Pressure drop from perforated ceiling panels.
Pressure drop from perforated ceiling panels.
(OP)
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).
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).





RE: Pressure drop from perforated ceiling panels.
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
RE: Pressure drop from perforated ceiling panels.
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.
RE: Pressure drop from perforated ceiling panels.
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
RE: Pressure drop from perforated ceiling panels.
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.
RE: Pressure drop from perforated ceiling panels.
RE: Pressure drop from perforated ceiling panels.
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?
RE: Pressure drop from perforated ceiling panels.
(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.
RE: Pressure drop from perforated ceiling panels.
RE: Pressure drop from perforated ceiling panels.
RE: Pressure drop from perforated ceiling panels.
I am in process of requesting that $124 SMACNA Duct Design Book for the library. Cheers.