, indeed, an acceptable option.
Would this work in a ducted system (assuming there is an adjacent equipment room for mixing to take place)?
Some people do suggest doing that, but I'm not very keen on the idea, for several reasons, even assuming it is legal. In some places it might not be permitted by code, for example if the "other room" is classified as habitable space: Code usually prohibits supplying air from one room into another,l or exhausting air from one room into another, if they are "living spaces" or "habitable spaces", even if there is nobody in there. For code, practical issues like that don't matter. Only the classification matters.
Would this work in a ducted system (assuming there is an adjacent equipment room for mixing to take place)? If not OK, is a closed plenum necessary and does that require additional calculation for size & avoiding static pressure issues?
A ducted system is generally a closed "loop", with an exit point for stale air, and an entry point for fresh air. The AHU itself is part of that, and has the return duct arriving on one side, with the supply duct leaving on the other side, everything sealed up air-tight. The return duct feeds the recirculated air coming back form the room(s) into the AHU, where it is cooled and dehumidified. That return duct generally is a plenum, and several feet upstream of the AHU is the point where the stale air is taken out thorugh the "exhaust" duct, to the outside world, then just before the AHU the inlet duct brings in the fresh air stream. You generally want about 6 feet between those two points, to prevent backflow in the wrong direction, and suchlike. So the stale air is sucked off first, then there several feet of duct or plenum, then the fresh air is pumped in, then maybe a couple more feet and it all goes into the AHU.
Finally, on baffle boxes. Do I understand correctly that you would, in a sense, set a speed limit for your entire supply system but allow that speed to fluctuate?
The only velocity that really matters, is the velocity through the registers: at the point where the air comes into the room, and at the point where it leaves the room. At those two points, the velocity must be low, and the flow smooth, for at least several times the duct diameter. But beyond that, inside the silencers and other ducting, the speed can be higher. Ideally, the highest speed is in the distant ducting, on the far side of the silencers from the room, and the lowest speeds towards the "room" side of the silencers. If you have seen my silencer box design, you'll not that the incoming airflow from the duct is immediately split into two parallel paths, which drops the air flow velocity by 50%, and increases the static pressure similarly.... but for each individual arm! The TOTAL airflow velocity is still the same, and therefore the total pressure is too. There are then additional changes in velocity/pressure along the rest of the path through the silencer, but once again, they are in parallel, not series, so the overall effect is minimal. Normally, I have a larger plenum-like region at the two ends of the box, where the CSA is similar to that of the actual "sleeve" that reaches through the wall or ceiling to the register, so the flow is slow and smooth for as long as possible before it hits the register.
I've seen it suggested that the CSA must gradually get larger as you get closer to the supply register to avoid wind noise issues.
Not gradually! There must be sudden, abrupt, drastic changes in CSA, to get the additional benefit of impedance mismatch: each time you have an abrupt change in CSA, you also have an abrupt acoustic impedance mismatch, which can gain you an extra couple of dB.
Hope that helps!
- Stuart -