My boss finally got his high velocity duct test contraption set up yesterday and showed us some interesting results. It’s the first time I’ve seen an example of static vs dynamic pressure in a working fluid. (even UF’s Fluid dynamics lab didn’t show anything like this)
the contraption is set up like this:
1/6 hp forward curved centripetal fan > 50′ length of 7″ diameter steel sheet metal duct > static regain diffuser (cone with inlet at 7″ and smoothly increases to about 20″ in 4′ of distance)> flow meter
there are 6 test ports along the 50′ length of pipe to test static pressure. He turned on the fan, and starting at the fan connection to the pipe, measured the static pressure at each test port:
length from fan: 0′ 10′ 20′ 30′ 40′ 50′ exit after regain diffuser
static pressure: .33″ .22″ .11″ .008″ -.1″ -.2″ atm (0″)
CFM flow at the meter: 870 cfm
Then he picked up the fan cutsheet and handed it to us. It reads 805 cfm @ 0.0″ water
so there are a few questions floating around now:
1) how does a fan move more air through a pretty high static pressure than it is rated to move at 0 pressure drop?
2) how do they test the fan?
3) there is obviously a ~.5″ friction loss through the pipe at ~870 cfm, but does the fan see .5″ or .3″ or is the fan actually seeing .1″ (.33+(-.2″))?
When the static regain diffuser is pulled off of the end of the pipe, and everything else is kept the same, the pressure readings look like this:
.46″ .39″ .32″ .22″ .11″ -.05″ ( the last test port was 2″ from the open-air discharge)
we can’t measure the air flow, but i bet it’s less than 870…
So the static regain diffuser is truly sucking air through the duct because of it’s conversion of dynamic pressure to static pressure, but does the fan see that? Or does the fan only see that the output static pressure is .33″ or .46″?
I don’t know… it’s going to take more research than I’m willing to commit at the moment.
