Does anyone know where I might find fan curves for typical residential HVAC blowers? CFM vs. static pressure. A friend in the HVAC business has offered me one and I want to find out if it would be suitable for a woodshop dust collector. The label on my Bryant furnace lists a 1/3 HP motor, but 9.8 amps max, current closer to a typical 3/4 HP motor.
Thanks,
Scott
Replies
If buy dust collector you are talking what the Brits call a "chip extractor", which connects to saws, planners, and the like and get the saw dust and deposit it into a container, it won't work.
If you are talking about an "air cleaner", which draws the air through a series of fine filters to remove the fine particate in the air, 1/3 to 1/2 hp are what are commonly used.
Scott,
I look at lots of fan curves and there is no "typical". If ypu have a curve that is not the curve for the fan in question, it is probably worse than having no fan curve at all. The shape of forward-curved centrugal blower preformance curves are similar, and this is the most common small blower type. Your friend in the HVAC business should have lots of curves available to him/her. Most vendors publish curves for their fans and blowers or at in the case of residential furnaces, tables of esp vs flow.
BTW, 9.8A at 115VAC is the FLA for a 1/2 hp motor, 13.8 is the FLA of a 3/4 hp motor.
Pssst. Tim, don't look now, I don't want anyone to notice how stupid I am: what the heck is a fan curve?
http://www.oneida-air.com/fancurves.htm
Scott: If you can't find the curve for the particular blower, I'd do the following. Get a curve for the same type of blower (centrifugal, squirrel cage, regenetive, etc. As close as possible to the same HP.
Measure the max static pressure. This can be done by blocking the exit with a tarp and some duct tape. Stick in a vinyl tube with colored water in it. Form a U-tube with the tubing and measure the difference in water heights. That (e.g 0.9 ") is the maximum static pressure in inches water column. That's one endpoint on the curve.
In the middle of the curve, pressure x flow rate with be roughly proportional to horsepower, within the same type of blower. So rescale the CFM scale so that (for instance) a 2-hp blower's cfm x midpoint static pressure is twice that of a 1-hp blower's cfm x midpoint static pressure.
Alternatively you could measure free air flow to very accurately get the other end of the curve, giving you the cfn scale. But measureing air flow is pretty hard. For small blowers, I like to time the inflation of a Hefty trash bag. It's rough, but easy to do in the field and there'll be no big errors. For large air flows (more than 100 cfm), there are pitot tubes, hot-wire anemometers, venturi meters, etc. None of which are cheap and easy.
Since a given pipe run has a certain pressure drop for every flow rate, you could just connect, for instance, 50 feet of 6" ductwork and measure the static pressure back at the blower. That determines what the flowrate is through the ductwork and gives you one data point. Repeat with different lengths or diameters of pipe to get one or two more data points. Draw a smooth curve through the data points.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
A fan curve is a plot of test data that consists of air flow along the "x" axis vs, static pressure along the "y" axis. It is based on manufacturer's test data, usually certified to some degree and is what you use to select a fan and/or motor (or to check that a supplied fan will perform as you need it to). A given fan will perform similarly at different speeds and so fan curves are usually plotted as a "family" of curves. There are conditions under which a fan will not perform predictably (surge) and these regions are also depicted on the graph. Also, a typical fan curve will have "power lines" on it as well, showing the horsepower required to make the fan operate at the point you are designing for, so that you can specify the motor hp as well as the fan.
Different types of fan/blower designs have different shaped curves. The types of designs are forward curved (FC) centrifugal (this is the very common squirrel cage fan), backward inclined (BI), BI air foil, vaneaxial and propeller. The most common types that are used in residential applications are FC and propeller.
Take the blower if it's free. It's no good for a dust collector, as stated above, but it's handy for making an air cleaner, and even as an exhaust fan for the shop, especially where you don't want to block a window, and/or if you want to hide it under a bench or up at the ceiling (with ducting to the outside). I power several smaller shop tools (radial arm drill press, bandsaw, buffing/polishing arbor) with old blower and appliance motors, which saved a few bucks in the days when you could still buy machines without motors. Regarding current, if you read the nameplate on the motor, it probably has a service factor (SF) of 1.25 or 1.35 with SFA of 9.8 or thereabouts (common practice on air moving equipment), which would explain why the air handler nameplate shows max. current higher than what is normal for a 1/3 hp motor.
Edit: Suitability for power tool use is predicated on it being a belt driven blower.
Edited 5/28/2002 10:34:29 AM ET by TDKPE
Been there, tried that.
Squirrel cage only works for awhile as a dust collector with a bigger (say 3 hp)motor at 3400 rpm, but once there is some weight from chips on the squirrel cage or you open up all the gates, the squirrel cage self destructs (one went flat-all the vanes bent flat, no air flow), the other produced shrapnel.
You can use the housing and weld up and balance a vane blower wheel, that works, but lots of work getting a homemade vane balanced.