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On what does the cfm of fan depend?

  1. Feb 14, 2015 #1
    On what does the cfm of fan depend ? how to gain maximum cfm from a 1800 rpm fan ?
  2. jcsd
  3. Feb 14, 2015 #2


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    Welcome to PF.
    Longer blades, a bigger motor and less obstruction to airflow will give a greater cfm.

    Are you sure it is a fan that you are considering? The difference between a fan and a blower depends on the pressure ratio between input and output. If it generates a pressure ratio of 1.1 or less, then it is a fan. At STP a pressure ratio of 1.1 is only about 1.5 psi.
  4. Feb 15, 2015 #3
    Blade efficiency, duct design, RPMs as you said, placement in the system. Remember CFM is just a mass flow rate, which depends on density, cross sectional area, and velocity. Ask yourself what are the variables within those parameters. You could also look into actuator disk theory and some other more advanced topics to get yourself an idea of what affects what.
    Last edited by a moderator: Feb 15, 2015
  5. Feb 16, 2015 #4
    i need to create suction to stick to walls using a small fan . hence i need to know if the 1800 RPM fan is enough to create suction so that the small box or case in which fan is placed will stick to the wall . it is vacuum cleaner concept but used to stick to the wall . and for this i guess i need sufficient CFM . and i am not using a motor for this . i am using a ready made 1800 rpm fan . suggest me someting
  6. Feb 16, 2015 #5
    suggest me a good fan for suction to stick on the wall . i am making a project on wall climbing car
  7. Feb 16, 2015 #6
    see this

    Attached Files:

  8. Feb 16, 2015 #7
    You need to do the analysis yourself and figure out the force exerted by the fan. Use F= mdot*V to start.
  9. Feb 16, 2015 #8
    and what is m and v exactly ?
  10. Feb 16, 2015 #9
    pick up your book and figure it out. Is this what passes for an engineering student these days?
  11. Feb 16, 2015 #10
    i know that stands for mass and velocity okay . but still i thought of knowing it from you
  12. Feb 17, 2015 #11


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    Staff: Mentor

    You've given us virtually no useful information about what you are trying to do, so even if we wanted to do your project for you, we couldn't. Please put some actual effort into this. We'll help more when we see you put more effort in.
  13. Feb 17, 2015 #12


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    @ Nilay Gala. The problem is really not maximum cfm. It is getting traction from the wheels against the wall. In post #6 you show a sloping wall.
    What is the maximum angle of the wall the car will climb?

    Consider a vertical wall. The car will slide down the wall unless the wheels have traction. What is the coefficient of friction between the wheels and the wall? What does the car weigh? From that you can calculate the minimum force needed to hold the wheels against the wall.

    That wall force results from the partial vacuum between the car and the wall. To have a partial vacuum requires a skirt to prevent high cfm and loss of partial vacuum. What is the area of the skirt against the wall? That lets you calculate the air pressure reduction needed to press the wheels onto the wall.

    If the skirt touches the wall there will be friction that prevents the car moving, but it will need a low cfm to hold on. If the gap between skirt and wall is too great, there will not be sufficient pressure for the wheels to grip. The gap must be selected to keep some flow through the fan because without airflow over the blade airfoil it will stall and not generate the pressure difference needed.

    So before any calculation can be made, you must find the total vehicle mass, the wheel to wall friction coefficient and the car's skirt area.
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