Exhaust duct air flow monitoring

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Old 04-26-16, 08:47 AM
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Exhaust duct air flow monitoring

I need to determine the most effective and reliable way of monitoring the air flow inside a 4" metal exhaust duct. The duct is used for exhausting corrosive gases from a vacuum pump and the duct has a 100 CFM fan at the top (on the roof of the building). I already have air flow monitors, which are typically used for fume hoods, but they have an attachment for use with duct work--essentially a fitting with a 1/4" O.D. plastic tube that secures to the duct. The airflow through the duct creates suction in the tube and pulls air passed a thermistor. The current delta required to maintain the temperature of the element is converted to the airflow. This is fine when it's just the exhaust fan running, but my concern is when the pump is running that it will push air the opposite direction through the tube and set off the low air flow alarm. So I need to ensure the air flow meter is satisfied with only the exhaust fan running and also with the pump and fan running simultaneously. The only time the air flow monitor should sound an alarm is when the exhaust fan cuts off (which should never happen).

Question is, what is the best way to connect the 1/4" O.D. plastic tube to the 4" metal duct to ensure there is always suction on the tube and turning on the vacuum pump does not significantly reduce or block suction? I'm thinking there should be some way I can fabricate a metal box with vacuum exhaust entering one end and fan exhaust exiting the opposite end and positioning the air flow meter's tube towards the bottom so air is always drawn out of it and never pushed into it. Like a venturi type of design. Or perhaps simply sticking the tube into the 4" duct will act like a venturi?
 
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Old 04-26-16, 09:19 AM
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The situation is very vague.

You have a 4" metal exhaust duct and you are monitoring or need to monitor air flow thru it. It appears that the fan should always be running so there should always be airflow.

With the type of sensor you want to use there may not be enough vacuum to operate the switch. The vacuum in the duct is created by the fan and the restriction at the pump end. In other words if you have a fan at one end and the other end is open.... there is minimal amount of vacuum to measure.

A thermistor measures temperature changes so that may not be the best type of sensor to use. They do make a type of hot wire sensor that mounts thru the duct and will monitor air flow. The system is used in cars to monitor intake airflow. Can be kind of expensive.
 
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Old 04-26-16, 09:30 AM
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How big is the vacuum pump? Seems to me unless it is *huge* the exhaust from it will be a small fraction of 100cfm. That being the case, I think it unlikely the vacuum pump will be able to pressurize the 4" duct at all unless the exhaust fan isn't running. The duct will always be under negative pressure with the fan running and the air will flow in the air flow sensor in the right direction.
 
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Old 04-26-16, 09:41 AM
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The pump is 29.7 CFM max. The whole point of the air flow meter is to detect if the exhaust fan cuts off or the vent loses suction, so I think it will work fine. Guess I'll find out pretty quick.
 
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Old 05-20-16, 08:03 AM
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I hooked up one of the sensors yesterday and there is hardly any suction if any, like you guys said. So, evidently this isn't going to work. The only other thing I can think to do is maybe install some sort of magnetic sensor on the fan to detect that it is spinning (same principal used to measure wheel speed on a car).
 
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Old 05-20-16, 08:35 AM
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The magnetic type RPM sensors don't have a very long detection range (0.1" was the largest I could find). Now I am considering an optical sensor. One I found on McMaster can detect up to 36" away. All I would need to do is put a piece of reflective tape on one of the fan blades and connect a meter to display the RPM.
 
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Old 05-23-16, 01:00 PM
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Does someone know of an online calculator for determining air flow when mating different sized ducts? What I'm trying to determine is how much FPM I need in the larger 4" duct in order to get at least 400 FPM in the hose I'm using. If it's a linear relationship, I can figure it out pretty easily by using the calculator I already found.

Based on a few numbers I plugged in, the CFM doubles for every doubling of duct area so apparently a pretty simple calculation. My hose is 6.38mm in diameter, so to get 400 FPM through the hose, I would need about 14 CFM through the 4" duct.
 

Last edited by mossman; 05-23-16 at 01:22 PM.
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Old 05-23-16, 02:37 PM
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While the relationship between CFM is as you describe, the wrinkle is that when you connect the small hose the resistance to air flow and hence the static pressure drop of your system will go way up. Your 100 CFM fan will be rated to move 100 CFM at a specified static pressure drop. Increase that drop and the fan's performance will drop off quickly and the resulting air flow will be lower than 100 CFM, maybe a lot lower.

There are table of SP drop for various sized ducts out there, and you will need to know the performance curve for the fan you have.
 
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Old 05-24-16, 12:05 PM
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I hear ya. I think the best thing to do is to put a manometer on the tube and see exactly how much suction I'm getting, then resize the duct fan accordingly.
 
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Old 05-24-16, 12:32 PM
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Mossman, you said "the CFM doubles for every doubling of duct area". I'm shaky on the calculation for air flow, but I believe the flow is related to the sq rt of the pressure, making it nonlinear. Was that a doubling of the duct diameter rather than area?

Bud
 
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Old 05-24-16, 06:43 PM
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I'm not certain. I was using an online calculator that didn't take pressure into account. It was just a FPM to CFM calculator and all I needed to plug in was FPM and area.

Turns out I wasn't calibrating the meters properly. I tried again today and they work like a charm. I was actually pretty surprised that they work. They should at $500 a piece. Thanks for the feedback!
 
 

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