Volume Flow Rate CFM question
Volume Flow Rate CFM question
(OP)
Hi guys and gals,
I have a quick question on volume flow rates, if air is flowing thru a louver, will the CFM be the same at the inlet and outlet of the louver? And, the only thing that will change is the velocity (ft/min) of the air from the inlet to the outlet of the louver? Am I thinking right?
Thanks in advance for your time.
Tobalcane
"If you avoid failure, you also avoid success."





RE: Volume Flow Rate CFM question
but i don't get "the only thing that will change is the velocity (ft/min) of the air from the inlet to the outlet of the louver?"
RE: Volume Flow Rate CFM question
The total pressure will change.
RE: Volume Flow Rate CFM question
rb1948:
Well would the louver restrict air flow? As in slowing it down? Does louvers have a % that is open? Say for an example that the louver is 90% open, does that mean the air velocity will be reduced by 10%?
MintJulep:
:) I see what you are saying, but if a duct ends at a louver, the geometry chages so it either slows the air down because so the obstruction (louver) or what comes in goes out. Back pressure is another concern...but Ill figure that out later...
Thanks
Tobalcane
"If you avoid failure, you also avoid success."
RE: Volume Flow Rate CFM question
The conservation of mass means that the same mass rate (i.e. lb/hour) of air flows on both sides, but the actual CFM will be higher downstream. The flow in SCFM will also be the same both sides (because SCFM is actually a unit of mass flow and not volumetric flow).
For normal HVAC applications I would expect the change in CFM to be very small.
Katmar Software
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RE: Volume Flow Rate CFM question
Thaks for responding. I should post my real intent, Im trying to model a duct and louver in a room in Flotherm, but it is difficult to model the louver. So does having the louver in the model make sense? Is the differance in CFM and ft/min befor and after the louver neglectable?
Tobalcane
"If you avoid failure, you also avoid success."
RE: Volume Flow Rate CFM question
The terms are being confused above. The louver "slows air down" in the sense that the velocity along the entire duct may be reduced from what it would otherwise be. That doesn't necessarily mean that air slows down between the inlet and outlet of the louver itself, though. Depending on the open area, the velocity could go up or down- but CFM should be about the same.
RE: Volume Flow Rate CFM question
RE: Volume Flow Rate CFM question
i think you're asking if i get X cfm with the louvre open, how much would i get if the lourve was 1/2 closed. personally (not being an HVAC type) i'd expect something close to X/2 ...
RE: Volume Flow Rate CFM question
Since the vent is not wide open, your model should include a resistance that's equivalent to the louvers. Some modeling methods avoid the complexity of a detailed louver model. The best approach depends on the importance of the vent and the data you have available.
1. Estimate a rough percentage open and model it as an equivalent planar resistance.
2. Measure the flow-pressure curve across the louvers using a wind tunnel, then use flotherm to create an advanced planar resistance to match.
3. If you don't have the hardware to #2, do it in software. That is, create a detailed model of the louver by itself in a numerical wind tunnel, then use the simplified 2D resistance in your room model.
4. Find published data for louvers. Possible sources:
-- The manufacturer
-- I.E. Idelchik "Flow Resistance: A Design Guide for Engineers"
-- HVAC forum on eng-tips
ko (www.ecooling.biz)
RE: Volume Flow Rate CFM question
So to answer your question, the cfm into the louvre should equal to the cfm out.
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RE: Volume Flow Rate CFM question
a freebie on my PE exam!!!
http ://www.ang elfire.com /planet/ga dgetforum/ index.html
RE: Volume Flow Rate CFM question
Ko99:
Thanks for the suggestions. That was what I wanted to do, but I have louvers that will direct air at certain directions and wanted to see how the reduction of air speed will affect the air flow within the room. If you are familure with flotherm, I am using the recirculator. With this model I can direct the air, but I (in its dialog box) can not implement resistance. I can use another feature that will restric the air, but when I add that right at the supply of the recirculator, it does not like it and over rides the resistance feature.
Thanks guys for the help, I have a better understanding of the situation, but I think I will break down and model in the louver. Pretty pics convice certain people anyways.
Tobalcane
"If you avoid failure, you also avoid success."
RE: Volume Flow Rate CFM question
ko (www.ecooling.biz)
RE: Volume Flow Rate CFM question
Thanks Ko99, I tried that yesterday, instead of collasping it towards the recirculator, I collasped it to the middle of the resistance volume. And guess what! The velocity is faster. Which makes sense.
If we looked at conservation of mass, yes the densities are the same but not the cross area (resistance).
M1=M2
v1p1a1 =v2p2a2 <densities are the same>
v1a1=v2a2 <at this point the CFMs equal>
v2=(v1a1)/a2 <when area becomes smaller the velocity goes faster and CFM does not chage>
Tobalcane
"If you avoid failure, you also avoid success."
RE: Volume Flow Rate CFM question
RE: Volume Flow Rate CFM question
Thanks for responding.
For my application, I have two really big fan cooling units for a realy small room. So once in steady state, the fan cooling unit is supplying and returning air so the flow profile looks well...fluid. In flotherm once I added the resistance, the velocity increased. I think this is because of a small room with high CFM fan cooling units.
Tobalcane
"If you avoid failure, you also avoid success."