Duct sizing question for HVAC gurus

[MacroLogos]

So you don't find a fan blade with a "foil" design (similar to how modern sail boats use air flowing over a sail's surfface to "pull" the boat, rather than using it to "push" it, and air plains use this same principle in wing design to create lift), to be creating a vacuum?

This should help with the difference between a fan/propeller blade that uses an 'airfoil' design vs an 'airscrew' design.

Airfoil - Wikipedia

en.m.wikipedia.org

Basic Propeller Principles (Part One)

The aircraft propeller consists of two or more blades and a central hub to which the blades are attached. Each blade of an aircraft propeller is essentially a rotating wing. As a result of their co…

www.flightliteracy.com

airfoil blades pull.
airscrew blades push.

There's a massive vacuum created right in front of an airfoil blade design, this pulls the plane forward. It is often mistaken as a force pushing the plane due to the other way propellers are designed, which is not using a 'foil' but using a 'screw'.

 

[Ozmosis]

I made some improvements. I did change the screened vent exit to 6" from 4". You could hear less woosh as the air left the vent outside, but that only helped a little bit. I found a bigger culprit that was a mistake on my end for not looking at the big picture. I noticed I got much better temp control with lower fan speeds at night when I used my 6" booster fan to bring fresh air in through the window, but during the day even if I lowered the AC I was running a higher ambient temperature. So I asked myself what was different and finally realized a big part of my issue. At night I was bringing air into the room and blowing it to the port at the bottom of the tent which was replacing the air leaving the tent and the room. I realized that I was keeping the bedroom door closed to keep some curious cats out of the room and keep the AC in. So during the day when I was not drawing outside air in the fresh air to replace what I was exhausting outside had to come under the bedroom door. The bedroom is only about 11' x 11' so quickly developed an air deficit during the day. This placed extra strain on the tent fan as not only did it have to blow air out of the tent while working against the carbon filter restriction, but it also had to draw air into the room to replace the air it was venting outside by drawing it in under a carpeted door. The quick fix was pretty cheap. I bought like a $3 hook and eye latch and installed it on the bedroom door to keep the door partially open with a 2" gap. So basically a 2 inch by 6 foot vent into the room. Once I started keeping the door partially open my temps started running 2-3 degrees cooler. Less strain on the fan as more air could be drawn into the room easily now. Fresh air, and the cats still can't get in. I didn't look past the tent and realize that blowing air out of the house changed the pressure dynamics of the whole room and to a smaller extent the whole house. Thought I'd post this information in case anyone else ever finds this thread with a similar issue.

 

[RootsRuler]

The filters level of pressure drop is the same be it on the suction side or the discharge side of the fan. That number does not change, a pressure drop is a pressure drop. How the fan reacts to the pressure drop is what's important. If on the suction side the fans performance is gutted because the fan and housing can not be loaded properly. If the filter is on the discharge side of the fan and the suction side is open with no more restriction than the tent vent panels present then the fan blade and housing can load up properly and make cfm and discharge pressure. With the fan loaded properly the fan can make better discharge pressure to combat the pressure drop restriction of the filter. Most of the company's suggest the filter on the suction side for ease of use,,, plain and simple. Some will show discharge into the filter as a secondary way of setup. But never truly touch on the sizing, pressure drop. Remember your garden is not producing stale air. Your plants are feeding on CO2 in your air and making O2. They are scrubbing and filtering the air. Is it dramatic? No, but that is what they are doing. In my setup the return air filter and duct is a foot away from the door of my office. So any humidity that is exhausted from the grow space is going right into the central A/C and improving the A/C units wet bulb performance a small amount. So outside the A/C unit is pissing like a race horse on a flat rock and maintaining a perfect RH for the wife and I in the home at around 40% and have a very good temp differential of 20 to 21F degrees. That means I have 70F air hitting the return air filter and the A/C is putting out 50F air at a central located register. With the evap coil pissing the condensate out of the unit,,, ie: dehumidification the condensate is carrying away impurities in the air as the water catches them and they flow out of the unit into one of the wifes flower gardens.

Curious.......you say that fan performance is degraded because of filter restriction on the frontside(input air) causing improper fan load. Most modern a/c's use a filter front design which would be in conflict to what you are saying would optimize fan performance. Is the difference the filter media that is being used in relation to the fan CFM being employed on that particular design? In another thread front side filtering was mentioned as a way to keep fan components cleaner. True or not?

Also, you say that with the input side unloaded, other than tent vents, the fan can load properly but you also say that pressure drop is the same be it suction or discharge with the same filter on either end. I'm assuming your increase in fan performance is what is yielding you the better discharge numbers. In essence the higher flow rates will flow more out of the filter. What I don't get is if the filter media has a flow limit how can the increased performance of the fan surpass that limit? Are you saying that the lower performance of , say an ACI T6 unit since you mentioned ACI fans, doesn't push the filter to its limit thus lowering net cfm? Because the filter is not being pushed to its CFM limits would the fan performance not match whatever level the filter is allowing it to flow on the frontside/input side of the fan vs being used on the discharge side? I'm sure there are some physics and aero laws that would explain this but I'm curious about your explanation and how that works.

BTW did you rinse your filters before hooking them up to mitigate carbon dust? I know you used the prefilter inside the chamber but putting it on the outside, IMO, would be better to keep carbon dust from escaping into the atmosphere. Prefilter on the inside or outside should have no difference in relation to airflow but I could be wrong.

ALSO, LOL, since we're on the subject of airflow, when in flower, to keep the girls in total darkness I have to cover my vent panels. I have two 6" vents that I plan on using as my input air but I need to connect a length of flex duct to kill whatever light is entering. I tested the airflow, no instrument testing just SOP testing by how much negative pressure the tent was under through visual. How do I calculate how much vent area I need to properly supply the tent and fan? I have an ACI T6 unit in the vent and am thinking of using your setup of filter on the discharge end.

 

[MedicalHydroponics]

This is the best information in a video I have seen, and should help with this discussion.

 

[RootsRuler]

This is the best information in a video I have seen, and should help with this discussion.

Thanks! Great vid!

 

[MacroLogos]

This is the best information in a video I have seen, and should help with this discussion.

Good vid, but there is some deeper science at play worth looking at... if you're into this stuff.

This guy does a couple other vids, but the intro is going to give solid insight into this topic.

 

[RootsRuler]

Good vid, but there is some deeper science at play worth looking at... if you're into this stuff.

This guy does a couple other vids, but the intro is going to give solid insight into this topic.

Great vid also! So going by what I've just seen the optimal setup would be to filter on exhaust side and add a lip on the fan housing input side that will give the Coranda effect which will decrease strain on the motor and help in air flow as the effect is drawing air in naturally aside from the mechanical draw the fan is making?

 

[Blastfact]

Curious.......you say that fan performance is degraded because of filter restriction on the frontside(input air) causing improper fan load. Most modern a/c's use a filter front design which would be in conflict to what you are saying would optimize fan performance. Is the difference the filter media that is being used in relation to the fan CFM being employed on that particular design? In another thread front side filtering was mentioned as a way to keep fan components cleaner. True or not?

Also, you say that with the input side unloaded, other than tent vents, the fan can load properly but you also say that pressure drop is the same be it suction or discharge with the same filter on either end. I'm assuming your increase in fan performance is what is yielding you the better discharge numbers. In essence the higher flow rates will flow more out of the filter. What I don't get is if the filter media has a flow limit how can the increased performance of the fan surpass that limit? Are you saying that the lower performance of , say an ACI T6 unit since you mentioned ACI fans, doesn't push the filter to its limit thus lowering net cfm? Because the filter is not being pushed to its CFM limits would the fan performance not match whatever level the filter is allowing it to flow on the frontside/input side of the fan vs being used on the discharge side? I'm sure there are some physics and aero laws that would explain this but I'm curious about your explanation and how that works.

BTW did you rinse your filters before hooking them up to mitigate carbon dust? I know you used the prefilter inside the chamber but putting it on the outside, IMO, would be better to keep carbon dust from escaping into the atmosphere. Prefilter on the inside or outside should have no difference in relation to airflow but I could be wrong.

ALSO, LOL, since we're on the subject of airflow, when in flower, to keep the girls in total darkness I have to cover my vent panels. I have two 6" vents that I plan on using as my input air but I need to connect a length of flex duct to kill whatever light is entering. I tested the airflow, no instrument testing just SOP testing by how much negative pressure the tent was under through visual. How do I calculate how much vent area I need to properly supply the tent and fan? I have an ACI T6 unit in the vent and am thinking of using your setup of filter on the discharge end.

A pressure drop is a pressure drop be it on the suction or discharge of the fan, blower or pump. It is a constant. The only thing that changes is how dirty the filter gets and increasing the pressure drop. Prefilters are to protect the filter from mechanical filtration and dirting up the carbon and they have there own pressure drop. I always rinse my carbon filter before use. I don't understand what your questions are and how they truly apply to the issue at hand. A fan, blower or pump makes differential pressure. If you rob the fan of supply air because of a pressure drop on the suction side it will never meet or make as much cfm or discharge pressure. If we leave the suction side open and allow the fan to load properly it will make much better cfm and discharge pressure. If we block the suction side of the fan we are throwing it right into a Low NPSH situation. ( Net Positive Suction Head ) which destroys the performance of said fan, blower or pump. Do the hand experiment I posted about earlier in this thread.

 

[MacroLogos]

Great vid also! So going by what I've just seen the optimal setup would be to filter on exhaust side and add a lip on the fan housing input side that will give the Coranda effect which will decrease strain on the motor and help in air flow as the effect is drawing air in naturally aside from the mechanical draw the fan is making?

Optimal is subjective...

There are gives and takes involved, and different people are designing different environments for different reasons.

However, the Coranda effect drew my attention as well. He posted a vid where the military applied that effect to create a hovering mobile platform worth looking at because it goes deeper into some key details which caused the project to get scrapped.

That addition to the intake port would be worth testing. It's not a matter of if it would help, but by how much. The "lip" curve design will require serious engineering to optimize it, however. I am not up to that...

 

[RootsRuler]

A pressure drop is a pressure drop be it on the suction or discharge of the fan, blower or pump. It is a constant. The only thing that changes is how dirty the filter gets and increasing the pressure drop. Prefilters are to protect the filter from mechanical filtration and dirting up the carbon and they have there own pressure drop. I always rinse my carbon filter before use. I don't understand what your questions are and how they truly apply to the issue at hand. A fan, blower or pump makes differential pressure. If you rob the fan of supply air because of a pressure drop on the suction side it will never meet or make as much cfm or discharge pressure. If we leave the suction side open and allow the fan to load properly it will make much better cfm and discharge pressure. If we block the suction side of the fan we are throwing it right into a Low NPSH situation. ( Net Positive Suction Head ) which destroys the performance of said fan, blower or pump. Do the hand experiment I posted about earlier in this thread.

I guess I'm confusing air pressure and air volume. If the filter is physically blocking the volume of air that can be pulled through the filter why would it be any different on the exhaust side? The physical limit has been reached in regards to how much air can flow through the filter.

I get that the air pressure drop is hurting the fans overall performance through different physical effects(bernoulli, etc.) but to what effect? 10% loss? 80% loss?

You're the expert. I'm not doubting your knowledge. Just trying to get a better grasp so that I can build a more efficient exhaust system that can better control temp and humidity.

 

[Blastfact]

I guess I'm confusing air pressure and air volume. If the filter is physically blocking the volume of air that can be pulled through the filter why would it be any different on the exhaust side? The physical limit has been reached in regards to how much air can flow through the filter.

I get that the air pressure drop is hurting the fans overall performance through different physical effects(bernoulli, etc.) but to what effect? 10% loss? 80% loss?

You're the expert. I'm not doubting your knowledge. Just trying to get a better grasp so that I can build a more efficient exhaust system that can better control temp and humidity.

One can not exist without the other. Look at the fan and pump curve I posted. Has head pressure goes up flow goes down. The point of no flow is max head pressure. The point of max flow is zero head pressure.

 

[MacroLogos]

If the filter is physically blocking the volume of air that can be pulled through the filter why would it be any different on the exhaust side? The physical limit has been reached in regards to how much air can flow through the filter.

Air pressure (- or +) determines how quickly air will move through the filter.

A greater air pressure can be created on the exhaust side than on the intake due to how the fan blades in these specific fans are designed to move air.

It is very similar to what causes a racing sailboat's sail to collapse... and then, the boat shifts from being "pulled" through the water by "lift" created on the surface of "sail", to being "pushed" by the air as high pressure builds up on the back of the sail. This kills the boat's speed... even though it still moves the boat.

 

[RootsRuler]

So if we extrapolate both, the pressure the fan will build is really what determines how much volume will move through the fan itself minus whatever restrictions like ducting/filters etc. hence why, in your experience and through measurements, doing it this way will yield a better air exchange rate within the tent itself.

Now......the reason people use charcoal filters is usually to mitigate smell. Air speed dictated by air pressure will dictate how effective the filter will be at reducing the smell since faster speed means less time the air has to interact with the activated charcoal or is this nominal within the scheme of things? Yes, people use them for both, this guy included, so I'm trying to find the balance between odor mitigation and temp/humidity control. Lower air speed, i.e. air pressure will help with odor mitigation while higher air speed will help in temp/humidity control. That is what I'm getting from this. The issue at hand is that when odor mitigation is needed so is a higher air exchange rate since more transpiration and more intense lighting in flower will raise both.

I'll ask again, how much vent do I need to properly vent a 5 x 5 x 80" tent if I set up the filter on exhaust and leave the input in open air meaning no curved edge for the Corana effect?

 

[Blastfact]

So if we extrapolate both, the pressure the fan will build is really what determines how much volume will move through the fan itself minus whatever restrictions like ducting/filters etc. hence why, in your experience and through measurements, doing it this way will yield a better air exchange rate within the tent itself.

Now......the reason people use charcoal filters is usually to mitigate smell. Air speed dictated by air pressure will dictate how effective the filter will be at reducing the smell since faster speed means less time the air has to interact with the activated charcoal or is this nominal within the scheme of things? Yes, people use them for both, this guy included, so I'm trying to find the balance between odor mitigation and temp/humidity control. Lower air speed, i.e. air pressure will help with odor mitigation while higher air speed will help in temp/humidity control. That is what I'm getting from this. The issue at hand is that when odor mitigation is needed so is a higher air exchange rate since more transpiration and more intense lighting in flower will raise both.

I'll ask again, how much vent do I need to properly vent a 5 x 5 x 80" tent if I set up the filter on exhaust and leave the input in open air meaning no curved edge for the Corana effect?

I run 6" systems on my 2x4 and 3x3 both tents 95" tall. If I was to go to a 4x4 or 5x5 the regardless of height I would move right up to a 8" setup. You never want to be in a position to where you have to run your exhaust fans wide open full tilt boogie. Each tent has a 6" x 12" intake mesh panel and I never run my fans above speed 6 maybe 7. So I never have issue with not removing odor and maintain humidity and temp just fine.

 

[RootsRuler]

I run 6" systems on my 2x4 and 3x3 both tents 95" tall. If I was to go to a 4x4 or 5x5 the regardless of height I would move right up to a 8" setup. You never want to be in a position to where you have to run your exhaust fans wide open full tilt boogie. Each tent has a 6" x 12" intake mesh panel and I never run my fans above speed 6 maybe 7. So I never have issue with not removing odor and maintain humidity and temp just fine.

Logical and will add it to my knowledge base so that the next time I set up a tent I have a better understanding in how to size my ventilation.

The tent vents open are great for veg but how do I vent air in when I'm in flower? I need to keep the girls in complete darkness and I can't do that with those vents open. Right now I have two 6" ports open to test and the tent is under a higher negative pressure than when I had all 3 ports open which is logical but what or how can I get enough vent to supply the fan if it goes to full open?

I think what I'm after is a formula to figure out how much vent I need to be able to properly vent the tent and not put too much pressure on the fan unit. Too much negative pressure in a room is not good for the girls.

I've thought about using the 8" ports and running 8" duct vertically to just above the floor to keep the light out and still have the vent capacity but I'm trying to keep as much room open since the entire tent floor space will be used for scrog with 6 plants in 7 gallon pots so not much room to add anything else. Adding two 8" duct runs is going to clutter up things in the tent.

 

[RootsRuler]

This is the square inches a 8" round port has.
8" x 8" x 3.1415 = 201.056 sq/in

This is the square inches a typical air vent port a tent would have.
12" x 6"= 72" sq/in

I have 3 ports meaning I have a total of 216 sq/in of open port that I have available to me if I use them for my input air. This would lead me to believe that one 8" port should provide almost the same amount of input air as the 3 rectangular ports on the bottom of my tent. I haven't factored in the reduction in input air due to the flex duct I will need to install to keep light out of the tent but I'm considering it a wash since the tent vents also have restrictions with the mesh they have over them.

But this calculation flys in the face of my real time testing with two 6" ports open which should provide me with the following:

6" x 6" x 3.1415 = 113.094

I have 2 ports open which should give me 226 sq/in of inlet air which should be more than the 8" and the lower vents but I'm not seeing that in real time testing.

What am I doing wrong?

 

[Ozmosis]

Your math appears to be off on the square inches on round ports. The 8" port should only be 8" x 3.14 or 25.12" For (2) 6 inch ports it would be 6 x 3.14 = 18.84 x 2 ports = 37.68

 

[MacroLogos]

Radius, not diameter.
3" x 3.14 = 28.27"
4" × 3.14 = 50.27"

 

[MedicalHydroponics]

IMHO pressure drop not an issue with less than 25’ ducts.
Seems this is going into HVAC engineering systems, wasn’t this a question about tent ducting?

 

[RootsRuler]

https://www.hunker.com/13417142/how-to-find-the-square-inch-of-pipe

inside radius x inside radius x 3.1415

I think where I messed it up is that it's supposed to be radius and not diameter. Always confuse the two. Macro caught that one.

"A radius of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length."

Since radius is a line from the center of the circle to the perimeter or edge that would mean the radius of an 8" port would be 4". Using the formula it should be 4" x 4" x 3.1415 = 50.264 sq/in. For a 6" port it would be 3" x 3" x 3.1415 = 28.2735

Each port vent is 72 sq/in. Now that I have the correct numbers that would mean each 8" port is a little less than 2/3 the sq/in of the vent flaps. All 3 vent flaps open would provide 113.094 sq/in of vent capacity. My tent has two 8" and two 6" round ports I can use for input air. Opening them all would give me 157.075 sq/in of vent capacity.

To approximate the same sq/in vent capacity of the tent vents I would have to keep both 6" ports and an 8" port open. This would be for a passive intake system.

I'm trying not to use a booster fan for the inlet but it looks like I may need to to get enough air in the tent.

I have a 6" pole mounted fan and an 8" Honeywell turbo force fan I hung off the roof inside the tent for air circulation. The 6" works fine as an air circulator but I think the 8" would work better as a booster. I'm going to have to rig up something to get the duct to fit around the back of the 8" fan. I guess I can set it on the floor with the duct attached to the rear of the fan and use it as the under canopy fan so that fresh air comes in from the bottom and pulls through the tent up to the fan mounted on the ceiling. Problem is there isn't much room on the floor as I plan on running six 7 gal cloth pots under a screen so there won't be much floor space to set things down on. Honeywell site says the fan can do 185 cfm at high setting so the fan should be more than sufficient at getting air in the tent. I'll need to tweek the speed of the fan to get the slight negative pressure I'm trying to achieve in the tent.