Flow rates

Jed_2

An air head hydronician, dabbling in highly unrecognizable theoretical mathematics, regarding the air flow through his head!

Mark_46

7\" CFM

Anybody have a formula for determining what the CFM would be for a 7" round duct between 1 and 3 inches of water column? In other words, CFM at 1, 1.1, 1.2, 1.3...and so on up to 3.0. Thanks.

realolman

try this

http://www.pipeflowcalculations.com/pressuredrop/index.htm

Brad White_9

Do you mean

total static pressure loss or rate of loss per 100 feet of duct, Mark? Sounds like a high pressure application, dust collection perhaps?

If this is a comfort conditioning application, may I assume it is at Gallaudet School for the Deaf?

Rule is that SP changes by the square of the volume.

If you have x flow and it loses 1.0" per 100 LF, at 2x flow it will lose 4.0" per 100 LF.

For your 7" diameter duct and from my Duct Calculator:

1.0" (per 100'): 560 CFM

1.5": 690 CFM

2.0": 800 CFM

2.5": 900 CFM

3.0": 1,000 CFM

Not the full array of increments you sought, but close enough?

Brad

Mark_46

Hi Brad

How did I know you would probably have the answer to this one? ). Yes, it is high pressure...Unico high velocity actually.

What I'm looking for is to ensure that the 7" plenums I have will not become the 'bottleneck' so to speak as I start to make changes to increase air flow. I want to make sure the 7" can do the job and reassure myself that it didn't need to be 9" plenums.

I now realize from your post that I overlooked the length factor. I dont have the lengths of the plenums. I will post later after I measure them.

At the end of the day I need about 375 CFM at 1.25-1.5 inches wc through a 7" plenum on the one system. And 625 CFM at 1.4-1.6 inches wc throug a 7" plenum on the other. Based on the numbers you posted, there shouldnt be a problem. Agree? Again, I will get the plenum lengths.

Mark_46

Here they are

Brad,

The attached file shows the plenum measurements.

Brad White_9

Hey Mark-

What helps the process is knowing how many outlets you have and the approximate flow in each. This will establish what the airflow is in each section. As air drops off, available pressure increases (static regain).

Can the first floor system at least be made into a loop (connecting the ends)? Easy way to "forgive" distance.

And do you have a pathological fear of 9 inch duct?

Re-post with branches and approximate CFM and I can get you a better idea. May I also assume that there are more elbows and offsets than your diagram shows, or is that about it?

Brad

Mark_46

Brad

LOL, no fear of 9" duct. The system is already done in 7" and I can't bear the thought of having to change it to 9".

I can tell you the first floor system has 15 outlets (+2 capped off in the basement for 'just in case heating'). The second floor has 12. But a few on the second floor are using orifice plates which are easily removed. I think they will have to come out anyway. Yes there are many branches...of miniduct. But the number of elbows in the plenums are basically as I drew it. I wouldnt be able to show where the miniduct takeoffs are right now. I would have to go back and diagram all that seperately.

Inserting a loop into the first floor maybe possible but tough. Prefer not to.

What happened here is the installer never checked or adjusted the flow. Frustrating and shabby, but true. The installer screwed us more ways than I can tell you. So Im now trying to get a handle on what the CFM is currently (and Im pretty sure its too low) and adjust up to where it should be. What I am in fact concerned with is that it may land up there are not enough outlets. Then I will have another fight on my hands to have more installed. It's exhausting and stressful.

Unico specifies about 250 CFM per ton. Thats where my crosscheck idea came into play before going any further...ensure the 7" is large enough to handle the required CFM. Otherwise, the installer is going to have even more problems than he already does.

I plan to follow Unico's recommeded method of measuring blower motor amperage draw and static pressure to determine flow. I have the amperage draws but am still futzing around with the best way to measure static pressure. I was going to go with a manometer/pitot tube but are expensive and cumbersome. So I think I will go with a magnehelic gauge and a few taps along the plenums. But I dont have all that stuff yet. Plus, looks like there are a few different types of taps that can be used for the plenum. Apparently some more accurate than others. Not sure yet.

Brad White_9

I might assume

that the outlets are reasonably spaced along the mains with the exception that the first few feet would not have any on account of noise.

How long are the runouts and may I assume attenuated? Those are worth what, 40 cfm each, IIRC?

Let me see if there is some time I can give you on this, no guarantees. Deadlines are looming.

As for measuring SP, any Magnahelic or a block manometer will serve you well especially if there is a dual scale. For a Magnahelic, I would get a 0-0.25" scale for fine velocity work (corresponds to about 2000 FPM) and a 0-1.0" one for up to 4000 FPM velocities. Your duct static pressure may require a 0-2.0" scale Magnahelic. If I had to get one, the 0-2.0 is probably the most common but do not expect much in the way of low-end accuracy.

Brad White_9

I did a quick check over lunch, Mark

I made some conservative assumptions:

*Full volume the full lengths (a "what if", assuming no outlet volume reduction along the way).

*Full radius die-stamped elbows, smooth.


*For First Floor (Zone 1), I split the flow 60-40, with 60% (375 CFM) in the longest branch of 28 feet after the tee, plus an elbow.

Here is a breakdown of Zone 1:

The first discharge piece (unit to and through the diverging tee) will absorb 0.60 inches right there. Full CFM in a 7" duct and losing 1.5 velocity heads at the tee. Slam-O. A wye fitting will cut that dramatically, the duct portions only amounting to ten percent of the 0.60 number.

The longer branch at 60% flow (375 CFM) would, with no reduction in flow along the length, take 0.14 inches including the elbow.

A typical runout of flexible duct and outlet? I have no idea but would assume 0.25 inches. Your Unico engineer can confirm this. Biggest loss is the discharge at velocity.

So the static pressure needed at the discharge of AHU-1 would be: 0.60 + 0.14 + 0.25 = 0.99 call it 1.0 inches, conservatively. That sets what the unit needs.

For AHU-2, again, full flow and no volume drop-off, 36 feet plus two elbows and a straight discharge: 0.475 call it 0.50 inches including the outlet.

With volume drop off I would expect actual numbers to be 80 percent for AHU-2. Not much credit, maybe 10% for AHU-1 given the intitial tee fitting loss.

But do check my terminal outlet runout assumption of 0.25 inches with Unico; that will affect the total numbers I came up with.

Hope this helps!

Brad

Mark_46

Thanks

Brad,

The outlets are probably not spaced as evenly as they should be.

Takeoff lengths I do not know, I'd have to measure.

Yes, the Magnehelic 2002AV is the one for my application. But requires the pitot tube.

Mark_46

Wye

Brad, could you recommend a duct brand for the wye? Web site? I've been trashing around online and find some like what I need but not exactly...7" splitting into two 7"ers out.

Brad White_9

Several

Lindab is one I like. Nice line of self-sealing double-gasketed slip fittings. United-McGill, Semco and others. There are some smaller companies that make industrial fittings (dust collection for example). Just seal the living bejeezus out of them.

Mark_46

Updated

Hi Brad,

Just to give you an idea of what the whole system looks like, have a look at the attached. After diagramming all of it, I am now a Visio expert ).

Brad White_9

You know

I did not figure the return ductwork in my calculations but it does count for something. I assume it is conventionally sized (0.07 to 0.10 friction per 100 feet) so should not amount to much.

As I stated, my calculations assumed full flow the whole length so there is some wiggle room.

Fix that Tee fitting and I would think that you are off to the races.

Mark_46

Hmm

Now that you mention return. You can see in the drawing I have one 12" @ 11' and 2 10" at...many feet. Id have to measure. But on the return side I've been thinking about the filters. I am using Glasfloss pleated types. They offer better filtration but also more drop. I just dont like those blue looking cheap mesh particle filters. If the pleated turn out to be too restrictive, back to junkers and once a month replacement.

When you say full flow the whole length, those calculations assumed no takeoffs? Is that correct?

Ya the Tee stinks. Ill eventually look into redoing it. But I just bought a 2002AV and pitot tube so now I can get some static pressure readings (as well as FPM) and get going.

By the way, one more uestion if I may. If i were looking at a basic digital home use thermometer, is the temp displayed dry bulb temp? The reason I ask is because the air velocity slide ruler requires that input to determine velocity from velocity pressure. ALong with baro, air density, etc.

Brad White_9

Filters, Dry Bulb, yup, we do it all!

Hey Mark-

Yes, by full flow, I mean that I did not take credit for the drop off of airflow along the way. The lenghts are not that long so it is conservative (in your favor in that real life will likely be less pressure drop). The alternative is to calculate each segment, and that would for me, be cruel

Filters- One thing I like to do is splurge on large filters. Filter return grilles too if I can, but large. Saves cleaning ductwork if you can stand looking at the larger grilles. Angle filters are a great way to compensate for limited flat-on face area. Filters are the largest variable pressure drop so it pays to be generous up-front IMHO.

I agree about pleated filters. If you can read a newspaper through a blue cheap-o filter, what will it capture, the New York Times?

Home digital thermometers- I like the kitchen types by the way, Kitchen Aid, Polder, others. Digital and for $15 pretty accurate. Yes, they read drybulb, all thermometers do unless you put a wet sock on one of them. No, not your pet Argyles with the holes in the toes, but a gauze sock made for the purpose. That will give you -are you ready? Sitting down now??

Wet Bulb Temperature. Oooh.

Do you have a psychrometric chart? Barometric pressure and air density mostly go to altitude. Not sure where you are in elevation, you probably told me. Temperature is the other density variant. Standard air (yeah right!) is at 70 degrees at sea level. Barometric pressure is 14.696 psia (29.921 inches Hg.) and density of dry air is 13.33 cubic feet per lb. according to my Psych chart.

If your air is 70 degrees and 50% RH, density is about 13.5 CF/lb.

Don't sweat the density stuff too much unless you are running hot air. Even then the difference is manageable. Not as accurate but close enough for what you are doing.

Hope that helps!

Brad

Mark_46

Good stuff

Its great 'talking shop' with you, Brad.

Yes of course, I wasnt asking you to calculate all the take-offs too. In fact, it was very nice of you to calculate the plenums. I wasnt expecting that either. But I think I will call Unico and get the specs and try it myself.

Ah ha, just wanted to make sure dry bulb temp did not differ from the readings of any common temp gauge.

The elevation of my town/city is 75 feet.

Im running hydro-air.

According to Dwyer, the gauge's FPM scale is calibrated assuming 'standard air' is being measured. If so, the FPM reading can be plugged directly into thier velocity flow charts. If not, the correction formula must be used.

But like you say, if I dont use the correction formula for air variances, how off could the readings be? Afterall Im only trying to set, or make sure, my systems are in the ballpark in terms of CFM. Which I dont think they are right now.