Air Pressure Drop Due To Security Bars

[hynhvac]

We are currently working on a military job which requires us to install security bars at every wall penetration. My boss has tasked me with finding the air pressure drop across each security bar section. Needless to say, there are a lot of wall penetrations.

The bars are 1/2 inch steel at 6 inches on center each way (horizontal and veritcal) and are welded to the frame and at the cross points.

I know that the drop for a specific section will be dependent upon the size of the duct, but could someone point me in the right direction? I pretty sure I am capable of writing a spreadsheet and/or Matlab program to simplify the calculation process once I have determines the approproate equations to use.

Mahalo in advance.

 

[nuhvac]

your pressure drop will be about equal to the velocity through the opening divided by 4005 and square that quantity. (V/4005)^2

 

[hynhvac]

is that for SI or IP?

 

[mauricestoker]

For IP, you might want to use the SMACNA Table 14-17, which matches the ASHRAE HOF if no reducers are included. Since it's a standard 1/2" daimeter rod spaced at 6", for a typical 6" section has 36 in2 and 6 in2 of area covered by the rod, the loss coefficient will always be the same, 30/36, loss coeffient would be the same (about 0.26).

Fitting loss would then be 0.26 * Vp.

I've done several of these, loss is small. Generally, the requirement typically applies only to duct greater than 96 in2, and then only between secure and non-secure areas. Some times it is requested that at least 180* in turns are included for acoustical security, which can end up being more loss than the manbars.

 

[nuhvac]

For my calc listed earlier those are IP units. There is a factor that i did not list earlier though. There is a coefficeint to that equation. That is something you could look up as it might be less than/ greater than 1 as I listed earlier. I use a 1 coefficent when I calc pressure drop across #3 (this corresponds to a 60% free area). If it is 54% free area i use a 3.1 coefficient. Calculate your free area and use accordingly. Either way it shouldnt amount to much unless your velocity is extremely high.