When reducing ductwork how much is lost?

[nukklehead]

Would like to take my 6 inch filter which has a 6 inch 452cfm vortex and reduce to 4 inches in about a 2-3 ft run to exit the foundation. When reducing this duct how much flow will I lose... ?? Or maybe down to 5 inch....,, i just dont want a 6 inch hole in my brick foundation. It cools/exhausts 4 x 8 room with 1000 hps ( not inline.. but have fans blowing up into hood to disperse the heat and cool bulb)
Ballast outside room. More interested in exhausting odor as temps are pretty stable in basement..
thanks

 

[BBoarderGirl]

Well as you probably know what your really doing by introducing a smaller diameter duct is increasing the total friction of your air system. Once you calculate the total friction for your entire duct run then you can compare to the nameplate data on your fan. If your friction loss is greater than what the fan was designed for then you will lose airflow as the fan motor has to push harder agaisnt the friction. If you can sketch up your entire system (with elbows, transistions total feet of straight duct) and get the fan motor data I could calc it for you. But my experience tells me that with the small amount of total ductwork involved in most grow rooms you will not lose a significant amount of CFM by transistioning at the end of the run to 4" for a few feet of straight duct.

 

[nukklehead]

Thanks BBG for the response ,,, I suspected as small as my situation is I wouldnt lose too much. I will give it a run and i guess if it fails I will try and try again... again thanks... as I see there a lot of electricians on board but not many HVAC guys

 

[messn'n'gommin']

Just a thought, but, is adding a second 4" hole nearby an option? Any ventilation system is only as large as its smallest aperture and the area of two 4" exhaust ports is only slightly less than one 6" port.

I would think with just one 4" hole you can probably add a bit to the noise level, too.

 

[Baba Ku]

As m&g mentioned, the smallest orifice is the bottleneck of the system. Small run or long run, it makes no difference if you choke down to a 4" from a larger duct the system will flow as a 4" system. Put the 4" in front of everything and it will be the same as putting it at the exit port. The length of a duct run does indeed add head and friction which reduces the flow (or the requirement of the fan to keep the flow up), but nothing compared to the loss of going to a smaller duct.

Just as a comparison, I will show you a 6" duct calculated for loss, and the same setup using a 4" duct.

Duct Type: ROUND
Duct Diameter (in): 6
Flow Rate: 250 ACFM
Duct Length (ft): 10
Viscosity (cP): 0.018
Inlet Pressure (PSIG): 0
Temperature (F): 85
Duct Material: FLEXIBLE DUCT (WIRES COVERED)
Duct Roughness (ft): 0.003
Total Duct Loss (inches Water): 0.309

Duct Type: ROUND
Duct Diameter (in): 4
Flow Rate: 250 ACFM
Duct Length (ft): 10
Viscosity (cP): 0.018
Inlet Pressure (PSIG): 0
Temperature (F): 85
Duct Material: FLEXIBLE DUCT (WIRES COVERED)
Duct Roughness (ft): 0.003
Total Duct Loss (inches Water): 1.789

Lots of other factors can be involved, but this shows how dramatic a loss difference there is between the two sizes of flexible ducts.

 

[BBoarderGirl]

That's good info Baba Ku, but to get the real skinny on the reduction in flow you would need to plot the flow points and drop on the fan curve for the actual fan. Your numbers show an increase in friction but depending on the fan it may or may not have that dramatic of an effect on the actual flow. Fans are sized to overcome varing amounts of friction at varing flow rates.

 

[TriWingGirl]

The only thing i believe you should reduce is the amount of equipment in a grow room.

I believe that the more air you have flowing the better you will cool.

I got rid of my old 4 in ducted hoods and got a 6 in x 6 ft light tube. it holds two lights. All the air flows though and cools both lights. I am also using ice cube chillers to cool the air coming out the end.

It is not cost effective to pump good air out of the house.

If you are going to use 6 in or 8 in ducting you need to not reduce it will not be as efficient as it could.

They make single light, light tubes, also 6 in. adapters for most hoods. Otherwise modify adapt. If you build it they will grow.

More air flow means better cooling

 

[Baba Ku]

That's good info Baba Ku, but to get the real skinny on the reduction in flow you would need to plot the flow points and drop on the fan curve for the actual fan. Your numbers show an increase in friction but depending on the fan it may or may not have that dramatic of an effect on the actual flow. Fans are sized to overcome varing amounts of friction at varing flow rates.

I was simply showing a comparison. I really don't think it much of a fruitful exercise to engineer for bad design. It's best to just know it is not a good move to make in most any circumstance. If we can't afford the proper equipment, we may just not have any business in this hobby. (shrug)

A powerful fan of a smaller diameter could well get your numbers back where you want them, at the expense of wear on the unit, and noise...which of course can be far more expensive than using proper ducting and fans.

 

[BBoarderGirl]

I'm not trying to 'engineer for bad design' I'm simply trying to help him understand that using 2 ft of 4" duct will probably be fine for his use which is not so much to control temp but to remove odor.

Yes he may get a little more noise and a little less CFM, but as with everything in life it's about give and take.

 

[Baba Ku]

I think maybe you should understand that the length of the run is not the problem, it is the choking it down so much. That is what I am trying to help the OP to understand by showing some math, and describing other problems that accompany choking the run down.
Probably be fine??...perhaps, perhaps not. Bottom line is it is better to not choke down an air run in just about any situation. It is always better to design to the larger ducting.
Always better to work smarter, not harder.

 

[Lazyman]

Others posted good info on the reducer, but know that a single 90* bend reduces CFM by 50%! Not too hard to reduce airflow to NIL in a poorly-designed ductwork setup.

 

[nukklehead]

I'm not trying to 'engineer for bad design' I'm simply trying to help him understand that using 2 ft of 4" duct will probably be fine for his use which is not so much to control temp but to remove odor.

Yes he may get a little more noise and a little less CFM, but as with everything in life it's about give and take.

Thanks to everyone for there input. Glad I finally got some responses and good information. I will be a little more specific. First of all there are "NO" elbows involved and this is a straight run. I have a new 6 inch vortex (approx 450 cfm) connected to a stealth box 200 ( anyone use these????) that is 6 inch. Vortex will be directly connected to box and have approx 2-2.5 foot ( very short) run to exhaust out of house. As noted earlier temps are relatively stable and exhaust is more for odor then temps. I am trying to preserve structural integrity of foundation "end plate" which is a 2 x 8 and didnt want to bore a 6 inch hole in it if possible. The more I think about it I think I will take a middle of the road approach and reduce to a 5 inch (rather then 4 inch) opening. i think this will do the trick Thanks and again does anyone know anything about the stealth air box systems??

 

[BBoarderGirl]

I have done a sample calc to show here. I did not have nukkleheads post about his system, but I used my own system as a model and added the reducer that he is considering. I think the important thing to consider is every fan is designed to overcome a certain amount of pressure. That pressure can take ANY form...elbows, reducers, duct etc. I hope showing how I actually derived my numbers instead of just showing a list of numbers will help clear this up.

 

[BBoarderGirl]

Here are the attachments

 

[guest 77721]

There's no question that reducing duct size will cause a SIGNIFICANT loss in airflow.

flow = K sqrt(pressure)*Area

let's assume the pressure doesn't change between the systems then

Flow(6") = K*Area(6")
Flow(4") = K*Area(4")

proportionally the flow will be reduced to Area(4")/Area(6") = 12.56/28.26 = 44.44%

so you're looking at a loss of 55.56%

The only saving factor is that the fan will increase pressure as it is blocked up so the loss won't be as bad. Let's say 25 - 50% would be reasonable.

What's interesting is that people buy a big fan and stick a reducer on it and can't figure out why it doesn't meet spec. A 6" fan with a 4" reducer is a 4" fan.

 

[Baba Ku]

A 6" fan with a 4" reducer is a 4" fan.

 

[robbiedublu]

Thanks to everyone for there input. Glad I finally got some responses and good information. I will be a little more specific. First of all there are "NO" elbows involved and this is a straight run. I have a new 6 inch vortex (approx 450 cfm) connected to a stealth box 200 ( anyone use these????) that is 6 inch. Vortex will be directly connected to box and have approx 2-2.5 foot ( very short) run to exhaust out of house. As noted earlier temps are relatively stable and exhaust is more for odor then temps. I am trying to preserve structural integrity of foundation "end plate" which is a 2 x 8 and didnt want to bore a 6 inch hole in it if possible. The more I think about it I think I will take a middle of the road approach and reduce to a 5 inch (rather then 4 inch) opening. i think this will do the trick Thanks and again does anyone know anything about the stealth air box systems??

What is this stealth box 200? The only thing google comes up with is car audio stuff.

 

[nukklehead]

What is this stealth box 200? The only thing google comes up with is car audio stuff.

www.airboxfilters.com/


Have only seen a few users, from what I have heard upside is changeable filters and stealthy, downside , need to replace filters every 6 mos. LMK i any one has a report on these and thanks for the previous info

 

[Dilligaf_]

Why not use a round to oval adapter and cut an oval hole in endplate?

 

[nukklehead]

Sweeeet Dill.... never thought about that... I knew there would be a duct man around here somewhere!!!..do they make a 5 incher???