Pressure drop from perforated ceiling panels.

[lippy]

Hello,
Here is the problem and what I think could be the solution based on the Bernoulli equation, but it seems too simple to be true.
We have say 1000 cfm(@0"sp) being delivered to a ceiling plenum of 10ft x 10 ft. with .50" holes spaced every 4". That means I have 900 .50" holes giving me a "thru" area of approx. 175 (sq. in.). Now if I compute V=Q/A, I get a velocity of 822 ft/min.
I want to know if I can use this velocity plugged into the equation ---delta(P)=(V/4005)*2. Then apply this delta(P) to my blower curve and acuratelly "de"-rate the blower delivery. Oh yea, I did validate the the 4005 number...Oh the memories. Thanks, Joe (lippy).

 

[Accystan]

Lippy:

I’m not sure I understand your question. Do you mean that when you plug 822 fpm into the equation the result is the static press. drop as seen by the fan, i.e. 0.042” wc, when 1000 cfm passes through 100 ft^2 of this particular ceiling. I’m not sure that this is correct. The formula:

P = (vel/4005)^2

gives velocity pressure. But surely this is not static pressure drop or as you put it “delta P”. I would guess that, as in all calcs. of this sort, you would have to multiply this calculated vel. press by a factor (K factor?). The perforated ceiling manufacturer should have this number.

My guess is:

delta P = velocity press. x K

But I’m interested to see some other people’s input here.


Cheers, Accystan

 

[lippy]

Accystan,
Your assumptions of the problem are correct.
I am glad you mentioned the "k" factor which is crucial when dealing with orifice obstructions in fluid flow. I will check if manufacturer of perf-ceiling panels (HunterDouglas) has such info. I, like yourself, wonder then if (K x Vel.Pres.) would be a proper value to apply in de-rating the fan performance. I guess I am after total delta P which I believe is made up of two components namely, static & velocity pressure changes. The thickness of the panels are only 1/16" so there is no friction factor like pipe. This is an actual application used in tissue culture rooms to get even conditioned air flow across all the shelves and shelf lights. Thanks for your participation.

 

[ChasBean1]

Lippy, all that would solve for is velocity pressure of air at 822 fpm (about 0.042 in. w.c.).

ASHRAE (1999 Applications, 51.5) suggests a leakage equation that relates pressure, area, and flow:

Q = 2610 * A * (dP)^5

Q is flow in cfm (1,000 cfm in your application)
A is net open area in ft2 (1.215 ft2 in your application)
dP is the pressure difference across the opening, in. w.c.

Solving for dP, you can estimate plenum pressure to be 0.1 in. w.c. Add pressure drop from the fan inlet and outlet duct work to the 0.1 in. expected across the plenum, and that would be the static pressure at which I'd size your 1,000 cfm fan.

Note that the above equation is the simplest I've found for this relation, but it has limitations, sort of like how the fan laws only approximate fan performance. It shows pressure drop as a square root relation, whereas it might be proven to have an actual exponent of 0.55 or 0.6 or something... but this is how I'd estimate.

As I always say, re-post if this works out to be nearly correct (but nobody ever does). Good luck, -CB

 

[lippy]

What about the the "K" factor for perforated plate like is found in this following link?

http://www.inres.com/Downloads/HVAC_RR.pdf

Here is the real life situation I am dealing with: I have to blower modules facing each other in a room 11'deep by 7'wide.
They blow 1800 cfm each (0"SP blower rating) at each other above the false ceiling and then this air has to go down thru the perforated part of the ceiling. This area part is 11'x 2' with some % open. I thought this is where the "K" factor comes in. So I thought I could use this formula:
(Delta P = K (V/4005)^2)like Accystan mentioned in a previous message. I figured that this would be an iterative process since the velocity would change due to the cfm change from re-rating the fan at the calculated Delta P. I'm doing this to estimate a true cfm thru this cooling circuit(ie: return side wall plenums with cooling coils) so I can be sure I am removing the heat from my tissue culture shelf lights. Your formula and mine without the "k" give much lower delta P's. Those "K" factors are huge.

 

[lilliput1]

You need to do pressure drop calculations for the entire air path (supply and return) as seen by the fan. Where is it drawing air from? Are there filters? See SMACNA HVAC Systems Duct Design. Table 14-17B shows pressure drop coefficent for various free area ratios and t/d ratios (plate thickness/diameter of perforated hole). Note Table requires the coefficient to be applied to the velocity pressure of the air in the duct (upstream from the perforated panel within the duct), not at the velocity pressure of the air through the hole.

 

[ChasBean1]

Lippy, the post you reference looks like interesting work and is pretty complex. I think the results largely show that room pressure is pretty constant throughout. Re-read the first paragraph on page 6. They note a K factor of 43, meaning the pressure drop is equal to 43 velocity heads. But (paraphrasing) 'when the airstream emerges, pressure changes on the order of one velocity head are produced.' I wonder if the analysis is accounting for the venturi effect as the air jets out of the hole... A change on the order of 'one velocity head' might be 2 x .042 or .084 inch (pretty close to 0.1 inch as per the ASHRAE estimation).

Do you have performance curves for the blowers? Is the plenum the only pressure obstruction, or are there duct work, filters, and coils? Is there anything actually in operation now and any measurements that can be taken?

 

[lippy]

I know there are other losses assoc. with the air circuit
(ie: blowers, refrig coil, duct bends, intakes). I am just concerned with calcualting what would be the pressure drop thru a perf. pattern as a function of %open area. You see the volume above this perf. panel is essentially the volume between it and a smooth hard ceiling about 16" up. It is pressurized with these blowers aimed at each other pulling
air up false side walls. This air came from the room below the false ceiling. The air in the false ceiling is turbulent, slightly pressured due to the blowers and perfs. I do not have a measurable consistant velocity leading into the perf surface. I quess I am struggling with applying some type of "K" factor to some velocity.

As Lilliput1 noted before;
THE VELOCITY VALUE TO USE IS THE VELOCITY UPSTREAM OF THE PERFS NOT VELOCITY PRESSURE OF AIR INDUCED BY THE HOLES. If this is true then do I calculate a velocity based on a completely opened area(ie: perf panels removed) and then apply those big "k" factors that people are publishing for certain percentage of opening? Thanks all.

 

[lilliput1]

It depends on the basis of the Table. When I calculated the parameter of your situation, the value was outside the applicable range of the particular SMACNA table I had refered to. However I think you can use the loss coefficients for entrance & exits. These would be applied to the air velocity in through the hole. The coefficient is 0.5 for the entry and 1.0 for the exit. So the loss would be 1.5 x the velocity head = 1.5 x (822/4005)^ 2 = 0.063" wg. This loss would be a small part of the overall loss. Typically I add minimum 0.25" wg pressure drop to my calculated total loss values. Your main losses would be pressure drop through equipment & fan system effect losses. If you don't have minimum 2.5 diameter straight run upstream & downstream of the fan there would be system effect losses as discussed & charted in SMACNA HVAC System Duct Design. You may want to create a spreadsheet tabulating resulting fan CFM as system pressure drop varies +/- from your calculation & as filters get loaded to see when you should alarm the system if it is critical that you do not go below a limit CFM.

 

[ChasBean1]

I agree w/Lilliput. See the last 3 posts. If you have a coil or any duct work, this drop across the perf. ceiling is nearly negligible. However, I don't mind going into the methods of figuring the loss, and this is a good discussion...

 

[lippy]

Thanks fellas. The discussion is good for me too.
I am in process of requesting that $124 SMACNA Duct Design Book for the library. Cheers.