Arkansas HVACR NewsMagazine September 2023

HVACR NewsMagazine September 2023

Tech News

VP = (V/4,005) 2

STEP 2: Next, we'll set the duct size on the Ductulator. The collar on this fan is 8” round, so let's t ry that. We line up 8” to the arrow under “Round duct diameter.” While holding the Ductulator still, we look at 600 CFM and note the corresponding velocity, which is around 1,720 FPM in this case. Also, we ought to note that the FR for galvanized metal duct is 0.55 (“W.C./100FT). Note: If your Ductulator is giving you values that are a few percentage points off, consider a phone app Ductulator. These digital Ductulators give precise calculations and are not subject to assembly tolerances (a duct wheel that is a hair off-center will give incorrect results). An app is easier to use and looks like this: “ASHRAE HVAC DUCT SIZER APP”

• VP = Velocity Pressure (inches of water column)

• V = Velocity (FPM – Feet per minute)

• 4,005 is a unit conversion constant 0.18 = (1720/4005) 2

Therefore, the velocity pressure in this example is 0.18. STEP 4: Now, let’s calculate the friction loss for our actual straight duct. Let’s start by adding the total length of the duct run (not including elbows). In this example, it is 10 feet. Keeping our Ductulator set at 8” round, let's look at what the friction rate is for 600 CFM under “Galvanized metal duct.” In this case, it is 0.6” of friction loss for 100’ of duct. But we aren’t using 100’ of duct; the actual straight duct run is 10’, so let's do the math so that we can convert it to our actual friction loss. That equation looks like this: PL SD = ADL x (FR /100) • FR = Friction Rate PL SD = 10 x (0.55/100) = 0.055” W.C. This means that 0.06” is our pressure loss for 10’ of straight duct Step 5: Next, we must calculate the loss for all the 90s in the duct run. To do that, we count up the total number of 90s that will be needed for the duct run. For this job, I am figuring 4 elbows. Once you have the number of 90s, we look at Table A6-1 in Manual Q, “Loss Coefficients for Elbows.” I am using “Elbow B” because the elbows that I can buy locally are 4-gore. I don’t have an elbow in front of me, so I will assume the worst-case • PL SD = Pressure Loss of Straight Duct in Inches of Water Column • ADL = Actual Duct Length in Feet

STEP 3: We need to convert our velocity of 1,720 FPM to velocity pressure, and the equation to do so looks like this: