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Thread: DC chip vs fine dust collection

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  1. #11
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    Just don't forget that apart from some basic training in HVAC systems I'm just a few pages of the book ahead of (some of) you guys - it's been so long since engineering school that i can't remember very much.

    The 'constant' factor in fan performance is perhaps that if you reduce it to the maths you can say that the fan HP is proportional to the CFM x pressure drop. (x = multiplied by) Using the word 'constant' in the mathematical sense of its just being a fixed number - the proportional sign in a maths equation can always be replaced by an equal sign and a constant. So you can say that:

    fan HP = a fixed number x (CFM x pressure drop)

    This tells us quite a lot, in that e.g. it shows that if you want to run a small bore system at very high air speeds (which creates a large pressure drop) so that you still get a decent CFM that it takes quite a lot of extra HP to overcome the extra pressure drop - even though you still will have only the same CFM available at the machine. It's also saying that the reverse is true - that if you want to maximise the CFM for a given fan HP you need to minimise the pressure drop, which means maximising the duct size. (with the proviso that we can't drop below 4,000fpm and still expect reliable chip and dust transportation - which is more or less the design basis of all of our dust collection systems)

    The catch is that the formula isn't all that useful in the everyday use sense for designing air systems. That's because the constant is a composite of several different numbers (fan efficiency, motor efficiency etc, and the pressure drop is sometimes expressed in terms of factors which relate to the system/ducting) which are not easily available without doing a lot of testing, and which unless you're very sharp with a pencil are easily got wrong. There's a more complete version of the formula is here http://tiny.cc/mqyh8 - plus some links and a good discussion of how the various factors influence system performance.

    The result of this is that engineers tend for convenience to use the sort of tabled information pointed to in the posts above (fan curves, estimated system pressure drop based on ft run of ducting) which numbers have long since been pretty much the standard throughout the ventilation industry.

    When for example you go into a 3,450 rpm fan curve or fan table and extract CFM and HP at the total system pressure drop you are expecting you are accessing what is probably a test derived number which by definition includes all of the above. Which means actually that you are using numbers based on e.g. a presumed typical motor efficiency.

    Your motor may not be quite the same. There may be small differences too if you are using an industrial makers fan tables to predict the likely performance of the actual fan you have. Similar issues arise too when estimating the pressure drop for your ducting from tables.

    Which is why these methods are close but not 100% accurate. My dust system for example is drawing a bit under 4HP with one gate open, whereas similar layouts in the US seem to draw something a little over 4HP. The total pressure drop is probably a little lower than most (short duct runs, few bends, 160mm ducting), so it's probably not down to the ducting being restrictive. It's very likely instead that the low amps/HP draw is actually the result of having used a high efficiency Euro ABB motor which (guessing) maybe draws maybe 6 - 8% less amps per HP output compared to an older design like the Leeson. (high efficiency motors are now required by EU legislation)

    Anyway.. It's all interesting, but in the end (unless we work for NASA) we're all in the same boat of practically having to rely on rule of thumb methods like these - and a fair amount of seat of the pants judgement. It tends to work out fine, because unless you are really skimping these numbers tend to have some cushion built in anyway.

    ian

    PS On adjusting system resistance so that your fan motor draws full load amps in normal use David. It can be done, and maximises the CFM delivered from a given fan. The potential problem is as you know that it could if care is not taken create a situation where there is an increased risk of overloading the motor by opening too many blast gates or whatever.

    So unless you're very aware of the issues there's an argument for sizing the fan and motor so that it delivers the CFM you need somewhere safely below full load amps to cover against unexpected eventualities. Ditto in the case of the fan, if you can run down the curve a bit in normal use it'll do better if faced with an increase in pressure drop when e.g. hooked up to a restrictive machine or whatever.
    Last edited by ian maybury; 02-19-2012 at 10:05 AM.

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