I see that the assumption for the hood entry is a flanged pipe entry, and that others will vary. How much can that assumption affect the result?
This can be significant. The best hood entry available is a bell mouth, and the worst is a sharp-edged orifice. If you eliminate the orifice from consideration then the flanged pipe entry used represents a reasonable middle ground guess. Eliminating the orifice from consideration gives a margin of error at 4000 FPM of plus or minus ½”. • How does the spreadsheet calculate the static pressure? The spreadsheet uses the straight friction loss method. It does NOT use static regain. This will induce a small error, but it is a much simpler method and is commonly used in the industry. Specifically, the sheet calculates velocity pressure using the formula (V/4005) squared, which assumes standard temperature air. All loss factors are published as a function of the velocity pressure. For example, the loss factor for a 90 elbow, in smooth stamped steel R/D >2.0, is 0.13. So the losses for the 90 elbows are a formula Q*VP*LF, where Q is quantity, VP is velocity pressure, and LF is loss factor. The
