motorized and volume dampers
Would there be any issues with putting a motorized damper and a volume damper in the same outside air duct? I am working on a project that requires both, and have them both shown on the same outside air duct, but have mark ups to change it to two seperate ducts, one duct with a motorized damper, and another with a volume damper. The cfm is the same for both.
I can't think of any reason why this was given back to me marked up the way it was. Any input would be greatly appreciated.
edit: Why would there be two outside air ducts required when I could use one duct with the two dampers in the same duct?
Outside air dampers
Maybe the volume damper is to be set for minimum outside air, and the motorized one only opens in economizer mode (100% OA). This is pretty common for big systems. I can't tell, from your description if you had the dampers in series...that wouldn't work. If you mounted the two dampers beside each other, it would work in one duct.
It sounds like your system would remain open to outside air even when it is shut off. That would be BAD in a cold climate. I would put a two-position damper in place of the volume damper, and shut it only when the unit is off.
There is a dedicated outside air unit supplying the outside air, so the air is heated.
The furnace is serving a laboratory, with hood exhaust. Your explanation has answered my question.
May have something to do with duct sizeing.How much air are you dealing with and whats the BUT rateing of the furnace? Also is there a requierment to maintain a positive pressure in the space?
When there is a manual balancing damper and a motorized damper in the same duct, they are expecting the motorized damper to open/close completely. When it is open, the balancing damper limits the maximum air flow.
When he changed to two separate ducts sounds like he changed to what HVACBoston posted.
If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?
Maybe I don't have this right, but I drink Natty Bo and my son makes me watch Super Special Dog on UTube.
I've used a single OA duct, or plenum, for multiple units, and any number of dampers would go on the duct, or plenum, as long as it was large enough to support max flow for all the units. If this is only for lab(s), then an economizer wouldn't apply, as it would be for 100% outside air. If you were returning air from the fume hood, I probably would have recognized the episode on Super Special Dog or the censored episode of Gilligan's Island where the professor finally unlocks the medicine cabinet from the Minnow. As you have a fume hood included, I'd guess that the hood may be desired to run as variable exhaust, but had constant volume supply. Since the exhaust varies, and the lab needs to maintain some differential variable volume compared to adjoining space (to maintain the NFPA 45 requirement of relative differential pressure), then some way of throttling up and down supply would be needed as a hood sash moves up and down.
What type of exhaust do you have matched with the supply? Sounds like the OA MOD is matched up against sash position on the hood, since you use "lab" and "fume hood" in the singular. If this is for a single lab with one VAV hood, the OA MOD would be taking the place of a pressure indepependent air valve or box. Or an architect is trying to do lab HVAC.
If exhaust is constant, there is no need for the MOD on outside air. If hood is VAV, either get rid of the MOD and install a pressure independent supply, like a Phoenix valve or TSI box. If a Super Special Dog approach is desired, set MOD min position at lab general exhaust plus 100 CFM for fume hood sash closed position, then modulate MOD from min setting to full open to correspond with fume hood sash position.
Really does sound like an architect trying to squeeze in one lab on the cheap.