Mechanical air flow switch or current transducer for fan proof?
For the last 30yrs or so here at our faciility they have used differential air flow switches to prove air flow/fan running. As i am going through and replacing the old equipment,sensors and controls to our new Tridium/bacnet network I am wondering if sticking with the differential air flow switches are worth sticking with. I have never used the CT's for fan proof before and would just like your opinions on whether or not it is worth it to use them over the differential switches.
At the same time i would probably throw one on the compressor at the same time and trend them both for energy usage.
From what I've seen so far airflow switches last longer. CT's are probably one of most replaced end devices I replace from day to day.
Ct's can be hard to adjust with fans that run at lower speeds a lot, on a vfd. I installed a new drive years ago and proofed the motor with a ct. I thought I had adjusted it pretty good. When we did the first pm after that, we discovered that the belts were gone and the ct was still showing proof. I say, if the wire is already there, use a diff switch. I've never seen many problems with the ones we use.
Originally Posted by khumphreys
1 + 1 = 3 ( *** for very large values of 1)
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Hmmm, interesting. Care to mention which make of CT's you see fail the most?
Originally Posted by xarralu
I only ask because I myself don't see many of them fail. OTOH, I don't see many mechanical air flow switches fail, either. In either case I don't see much for failures until they're in the >15 year age range.
On one recent project, a controls upgrade project, we replaced a bunch of CT's (>100) that had been in place since the mid 1990's. They were still working. It was just part of the upgrade contract that called for replacing all sensors that caused us to replace them.
I'm not contesting your experience, I'm just wondering if you might be seeing some particular brand I might want to avoid.
Ditto. We rarely replace current switches. (RIBXGTAs)
Originally Posted by osiyo
We use them on the vast majority of our projects. They seem to work well and save costs on new projects. This is reflected on bid day and customers seem to respond….
VFD rated units I have tested, never impressed. Installed upstream of a VFD and adjusted properly, they work fine. All and all, my experience with switches has been good. We adjust them close, and more often than not return to the site to decrease the setpoint.
Proper install / adjustment is key with both.
If differential pressure is high enough for a DP switch its a more proof positive way to prove a fan is operating. I've had problems with small fans using DP switches because of very low differential pressures. In those cases a CT would be better as long as it is not a critical fan.
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"Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
A DP that is piped out on both sides will far out last a CT. Furthermore a CT that isn't properly adjusted, some cant be, will not reflect a broken belt or other malfunction that allows the motor to spin but not the fan.
I prefer the dp over the ct. I have seen the DP's fail but mostly in units that didn't receive proper maintenance and were very dusty. I think they also help if adjusted correctly to send notification that there is an "issue" with airflow like a coil that's froze, failed fan, dirty filter to name a few situations.
I agree with "samwinter" on this one, you could use both wired in series to one input if it's an ultra sensitive install.
Originally Posted by samwinter
I like DP both for proving and alarming to dirty filters and coils.
With that said we end up putting in Veries Hawkeye H940 CT's because they come with built in relay at a very low cost and can be adjusted to show for a broken belt. Per unit it is not a huge price adjustment but on large installs(8 or more AHU) the cost of wiring conduit parts and time can make a decent difference as it only requires a 2 conduit wire instead of 4 and reduces the cost of conduit to a DP.
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Just wanted to say that this type does work well enough for fans without VFDs. right price range.
Originally Posted by XcelTech
Leave existing, working DP's in place. This a digital, or binary signal. Best for flow proof, especially with a VFD.
Current switches (binary) marketed to work with VFD's are a PITA, IMHO. Please don't call a current switch a CT, you are insulting the CTs.
Current Transducers (4-20ma analog) designed for low frequency use will provide reliable values, but you will need to profile the system and set-up your software as needed. Remember to install the CT between the drive and the motor. Electricians call these donuts. If you don't go with the cheapest available, they will probably last a very long time.
Be careful with Variable Frequency Drives, they work by reducing the frequency (and voltage) available at the motor, and a lot of sensors are expecting 60Hz. If you put a sensor before the drive, you will read loads from the electronics, cooling fans, relays, etc.
With higher-end drives, I would simply connect to the 4-20ma output of the drive that provides motor current - this will take into account all three phases, and you don't need to buy the CT
For trending energy usage a VFD may even know what the KW load is. Your results may vary.