2013



Results 1 to 15 of 15
  1. #1
    Join Date
    Nov 2007
    Location
    singapore
    Age
    35
    Posts
    37
    Rep Power
    0

    Thumbs up formulae to calculate the difference in airflow


    hai friends,
    can you pls tell me the air flow (CFM) difference between the fresh air supply and exhaust air for a room is there any formulae to calculate i hope u will uncerstand my question whats my doubt is if suppose the fresh air supply for a room is 400 cfm how much should be exhaust how to calculate this anyone pls i need it very badly pls



  2. #2
    Join Date
    Mar 2008
    Location
    England
    Posts
    188
    Rep Power
    9

    Re: formulae to calculate the difference in airflow

    Dont fully understand your problem, but lets start with basics. Decide if you are trying to maintain a positive or negative pressure within the space. If this is just for general ventilation for human occupancy then you need to have some basic information as to the activity taking place within the room.
    Lets take a simple example, office with the need to provide just adequate ventilation to maintain acceptable conditions, lets say you need 4 to 6 air changes per hour.
    your room is say 10ft x 10ft and 8ft high this gives you 800 cubic feet muliply this by 6 then you need 800 x 6 = 4800 cubic feet per hour to provide the required air volume to satisfy the design criteria.Select your fan by dividing 4800 by 60 to give you 80CFM ( cubic feet per minute)
    As I said at the outset if you need a possitive pressure within the space you will need to calculate the approximate infiltration ( door frames windows etc)amount to balance input v extract air quantitiy

  3. #3
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    53
    Posts
    5,510
    Rep Power
    23

    Re: formulae to calculate the difference in airflow

    Quote Originally Posted by balaji View Post
    hai friends,
    can you pls tell me the air flow (CFM) difference between the fresh air supply and exhaust air for a room is there any formulae to calculate i hope u will uncerstand my question whats my doubt is if suppose the fresh air supply for a room is 400 cfm how much should be exhaust how to calculate this anyone pls i need it very badly pls
    Extraction = pulsion if no special requirements are needed.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  4. #4
    Join Date
    Nov 2007
    Location
    singapore
    Age
    35
    Posts
    37
    Rep Power
    0

    Re: formulae to calculate the difference in airflow

    your room is say 10ft x 10ft and 8ft high this gives you 800 cubic feet muliply this by 6 then you need 800 x 6 = 4800 cubic feet per hour to provide the required air volume to satisfy the design criteria.Select your fan by dividing 4800 by 60 to give you 80CFM ( cubic feet per minute
    thanks for your reply scott but my question is if the supply fan is 80cfm how much should be the exhaust fan to maintain positive pressure within the space is there any formula to calculate this

  5. #5
    Join Date
    Nov 2007
    Location
    singapore
    Age
    35
    Posts
    37
    Rep Power
    0

    Re: formulae to calculate the difference in airflow

    what do you mean extraction=pulsion peter can you pls brief it

  6. #6
    Join Date
    Nov 2007
    Location
    singapore
    Age
    35
    Posts
    37
    Rep Power
    0

    Thumbs up Re: formulae to calculate the difference in airflow

    Quote Originally Posted by balaji View Post
    your room is say 10ft x 10ft and 8ft high this gives you 800 cubic feet muliply this by 6 then you need 800 x 6 = 4800 cubic feet per hour to provide the required air volume to satisfy the design criteria.Select your fan by dividing 4800 by 60 to give you 80CFM ( cubic feet per minute
    thanks for your reply scott but my question is if the supply fan is 80cfm how much should be the exhaust fan to maintain positive pressure within the space is there any formula to calculate this
    to the depth of my knowledge what i feel is if suppose a room needs 400 cfm positive pressure the fresh air supply should be 800 cfm and exhaust air should be 400 cfm so that room will be maintained at 400 cfm, is it right anybody pls ill be happy if there is anyformula to get accurate results

  7. #7
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    53
    Posts
    5,510
    Rep Power
    23

    Re: formulae to calculate the difference in airflow

    Quote Originally Posted by balaji View Post
    to the depth of my knowledge what i feel is if suppose a room needs 400 cfm positive pressure the fresh air supply should be 800 cfm and exhaust air should be 400 cfm so that room will be maintained at 400 cfm, is it right anybody pls ill be happy if there is anyformula to get accurate results
    400 cfm is a mass flow, not a positive pressure.

    If there are no pressure requirements, what you blow in the room must be sucked out the room, equal amount.
    If you want positive pressure, then you must blow more than you extract.

    Vice versa if you want negative pressure.

    There are no formulas for this, it solely depends on the airtightness of the room. A room which is perfect gastight will only need a small difference between extraction and pulsion. If you have huge leaks, then the supply fan will be double or triple the size of the exhaust fan.

    If you want a precise pressure difference - avoiding dust enters the room - then you must measure the DP and send this signal to a VFD which controls the exhaust.

    You then need also some pressurized room before entering the process room but this is going beyond your question I suppose
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  8. #8
    Join Date
    Nov 2007
    Location
    singapore
    Age
    35
    Posts
    37
    Rep Power
    0

    Re: formulae to calculate the difference in airflow

    thanks for your reply peter your answer satisfies me only a little bit anyway ill workout in this matter to get accurate results

  9. #9
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    53
    Posts
    5,510
    Rep Power
    23

    Re: formulae to calculate the difference in airflow

    This is because nobody ever learned you the basics of this.
    The ppdf I gave you is the only proper way like it should be done.
    Everything starts with school and what you have learned can then be applied in the real world.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  10. #10
    Join Date
    Jan 2009
    Location
    Earth
    Posts
    76
    Rep Power
    6

    Re: formulae to calculate the difference in airflow

    Hello there Peter, if it is not much trouble can you give me or tell me where to get a copy of the airflow pdf you talking about?

    thanks and good day

  11. #11
    Join Date
    Jan 2008
    Location
    India
    Age
    60
    Posts
    178
    Rep Power
    7

    Re: formulae to calculate the difference in airflow

    If you are looking at the pressure to be maintained then you will have to calculate the leakage rate and suplement with exhaust. The leakage rate calculation methodology is given in clean room design books shall send you the details later

  12. #12
    Join Date
    Jan 2008
    Location
    India
    Age
    60
    Posts
    178
    Rep Power
    7

    Re: formulae to calculate the difference in airflow

    Dear Balaji
    Just check this site will be of help
    http://www.csgnetwork.com/airexchangecalc.html

    Supply and exhaust requirement is based on the pressurization level or if any extract hood is there in conditioned space you may have to add additional fresh air to compensate the exhaust air.

  13. #13
    Join Date
    Jun 2006
    Location
    British West Indies
    Posts
    557
    Rep Power
    9

    Re: formulae to calculate the difference in airflow

    when you are dehumidifying, aka running cooling, you want the fresh air to exceed exhaust

    you want the building to be overall positive a rule of thumb is 10% more fresh air than exhaust air

    if a general contractor cries about having to adjust door closers it is music to my ears

    do not worry about the imbalance in individual rooms supply what you have to and adjust the return to be the total supply less the fresh air portion
    Take the V out of HVAC and you are left with a HAC job

  14. #14
    Join Date
    Jun 2006
    Location
    British West Indies
    Posts
    557
    Rep Power
    9

    Re: formulae to calculate the difference in airflow

    the above advice assumes you already know the flow to the rooms to control sensible cooling and are just working out air changes as your method of determining how much of that supply air has to be fresh air

    I hope you are not working out the total airflow to the room using so many air changes, that is something that a British Quantity surveyor or Architect would do when he meddles in things that are beyond the realm of his expertise
    Take the V out of HVAC and you are left with a HAC job

  15. #15
    Join Date
    Jun 2006
    Location
    British West Indies
    Posts
    557
    Rep Power
    9

    Re: formulae to calculate the difference in airflow

    you work out the air flow to control temperature

    It is based on how much sensible cooling the space needs and how much cooler the air out of the diffusers is with respect to the room air temperature

    This air also needs to have a low enough dew point to control humidity in the room

    Most likely the supply air is a mixture of return air and fresh air that has been cooled down and dehumidified to the supply air temp and dew point that you need.

    Your return duct draws in the toal supply volume, less the amount of fresh air.

    Suppose you supplied 100 CFM to a room, and 20 CFM out of that 100 CFM was fresh air. Your return duct should 'pull' 100-20 = 80 CFM out of that room, to eventually mix with more fresh air and get cooled off again
    Take the V out of HVAC and you are left with a HAC job

Similar Threads

  1. Formula to Calculate
    By suny in forum Fundamentals
    Replies: 7
    Last Post: 09-08-2007, 02:38 AM
  2. A weird question about airflow measurement
    By Vincent Yu in forum Fundamentals
    Replies: 18
    Last Post: 16-10-2006, 10:02 AM
  3. Help to calculate Heat Exchancger
    By Carode in forum Fundamentals
    Replies: 1
    Last Post: 17-12-2003, 11:57 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •