Michael,
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Flow x pressure = horsepower
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All 3 have to be considered together.? Sure, big pipe reduces pressure losses but at the same time reduces air speed.? Chunky materials can settle out on longer horizontal runs.? Speed is just as critical.? Most loss is produce by fittings and transitions and poor design.? Avoid 90's and couplings and keep hoses short. Don't neck down.? Seal even small leak points because they add up.? If this is a hobby shop with only one machine running at a time, size for the largest I.E.? belt sander. 5hp seems generous for most shops of double car garage size.? The performance curve is king.
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Side note: Cement plants often have these giant 60 degree incline-decline ducts sticking up above most other equipment.? This design reduces accumulation of dust and maybe even blockage or possible collapse of the structure by overloading versus long horizontal runs.? A funny looking but cost efficient design.? A best balance point of compromises.
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Jonathan Smith