Obstruction in front of louvers

[Ron757]

For a project I'm working on, there is a 8.75' x 6.5' acoustic intake louver that might have a 3' tall planter located 10" from the louver. The airflow is 13,000 cfm and the face velocity is 508 fpm. Is there enough room between the planter and the louver? Is there a minimum distance that objects need to be separated from a louver?

There is also have a 8.75' x 6.5' acoustic exhaust louver that might have a sign covering the whole louver and be 2' in front of the louver for aesthetic purposes. The air flow is 13,000 cfm and the face velocity is 508 fpm. Is this enough distance from the louver to get the proper airflow? And again, is there a minimum distance for objects in front of a louver?

 

[SAK9]

Tell the architect that you need to increase the louvre area by 50% or move the planter by 3 ft!

 

[Ron757]

Is that because of the pressure loss by the planters would be excessive or is there another reason? How much pressure loss would the planters cause?

How about the sign in the second example? Would the sign cause a problem?

 

[IRstuff]

There are a number of possible issues, for which you need to consult someone familiar with HVAC:

> 13000 cfm / (8.75' * 6.5') = 228.6 fpm, the lower flow is good, less of an issue with noise and leaves getting sucked into the louvers
> noise
> hypothetically, you need something like 1 object diameter's separation for the flow to return to normal:

https://github.com/timhutton/latticegas.

Otherwise, you will likely get a noisier system

TTFN (ta ta for now)
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[ChasBean1]

It's 229 fpm as per IR.

The questions:

Yes.
No.
Yes.
No.

 

[willard3]

Stop looking here for free Engineering and hire an Engineer who does this.

 

[Ron757]

What is meant by "1 object diameter's separation"? What is the definition for "object". I am am a pump and piping engineer so I understand such things as 10 pipe diameters needed in front of a flow meter is required to straighten out the flow. Is this basically the same thing? I tried to estimate the pressure loss for the sign scenario by assuming that the flow around the sign would be equivalent to a 90 degree bend though a 50" diameter pipe flowing at the same velocity as around the sign. I got less than 0.1 wc. Is this a valid method?

 

[Ron757]

It is also helpful to know that this is ventilation for a emergency generator in a building. It would not be a constant ventilation but would happen approximately for a half hour every two weeks for testing. And during emergency conditions.

 

[SlideRuleEra]

I am am a pump and piping engineer... Is this basically the same thing?

Yes, but since air is compressible and liquids are not, the problem is complicated, not basic. The fan type and operating characteristics are more important than louver dimensions. Is there one fan, or two (one intake, one exhaust). The technical information from the fan manufacturer needs to be evaluated to see how sensitive fan operation is to airflow disruption.

IMHO, the planter in front of the intake is a small problem. The sign just outside the exhaust is a big problem.

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[Ron757]

Thanks SlideRulerEra for the information. Yes, I realize the some differences between liquids and air. I used a chart that takes into account pressure, temp and flow in determining the pressure loss. But not familiar with effect on fans, etc. I am from the slide ruler era too. I am helping someone else on this so I will have to find out the location of the fans. I think this problem is interesting.

 

[SlideRuleEra]

Ron757 - You are welcome. The common term for the manufacturer's technical information is "fan curves". Here is a summary of "The Basics of Fan Performance Tables, Fan Curves, System Resistance Curves and Fan Laws".

Stay in practice with the slide rule... I was teaching a grandchild how to use one last month.

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