Originally Posted by PepeLapiu
I don't agree with your thinking Jack
Let's say you have two fans.
Fan A produces 50 cfm @ 1mm
Fan B produces 50 cfm @ 2mm
And you have a rad that produces 0.75mm of pressure drop @ 50 cfm.
Your thinking is that both fans will perform the same, with fan A creating 0.25mm of residual 'useless' pressure while fan B will produce 1.25mm of residual 'useless' pressure. As both fans will push the same 50 cfm through the rad.
Am I correct in assessing your logic?
If so, there is no such thing as too much residual pressure. If fan A, for example, has 0.25mm of residual pressure, it will increase the cfm until the pressure created by the fan is in equilibrium with the pressure drop created by the rad.
So fan A would probable push more cfm, 55 cfm for example, at 0.85mm, while the rad is creating 0.85mm of pressure drop at that same 55 cfm.
And so, since fan B has more residual pressure, the cfm will increase more than with fan A, until the pressure created by the fan is met by equal pressure drop from the rad, at a given cfm.
In other words, the residual pressure of the fan is converted into airflow until the fan creates the same pressure as the pressure drop of the rad. More pressure from the fan is always converted to airflow if it's not met by pressure drop.
If the fan pushes harder against the rad, more air will be pushed throught.
There is no such thing as a fan pushing too hard.
There is not some ghost pressure that gets pushed out of the rad into thin air.
This is a subject oft covered in my classes so I will borrow from that.
Think of it this way. Put a fan on the top of your desk, plug in .... how much SP will it create ? Zero....the fan does not create pressure, the resistance does, the fan can't create what isn't there. What is the load on a hoist in the example I gave .... it's 0 until ya hook a weight to it. Same deal..... without the weight, the hoist has no load, w/o resistance to air flow, the fan has no SP. look at any fan curve ..... where you see the point where the curve hits the X axis, what is the SP ? It's 0.
Here's where your thinking went off track
'Let's say you have two fans.
Fan A produces 50 cfm @ 1mm
Fan B produces 50 cfm @ 2mm
That comparison bears no resemblance to the conditions I presented. You are arguing a position which "isn't on the table" so to speak.
If the resistance at a given cfm is say 1.25 mm H2O and two fans are capable of providing say 65 cfm @ 1.25mm, then there will be no discernable effect whether one fan's maximum SP is 1.50 or 1.75 when the resistance provided by the system at the given flow is 1.25 mm.
You have taken the specific data set I provided, argued against it and then substituted a different data set to argue a point which fits a predetermined misconception. I have to take issue with this for the following reasons:
a) I had two fan curves intersecting at a common point .... you answered by creating two different points
b) This ignored the fact that, by definition, intersection points have the exact same data sets.
c) I referred to maximum SP as being irrelevant .... you countered with points other than the max SP
d) Advertised fan data of Air Flow: 50 cfm / SP: 1.00mm almost never means 50 cfm @ 1.00 ..... it generally indicates max flow = 50 cfm (@ 0 SP) and Max SP = 1.00 mm (@ 0 flow)
So lets take both of our examples and adjust it a little bit to fit the curve I have modified (changed the x and y axis numbers to get them outta 1000 cfm / ft of water) from one of my lesson curves:
Your example .... had same flow and different SPs
Fan A produces 50 cfm @ 1.37 mm (red)
Fan B produces 50 cfm @ 1.50 mm (blue)
My example ... had intersecting points
Fan A produces 65 cfm @ 1.25 mm (red)
Fan B produces 65 cfm @ 1.25 mm (blue)
All 4 of the above points are represented on the fan curves above so how does Fan B having a higher SP at 50 cfm (or the 0 cfm from my example) change the fact that they both have the same exact SP at 65 cfm. Now let's look at my original statement and adjust for the curve data
"If the resistance at a given cfm is say 1.25 mm H2O and two fans are capable of providing say 65 cfm @ 1.25mm
, then there will be no discernable effect whether one fan's maximum SP
is [1.63] or [1.79]. "
In the curves presented, did the curve with maximum SP of 1.79 do any better than the one with a max SP of 1.63 at the design SP of 1.25 ? No, as indicated in the original statement, they intersect at the exact same point.
While it is a common misconception that B is somehow better than A, it simply doesn't hold up, at least not in the example I had presented
. "A" can be better in certain instances, "B" can be better on others, However, both are the exact same in my example regardless of max SP, at design conditions And A is actually better at other than worse case conditions. Here's why:
1. You design your cooling system for the worse case
condition ...the largest load it will handle.
2. You determine the radiators you need based upon your cooling needs, flow requirements and noise (aka fan rpm) you can live with.
3. When you come up with needing 65 cfm at 1.25 mm, the design point representing that point on the curve is the worse case
load condition which your fans are expected to meet at max rpm. What the fan does at it's advertised maximum SP 1.63 or 1.79 is of absolutely no relevance whatsoever because the fan will never see that point on the curve. It will also produce 0 flow and that won't provide much cooling which is one of the reasons I fail to see the relevance.
4. The problem is fan B will be advertised as follows ... CFM = 95, SP = 1.8 mm and people, unless a curve is also published (very rare indeed), will actually assume it can deliver that...it can't. It can only deliver what is on the curve.
5. It doesn't "ride out the curve" to a higher number as you suggested as in the example given
, both curves "ride out" and intersect at the exact same point. At the design point in the original example (as indicated by System Curve 1), which fan you have A or B makes absolutely no difference whatsoever as both deliver the exact same cfm at the exact same SP.
6. Fan B has a higher SP to the left of the design point ..... so you would be right if the fan was operating on this part of the curve (System Curve 2)..... but a) that was not the condition stated and b) can it ? The Radiator won't grow any thicker and the fans can't go any higher than max rpm. If you designed on the "worse case condition", how can ya system ever be faced with "worse than worse case conditions" ? Conversely if ya system was akin to the one indicated by System Curve 3, Fan A is the better choice.
7. However, when the system is not at its worse case condition with regards to loadings, you will likely run at lower rpm. When that happens, the point of intersection will move down and to the left. And when that happens, at lower speeds, you will be operating to the right of the intersection point where fan A has the edge.
My curve was taken from a typical large air movers as is used in buildings from one of my lesson plans and is much more "regularly shaped" .... Our little 120 fans have more "anomalies" so let's use one of Martin's examples
I said that what fan has the higher max SP is meaningless and that only the SP at the design point is significant. This is clearly demonstrated in Martins curve. Following your argument, the Cougar Vortex must be the better fan
because at certain points other than the design point
it has a higher (and flow rate)....
1. It is clearly not better at 22 cfm and 0.058 SP as the two fans have the exact same flow rate and SP
2. It is clearly not better at 47 cfm and 0.030 SP as the two fans have the exact same flow rate and SP
3. It is clearly not better in the system tested as the AP-15 is delivering 37 cfm across the rad to the Cougar's 34 cfm despite the Cougar having the higher max SP
Fitting that real curve data in place of the assumed data in our original statements
My original statement with arbitrarily assumed was:"If the resistance at a given cfm is say 1.25 mm H2O and two fans are capable of providing say 65 cfm @ 1.25mm, then there will be no discernable effect whether one fan's maximum SP is 1.50 or 1.75 when the resistance provided by the system at the given flow is 1.25 mm."
Substituting Martins data for the assumed stuff, the statement still holds .... twice"If the resistance at a given cfm is say 0.058 in H2O and two fans are capable of providing say 22 cfm @ 0.058, then there will be no discernable effect whether one fan's maximum SP is 0.080 or 0.084 when the resistance provided by the system at the given flow is 0.058 in."
....... Martin's curve shows this to be absolutely correct."If the resistance at a given cfm is say 0.030 in H2O and two fans are capable of providing say 47 cfm @ 0.030, then there will be no discernable effect whether one fan's maximum SP is 0.080 or 0.084 when the resistance provided by the system at the given flow is 0.030 in."
.... Martin's curve shows this to be absolutely correct.
Your statement was
"Fan A produces 50 cfm @ 1mm
Fan B produces 50 cfm @ 2mm
So fan A would probable push more cfm, 55 cfm for example, at 0.85mm, while the rad is creating 0.85mm of pressure drop at that same 55 cfm."
Again, I was referring to maximum SP, so your answer was a bit "off topic" but when substituting Martins data, your statement is still not valid for the majority of the curve
"GT AP-15 produces 15 cfm @ 0.055 in
Cougar Vortex produces 15 cfm @ 0.068 mm
GT AP-15 produces 55 cfm @ 0.010 in
Cougar Vortex produces 55 cfm @ 0.025 mm
So your conclusion that the Cougar Vortex would probably push more cfm while the rad is creating 0.40 in of pressure does not prove out....the AP-15 produces an extra 11 cfm at that point. The AP-15 will ride out the curve and produce 37 cfm @ 0.047 to the Cougar's 34 cfm @ 0.040 despite it having a higher maximum SP.
The original statement is therefore clearly borne out in both my classroom and martin's example . The ONLY thing that matters is what the SP is at the design point.... being bigger at other points on the curve, especially at the point of max SP (and 0 flow) is meaningless.Edited by JackNaylorPE - 4/7/14 at 12:54pm