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Discussion Starter #41
While you are right, the larger pump would use less voltage to achieve the same vacuum power with a bigger piston. It’s not just motor size, think about woofer size, the bigger the woofer the less power is needed at lower frequencies.

Also less heat and less resistance, bigger bearings having less restriction. It all adds up but I imagine the biggest effect comes from lasting longer and needing less maintenance.


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We design mining equipment systems that pump slurry under pressure for thousands of linear feet at varying amounts of lift, usually do not pull vac (our centrifugal pumps don't like to prime). Centrifugal pumps have more in common with a fan than piston pumps do. The idea is that there is less work being lost to friction that come from higher velocities. Immediately after the pump we also put a reducer, actually as an increaser, to further slow the velocity to just above where the slurry remains in suspension. Through many years of experience, this recipe has proven itself over and over. Less power is used and pumps and piping lasts longer. That's the way it's done (correctly) in the mining industry.

If given the choice between too small, theoretically perfect, or too big, it is better to go somewhere bigger than theoretically perfect.

I hope I was effective in explaining the reasoning.

I know that this talk of pumps was not intended to say that dual fans will have no benefit. It was an analogy. Perhaps I should not have mentioned it because of the distraction potential.

Spinning 2x 92mm fans at 1/4 the speed will flow similar amounts of air in cfm (not static air pressure which is rather unimportant in an open flow situation that cooling requires), will use half the power, the fans will last longer and be nearly silent, all benefits.
 

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We design mining equipment systems that pump slurry under pressure for thousands of linear feet at varying amounts of lift, usually do not pull vac (our centrifugal pumps don't like to prime). Centrifugal pumps have more in common with a fan than piston pumps do. The idea is that there is less work being lost to friction that come from higher velocities. Immediately after the pump we also put a reducer, actually as an increaser, to further slow the velocity to just above where the slurry remains in suspension. Through many years of experience, this recipe has proven itself over and over. Less power is used and pumps and piping lasts longer. That's the way it's done (correctly) in the mining industry.

If given the choice between too small, theoretically perfect, or too big, it is better to go somewhere bigger than theoretically perfect.

I hope I was effective in explaining the reasoning.

I know that this talk of pumps was not intended to say that dual fans will have no benefit. It was an analogy. Perhaps I should not have mentioned it because of the distraction potential.

Spinning 2x 92mm fans at 1/4 the speed will flow similar amounts of air in cfm (not static air pressure which is rather unimportant in an open flow situation that cooling requires), will use half the power, the fans will last longer and be nearly silent, all benefits.

Wouldn’t the compression of pushing a 92mm fan through a 80mm hole effect the expected CFM flow? Also with the shroud restricting the movement of the air flow be affected by the static pressure as well?


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Discussion Starter #43 (Edited)
Wouldn’️t the compression of pushing a 92mm fan through a 80mm hole effect the expected CFM flow? Also with the shroud restricting the movement of the air flow be affected by the static pressure as well?


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I wondered that myself so cfm was allready talked about. The stock fan is rated at 52 cfm. These two Noctua fans I have selected, when run at their lowest speed, push a similar 50 cfm (25 each) through the now clear 80mm hole that no longer has a fan motor in the center.

It is also noteworthy that a 92mm fan does not flow air through the entire 92mm because the motor obstructs the center.

Static air pressure is irrelevant in an open flow situation where we are not pushing against anything. Static air pressure is measured by blowing into a duct and closing the end off until you reach max pressure. Closing it off beyond that may well result in a decrease in pressure from the max. We are interested in moving as much air across the heatsink as we can, not inflating balloons (so to speak).

The Behringer NU series has enough slots in the front panel to allow 100 cfm (2 stock fans in the NU6000) to flow openly.

There may be an increase in static pressure if 2 sets of 2x 92mm fans were to be run at full speed if the cfm exceeded the flow capabilities of the slots in the front panel.
 

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Hydraulic equations aren't applicable to fans, as the assumption is for incompressible fluids.


Standard hydraulic power equations. The link below is for pumps but it’️s basically the same for fans.

https://www.google.com/amp/s/www.engineeringtoolbox.com/amp/pumps-power-d_505.html
When I meant the same I just meant the principle is the same. Power is based on pressure, flow, and pump/fan efficiency (and motor efficiency). Doesn’t matter if it’s fans or pumps just the constant and units change.

Here’s the equation for fans.

https://www.google.com/amp/s/www.engineeringtoolbox.com/amp/fans-efficiency-power-consumption-d_197.html
 

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Hmm, no mention of blade air friction loss so I guess it's not a significant factor.


When I meant the same I just meant the principle is the same. Power is based on pressure, flow, and pump/fan efficiency (and motor efficiency). Doesn’️t matter if it’️s fans or pumps just the constant and units change.

Here’️s the equation for fans.

https://www.google.com/amp/s/www.engineeringtoolbox.com/amp/fans-efficiency-power-consumption-d_197.html
Any friction losses like that would be taken into account in the fan efficiency. Basically any energy converted from motion to heat would be considered the efficiency losses in the fan. The equation assumes you have the fan curve so you can look up the efficiency at the operating condition that you are calculating power for.
 

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Ok then you'd have to have the curves for the fans in question to see if there are any meaningful differences between the different fan sizes.

All highly academic though, as these fans use on the order of a watt or so.


Any friction losses like that would be taken into account in the fan efficiency. Basically any energy converted from motion to heat would be considered the efficiency losses in the fan. The equation assumes you have the fan curve so you can look up the efficiency at the operating condition that you are calculating power for.
 

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Very nice work.

I think you've done as good a job as possible directing airflow in the volume available, but it's still a lot of restriction relative to a normal installation.

Any idea what the net cfm is?
 

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Discussion Starter #53
Very nice work.

I think you've done as good a job as possible directing airflow in the volume available, but it's still a lot of restriction relative to a normal installation.

Any idea what the net cfm is?
The 2 Noctua 92mm fans running with the ULNA are rated at about the same CFM as the stock OEM fan, right around 50CFM. Without the fan motor in the center of the 80mm opening, the flow path into the case is actually greater than with a 80mm fan.

I'm actually running at full speed without the LNA or ULNA adapters and it's almost completely silent. Now I have to replace the fans on my ent. center cabinets with Noctua.
 

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Without the fan motor in the center of the 80mm opening, the flow path into the case is actually greater than with a 80mm fan.

I'm thinking of the restriction caused by the fans blowing almost straight into the dividing wall.

But I guess you can get a pretty good idea just by feeling the sir flow with your hand.
 

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I'm thinking of the restriction caused by the fans blowing almost straight into the dividing wall.

It is not ideal. They would be a lot more efficient sucking air rather than blowing. They should still be sufficient though.
 

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Discussion Starter #57 (Edited)
I'm thinking of the restriction caused by the fans blowing almost straight into the dividing wall.

But I guess you can get a pretty good idea just by feeling the sir flow with your hand.
These fans have pretty high static pressure especially at full speed. You can def feel the air across the heat sink and even coming out the front grill. Air changes direction pretty readily. If it were backing up it'd increase the fan noise. Without doing a flow analysis, my gut tells me there is no significant loss especially since I have the fans running at full speed which results in lots more CFM (76 vs 50 ULNA or 52 OEM fan).

It is not ideal. They would be a lot more efficient sucking air rather than blowing. They should still be sufficient though.
The flow through the dual fan adapter might be better in the other direction (fans reversed) but you'd get far less over the heat sink. Think of a cup of coffee...you cool it off by blowing. The opposite just won't work.

A mentor once told me, "That's the difference between a suck and a blow." :D

back to front airflow is idiotic.
Not sure what you are saying here. Reverse air flow perhaps? All Behringers flow back to front.
 

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Not sure what you are saying here. Reverse air flow perhaps? All Behringers flow back to front.
Out of the factory they do, yes. What Not is saying is that is against all supported cooling methods in rack mounted equipment (I hope he will correct me if I'm interpreting his response incorrectly).

Rack mount equipment residing in co-locations (for those that aren't aware, datacenters of computers) are all designed to evacuate hot air to the back, or the "hot aisle". Cool air enters from the front, or the "cold aisle". There is a separation in datacenters to keep the cold aisle producing cold air, and the hot aisle to evacuate hot air.

So in the case of the Behringers the air flow has always been backwards, per rack assembly methodology.
 

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Not sure what you are saying here. Reverse air flow perhaps? All Behringers flow back to front.
Yes, and they're stupid for doing it. that's all. Especially when industry standard is front intake, you end up sucking hot air from other gear in the cab.

Depending on which amp you have, might want to compare with front to back airflow.
 

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Discussion Starter #60
Out of the factory they do, yes. What Not is saying is that is against all supported cooling methods in rack mounted equipment (I hope he will correct me if I'm interpreting his response incorrectly).

Rack mount equipment residing in co-locations (for those that aren't aware, datacenters of computers) are all designed to evacuate hot air to the back, or the "hot aisle". Cool air enters from the front, or the "cold aisle". There is a separation in datacenters to keep the cold aisle producing cold air, and the hot aisle to evacuate hot air.

So in the case of the Behringers the air flow has always been backwards, per rack assembly methodology.
Yes, and they're stupid for doing it. that's all. Especially when industry standard is front intake, you end up sucking hot air from other gear in the cab.

Depending on which amp you have, might want to compare with front to back airflow.
You are correct. Back to front is contraventional. It'd be pretty involved to turn the air flow around on a Behringer. Easiest way might be with another fan inside...but meh.

Are other pro amps back to front? Is that a pro gear thing or a Behringer thing?
 
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