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Thread: Airflow / Temperature / Wattage table (and calculation).

  1. #1

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    Default Airflow / Temperature / Wattage table (and calculation).

    We've all seen the room volume/air flow calculation* and, hopefully, we've all realised that it's next to useless for controlling temperature when high-Wattage equipment is used in small spaces.

    To address that, I made a table which shows how much air flow is required to maintain a given temperature rise above ambient.

    It's calculated using the basic heat transfer/dissipation equation, which is a standard method for estimating the cooling requirements of hot things in small enclosures (it's often used in relation to electronic devices, for instance).

    I hope it's useful

    Short (memorable?) link to this post: http://is.gd/airflow (I made it as easy as I could!)

    If you've found this post helpful then, please, do pass that on to when people ask the age-old, "how do I work out air flow?" question.

    Short link directly to the table: http://tinyurl.com/airflow-temperature




    *The room-volume method is derived from calculations used for building ventilation. For those who don't know it, it's 20 x room_volume per hour.




    My original post is below here. I'm leaving it because, it has good keywords for the search engines and, it may be useful if people want to copy/paste the formulae but, I've removed the old tables to reduce the page size a bit.



    [edit 26-10-11]

    To make it easier for everyone I've created a table which can be used to look up lamp Wattage and temperature rise and then read off the required air flow.

    <snipped table>

    And here's one to convert CFM to m³/Hour...

    <snipped table>

    Hope it helps.

    [/edit]

    Calculating the airflow required to maintain a stable temperature, and so determine the size of fan needed, is surprisingly simple.

    It boils down to CFM = 3.16 x Watts / DT(°F)

    Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
    DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.

    So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.

    An example is:
    Ambient temerature = 20 °C
    Target temperature of the enclosure = 25 °C
    DT in °C = 25 - 20 = 5
    DT in °F = 5 x (9 / 5) = 9
    Lighting Wattage = 250 Watts

    Plugging these values into the equation gives:

    CFM = 3.16 x 250 / 9 = 87.77

    This represents the actual throughput required but it doesn't take account of the static pressure necessary to overcome the system impedance (how hard the fan has to suck or blow). But for a free-air system with no ducting or filters it should be fairly accurate.


    For those who want a bit more info on how to work it out, here it is.

    First, you'll need to know the amount of heat that needs to be dissipated.

    The general equation for heat transfer is:

    q = Cp x W x DT

    where:
    q = amount of heat transferred
    Cp = specific heat of air
    DT = allowable temperature rise within the enclosure
    W = mass flow

    Mass flow is defined as:

    W = CFM x Density

    DT is the difference between the ambient air (room) temperature and the target temperature inside the grow space in °F.

    At sea level the density of air is 1.2041 kg/m3 (at 20°C) and the specific heat capacity (under typical room conditions) is 1.006 kJ/kgC. After doing some substitution and conversion this gives:

    CFM = 3.16 x Watts / DT(°F)
    Last edited by Grub; 04-03-13 at 09:57 AM. Reason: OP requested. :)

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  3. #2

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    clever guy, but no offense but you have just made my brain take a shit!!!!!! lol

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  5. #3

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    Quote Originally Posted by gingerbread man View Post
    you have just made my brain take a shit!!!!!! lol
    Hee hee. Sorry about that.

    Maybe I should have put the easy (important?) bit (the final calculation) at the top, and then the rest until afterwards for those who wanted to know more?

    Quote Originally Posted by gingerbread man View Post
    clever guy
    Nope. (Hint: the clue is in the name.)

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    Let me guess............you are selling calculators and you are going to confuse all the poor stoners by putting up hard sums so that they buy said calculators?

    Will have to spend a bit of time working this out. Cheers Anonymiss.


    PS. Tell your brother - ANONYMOUS - I said hi!!

  8. #5

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    Quote Originally Posted by nuube View Post
    Let me guess............you are selling calculators and you are going to confuse all the poor stoners by putting up hard sums so that they buy said calculators?

    Will have to spend a bit of time working this out. Cheers Anonymiss.


    PS. Tell your brother - ANONYMOUS - I said hi!!
    I was thinking of abacuses. I think those would be far more fun... Ooooh! Look at the shiny beads!

    Seriously though, it's not that difficult (at least not now that Grub kindly edited the post for me after I realised I'd gone about it the wrong way initially), and the only bit you need is the first CFM = 3.16 x Watts / DT(°F) bit. The rest is just there for the completists really.

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    mate my brain jus turned inside out

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    Awww my brains hurt pass the bong

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  12. #8

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    ANONYMISS: could you work out mine for my i have a 1x600watt hps

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    er what about the duct lenght % per foot you lose on runs and % you lose on bends ? & % you lose in air resistance for the cf ? there online cals for this as well some where so ive read and a great artical this mth in urban gardener
    other thing which is very important is fan placement these fans are made to push air not pull so you need to always have to longer duct run on the push side

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  15. #10

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    Quote Originally Posted by stone14 View Post
    ANONYMISS: could you work out mine for my i have a 1x600watt hps
    Sure. What's your ambient (room) temeparature, and the target temperature in the grow space?

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