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  1. #1

    Calculating the required CFM

    Could some one please tell me how to calculate the required CFM for a room.

    Is it Calculated like - Required CFM = (Room Volume in cubic feet)
    divided by
    (Minutes per air change in the room)

  2. #2
    Join Date
    Nov 2006
    Location
    Athens, Ohio
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    1,753
    It is not as simple as you suggest.
    It is a combination of factors. Among them are exterior wall dimensions, ceiling dimensions, insulation of those surfaces, size of windows and skylights, compass heading of the windows, # of people in the room and the typical activities they will be doing (exercising, sleeping, cooking), heat generating appliances in the room, lights (incandescent, florescent, other), floor size and insulation, forced ventilation, etc.

    It is not something which can be learned in a couple of minutes.

  3. #3
    Thank you very much for the reply kdean. I'm new to the field of HVAC and my purpose of calculating the CFM is to define it as a boundary condition to my CFD simulation.

    My CFD domain is a simple room with three people whose breathing ( Inhalation of O2 and exhalation of CO2 ) is modeled for the simulation. Now the objective of the simulation is to see the effectiveness of ventilation to the room in reducing the concentration of CO2.

    Could you please provide your suggestions on this

  4. #4
    Join Date
    Jul 2011
    Location
    East coast USA
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    961
    Quote Originally Posted by saisanthoshm88 View Post
    Thank you very much for the reply kdean. I'm new to the field of HVAC and my purpose of calculating the CFM is to define it as a boundary condition to my CFD simulation.

    My CFD domain is a simple room with three people whose breathing ( Inhalation of O2 and exhalation of CO2 ) is modeled for the simulation. Now the objective of the simulation is to see the effectiveness of ventilation to the room in reducing the concentration of CO2.

    Could you please provide your suggestions on this
    just saying...for a simulation use only and no other real life situation use, 1 cfm per sqft.

  5. #5
    Thanks for the suggestion dlove but the simulation should of course adher to a real life context so could you please suggest if some guide line is availble to calculate the required ventilation rate in CFM to a room.

    Best Regards,
    Santhosh

  6. #6
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
    Posts
    2,470
    The National Fuel Gas Codes requires .35 ACH(air changes per hour) for homes today. That equates to approximately 47cfm per 1000 sq.ft. This does not have anything to do with the amount of cfm for heating and cooling loads. This is the amount of air introduced to the duct systems from outside.
    captain CO

  7. #7
    Join Date
    Feb 2012
    Location
    Upstate SC
    Posts
    84
    For commercial space, 1.5 CFM planned infiltration per occupant is the guideline here. Not sure if that is ASHRAE driven or not.

  8. #8
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    1,753
    By your question it seems you are looking for not a CFM for heating and cooling load, but only a rate for ventilation. Is that correct?

  9. #9
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
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    Perhaps this site will help.
    http://www.windmaster.co.za/vbcalculations2.htm
    Though it is for turbine ventilators it still has calculators for ACH.

  10. #10
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    961

  11. #11
    Kdean and dlove thanks for your suggestions.

    Kdean as you mentioned it Im looking for a ventilation rate and the main focus in the simulation is to provide the room with such ventilation rate that can reduce the CO2 concentration in the room.

    The floor area of the room is 24*16 = 384 sq.ft, it's a small office room accomodating 3 people. The ASHRAE guideline says that the acceptable CO2 concentration in a room can be 1000 ppm. I have right now a CO2 concentration of 2564 ppm.

    My objective is to bring this down by providing adequate ventilation to the room.

    The room has no natural ventilation and it should absolutely relay on mechanical ventilation. Since it is an office room, Im considering 8 air exchanges per hr.

    Could you please comment on this

    Best Regards,
    Santhosh

  12. #12
    [QUOTE=saisanthoshm88;12837721]Kdean and dlove thanks for your suggestions.

    Kdean as you mentioned it Im looking for a ventilation rate and the main focus in the simulation is to provide the room with such ventilation rate that can reduce the CO2 concentration in the room.

    The aim of ventilation should be to dilute or replace indoor odors or objectionable off-gassing. in order to do this with unconditioned outside air the outside air must be "cleaner" then the indoor air.
    Next - how to determine when the indoors needs "cleaning". CO2 is not an objectionable gas. Man has decided that the measurement of CO2 can be used as a surrogate to determine the concentration of "objectionable" gasses and thus control more or less ventilation.
    Building codes or governing authorities may determine the amount of outside air to be introduced depending on the type and quantity of occupancy.

  13. #13
    Join Date
    Nov 2007
    Location
    Central Ga.
    Posts
    44
    If the task is to simply lower the CO2 to below 1000 ppm and maintain that concentration, with no regards to temperature or humidity, I think it would be rather simple. A 100% outside air variable speed fan, say 500 cfm maximum flow, and a barometric relief damper. Full fan flow for purge, once set point is achieved control the fan speed to maintain required CO2 level. There would not be a hard fast ventilation rate number ... just the minimum to achieve set point.

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