Quote: My previous estimate had been that it should require 10 horsepower or less. Have I missed something?
Yup, your calculations are extremely simplistic. You don't take into account the flow. What you think is taking into account the flow is just taking the volume of air every second seperately, and compressing it to 5psig and then moving on to the next volume of air, and compressing it. You are not actually taking the account the continual flow.
Yes I am. The math I used is for a continuous situation, the kind you can do calculus on. It's similar to the basic equation for air drag on a moving body, in that the faster the air flows through the intake pipe, the more power it takes to apply a given pressure to it.
Originally posted by Rara: Imagine if you will you have your ordinary shop compressor ... Your calcs don't even assume there is a small leak, just that it keeps filling up new tanks every second . . .
Yes they do. You're mistaken. If it weren't for the rate of flow being accounted for, the power consumption over time would average out to zero.
Originally posted by Rara:The actual calculations are far from trivial, and far more than I even want to be reminded of this late at night (I hated fluids . . .)
Calculating it exactly is complicated, but it is simple enough to calculate the minimum amount of power that must be consumed regardless of the details of a particular design -- namely, the amount corresponding to the true work done on the fluid itself in compressing it. It's the number below which the compressor would have to be a perpetual motion machine. And since compressor makers advertise an efficiency figure which is measured relative to that ideal minimum (at least, I hope that's what they're doing if they're honest)... then just dividing by that efficiency figure gives you a shortcut to the genuine power consumption. Centrifugal devices seem to claim numbers in the range of 70 to 80 percent.
