Calculating Duct Sizes for Swamp Coolers

[luca5371]

So, this should be an easy one but it's been so long since Fluids in college I've forgotten.

I'm redoing the ducting in my house and need to figure out what diameters I need for the ducts. The cooling system used where I live is a swamp cooler. For those of you not familiar with them, they're basically giant fans that suck outside air through thick (12" in my case), wet, cardboard-like pads. They then blow this cooler air into the house. They rely primarily on air-flow to cool so you have rather large cfm's (~4400 for the one I own).

I know what the cfm output of the cooler is. I know the basic layout of the ducting (lengths, bends, y's, etc...). I know how big of an area/volume each vent must keep cool. I know the basic diameters of the ducts I'm going to use. How do I figure out what comes out of each duct (flow rate or percentage of overall flow)? What else do I need to know to do this?

Thanks in advance.

 

[Navin A S]

The basic equation you need to use is the Darcy-Weisbach equation. This equation takes into account the viscosity of the air, the diameter of the pipe, and the length of the pipe, and combines it with the pressure drop that occurs due to friction. You can then use this equation to calculate the flow rate of the air through the ducts. You also need to take into consideration the fittings, such as bends and Y-junctions, which will affect the pressure drop and therefore the flow rate.