In an industrial application I need to monitor air flow and pressure/vacuum in a dust collection system (it is just like a huge vacuum cleaner) to understand if the filters are working okay or not.
Would any one have any experience in such an application to share or any suggestion for this type of application?
You can use a differential pressure gauge across the filters. This should give you an indication if they are getting clogged - the higher the pressure differntial, the greater the blockage in the filter. This can be inaccurate if the filter material can tear as it reduces the pressure differential and gives a false reading. Air flow can also be measured using differential pressure, or by using an axial turbine generator.It is also possible to use a hot wire velocity sensor where the cooling is proportional to the air flow.
Dear Mark, Thank you for reply. I think the biggest problem on measuring the pressure is the dust in the environment. Would you have any suggestion for which type of sensor I can use as differential pressure gauge in dusty environment ?
It really depends on the pressure and vacuum levels you expect to see.
Pull down an automotive sensor catalog. There are probably several hundred different types of pressure and vacuum switches and sensors used in automobile engines. See if you can find something that matches up.
Does it have to be electronic? I cant remember the name of them, but they are a bit of clear plastic tube filled with green water in a U shape, and a scale behind, maybe 2 of them can be used to measure the pressure differential, low tech solution but they aren't going to clog up with dust to quickly.
I read in sci.electronics.design that ZForce wrote (in ) about 'Air flow and vacuum/pressure sensors in a dust collection system', on Sun, 23 Nov
2003:
Manometers. I suppose they are called personometers now.
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Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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Should it really make much difference? There will be practically no air flow through the hoses leading to this sensor, unless the filter fails. If you have a hood with the open end facing downstream, to create a "dust shadow" around the test point tap, and the operator who replaces the filter is trained to blow out dust from the test point as part of the process, it should surely be OK for a long time?
What is the nature of the dust? (Is it sticky, corrosive, abrasive, highly reactive, etc?)
Another approach to this problem is to cut two portholes in the pipe, one before the filter and one after, and use a tough rubber diaphragm with a strain gauge to measure approximate absolute pressure.
You can also infer the state of the filter by looking at the *output* flow rate from the fan that's creating this partial vacuum. The apparatus to do this can be nothing more complex than a swinging lid over the output pipe, and a light gate...
My central heating system (gas), uses a simple vacuum switch to sense pressure differential from the exhaust fan's input vs. output (to decide if the exhaust fan is actually functioning).
In effect, it's a switch that changes state when there's a pressure differential. Very simple and effective if you're only looking for good/bad state (not pressure readings). I'd expect the differential could be tuned in some models to determine the threshold for a "clogged filter".
The standard differential pressure gauge used to monitor overspray filters in paint booths is just an oil-filled manometer. I don't remember the differential pressure range they work in, but they can't be too expensive.
Gauges like this usually aren't bothered by dusty air because there is virtually no airflow in thru either of the sensor openings.
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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
One machine I worked on had a flow sensor made from a piece of tin plate about one inch square attached to a microswitch arm that sat in the air flow near the fan. when the fan was on and air flowing it made the plate move and operate the switch.
Changing the size of the plate would change the ari flow required to operate the switch. Gravity balanced this unit but a spring might be just as effective. Have a look at the dust indicator that is fitted to several models of vacuum cleaners and maybe add a magnet and a reed (relay) switch to operate an idiot light.
Are you talking about a dust collector for a woodworking shop or similar application?
How many CFM's does the fan move?
What's the nature of the dust (hard, soft, wet, dry, dangerous, harmless, etc.)?
What degree of accuracy do you need?
What's the ultimate purpose of the pressure measurement? For example, do you just want to trigger an alarm to have an operator change the filter?
There are problably over a dozen ways you can measure airflow. From shed-vortex sensors to simple linear plungers in the airstream. To narrow this down to a small set suitable for your application you have to specify the constraints/application with a bit more detail.
Example 1: A typical home/industrial air conditioner filter will bend as it clogs. You could place a plunger touching the back of the filter to measure this deflection. The firmware could auto-calibrate and "learn" about what a good filter looks like and trigger an alarm when the filter is clogged.
Example 2: Use a radial impeller in the airflow. All air must pass through it, and, in order to do this, the impeller will/must rotate. Debris will not affect it if designed properly. Generating some sort of a proportional airflow number is pretty simple.
Example 3: A piece of piezoelectric film sandwiched between two metal plates. Arrange this perpendicularly to the airflow and mounted with brackets such that the plate is in the center of the air stream. This is located prior to any air filtration. Larger debris will impact the plate and generate voltage pulses. Flow/no-flow determination is very simple. With more knowledge of the material being vacuumed it might be possible to generate coarse velocity/volume data as well.
Example 4: Bond a heating element and a thermistor to a common piece of metal (maybe inside a sealed tube). Insert this into the duct. With no airflow the heater will stabilize at a certain temperature. As air flows, it will be cooled. Measure with the thermistor.
Like I said, there are so many ways to do it...
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Martin Euredjian
Thank you all for your inputs. As I mentioned in the original posting, I'll try to control a dust collection system. One of the major problem was detecting the air flow and/or vacuum/pressure/depresion in the system. Now assumed that we solved this detectin problem. The next step is shakeing the blocked filter. I think an electromagnet would do the job. What I need to have is an electomagnetic actuator that can shake (translate 25kg filter over 20cm with 2-5Hz.) the filter.
My questions are related to design/calculations of electromagnetic actuator. Assume that I will use 240v AC line voltage. (1) How to calculate number of turns to have 25kg pulling or pushing force? (2) Diameter of the wire to be used in the actuator coild? (3) Diameter of the coil ?
Rather than trying to shake a huge thing like that, how about just raising it a few inches and dropping it? Kinda like when you whap the coffee filter basket against your hand to dump out the grounds.
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