Airflow measurements

[Schreck]

The benefits of using a bell mouth shape for the outlet of a top hat style separator were discussed extensively in this thread:  http://www.jpthien.com/smf/index.php?topic=550.0

One of the charts from that thread presents a variety of duct end treatments that improve airflow over a bare duct end.  Improvements are presented in terms of percent velocity pressure, which is difficult for most people to understand. The bell mouth showed the lowest losses of the six types listed. 

In the course of measuring the performance of a 2 HP blower, I realized that I could measure the impact of three end treatments from the chart: the unflanged pipe, the flanged pipe and the unflanged cone - all using materials at hand. 

[Schreck]

The flanged pipe is simply a piece of 1/2" foamboard fitted to the pipe end by friction.  It is not sealed whatsoever.  When slipped back 2-3" from the pipe end, it represents the unflanged or "raw" pipe end in the charts and tables below.  The unflanged cone is either a 5" to 6" inch reducer (fitted as an increaser) with the short, straight 6" piece intact, or a 6" to 8" reducer with the 8" section removed, which approximates the two-cone end treatment.

The blower is a 1984 Grizzly with a 240 volt 2 HP motor, 12x3 inch impeller and a pourous polyester filter bag (freshly washed to save my lungs).  The blower has a 5" inlet and the tests were done to see if it could support 6" ducts. A 6 inch inlet is shown below.

[Schreck]

All measurements were made with a pitot tube installed in the center of a 10 foot straight duct, roughly 6 feet from the blower, with a digital micromanometer.  A true RMS kW meter measured the volts, amps, power factor and watts drawn by the blower.

The first measurement is for a raw pipe end, which serves as the baseline.  The second measurement is for the flanged pipe, followed by the reducer installed as an increaser. 

This first chart shows the impact on velocity (in feet per minute), static pressure (as inches of water column) and motor amps for the blower with the 5" inlet connected to 5" ducts.
The second chart shows the impact when 6" duct is connected to the 5" blower inlet.
The third chart shows the impact with 6" duct connected to a 6" blower inlet.

[Schreck]

In each case the flanged pipe showed higher velocity and lower static pressure than the raw pipe end.  When the reducer was tested, it showed a further increase in velocity and reduction of static pressure. 

Note how little the motor amperage changes.  Measuring changes in motor amperage is often cited as a way to determine whether more work is being done by the blower, which generally means it is moving more air.  The changes I measured are very small, which would (incorrectly) indicate small changes in airflow.  As motors become more loaded, the amperage increases but so does the power factor and it is the product of the amperage and power factor that reflects the change in work being done.

The table below summarizes the data behind the charts and it includes watts and power factor.  My blower motor is showing a very low power factor: 0.65 when loaded versus 0.80 or higher for good quality modern motors. 

[Schreck]

The last chart shows the cfm calculated from the velocity.  Since the velocity readings were taken in the center of the duct, a 0.90 factor was applied when calculating CFM.  Watts are also shown, along with percent improvement versus the baseline case for each duct configuration. 

Keep in mind that these tests were made on 10' to 15' of duct, with no or only one elbow, and the percentage improvement may be lower when a longer duct system is installed.

At some point I'll find a bell mouth and perform some more tests.  In the mean time, the reducer fittings offer a good performance boost without being too wide, so the likelihood of bypass is reduced versus a wide bell mouth flange when used on the outlet of a top hat.  Reducers are commonly available at home centers at modest cost.

[Schreck]

I tested the 2 HP Grizzly blower to see whether it could support 6" ducts.  The blower has a 5" inlet and I tested its performance with 10 or more feet of 5" duct and again with a 5" to 6" increaser and 10 or more feet of 6" duct.  Then I made a 6" inlet for the blower and repeated the tests.  The first chart shows the CFM versus static pressure (fan curves) for all four combinations of duct and inlet.  All of these measurements were made with a raw duct end, i.e., no flange or reducer fitted as an increaser. The performance appears to be similar with the exception of the 5" inlet driving the 6" duct. 

The second chart shows the real impact of the change in duct size.  With the larger duct, velocity drops to the point where any further resistance in the ductwork, separator or filter would result in a velocity too slow to collect chips and sawdust.

[Schreck]

I used a 1/2" radius router bit to round over the inside of the 6" plywood inlet plate, thinking this might be beneficial.  The original steel inlet plate brought the 5" steel inlet pipe into the blower housing by about 1/2", so it terminated much closer to the impeller than my plywood 6" inlet. 

The chart below includes the rounded-over 6" inlet and adds measurements for the other inlet-duct combinations at low flow/high SP conditions.  These measurements were made by partially blocking the end of the duct with a flat piece of foam board.

The 6" inlet rounded - 5" duct measurements show degraded blower performance.  The result of rounding over the inside face of the plywood inlet was to make the blower less efficient by increasing the distance between the impeller and the housing.  I could correct this by extending the 6" duct further into the plywood inlet, closer to the impeller.

[Schreck]

For one of the duct configurations I measured velocity with both the pitot tube and a pinwheel meter.  This was a 10 foot long 5" duct with a 90 deg. elbow, 5 feet of duct and another 90 deg. elbow.  The pinwheel meter showed 6393 feet/minute while the pitot tube measurement showed 4660 feet/min. which is 27% lower.  Could part of the reason for the discrepancy be that the pinwheel meter partially blocks the end of the duct? Other tests where I partially blocked the end of the duct showed reduced velocity at the pitot tube, so I don't think this is the case.  The inlet to an elbow was probably not the best place to try this measurement...

[Hoota]

In each case the flanged pipe showed higher velocity and lower static pressure than the raw pipe end.  When the reducer was tested, it showed a further increase in velocity and reduction of static pressure. 

Note how little the motor amperage changes.  Measuring changes in motor amperage is often cited as a way to determine whether more work is being done by the blower, which generally means it is moving more air.  The changes I measured are very small, which would (incorrectly) indicate small changes in airflow.  As motors become more loaded, the amperage increases but so does the power factor and it is the product of the amperage and power factor that reflects the change in work being done.

The table below summarizes the data behind the charts and it includes watts and power factor.  My blower motor is showing a very low power factor: 0.65 when loaded versus 0.80 or higher for good quality modern motors.
[quote ] The load current of an induction motor shafted to a fan inside of a housing is purely based upon how much the output flow is restricted.  A completely open output causes the motor to draw maximum current, maybe even exceeds the rated load stamped on the tag.  Always restricted the output of the fan housing slightly to prevent burning the windings to toast. When the fan housing output is restricted, the shaft spins slightly faster (less load) and load current declines.  Should the intake be restricted, there is less air mass available to the fan blade to draw upon, motor rpm is increased and load current will reduce (slightly).  Measurements should use pressure deltas inlet vs outlet and mass flow volume (ft^3/min?).  This type of problem is a mass flow and finding the solution on the curve is a differential equation.  Set up the parameters and Excel has Goal seeker app to solve differential equations for these particular problems.     

[Schreck]

[quote ] The load current of an induction motor shafted to a fan inside of a housing is purely based upon how much the output flow is restricted.  A completely open output causes the motor to draw maximum current, maybe even exceeds the rated load stamped on the tag.  Always restricted the output of the fan housing slightly to prevent burning the windings to toast. When the fan housing output is restricted, the shaft spins slightly faster (less load) and load current declines.  Should the intake be restricted, there is less air mass available to the fan blade to draw upon, motor rpm is increased and load current will reduce (slightly).  Measurements should use pressure deltas inlet vs outlet and mass flow volume (ft^3/min?).  This type of problem is a mass flow and finding the solution on the curve is a differential equation.  Set up the parameters and Excel has Goal seeker app to solve differential equations for these particular problems.   

Thanks for that explanation, Hoota.  The first measurement I made was motor amperage to be sure I had enough restrictions.  At 9.5 amps/240 volts, the blower is drawing almost as much as the nameplate of 10 amps/220 volts.  I will measure the blower without the filter ring, 5" flex hose and outlet flange fitting and post the results.  I'm less interested in the pressure across the blower (inlet to outlet) than its capabilities on the suction side, because my eventual installation will have an outlet plenum and pleated filter.  My immediate decision is whether to install 5" or 6" ducts. 

[Schreck]

6" inlet extended into the blower housing

I modified the 6" plywood inlet so the pipe extended 1/2" into the blower housing.  I then measured airflow and motor performance with both 5" and 6" ducts.

The charts below show data for the 6" plywood inlet plate, initially with the pipe ending flush with the inside face of the plywood, then with the plywood rounded over, and finally with the pipe extended 1/2" into the housing (6" tight inlet).  The tight inlet showed the best performance and the highest efficiency.  The best inlet/duct size combination was to use the 6" tight inlet to draw from 5" duct through a 5-6 reducer. 

[WorksInTheory]

Awesome data - makes my head hurt so still digesting. I have been asking this question around these parts and will ask you here - what are your thoughts on going to 8" intake?

[Schreck]

Just saw your post.  An 8? inlet would not be helpful for this blower.  The relationship between the impeller and the housing affects blower efficiency.  The distance between the impeller and the inlet would increase because there is a profile to the blades of the impeller. See pix in reply 6 above.