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  1. #1
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    When I check for proper airflow, I have several
    ways I check it. I measure the air speed with
    a velometer and convert to CFM, measure with a
    flow hood, and even checked using temp rise method
    on electric heaters (gas too on rare occations, gets
    pretty close. I use this as a last resort).

    The only time I pull out my magnahilic (spelling?)
    is if I measure low air flow and need to determine where
    my resistance is.
    hmmmm back up.... I have used it on rare occation to traverse the duct with a pitot tube and calculate air flow
    on a very unusual duct job.

    I've asked a duct balancing man (all he does is balance BIG jobs..... Schools, hospitals, etc).
    Everytime I'm around him I try to absorb as much info as I can in a short amount of time.
    errrr back to what I was trying to say....
    I talked to the balancing man about measuring static pressures and if I could use it along with the fan curve
    in the manufacturers data to determine CFM.
    He told me the fan curve is used by engineers to design the duct system. You can't use it to determine CFM.
    He says he checks with a flow hood or traverses the duct
    with one of his high tech gadgets (he showed me some of them..... those meters made me want to drool.... too bad they cost a small fortune. He was talking to me one time about a hospital he balanced where they wanted so many pascals of possitive pressure in the operating rooms. It couldnt' be any more, or any less... he had to get some highly sensitive meters to measure it... he didnt' have it with him at that time... arghhhh there I go getting side tracked again.....

    Anyways...
    he said you can't determine CFM using fan curves or available static on nameplate.
    He said you have to measure it.
    He gets the static pressures to put in his report so the engineer will be happy.
    He said once the proper airflow has been determined and you have the static pressures wrote down on the unit.... then if you have a prob later you will know what the original setup was quickly. As in if some liner come loose, then that ducts static would be higher than what was originally recorded.

    All this to ask...
    is there something I'm missing?
    I see lots of posts on here about checking the static
    but I'm not seeing what it will tell you.

    I know how to measure it... just not sure how you guys are applying it.

    One of the biggest reasons I'm asking this is on account of
    the ECM motors out there....
    If I measure the air flow with the velometer or flow hood,
    its bound to come out what the blower is set up for...
    but what if there is a prob with the duct that could
    have the motor operating out of its normal operating range.



    [Edited by wormy on 05-04-2005 at 07:53 PM]

  2. #2
    I would go back to Manual D and use the equations given.
    Nameplate static (IWC) is given.
    Subtract device pressure losses and you get ASP
    Friction rate = PD x 100
    --------
    TEL

    I would simply go back to basics, if possible, to find lengths of S/A and R/A duct and available static pressure.
    You will be certain what CFM you'll get and then ...yes, you must measure it if you need it to be exact and constant.

    Every time I check it on a dual or multi speed fan blower, it is within + (-) 5% and usually dead on .

    V/S settings can be trimmed nicely and I find that they are very accurate in relation to what the manufacturer claims.


  3. #3
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    Chillbilly,
    Sounds like you are talking about designing the duct system with the available static in mind.

    I was wondering more on the lines of...
    I just walked up on a system and, for instance,
    found my amperage low on my blower.
    I want to check air flow.
    I would normally get out my velometer, run it across
    the return grille and calculate CFM.
    If it was low, I end up checking the most common stuff.. i.e. dirty filter, dirty evap, dirty blower.
    If those check out ok... I use the magnahelic to find where the most resistance is.
    These steps work pretty well on a standard blower setup.

    But if I run across an ECM motor, and question the ductwork.... how would I make sure the static is in the right range? Do I just measure it and compare the nameplate? If supply static plus return static equals nameplate static... then is the blower working under ideal conditions... i.e. ducts not too small

    I feel there is something I'm missing... and dont' know how to ask the right question to get what I think I'm missing........ hmmmm I'm confusing myself hehe

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Checking the ESP,accuratly and in the right places,then read your cfm on the fan data chart,taking ALL the footnotes into account properly, is very accurate .We have tested it against a calibrated flowhood,in our "lab".Be sure to use the correct fan speed,on the chart.

    If you test flowhood,versus ESP/fan chart,in the field the difference is likely duct leakage.

    The guy your taking to is likely correct,but he's working on "built up" equipment,adjustable pulleys etc.,basicly custom built fans.It's also his goal,to determine actual cfm delivered to the space,ESP/fan chart,doesn't tell you what delivered to the space ,just what's going thru the equipment.It is only accurate,on clean blowers,coils,etc.,though you can Kinda adjust for the dirt,with experience,and it is a much quicker method.

    Flowhood tells you what's delivered to the space,but more could be going thru the coil,do to duct leakage.



    ASP X 100 / TEL = FR ,and yes that's for design,hard to work with in the field,unless the TEL is Known.

    [Edited by dash on 05-05-2005 at 09:36 AM]

  5. #5
    Join Date
    Feb 2005
    Posts
    344


    I know how to measure it... just not sure how you guys are applying it.

    One of the biggest reasons I'm asking this is on account of
    the ECM motors out there....
    If I measure the air flow with the velometer or flow hood,
    its bound to come out what the blower is set up for...
    but what if there is a prob with the duct that could
    have the motor operating out of its normal operating range.



    [Edited by wormy on 05-04-2005 at 07:53 PM] [/B][/QUOTE]


    I don't know about the rest of the OEMs but RUUD/RHEEM have an error code to indicate the blower is operating beyond the design limits. Also there may be a "huffing" sound indicating the ECM is operating at it's high limit.

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    VS or PSC,the nameplate may not indicate the ESP,that the equipment can handle,I'd look at the fan chart,notes ,etc.,to decide.

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