ECM blower manufacturer's cfm table accuracy
I've seen a few comments here and there about how accurate the cfm tables are, or aren't from manufacturers on ecm blower equipment. I've also heard they're far more accurate than what a tech can measure out in the field, assuming he has a fairly accurate static pressure measurement.
Curious what you're thoughts and findings are?
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
Comments
-
Oh boy this will be a can of worms… I'm sure there are some less than accurate claims made by manufacturers however when you get to the unitary stuff from the big name manufacturers, I have seen how stringent the testing requirements are, and frankly I trust those tested values more than an unknown tech measuring in the field with unknown methods.
1 -
Are you talking about the Calculated CFM that the t-stat gives. Then NO its calculated, close a duct and that number wont change much if at all. There is NO static pressure sensor in the system to actually measure the static. Thats the computer saying were turning X RPM's so that SHOULD be Y CFM.
1 -
No, the cfm charts in the documentation for the equipment that shows specific ecm at different total static pressures.
As an example because I have it handy.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
0 -
Thats should be accurate or damn close when NEW. Now a little dust drops that number quickly!
1 -
Makes me curious how fast that squirrel cage is spinning at a static of 0.9".
Hopefully they come very very well balanced……..
I'd actually be curious what the speed difference is between 0.1 and 0.9. I'd think it's enormous.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
0 -
They have to cypher in motor power a well as RPM to guestimate airflow. As a general rule, I take them at face valve; if I were investigating I would measure independently.
I have to admit I'm skeptical, but I see nothing that indicated they can't be reasonably accurate. The new axial fans are pretty neat.
0 -
With a centrifugal blower, isn't power power regardless of rpm?
That's actually had me wondering for a while because I've heard that high static makes them use more power, yet that's literally what it's using to control them, no? So shouldn't actual power input be relatively constant across the board unlike a PSC motor?
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
0 -
@ChrisJ said:
"shouldn't actual power input be relatively constant across the board unlike a PSC motor?"
I don't know how the control logic works, but I can imagine the power input might vary depending on external duct conditions that make it easier or harder for the blower motor to reach a given RPM.
We had an overheating problem with our church furnace that turned out to be due to the HVAC installer who put in too small a return plenum, resulting in a TESP of 1.4 inches WC across the furnace, where the mfr's spec max was 0.5 inches WC.
I checked the blower output table that came with the furnace, and our TESP was so high, it was literally off the chart (which I think went to 1.1 max). I had to make a graph and extrapolate to find what the probable CFM was at 1.4 TESP.
Doing the math on observed/measured temperature rise across the furnace, it looked to me like the predicted CFM at 1.4 TESP was fairly accurate. I think it was 25% below the required minimum CFM at 0.5 TESP.
In that case, I imagine that the power to the blower was maxed out, but with RPM below what it would have been with a lower TESP.
But in other cases, with larger ducts and a lower TESP, I could imagine the blower motor running at higher RPM's but not having to working as hard, so maybe at a lower power input.
Isn't that the point of ECM motors, that the power input can be reduced if conditions don't require them to run maxed out?
0 -
High static causes lower amperage.
run a squirrel cage fan out of the cabinet and it will probably over amp and cut out on overload.2 -
That part I understand.
From what I've heard ECM motors are usually programmed to increase rpm until they draw the correct amount of power for the cfm setting. If that's true why would high static cause them to fail?
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
0 -
surging.
with excessive static. Beyond what the ECM is designed for it spins faster and faster eventually causing the blade to slip. That drops the static for a moment then it corrects. All this up down drives the motor slower faster several times a second.
they don’t like it b1 -
What is a fan surge?
At a certain moment, the fan will not be able to deliver the required flow and pressure in which case the fan will surge and the airflow direction will reverse. The system pressure will decrease until the fan recovers and the process will start all over again. This phenomenon is called surging.1 -
Sorry, I was thinking about vane axial blowers. I just swapped the motor out on a 10 ton rooftop, that's what it had. I always got to stop & think about which type of blower I'm working on, to figure out how it responds to static etc.
2 -
I wonder if static pressure would be the same if the blower and evaporator swapped places.
0 -
For most of these types of blowers power is:
Power=Cfm * deltaP / BlowerEfficiency / MotorEfficiency
If you look at that, blower efficiency and motor efficiency are based on design so easily characterized and only effected by production tolerances. These do change with RPM, but RPM is measured on ECM units, so it is easy to look up.
The only unknown is delta P which is roughly proportional to motor torque, thus motor current which is measured on ECM units.
With a bit of characterization, it is easy for a blower to know the CFM it is running at and and have internal logic to adjust RPM to maintain constant CFM.
So all that to say that I would trust those curves to be accurate within reason.
A couple of things you can take away from the above equation. If you double the pressure drop, the power about doubles. This is why something like high velocity air handlers use so much power.
Another thing to note on a typical install. When you turn the blower down, both CFM and delta P drop a lot. This is why a typical 1000CFM air handler when running in circulation mode at around 400CFM uses something like 40W of power.
PSC blower follow similar laws except there adjusting speed significantly drops efficiency. Basically the way you slow those motors down is by adding additional slip which is straight loss. So an PSC blower on low will use close to the same power as at high speed.
These rules also apply to circulator pumps. When you dead head an ECM power, GPM drops to zero thus fluid power also to zero. The only power input is to supply pump and motor efficiency thus electrical power is very low.
0
Categories
- All Categories
- 86.8K THE MAIN WALL
- 3.1K A-C, Heat Pumps & Refrigeration
- 56 Biomass
- 425 Carbon Monoxide Awareness
- 108 Chimneys & Flues
- 2K Domestic Hot Water
- 5.6K Gas Heating
- 105 Geothermal
- 160 Indoor-Air Quality
- 3.6K Oil Heating
- 69 Pipe Deterioration
- 960 Plumbing
- 6.3K Radiant Heating
- 385 Solar
- 15.3K Strictly Steam
- 3.4K Thermostats and Controls
- 54 Water Quality
- 43 Industry Classes
- 47 Job Opportunities
- 18 Recall Announcements