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

  1. #11

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    28-30oC room temp....
    target low 20's-26max (there oC)
    atm cant get below 34oC 2foot away


    (im drawing in cooler air from room below, sorry dont no the temps but im guessing still mid 20's

  2. #12

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    i want my lamp to be cool or just warm to the touch

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  4. #13
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    OK going to be cheeky like stone14.

    I have a shed ambient temps are around 15*c the grow tent will have a 400w bulb looking for high 20's temps. The grow will continue til christmas when the temps in the shed will be about 5c or so. I know I am been cheeky but if you show me how to work that calculation out I could do the others myself.


    Thanks in advance.

  5. #14

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    Quote Originally Posted by G-Man View Post
    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
    The calculation provided above will only tell you how much airflow you need to achieve in order to dissipate a given amount of heat.

    System impedance will always be a factor when it comes to fan selection, and to work it out in detail you need to know the impedance curve of the system in question and then relate that to the static-pressure:flow-rate curve of the fan.

    This graph shows some (imaginary) system-impedance/flow-rate/pressure curves. Find the desired flow rate on the bottom, go up until you hit the relevant impedance curve, and then go left to determine the static pressure required to achieve that flow rate. (This is for illustration purposes and so doesn't have any actual values attached.)



    From this, it's obvious that a higher impedance system will need a higher pressure to achieve a given flow rate.

    This next graph shows the pressure:flow-rate curve for a typical (but imaginary) fan. (Accurate data for specific fans should be available from the manufacturers of those fans.)



    This graph shows the throughput a given (imaginary) fan will achieve through systems of differing impedance. The intersections between the fan curve and the impedance curves will give the flow rate and pressure for this particular fan in those systems.




    Although it's possible to calculate the impedance (losses) caused by ductwork it is fairly complicated. It's definitely not impossibly difficulty though, but I haven't spent much time on it yet. If (when?) I do then I'll post a short how-to, or at least some rough correction factors.

  6. #15

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    Quote Originally Posted by stone14 View Post
    28-30oC room temp....
    target low 20's-26max (there oC)
    atm cant get below 34oC 2foot away


    (im drawing in cooler air from room below, sorry dont no the temps but im guessing still mid 20's
    You'll never be able to get the temperature below that of the incoming air, regardless of air flow rate.

    Assuming that the incoming air is at 22°C and the target temperature is 25°C then to dissipate 600W of heat you'll need a flow rate of 351 CFM. The lamp envelope itself will still be hot though.

    Incoming air = 22°C
    Desired temperature = 25 °C
    DT = 25 - 22 = 3 °C
    DT °F = 3 x 9 / 5 = 5.4

    3.16 x 600 / 5.4 = 351.1 CFM
    Last edited by Anonymiss; 28-06-11 at 03:24 PM.

  7. #16

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    ok cheers for that, my 5" ruck suck 395m3/hr and still temps are 34+ will seal off all my passive and just intake from down stairs, also my cf must be restricting my air flow, i also had 4 90degree bends in my ducting before it reaches the fan

  8. #17

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    Quote Originally Posted by nuube View Post
    OK going to be cheeky like stone14.

    I have a shed ambient temps are around 15*c the grow tent will have a 400w bulb looking for high 20's temps. The grow will continue til christmas when the temps in the shed will be about 5c or so. I know I am been cheeky but if you show me how to work that calculation out I could do the others myself.


    Thanks in advance.
    You have to plan for the worst case (i.e. the warmest incoming air), so I'll do the calculation for 15 °C ambient. When it's colder you'll need to reduce the fan speed to compensate.

    At 15 °C ambient:
    Incoming air = 15°C
    Desired temperature = 28 °C
    DT = 28 - 15 = 13 °C
    DT °F = 13 x 9 / 5 = 23.4

    3.16 x 400 / 23.4 = 54 CFM

    And at 5 °C ambient:
    Incoming air = 5°C
    Desired temperature = 28 °C
    DT = 28 - 5 = 23 °C
    DT °F = 23 x 9 / 5 = 41.4

    3.16 x 400 / 23.4 = 30.5 CFM

    The relationship between fan RPM and CFM is linear. So if a fan running at 100% gives 54 CFM then reducing it to about 60% will give 32 CFM.

  9. #18

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    Quote Originally Posted by stone14 View Post
    ok cheers for that, my 5" ruck suck 395m3/hr and still temps are 34+ will seal off all my passive and just intake from down stairs, also my cf must be restricting my air flow, i also had 4 90degree bends in my ducting before it reaches the fan
    395 m³/hour is only 232 CFM

    To convert CFM to m³/hour to CFM multiply by 1.7
    To convert m³/hour to CFM divide by 1.7

    351 CFM in m³/hour = 351 x 1.7 = 596.7 m³/hour

    You definitely need a bigger fan!

    And yes, ducting and especially a carbon filter will increase the impedance and so reduce the airflow.
    Last edited by Anonymiss; 28-06-11 at 03:44 PM.

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  11. #19

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    Quote Originally Posted by Anonymiss View Post
    395 m³/hr is only 232 CFM

    To convert CFM to m³ to CFM multiply by 1.7
    To convert m³ to CFM divide by 1.7

    351 CFM in m³/hour = 351 x 1.7 = 596.7 m³/hour

    You definitely need a bigger fan!

    And yes, ducting and especially a carbon filter will increase the impedance and so reduce the airflow.
    just too much mathes for me to work out, lol but thanks for doing it for me. ahh so i do need a biger fan, so i will have to buy a new 8" 760m3/hr fan, but my cf has a max of 480m3/hr..... will still be ok or wil i have to buy new cf also ?

  12. #20

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    thanks for the maths, guess i will still have to wait till end aug then, cant afford new fan and filter ...................

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