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Thread: Return Fan Setup

  1. #1
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    Return Fan Setup

    Hello,
    I would like to know the answer to the following. I've never been able to grasp the return fan setup process completely. An AHU (no diversity) rated at 11475 CFM supply with an OA design of 2870 and a return fan rated at 8605 CFM (all per schedule). Is the actual return air fan value of 8605 CFM correct? I always tell myself there has to be some relief going on (because the OA and Relief damper open to the same %) and question if the return fan should actually match the supply fan value allowing for 2870 relieving. Or is it in fact just neutral/slight positive at the relief with these design values? Thank you for the time

  2. #2
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    Welcome aboard Justin you will love this site. The answer to your question is yes the return fan should be delivering 8605 within the specified balance tolerance (normally +/- 10%). If properly set up under normal conditions there will be no exhaust from the AHU because the OA is used to provide a positive building pressure and fresh air. The damper positions should allow you to make the adjustments to accomplish the desired results. The outside air quantities normally cannot be direct measured accurately but you can get close enough for the system to work properly. In most cases you can traverse the return fan CFM and the total supply CFM and the difference is the outside air quantity. In the cases where you can't do that you can normally get close enough using temperatures.
    I had one system where I had them add a piece of duct to the AHU outdoors for me to traverse but that was because they wanted to operate a system designed for 75F @ 50% RH at 65F @ 50% RH and we had to know what the AHU would do on the nose.
    No man can be both ignorant and free.
    Thomas Jefferson

  3. #3
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    Thank you for the welcoming and the response. I just tend to get brain fog with the positive pressure plenum static and the exact location of that measurement. Especially on RTU's with internal return fans. The externally installed RAF's make more sense to me as the pressure would have to be positive where the relief and return meet to avoid OA being pulled in through relief.

  4. #4
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    The setup and configuration can be a challenge for a TAB guy. After a while you get a feel for how to start the setup which helps. Most designs have little or no consideration of how the TAB is going to be done. Most design engineers don't know or maybe don't care what the limits are of TAB instruments. I have been ask to traverse on a fan elbow outlet right on top of the fan. Aside from the fact the traverse would have been useless the pitot tube would have hit the fan wheel.
    No man can be both ignorant and free.
    Thomas Jefferson

  5. #5
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    Too often I see these complicated situations myself, glad I'm not the only one. I tend to see matching RAF values matching SAF values...what is the correct setup when dealing with this? so 11475 SA and 11475 RA with 2870 OA...2870 relief? or is this an engineering mistake?? Thanks

  6. #6
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    In your case in post #5 the engineer probably didn't word the spec right. One thing to remember is buildings that are occupied with live people have to have fresh air which has to be drawn in or in some cases blown in with an outside air fan. The return air at the supply fan does match the supply air because it has to. The difference between the RAF CFM and the total return air CFM is the outside air required and measured when balancing.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #7
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    In typical applications the inlet cfm to the return fan should equal the output of the supply fan.

    Only changes would occur between the 2 fans.

    Only time when you will find the output of the supply fan greater then the inlet of the return fan is when their is an excess of OA that is being by a different system lacking its own means to make up air


    Sent from my iPhone using Tapatalk

  8. #8
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    When the return fan CFM is specified less than the supply fan CFM by the amount of the outside air required the return fan's job (in non econ mode) is to reduce the negative static created by the return duct system on the supply fan. The supply fan draws in the outside air and to do that the inlet static has to be negative. The outside air is not drawn in or go through the return fan.
    No man can be both ignorant and free.
    Thomas Jefferson

  9. #9
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    My confusion comes from 1 thing...the engineer always seems to match the RAF CFM with the SAF CFM and of course give an OA design. So when I look at the schedule all I see is matching numbers and think that the engineer accounted for relief (matching the OA essentially). I need clarification on intent with such scenarios. And lastly, where would I expect the positive pressure on a RAF that is built into the RTU/AHU? My boss would say the plenum but I only think of the mixing plenum (oa plus return)

  10. #10
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    Relieving air when not economizing is a complete waste of energy. If that is necessary for air quality you should and almost certainly would have an ERV to recover as much of the energy as possible. If the return fan is specified to deliver the same amount of air as the supply fan then the return fan has to draw in the specified amount of outside air. Since the return fan in your situation is sized for the exact amount of air returning from the building not including OA then the duct work should be designed for and have the dampers needed for the supply fan to draw in the outside air. If the OA damper is located between the two fans the supply fan draws in outside air. If the OA damper is located on the inlet side of the return fan then the return fan draws in the OA. Given the air quantities specified in your case the OA damper is located between the two fans.
    The pressure between the two fans is negative. Don't get your hopes up of measuring an exact pressure between the fans because of the turbulence but in will measure negative.
    No man can be both ignorant and free.
    Thomas Jefferson

  11. #11
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    Quote Originally Posted by WAYNE3298 View Post
    Relieving air when not economizing is a complete waste of energy. If that is necessary for air quality you should and almost certainly would have an ERV to recover as much of the energy as possible. If the return fan is specified to deliver the same amount of air as the supply fan then the return fan has to draw in the specified amount of outside air. Since the return fan in your situation is sized for the exact amount of air returning from the building not including OA then the duct work should be designed for and have the dampers needed for the supply fan to draw in the outside air. If the OA damper is located between the two fans the supply fan draws in outside air. If the OA damper is located on the inlet side of the return fan then the return fan draws in the OA. Given the air quantities specified in your case the OA damper is located between the two fans.
    The pressure between the two fans is negative. Don't get your hopes up of measuring an exact pressure between the fans because of the turbulence but in will measure negative.

    I agree with Wayne, but it sure would be nice to see a set of drawings, a sequence of operation, and to know what the unit serves.
    Fixed O/A? CO2 monitoring? Economizer? Variable Volume? On and on......

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  13. #12
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    A case I have right now is a VAV system with a fixed min OA of 8600, I am sure it has econ and maybe CO2. The supply matches connected VAV totals of 26000 CFM...this would leave 17400 return from the grilles however the schedule says 26,000 RAF CFM and on top of it all the connected return grilles only total up to 15350. So, based on the engineers numbers it seems as if he/she has built in relief into the RAF CFM...unless that scheduled value is just an economizer amount of CFM but how am I supposed to assume that? Being that the RG's fall short I will set it up to the lower end of 17400 and still be in tolerance (90% or so) with an increased OA (105% or so) to satisfy the supply and have 0 relief...just stumped on why the engineer puts 26000 on the RAF

  14. #13
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    Per post #12 the first thing you need to do is get the specified control sequence and thoroughly understand the entire intent. We could probably guess pretty close but that isn't good enough. If all else fails call the engineer.
    No man can be both ignorant and free.
    Thomas Jefferson

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