Timeline for Duct static pressure vs ambient air pressure
Current License: CC BY-SA 3.0
12 events
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| May 28, 2015 at 3:13 | comment | added | Ryan Smith | @rith87: I would anticipate your operating range to be in the range of 0.1-2 in w.c. (25-500 Pa), with 2 in w.c./500 Pa being the extreme upper end (I would be surprised if the unit were capable of operating with that kind of static) and 1 in w.c./250 Pa being the more probable upper end with the system closed. I would anticipate a normal variation to be between 50-150 Pa depending on how many diffusers/dampers you have closed. | |
| May 26, 2015 at 9:17 | comment | added | rith87 | @RyanSmith: I am considering the BMP180 and the LPS25H; both of which have noise in the order of Pa and both expose an I2C interface. Their operating ranges are fine, 300 - 1100 hPa and 260 - 1260 hPa respectively. I guess my main question here is what is the difference in static pressure when the system is on, with registers closed vs with registers opened. Or when the system is on vs off. Do you expect it to be something in the order of 0.1 Pa, 1 Pa or 10 Pa? | |
| May 26, 2015 at 3:10 | comment | added | Ryan Smith | On sensor selection: pick one that has the range and accuracy you need. 1 in w.c. is approximately 250 Pa; if the supply fan is capable of supplying at 0.5 in w.c. external static pressure, a sensor with a maximum range of 0-250 Pa (or even 0-500 Pa, 2 in w.c. is probably beyond the normal operating envelope of a residential fan) would work, though some of this will depend on how you are taking the measurement (analog 0-5V/4-20mA? SPI/I2C?). | |
| May 26, 2015 at 3:07 | comment | added | Ryan Smith | Referring back to the fan curve link from above, the fan can only provide specific airflow at a given static pressure (read the other way, at a specific flow rate, the fan is capable of overcoming a certain static pressure). As Joel mentioned, the pressure will stabilize almost immediately. This occurs somewhere along the fan curve based on the system pressure (internal and external static pressure). Practically, the typical 10-20% duct leakage will keep the fan operating in a stable part of its curve because you will never have a truly closed system. | |
| May 25, 2015 at 20:05 | comment | added | Joel Keene | The static pressure will stabilize in the duct within seconds of the air handler turning on. It should not vary over time except in extraordinary circumstances. | |
| May 25, 2015 at 3:51 | vote | accept | rith87 | ||
| May 25, 2015 at 3:51 | comment | added | rith87 | The total pressure is the same at a single point, regardless of whether the diffusers are opened or closed? I'm not sure if I agree. For example, if all the registers are closed, then the dynamic component is 0 and the static component will vary based on how long the air handler is running. Am I making sense? | |
| May 25, 2015 at 3:47 | comment | added | rith87 | Thanks for the super prompt replies. A standard pressure transducer? What do you think is the accuracy that I require? 1 Pa, 10 Pa? What is the best way to calculate this value? Estimating the kinetic energy of the air in the ducts? | |
| May 25, 2015 at 2:19 | comment | added | Ryan Smith | (con't) Whether the diffusers are open or closed, the total pressure is the same at a single point (because the "energy content" of the fluid is the same), but with all of the diffusers open, the static component is lower while the dynamic is greater. | |
| May 25, 2015 at 2:19 | comment | added | Ryan Smith | @rith87 You are correct: static pressure is measured tangentially to the direction of the airflow. A standard pressure transducer mounted perpendicularly to the duct would measure the static pressure in the duct. A pitot static tube can measure both the stagnation pressure and the static pressure simultaneously and is used to derive the dynamic pressure. | |
| May 25, 2015 at 1:31 | comment | added | rith87 | Thanks, the CaptiveAire link had a good introduction. So, correct me if I'm wrong but if you closed all the diffusers, the pressure measured could be the same as if all were open since the increase in static pressure is offset by the decrease in dynamic pressure (since velocity decreases). So, what is the best way to detect high static pressure in the ducts? Orient the pressure sensor tangential to the air flow? Regardless, Thanks for the answer, Ryan. | |
| May 23, 2015 at 13:51 | history | answered | Ryan Smith | CC BY-SA 3.0 |