If two blowers operate in series within the same system, but the upstream blower is delivering more cfm than the downstream blower…what would happen to the downstream blower? How will it behave? (If you simply say “it will be overwhelmed”, what will physically happen? Can I damage the downstream blower and/or motor? Or, will it try to compensate by “walking out” on the fan curve? And, what happens if it tries to “walk off the curve”?) - Jinx
WAG: a back-pressure will develop in the intervening area; the downstream blower will move a little bit more air than it should, and if there’s a good power supply on the fan, it will back off the current to the motor. This overdriving may damage the downstream motor, or it may extend its life (?). The upstream blower will be pushing into an overpressured channel and its motor will definitely experience more wear.
Why not take the downstream blower out of the chain?
Yeah, that’s what I’m thinking, but I need more info to justify. Thanks!
I think both blowers would stay on their fan curves, though perhaps go to points on the curve beyond the ends as the manufacturer prints the curves.
And it depends on what kind of blowers they are, but I think most blower motors will experience less wear if you impede the movement of air through that blower, not more. Throttling a blower usually reduces the torque and gets the motor closer to freewheeling, and I am pretty sure reduces the force on the impeller that’s perpendicular to the shaft axis. Block a vacuum cleaner and you’ll hear it speed up (though vacuum cleaner blowers, being somewhat unusual in having universal rather than induction motors, will spin quite fast with low load and that does wear them down faster).
This is a contradiction in terms, isn’t it? A fan can’t stay on the curve if its’s beyond what is printed by the manufacturer. My experience resides more with centrifugal pumps, and you just don’t do that. But, I’m not sure what happens if you allow a pump to deliver all it can - and more! You probably overload and burn out the motor, I wager. - Jinx
Well, I think of a fan curve per se as being the relationship between the pressure and flow for a given shaft speed - what you get from the manufacturer is a printed record of their representation of the part of the curve they expect you to use.
Any time you leave a disconnected fan in the breeze and it sits there slowly spinning, you’re operating it off the printed part of the curve.
Blower manufacturers usually don’t expect you to connect them in series, or operate them in parts of the true fan curve that the manufacturer didn’t print, but that doesn’t necessarily mean you’ll regret doing it.
Here’s a sort of a real life example - one time we started up a cleanroom and found that the 50 hp fan for one section of the room had terrible performance - low flow and enormous electrical current drain. Turns out the installers couldn’t fit the duct the way the shroud was mounted around the fan wheel, so they removed the shroud and flipped it and reinstalled it. Now the shroud looks like the wheel should spin the opposite way. So they also flipped two of the three phases to make the motor spin the other way. Well, this fan had an airfoil type wheel, in which the blades are hollow sheetmetal shapes that look like a curvy airplane wing, and unlike the radial bladed fans the installer was experienced with, this kind of wheel only works right spinning in one direction. So we tried flipping the phases back, and we found the fan operated pretty well - a little less efficient, but air flow and pressure and electrical consumption were all about what we originally expected anyway. The ductwork would have been enormously expensive to redesign to fit the “proper” shroud orientation, so we left the fan set up with the pulley spinning in what was obviously “the wrong direction” if you just look at the fan from the outside.
By the way, I crawled through about a hundred feet of duct and elbows and whatnot high above the factory floor so I could feel the airfoils to figure this problem out. I bet now the safety people wouldn’t let me.
Holy cow! Now that’s dedication! Those must have been some wide-ass ducts that you could slip through the elbows! (Were there dampers to get past, too?)
And, I don’t know how you didn’t suffocate. When men enter underground pipes, for example, there’s always fresh air being “pumped in” by compressor. Also, every now and then, I catch a news story about people digging narrow holes (for various purposes) maybe just 10-15 feet down and there’s not enough oxygen anymore, although the danger does not seem apparent. You’d think fresh air would naturally move in, but I guess the CO2 just won’t be displaced so easily.
Crazy, man!
- Jinx
All air ducts are large enough to allow easy passage.
Or so Hollywood implies.
WAG It all depends!
Depending on duct sizes, lengths, CFM, etc. etc. i.e. design details of entire system.
It is not unusual to add a booster fan at the end of the first duct to boost the flow to the ultimate end of the line.
Pretty much. They make two stage centrifugal blowers, where the first blower pressures up the input for a second blower on the same shaft. The inevitable differences in the efficiency of the two fans doesn’t necessarily spell catastrophe for the motor.
CFM will increase slightly, but static pressure in the duct will increase significantly.
:: can’t stop giggling at thread title ::