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MacLaddy

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I'm working on my first engineering project in the real-world and I'm hitting a bit of a snag. It's regarding pressure loss analysis in an HVAC system. We have a system that is capable of providing a maximum of 2.0-in.wg pressure at the air handler, but we are finding very little air movement at the end of the duct. Using the methods that I lay out below, I am finding a final number that is not realistic. I'm hoping somebody can provide me with some expertise. It's very difficult to find any information online that agrees.

The attached PDF file shows a sketch of the ductwork coming off of the air handler. The circles represent sections and the squares represent fittings. I have also attached an Excel Spreadsheet to hammer out most of my calculations. On the PDF there is a datum line perpendicular to section 5 on page 1 and page 2 to show how they connect.

Column L shows some of the assumptions I have made regarding the material. The material is galvanized sheet metal (Spiral circular duct), and I have used the Moody Chart for the friction factor f.

For the Major Losses (Column H), I have used the following formula.

$$ h_{Lmaj} = f\frac{L}{D}\frac{V^2}{2g} $$

Finding the minor losses has proven to be the more complicated part...and the place I believe I am making a mistake. For all of the items listed EXCEPT the 45° Reductions, I have used the values on this webpage to find the K Factor.

https://neutrium.net/fluid_flow/pressure-loss-from-fittings-2k-method/

For the two 45° Reductions, I used step 3.1 on the following webpage and calculated the Square Reduction K value for turbulent flow and then multiplied it by the steps in 3.2. (assuming a 45° angle)

https://neutrium.net/fluid_flow/pressure-loss-from-fittings-expansion-and-reduction-in-pipe-size/

Once the K Values were found I used the following equation to calculate the minor losses.

$$h_{Lmin}=K\frac{V^2}{2g}$$

You can see in the h(Lminor) column that I am getting some very large numbers. I suspect that my error is in the Tee, Run Through method, but I couldn't find any source for this online other than that website above.

If you follow over to Column K you can see the total of all pressure losses in the system. I calculated this at 6-in.wg, which...of course...isn't possible.

I appreciate any help that can be offered. I'm really at a bit of a loss here.

Thanks,

Mac

The attached PDF file shows a sketch of the ductwork coming off of the air handler. The circles represent sections and the squares represent fittings. I have also attached an Excel Spreadsheet to hammer out most of my calculations. On the PDF there is a datum line perpendicular to section 5 on page 1 and page 2 to show how they connect.

Column L shows some of the assumptions I have made regarding the material. The material is galvanized sheet metal (Spiral circular duct), and I have used the Moody Chart for the friction factor f.

For the Major Losses (Column H), I have used the following formula.

$$ h_{Lmaj} = f\frac{L}{D}\frac{V^2}{2g} $$

Finding the minor losses has proven to be the more complicated part...and the place I believe I am making a mistake. For all of the items listed EXCEPT the 45° Reductions, I have used the values on this webpage to find the K Factor.

https://neutrium.net/fluid_flow/pressure-loss-from-fittings-2k-method/

For the two 45° Reductions, I used step 3.1 on the following webpage and calculated the Square Reduction K value for turbulent flow and then multiplied it by the steps in 3.2. (assuming a 45° angle)

https://neutrium.net/fluid_flow/pressure-loss-from-fittings-expansion-and-reduction-in-pipe-size/

Once the K Values were found I used the following equation to calculate the minor losses.

$$h_{Lmin}=K\frac{V^2}{2g}$$

You can see in the h(Lminor) column that I am getting some very large numbers. I suspect that my error is in the Tee, Run Through method, but I couldn't find any source for this online other than that website above.

If you follow over to Column K you can see the total of all pressure losses in the system. I calculated this at 6-in.wg, which...of course...isn't possible.

I appreciate any help that can be offered. I'm really at a bit of a loss here.

Thanks,

Mac

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