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  1. #1
    Join Date
    Apr 2006
    Posts
    8

    Confused

    Would it be possible for anyone to define the term 'fan external static pressure'?
    As I understand it this is the pressure generated by the fan as measured at the fan outlet, would that be correct?

    Also, if that is the case, how would the 'AHU external static pressure' be derived? Would this be measured at the fan outlet within the AHU or at the outlet of the AHU itself? Thanks everyone - I appreciate your help!!

  2. #2
    Join Date
    May 2004
    Location
    Illinois
    Posts
    2,843
    "External Static" is the difference in pressure between the fan inlet and outlet.. i.e. everything external to the fan.

    Same thing for the AHU.
    "If I had my life to live over again, I'd be a KA." - Albert Einstein

    It's later than you think.

  3. #3
    Join Date
    Feb 2006
    Location
    SouthEastern Virginia
    Posts
    1,075
    ralphtheplumber is correct.

  4. #4
    Join Date
    Apr 2006
    Posts
    8
    Thanks guys, but the situation that I have is that I have some AHU details (it is a fresh air AHU, not a recirculating AHU) and the in the 'Fan Details' of the Technical Data it tells me that I have;

    'Ext Static Pressure' of 1000pa
    'Total Pressure Rise' of 1800pa.

    If I am understanding you correctly that would mean that when the AHU is up and running and I take a static pressure at the fan intake it may read -300pa and if I take a static pressure at the fan discharge it may read +700pa, this, as you say, would provide the 'Fan External Static Pressure' of 1000pa as stated in the Technical Details yeah?

    But if that is the case where is the 'Total Pressure Rise' of 1800pa coming from?

    Thanks for your help guys, just getting on the ladder so I realy appreciate it.

    Ta.

  5. #5
    See if this website helps:

    http://www.delta-t.net/flow/pressure.htm

  6. #6
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by starter
    Thanks guys, but the situation that I have is that I have some AHU details (it is a fresh air AHU, not a recirculating AHU) and the in the 'Fan Details' of the Technical Data it tells me that I have;

    'Ext Static Pressure' of 1000pa
    'Total Pressure Rise' of 1800pa.

    If I am understanding you correctly that would mean that when the AHU is up and running and I take a static pressure at the fan intake it may read -300pa and if I take a static pressure at the fan discharge it may read +700pa, this, as you say, would provide the 'Fan External Static Pressure' of 1000pa as stated in the Technical Details yeah?

    But if that is the case where is the 'Total Pressure Rise' of 1800pa coming from?

    Thanks for your help guys, just getting on the ladder so I realy appreciate it.

    Ta.
    or it might be:

    pressure at the fan intake it may read -800pa and if I take a static pressure at the fan discharge it may read +1000pa,





  7. #7
    Join Date
    May 2004
    Location
    Illinois
    Posts
    2,843
    Originally posted by starter
    If I am understanding you correctly that would mean that when the AHU is up and running and I take a static pressure at the fan intake it may read -300pa and if I take a static pressure at the fan discharge it may read +700pa, this, as you say, would provide the 'Fan External Static Pressure' of 1000pa as stated in the Technical Details yeah?
    Yeah.

    Except you need to be clear whether you're talking about fan esp or unit esp. Read your specs carefully.

    Originally posted by starter
    But if that is the case where is the 'Total Pressure Rise' of 1800pa coming from?
    There is an additional 800pa allowance for the pressure drop through the filters, coils, hx's, etc. That's the "internal" part of the "total pressure rise".

    That, plus the 1000 "external" gets you back to 1800.

    Keep in mind that the pressure drop through the unit may not be exactly 800. It will change depending on how dirty the filters are, the amount of condensate running down the coil, etc.. 800 is just the number that the engineer used.
    "If I had my life to live over again, I'd be a KA." - Albert Einstein

    It's later than you think.

  8. #8
    Join Date
    Apr 2006
    Posts
    8
    Thanks ralphtheplumber, jacob perkins, Rudy and duke of earl, so the External Unit Pressure is 1000pa (the difference between the static pressures measured at the intake & at the discharge of the AHU) but the actual Fan Total Pressure Rise of the fan itself will be 1800pa so as to overcome the restrictions within the AHU and still provide a External Unit Pressure of 1000pa. Got it!

    The only thing I need to do now is find out why, 'External Static Pressure' and 'Total Pressure Rise' are listed specifically in the 'Fan Details' section of the AHU Tecnical Data breakdown for each component, I suppose it would've helped me if they'd put in 'AHU External Static Pressure' rather than making it so I was led to believe it was the 'Fan External Static Pressure', but I suppose that's something you get to understand with experience!

    Thanks very much once again.

  9. #9
    Join Date
    Apr 2006
    Posts
    8
    Just one more thing though, I've since been handed the details for the Extract Fans, which are not in any 'housing' as such like the supply fans are within the AHU.
    Looking at the Extract Fan details in the Technical Data the 'Ext Static Pressure' is stated as being 1500pa but the 'Total Pressure Rise' is 1580pa.

    If I understand peoples answers to my original question correctly then due to these fans not being in any sort of housing as such, then the 'Ext Static Pressure' and 'Total Pressure Rise' for these Extract Fans should be the same should they not? So why would there be an 80pa difference when there is no resistance from any in-line items (coils, filters etc.) to overcome as there is for the supply fans within the AHU's?

    Going from the answers kindly provided to me in this thread I am assuming that the 80pa difference may exist to overcome ductwork friction losses over the very short lengths of duct between the fan and the Ext Static pressure tap in points or/and losses across the flexible connections at the intake and outlet?

    Would I be right to assume that this is why the additional 80pa is provided?

    Once again thanks in advance!



  10. #10
    Join Date
    Apr 2002
    Posts
    11,808
    Extract, you must be from the UK. Only hear Brits use that word

  11. #11
    Join Date
    Apr 2002
    Posts
    11,808
    got a link to the fan in question?

  12. #12
    Join Date
    Apr 2006
    Posts
    8

    Smile

    Tomato/Tomato eh? What would the rest of the world call Extract?

    I haven't a link unfortunatley, I'm kind of hopeful that I've got a handle on it anyway thanks - my assumptions sort of make sense to me anyway!!






  13. #13
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by starter
    'AHU External Static Pressure' rather I was led to believe it was the 'Fan External Static Pressure', but I suppose that's something you get to understand with experience!

    Experience may be a common factor here but the use and precise meaning of these terms will change as you travel deeper into the swamp...

    If you dont have a web link to these spec sheets, then tell us what you are trying to determine....provide some context to the question.

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