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Question about centrifugal blower airflow vs power

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  • Question about centrifugal blower airflow vs power

    I recently made the inlet screen and discharge damper for this centrifugal blower in the AgTech shop.



    That got me thinking about the late '60s, when I worked for Avco-Lycoming. We were making mobile test stands for the US Army bases where they repaired our turbine engines. One version of the stands we built used an air-dynamometer, which was similar (but MUCH larger) to the one in the picture. The volute was about five feet tall, and the impeller was about thirty inches in diameter. IIRC, it was good for about 800HP, but it's been a long time. Seems like the engine was a T-53.

    I seem to recall the outlet being left wide open all the time, and that we controlled the load on the engine by opening/closing the inlet to the blower. More airflow through the blower increased the load on the engine. Would it have worked that way?

    I've never seen the one at AgTech in operation, so I don't know exactly what they do with it.
    Last edited by winchman; 11-09-2013, 07:00 PM.
    Any products mentioned in my posts have been endorsed by their manufacturer.

  • #2
    You are absolutely right, the bigger the opening on the inlet side the bigger the load on the motor. I spent many years installing industrial dust collectors and after watching the damper being opened and closed from the control center, you could watch the amperage go up on the motor when the damper was opened.

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    • #3
      Rotary screw compressors (Sullair) utilize a large flapper valve to cover/uncover the air inlet. Cover to unload the compressor, uncover to load. What goes in must come out------

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      • #4
        I would imagine that it would work either way- block either the outlet or the inlet, and without airflow the load on the motor would lessen. Block the inlet, maybe it would unload even more, as there could be less air being dragged around in the volute because of the degree of vacuum.

        Reminds me of how a regular vacuum cleaner works. It works the hardest when there is the most airflow through it- no clogged hoses, clean filter-

        Back to the blower- I've always thought that the way to get the most out of a blower (talking about a turbocharger now) is to throttle the input. The fan could come up to speed before it's required to have air flow, then when boost is needed the inlet gate is opened. The fan could spend its idle time stealing less power, yet be at a higher rpm ready to pump.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Originally posted by studentjim View Post
          You are absolutely right, the bigger the opening on the inlet side the bigger the load on the motor. I spent many years installing industrial dust collectors and after watching the damper being opened and closed from the control center, you could watch the amperage go up on the motor when the damper was opened.
          +1 This reality seems counter intuitive from a layman's standpoint but it's true...that's how it works.

          Stuart

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          • #6
            The same applies when you have a furnace blower and want to use it to cool a motorcycle engine, while balancing the carbs. Restrict the air flow in, otherwise the motor will pull excessive current.

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            • #7
              I found that a squirrel cage furnace blower would overload and overheat when operated in the open air. I learned that backpressure from the furnace and ductwork actually reduces the load on the motor. So you can throttle the inlet or outlet to reduce the airflow and reduce the motor load. I still prefer to throttle the intake.
              Don Young

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              • #8
                Yep....

                The theory behind it is that it takes energy to accelerate and move air. Since the blower is essentially constant speed, the more air moved, the more the load, because it is moved at the same speed more-or-less regardless of how much is moved, taking more energy.* Can be more energy than the motor can supply. At least the motor is attempting to do that, and it will "try" until slowed to the torque breakdown point.

                When you cover the inlet, less air is moved, or no air is moved, and conditions move toward no-load.


                * if the blower were spun slower, or if it slowed down a lot under load, then the motor might "find it's own speed" to just equal the available HP. This happens in an regular desktop open propeller fan, which is designed for opwn flow, with the ability to tolerate a lot of "slip".
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  Looks like an insulation blower motor.
                  "Let me recommend the best medicine in the
                  world: a long journey, at a mild season, through a pleasant
                  country, in easy stages."
                  ~ James Madison

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                  • #10
                    Originally posted by winchman View Post
                    I recently made the inlet screen and discharge damper for this centrifugal blower in the AgTech shop.

                    I do not have an informed position on whether throttling the inlet
                    or the outlet is best.

                    However, I can say with a good deal of confidence that the efficiency
                    of the blower with that flat piece of expanded mesh in a sheet metal frame
                    mounted flush to the inlet on the scroll housing will be WELL down from
                    what could be achieved with a bell-mouthed, velocity stack-shaped inlet
                    tube of some length with an over-sized, dome-shaped mesh grill to keep
                    birds, children and small pets from being drawn in.

                    Maybe maximum efficiency is not the object in this case.

                    The workmanship on the parts is exemplary.
                    Last edited by EddyCurr; 11-10-2013, 07:28 AM.

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                    • #11
                      They asked me to make the inlet screen several weeks before they asked me to make the damper. I had no idea what either part was for, or that they were going on the same thing. They just brought me sketches with dimensions on pieces of cardboard, and told me they needed it yesterday.

                      They say they're happy with the way it works now, but I wish I had known earlier that I could have combined the functions into one assembly.

                      Working with the machinists at Avco-Lycoming was a real treat, and they gave me an appreciation for top notch work. They machined the impellers for those air-dynos out of big chunks of aluminum on MANUAL machines.
                      Any products mentioned in my posts have been endorsed by their manufacturer.

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                      • #12
                        Originally posted by winchman View Post
                        They asked me to make the inlet screen several weeks before they asked me to make the damper. I had no idea what either part was for, or that they were going on the same thing. They just brought me sketches with dimensions on pieces of cardboard, and told me they needed it yesterday.

                        They say they're happy with the way it works now, but I wish I had known earlier that I could have combined the functions into one assembly.

                        Working with the machinists at Avco-Lycoming was a real treat, and they gave me an appreciation for top notch work. They machined the impellers for those air-dynos out of big chunks of aluminum on MANUAL machines.
                        Was it "billet" aluminum!!!!!!!!
                        How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                        • #13
                          Oddly enough, I was thinking about this when adding sugar to my coffee this morning. I didn't have a spoon so I was moving the cup around to set up some fluid dynamics in an attempt to get some of the sugar mixed in. It occurred to me that air flow is little different than fluid flow, and it can either be enhanced or interfered with depending on the size and shape of the guide, surface effects, etc. I thought about this quite a bit when I was designing my transmission line subwoofer box a few weeks back. It also came up when I made a grille to cover the fan in my thermoelectric cooler module. In that case the airflow was restricted quite dramatically with the grille in place.

                          The goal in many of these cases is to keep foreign material out while not interfering with air flow rates. It's pretty easy to think of putting a grill in front of something, but it's much more modern or up-to-date to use a flow dynamics approach to come up with a solution. Probably much more time consuming and expensive as well
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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