Arkansas HVACR NewsMagazine September 2023

HVACR NewsMagazine September 2023

Tech News

= 4 x 0.5 x 0.18 = 0.36” W.C. pressure loss for 4 elbows

radius to diameter column, which is a loss coefficient of 0.5 per elbow.

Step 6: We are almost there. We just need to take the wall cap exit into account. In this case, I referenced Table A6-10 in Manual Q. The most conservative, or largest, exit pressure loss coefficient is 1, so that’s what I will assume for a wall cap exit.

Note: A 45° elbow pressure loss coefficient can be estimated to be half the value of a 90° elbow pressure loss coefficient. Now, we can solve for the loss of each elbow, which looks like this:

PL Ci = C i x VP

• PL Ci = Pressure Loss Per 90° Elbow • C i = Pressure Loss Coefficient for Fitting i, dimensionless

VP = Velocity Pressure

•

PL C = 0.5 x 0.18 = 0.090 “W.C.

Therefore, 0.090 is the loss per 90° elbow, so if we want to find the loss of all our elbows, we just slightly adjust the math. Since we have just one single duct run with the same diameter (i.e., same velocity, therefore same velocity pressure) and fitting type (4 elbows), our total fitting losses equate to:

PL C-Exit = C Exit x VP

PL C-4 Elbows = (∑C’s) x VP = 4 C-Elbow x VP

• PL C-Exit = Total Exit Pressure Loss