There's two concepts here, Anthony, that often get confused and muddle together. One is the total flow rate of the system: how muc hair you need to move through the room every hour, to ensure even mixing, good temperature distribution, proper humidity control, good control of stale air, etc. The other is how much FRESH air you need to add to that, to keep people alive, and even more important, how much STALE air you need to remove in order to keep people alive.
On that second point: Many people assume that the reason for bringing in fresh air is to provide enough oxygen: that's partly true, but it's not the real reason. The REAL reason is that you need to get rid of the CO2! You exhale a large volume of CO2 with every breath, and it builds up fast in a sealed room. Since the concentration of CO2 in air is very small (about 0.04%), you don't need to add a lot before the concentration gets up high. On the other hand, the concentration of oxygen is about 21%, so even if you use up a LOT of oxygen as you breath, there's still plenty there. You will die from CO2 poisoning long before you die from oxygen starvation, and CO2 rises very fast in sealed spaces, much faster than oxygen decreases. And you pass out from CO2 poisoning long before you pass out from lack of oxygen. And way before that, at much lower Co2 concentrations, you start to feel bad, suffer headaches, nausea, vision problems, attention problems, ... way before your body even notices the slightly lower level of oxygen. You can die in a room that has plenty of oxygen available, if the CO2 is high enough. You can live with only 15% oxygen in the room (6% points down from normal), but 6.04% CO2 (6% points up from normal) is not something you want to have in your room, if you plan to live....
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So firstly, you need to REMOVE enough CO2 (stale air) to keep people alive, and in order to do that, you need to provide enough fresh air to replace the stale air that you removed.
And that's the reason why you also want to move a much larger volume of air around the room, and through the room: to ensure that the fresh air comes on and mixes with the room air, so that the concentration of both CO2 and oxygen stays about the same THROUGHOUT the room. Consider this: If you sit still in one place and there is no air movement at all in the room, then you create a "bubble" of atmosphere around you that is high in CO2, low in oxygen, high in humidity, high in temperature, etc... while over in the corner of the room, it is cooler, dryer, and with a property balanced mix of CO2 and O2.
So, firstly you need to ensure that you are bringing in enough air and removing enough stale air to keep everyone alive and comfortable. That's what you are talking about. Also sometimes called "make-up" air in HVAC terminology. Yes, you need ducts in your HVAC system to do this. First, a duct that dumps some of the room air overboard, into the outside world, and ANOTHER duct that brings in the same amount of fresh air from the outside world.
That's the first part. That is based on occupancy: how many people there will be inside the room.
Then thees the SECOND part, which is just recirculating air through the room to ensure good mixing, so that temperature, humidity, CO2, and O2 are pretty much contant throughout the room, with no "pockets" of hot, humid, "bad" air, nor of cold, dry, "good" air. The general rule here is that you need to replace the entire volume of air in the room, every ten minutes or so. In other words "6 room changes per hour". Some people recommend more than that, such as 8 room changes per hour (roughly one every 8 minutes), or even ten per hour (once every six minutes). But for a typical studio, 6 per hour is about right. And certainly no less than that (for example, 4 per hour would not be enough).
Therefore, you need to calculate the TOTAL air volume in ALL of your rooms, multiply that by 6, and that's how much air your AHU must be able to move, under normal circumstances, on average. That's NOT the maximum amount when it is running full bore! It's the normal amount when running normally. You also need some "headroom" in the system, for dealing with difficult days....
So when you dimension your HVAC system, your AHU must be able to move enough air to replace the entire volume in all of your rooms, 6 times per hour.
That is your flow rate.
Then IN ADDITION to that, you need to have your "make-up" air ducts, that bring in a much smaller volume of fresh air, mix it with the recirculating air, and also take some of the recirculating air and dump it overboard. That might or might not need an additional fan to drive the make-up air, and you might or might not decide to add an HRV to the make-up air system, to recover some of the energy that would otherwise be lost from dumping the stale air.
Then, when you have figured out all of that, only now can you start dimensioning the ducts that fo to your rooms, and the size of your silencer boxes and registers. Because only at this point, after you have done all the above math, only now do you know how much air is flowing through it! You cannot decide on duct sizes or silencer boxes BEFORE this point, because there's nothing to base it on!
So, you know how much your air flow rate is (cubic meters per hour) through the AHU, ducting and silencers: and you also know the maxim air flow velocity (about 1.5 meters per second), so now you can do the math to figure out what the duct size must be to get THAT speed at THAT flow rate. The speed cannot be higher, or it will create noise just due to the movement of air. So that's your limiting factor here. The speed can be lower if you want, and that's great.
So, with that info in hand you can now calculate what size ducts you need, and what size registers you need, and therefore what the interior dimensions of your silencer boxes need to be, since the air flow speed must suddenly drop by at least half at the entrance, and double at the exit (at least).
So now, with all THAT in hand, you can finally calculate the last piece of the puzzle: what your static pressure will be, for the duct, silencer, and register system that you have designed. Then you need to check that calculated static pressure against the specs of the AHU units you are considering, and find one that can handle that much static pressure. If none of the ones you are considering are able to handle that pressure, then either you need to look for a larger AHU and run it on a lower setting, or you need to modify your duct/silencer/register system to decrease the static pressure such that it is within the range of the AHU. You CANNOT run an AHU into a system where the static pressure is too high: that will cause the fan blades to stall, or at least work outside of their design range, thus causing the fan motor to work much harder than it needs to, but it still won't be able to move the correct volume of air at the correct speed, the fan will overheat, the life span will be shortened, the fan will fail, and apart from that, even while the fan is working, the rooms will not be cooled or dehumidified properly, and you won't be supplying enough air, either in re-circulation of in make-up.
So there's a lot to take into account here. Normally when I design a studio, I spend as much time on the HVAC system as I do on all of the rest! It's a big deal, and getting it wrong is an even bigger deal...
We are setting up a ducted system in our live room and control room. One unit feeding both rooms with dampers, silencer boxes etc.
Fine, but are you going to adjust those manually, or are you going to have a system controller with sensors to do that for you automatically? If you opt for a manual system, you will have to be adjusting the dampers, flow rates, flow speeds, and temperatures constantly, yourself.
I hear that the HAVC contractors just add a small duct from the outside then attach to the main unit in the ceiling to bring fresh air into the circuit.
Right, and the also add a similar sized duct (generally a bit larger) to exhaust the stale air from the system. And BOTH of those ducts must be sized correctly to provide the correct flow rates without increasing the static pressure too much.
Do I let them know that that we need a minimum of 5 cubic metres of fresh air for our system?
No. Either you find an HVAC contractor who already understands studios and has done several, successfully, and provides references for you to go check, or you find someone to design the system correctly for you, and hand over that design to the contractors, with the instruction: "build it like that". If your contractors have no experience in doing HVAC systems for studios, then they won't understand the need for low velocity, high volume, low noise, silencer boxes, insertion loss, etc. They'll do it like a normal house, office, shopping center, church, school, etc, and it will be too noisy to be useful, and won't control the humidity and temperature correctly. There's a lot of reasons why that is, but too many to go into. HVAC is a much, much bigger issue than most studio builders realize, and much more different for studios than most contractors realize... unless they have done it before, successfully.
First rule of thumb: don't let an inexperienced contractor with no studio experience design your system!
- Stuart -