- #1
Nilay Gala
- 10
- 0
On what does the cfm of fan depend ? how to gain maximum cfm from a 1800 rpm fan ?
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Discussion Overview
The discussion revolves around the factors influencing the cubic feet per minute (CFM) of a fan, particularly in the context of using an 1800 RPM fan for a project involving suction to adhere to walls. Participants explore various aspects of fan design, efficiency, and application in creating sufficient suction for a wall-climbing car concept.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
- Homework-related
Main Points Raised
- Some participants suggest that longer blades, a bigger motor, and less obstruction to airflow can increase CFM.
- Others mention that CFM is a mass flow rate dependent on density, cross-sectional area, and velocity, and encourage consideration of variables within those parameters.
- One participant raises the distinction between fans and blowers based on pressure ratios, questioning whether the device in question is indeed a fan.
- A participant expresses the need to create suction with a small fan and questions if an 1800 RPM fan is adequate for this purpose.
- Another participant emphasizes the importance of analyzing the force exerted by the fan using the equation F= mdot*V, prompting further inquiry about the variables involved.
- Concerns are raised about the lack of detailed information provided by the project initiator, suggesting that more effort is needed to clarify the project requirements.
- A later reply discusses the necessity of calculating the traction needed for the wall-climbing car, including factors like the angle of the wall, coefficient of friction, and vehicle mass.
- Participants note that achieving a partial vacuum requires careful design considerations, such as the skirt area against the wall and airflow management to prevent stalling of the fan.
Areas of Agreement / Disagreement
Participants express varying views on the factors affecting CFM and the specific requirements for the wall-climbing car project. There is no consensus on the adequacy of the 1800 RPM fan or the best approach to achieve the desired suction.
Contextual Notes
Limitations include the lack of specific details about the project, such as the weight of the car, the coefficient of friction, and the design of the skirt, which are essential for accurate calculations regarding suction and traction.
- #2
Baluncore
Science Advisor
2025 Award
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Welcome to PF.
Longer blades, a bigger motor and less obstruction to airflow will give a greater cfm.
http://en.wikipedia.org/wiki/Centrifugal_fan
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.
http://en.wikipedia.org/wiki/Centrifugal_fan#Difference_between_fans_and_blowers
Longer blades, a bigger motor and less obstruction to airflow will give a greater cfm.
http://en.wikipedia.org/wiki/Centrifugal_fan
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.
http://en.wikipedia.org/wiki/Centrifugal_fan#Difference_between_fans_and_blowers
- #3
HuskyNamedNala
- 148
- 22
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.
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- #4
Nilay Gala
- 10
- 0
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
- #5
Nilay Gala
- 10
- 0
suggest me a good fan for suction to stick on the wall . i am making a project on wall climbing car
- #6
- #7
HuskyNamedNala
- 148
- 22
You need to do the analysis yourself and figure out the force exerted by the fan. Use F= mdot*V to start.
- #8
Nilay Gala
- 10
- 0
and what is m and v exactly ?
- #9
HuskyNamedNala
- 148
- 22
pick up your book and figure it out. Is this what passes for an engineering student these days?
- #10
Nilay Gala
- 10
- 0
i know that stands for mass and velocity okay . but still i thought of knowing it from you
- #11
russ_watters
Mentor
- 23,867
- 11,317
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.
- #12
Baluncore
Science Advisor
2025 Award
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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 let's 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.
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 let's 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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