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Air flow measurement with multiple fans

  1. Jun 4, 2015 #1

    My company builds machinery with multi-zone hot air dryers. Each dryer has 2-3 supply blowers that push air through an electric heater and then through a duct into a plenum (one plenum for each dryer zone) at the top of the dryer. Each plenum is fitted with multiple air nozzles that distribute the heated air into the drying chamber (and onto the product we are drying). At the bottom of the dryer, we have a single exhaust plenum that spans the entire length of the dryer. We use a single exhaust blower to remove air from this exhaust plenum (and the dryer as a whole). The exhausted air contains solvent vapors from the product that we are drying. This dryer has an opening at its "entrance" that allows us to convey the product that we are drying into the dryer, and it has an opening at its "exit" that allows us to convey the product out. We control our supply and exhaust blower speeds with variable speed drives, and we try to choose our speeds so that that exhaust blower removes slightly more air than the supply blowers are supplying. This way we make sure that we have a slightly negative draft from the room into the dryer (so solvent-laden air does not enter the room where the operator is). We have been building this type of dryer for a long time and it suits our needs in terms of its ability to dry the product and its flexibility/adjust-ability to deal with different types of products. While the dryer does what we need it to do, I am coming under pressure from my customers to measure and document different performance characteristics such as air flows (in CFM) in my supply ducts and my exhaust duct. I am finding my lack of theoretical knowledge to be hampering my ability to evaluate flow measurement devices that might work for me. I would like to be able to answer questions like the following:

    My exhaust blower that is rated for 2063 CFM @ 1.5” SP @ 1719 RPM. My two supply blowers rated for 1031 CFM @ 1.5” SP @ 1750 RPM. (These are the operating points shown on the fan curves provided by my blower manufacturer). It turns out that I am getting my desired drying effect with an exhaust blower speed = 1200 RPM, Supply #1 Blower Speed = 660 RPM, and Supply #2 Blower Speed = 780 RPM. I would really like to know the resulting CFM that each supply blower is sending into the dryer and the CFM that the exhaust blower is removing. Do I have enough information to calculate this? If not, what other information would I need?

    If anyone here can help me with the above question, I would really appreciate it. Also, if you could recommend a book that I could read that does a good job of explaining the theory behind situations like this where multiple blowers are pushing air into and/or removing air from a single chamber, I would really appreciate it. I do not have an engineering degree, so I am hoping for something that covers the theory but is not impossible to understand for someone who may not have all the prerequisites.

    Any advice will be greatly appreciated.

    Thanks in advance,
  2. jcsd
  3. Jun 6, 2015 #2


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    Staff: Mentor

    The pipe system will have an impact on air flow. To estimate this you would either need a detailed model of the system and the room, or some other method to estimate air flow.

    There are commercial system that can measure air flow. And probably also the concentration of solvent transported, which could be an interesting number on its own.
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