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I am trying to design/install/eval/validate a sensor so that the installed accuracy is at least +/- 5% (would like +/-3%) in a square duct. I have less than the normal L/D available. I have a single point FCI thermal mass meter, but would be willing to buy a new multipoint meter, if it is unlikely to work.
So far I haven't found data validated L/D requirements for flow meters mounted in rectangular ducts for my complex geometry (details below). I've also had no luck finding validated rectangular duct data on converting a highly asymmetric flow profile to a flatter profile, except for material on wind tunnels. [They use and antiswirl (honeycomb or tubes) screens (flattening) AND a specially designed converging section.]
I will admit my experience is with instruments in circular ducts/pipes but given the data and practices of AGA and ISA, I wonder if the L/D requirements Vs the achieved accuracy quoted for multipoint arrays in rectangular ducting are too optimistic. IE - Ebtron meter.
I've Googled & lit searched till my eyes have glazed over. If someone can direct me to literature with real data, or a referral to a rectangular duct flow expert (wind tunnels?), I would be grateful.
Lastly, it has occurred to me that if I used elbows with turning vanes, would I consider the total straight run as being nearly the same as one straight run? I imagine that the flow out of the elbows would be fingers of flow, but less likely to be skewed to the wall. Any idea how poor an approximation that is?
Thanks in advance for your help
Gary
Background:
I performed PITOT traverses @ the meter location for the current configuration using the T-rule:
-- Centrifugal blower (side outlet @ bottom elevation, aka arrangement 1)
-- a divergent section (~8 deg half angle),
-- a straight section 6D (equiv) of 18x18 duct (meter 2.4D upstream 3.5 D downstream)
-- a double elbow section. The radius elbows (R=1) form a "U" w/ 1.5D straight run between them, which directs the duct back under the elevated blower
-- a final straight section of 6D before I enter the process equipment
Five sets of 5x5 PITOT traverses, at different flow meter readings, established that the profile at the current meter location is highly inconsistent. Sometimes flow hugged the duct so tightly the traverse (per the equal area method) misses the highest flow against the wall. Other times I see the velocity is relatively flat along one axis and parabolic along the other axis.
The vendor for the thermal "mass" meter (FCI) and a competitor (Kurtz) both agree the current location is a tough place for a meter. They think moving it to the last straight section and calibration from Pitot traverse data is more than adequate to make a single point meter accurate. However my Pitot traverse makes me believe they trivialize the impact of square/rectangular duct geometry, especially in light of the fact both would recommend installing flow conditioners to reach rated accuracy, if it were circular duct. Would think that HVAC vendors are more realistic by providing a "station" which incorporates some flow conditioning or multipoint like Ebtron, but so far cannot find data to back up their claims.
Even with the sensor in final straight run, I fear that the skewed flow, the short straight runs and the double elbow will generate a profile too inconsistent to establish a known accuracy, without some other mechanical change.
So far I haven't found data validated L/D requirements for flow meters mounted in rectangular ducts for my complex geometry (details below). I've also had no luck finding validated rectangular duct data on converting a highly asymmetric flow profile to a flatter profile, except for material on wind tunnels. [They use and antiswirl (honeycomb or tubes) screens (flattening) AND a specially designed converging section.]
I will admit my experience is with instruments in circular ducts/pipes but given the data and practices of AGA and ISA, I wonder if the L/D requirements Vs the achieved accuracy quoted for multipoint arrays in rectangular ducting are too optimistic. IE - Ebtron meter.
I've Googled & lit searched till my eyes have glazed over. If someone can direct me to literature with real data, or a referral to a rectangular duct flow expert (wind tunnels?), I would be grateful.
Lastly, it has occurred to me that if I used elbows with turning vanes, would I consider the total straight run as being nearly the same as one straight run? I imagine that the flow out of the elbows would be fingers of flow, but less likely to be skewed to the wall. Any idea how poor an approximation that is?
Thanks in advance for your help
Gary
Background:
I performed PITOT traverses @ the meter location for the current configuration using the T-rule:
-- Centrifugal blower (side outlet @ bottom elevation, aka arrangement 1)
-- a divergent section (~8 deg half angle),
-- a straight section 6D (equiv) of 18x18 duct (meter 2.4D upstream 3.5 D downstream)
-- a double elbow section. The radius elbows (R=1) form a "U" w/ 1.5D straight run between them, which directs the duct back under the elevated blower
-- a final straight section of 6D before I enter the process equipment
Five sets of 5x5 PITOT traverses, at different flow meter readings, established that the profile at the current meter location is highly inconsistent. Sometimes flow hugged the duct so tightly the traverse (per the equal area method) misses the highest flow against the wall. Other times I see the velocity is relatively flat along one axis and parabolic along the other axis.
The vendor for the thermal "mass" meter (FCI) and a competitor (Kurtz) both agree the current location is a tough place for a meter. They think moving it to the last straight section and calibration from Pitot traverse data is more than adequate to make a single point meter accurate. However my Pitot traverse makes me believe they trivialize the impact of square/rectangular duct geometry, especially in light of the fact both would recommend installing flow conditioners to reach rated accuracy, if it were circular duct. Would think that HVAC vendors are more realistic by providing a "station" which incorporates some flow conditioning or multipoint like Ebtron, but so far cannot find data to back up their claims.
Even with the sensor in final straight run, I fear that the skewed flow, the short straight runs and the double elbow will generate a profile too inconsistent to establish a known accuracy, without some other mechanical change.
