For measuring velocity directly the ideal tool is a hot wire anemometer. This is a thin probe you insert in the flow. Inside the probe and exposed to the air is a length of wire heated by the instrument. Airflow cools the wire and the change in resistance is measured and converted to display velocity. Because its a thin probe it has little effect on what you are trying to measure--unlike the fan type which has a big effect on the airflow. Those cheap pocket-size fan anemometers are really only good for relative measurements in your own system. Not as a reproduceable measurement to compare with others, or with fan specs.
Yes, you pretty much have it right. I have a flat plate with my gauge hose connected to it. I placed this against the inlet side of my fan, with the outlet of the fan open to atmosphere, and no filter in use. After testing at the inlet, I hook up my manifold, which is a run of about 12 feet of 6” duct, and then test at each of 5 gates.
All tested at 3” on my simple manometer. It could be changing, but I can’t see any movement.
I did find a site which shows how altitude affects vacuum systems, and from what I can see, at 5,000 feet, I will have about 16.8% less vacuum that at sea level. So, guess that means that my 3” here at altitude would equate to about 3.4” at sea level.
To answer a few of your questions, the HF is 2hp (really 1.5) and I think the impeller is 10”. I can’t imagine any leaks because all I have in the system is the impeller housing and a motor??
Hope that sheds some light on things.
Last edited by Vaughn Kaloust; 03-12-2013 at 8:31 PM.
I certainly don't mean to insult your intelligence, but is it possible that the motor could be wired so that it is turning backwards? Is this a new unit, or did you purchase it from a previous owner? With the inlet and outlet open does it put out a lot of air? Do not run the motor this way for more than a few seconds so that you don't damage the motor.
Are you comfortable working around electric wiring? If so, I would disconnect the motor plug from the wall and open the motor junction box and inspect the wiring. Is it connected in accordance with the wiring diagram usually found inside the J-box cover? Are you running the DC on 110VAC or 220VAC? Are you using an extension cord?
The motor current can be measured by clamping the meter around either one of the wires coming into the motor. Set the meter on the highest AC Current range and start the motor. How does the reading compare with the current rating on the motor label?
It has been my experience that in trying to troubleshoot a problem, it is best to eliminate each possibility one at a time. I hope this helps. The more information we have, the easier it will be for us to help.
This is a new 110v unit that was pre-wired. I have it on a 20 amp circuit, and it seems to be running fine. Seems to pull lots of air.
I am fairly familiar with both AC and DC circuits, and I have measured the amps and all seems good.
I may be giving the wrong impression about my system. I do not know if my system is performing well or not. It may be working great, but I have nothing to compare it to. What I am trying to find out is what would a good benchmark be for my system at my altitude?
I wanted to start with static pressure, and when I have that as good as I can get it, I will move on to CFM.
Vaughn,
If you are measuring this SP at the fan inlet with the manifold connected as you describe (6" duct all the way to the machine), you should be pulling around 875-950 ACFM, assuming 5,000 ft AMSL. Your SP at the hood (near the opening) will be around 2"wg. You really don't have that much loss in the 6" duct. Your amp draw will be low due to less power required to move the lighter air.
Remember that SP and CFM are related for a given system. If you change one, you change the other. You can't really optimize one, then move on to the other to optimize it.
If you have the manometer, the next logical step to get CFM is to use a pitot tube. The 12" mini from Dwyer would be perfect for your needs. You can also use the pitot tube to measure SP, although sticking the hose to a hole on the side of the duct will work too.
Mike
Edit: I am not affiliated with Dwyer, just familiar with their products and use them.
Thanks Michael, but I am not measuring SP at the fan with the manifold connected. I am measuring it at the fan only, with the inlet connected to the manometer only. Here is a simple drawing of my setup.
I have also done the same test with the manifold connected, but I am not measuring at the fan. I am simply moving my plate to the gates that are connected to the manifold. I also get the same 3" readings at each of those outlets, and you are correct that this seems right, as there is very little loss in the 6" duct.
No air is flowing in any of my test as the ducts are completely plugged and connected to my manometer only.
I am looking at the Dwyer gauge, and as soon as I am sure I have good SP I will move on to the CFM question.
Hi Vaughn,
I am still puzzled by the low SP. With the inlet and outlet open, what motor amperage did you measure? What is the motor nameplate rating? Under those conditions, you should see motor amperages at least as high as the nameplate rating and probably higher. If the amperage is low, I would suspect a problem with the motor, or possibly something related to the incoming line, such as a faulty breaker, bad connection, other devices on the same circuit, etc.
The SP and CFM are interrelated, and having a low SP will make it difficult to obtain a high CFM without the SP to overcome system losses. I notice in the new testing that WOOD magazine reported in the current issue that they list 800CFM @ 5.5"WC Static Pressure as being the minimum they recommend for a typical shop layout, tho they do indicate that shorter runs can be accommodated with lower numbers.
OK, I measured the amperage in both conditions. With no load (all inlets closed) I am pulling 7.9 amps. This seems ok to me based on what I have read. With a completely open system, I am only pulling 8.5 amps. This seems wrong because the DC should be working at maximum load. I would expect at least 14 to 18 amps. I have read others with the same HF dust collector often tripping 15 amp circuit breakers, so I know that similar systems often pull about 15 amps.
I have this on a brand new dedicated 20 amp circuit, and have never tripped it. I guess I never will if it only pulls 8-9 amps. Lol
Maybe the air is so thin at 5,000 feet that this is normal?? Or maybe something really is wrong.
Is there anyone at a similar altitude that can that can compare readings with me?
Paul, there is no plate on the motor, but I have read a lot on forums about this DC and know it is called a 2hp, but is actually about 1.5 hp. It also seems to pull just about 15 amps in normal operation for most people that have the same unit.
Again, remember this seems to be moving a lot of air, so I don't even know if I have a problem?
Hi Rod, I agree that 3" WC seems too low. I can check the impeller, but if it were running backwards, wouldn't I get pressure and not vacuum at the inlet side of it? I would think that would be the case anyway?
I agree with Rod, the SP is way too low, even for your altitude. I checked the HF website and the 2Hp Model 97869 is listed at 20A peak. That tells me that it should be pulling a lot more than 8-9 Amps with the inlet and outlet open, in spite of you altitude. Are you sure you are at ~5000 ft. and not 65000 ft? :-)
I would check the motor wiring against the diagram that should be under the j-box cover. It sure sounds like the motor is running backwards to me. Please let us know what you find.
Vaughn,
There are threads on here about the wheel running backwards on the HF units.
I am not sure by your diagram how you are measuring SP? SP acts in all directions and is usually measured perpendicular to airflow. Simply drill a hole in the side of the duct and cover it with the open end of the tube. Make sure the other end of the manometer is vented to ambient. The reading is the total displacement between the two columns of water (+ on one side, - on the other), not just how much the fluid moves on one side. If you are putting the tube in the inlet or inside the branch duct, you will get wacky readings. Just cover the drilled holes with duct tape when you are done. A 3/16" hole works well with the Dwyer mini, you need a 3/8" hole with the full size pitot tube. Just saying this so you only have to drill the holes once if you get a pitot tube in the future.
Shut all your gates and measure the SP, it should be maxed out and well above 3"wg. Your amp draw will also be minimal (not moving any air). Next open all the gates and read it again, you should have a much lower SP and much higher amp draw. I don't know that you will be able to trip the breaker or get FLA at your elevation. The HP requirement is less due to the thinner air.
Sorry Paul, there seems to be no wiring diagram anywhere on the motor.
I am not sure if the impeller is turning the right way or not. I just checked it, and the best I can do is to show it in a picture. Hopefully someone can tell if this is the correct direction.
Michael, You may have found my mistake. As I show in my simple drawing, I am not adding the amount the manometer rises on one side, to the amount it falls on the other side. If that is how it is supposed to be done, I have screwed this up, and I am actually getting 6” WC. Is this correct? I mean are you supposed to add how much the water drops on one side to the amount it rises on the other side?
This may sound a little wacky, but could you post a pic of your wiring in the J-box? Maybe we could figure out how to reverse the direction of the motor for a test. I am not familiar with curved bladed impellers, so I don't know which way it should rotate.
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