Where are you in the world? (ie what Building Codes do you follow?). In Canada we use 0.75cfm/ft2 for underground garages as a minimum (from our National Code). In the US I've heard up to 1.5cfm/ft2 from some of the model codes (don't quote me on the US number as I've only heard it through these forums).

 

i am from ontario.. and yea i read some where it is 0.5cfm/sqft. but that is not the problem now i couldn't find a way to size the motor of the exhaust fan based on that number.. any ideas? thank you Chris ,really appreciated  

Don't know if this helps you...
http://www.engineeringtoolbox.com/garage-ventilation-d_1017.html

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

You are dealing with a first order differential equation which if you are a degreed ME know about when you took differential equations in college.  The general solution is:
  C2=G\Q-(G/Q-C1)exp(-Q/V(t2-t1))
  whereby C2 and C1 are the final and initial    concentrations (mg/cu.m.)of CO; V is the volume space of the garage;G is the generation rate(mg/min) of CO and make that value constant; Q is the flow rate (cu.m/min) of fresh air to dilute CO to a C2 concentration.
With the general equation, you can work out at least three separate cases: 1) C1=0 at t=0,G>0 2)t>>t1,G>0 3)G=0.
For your situation use case 1) to calculate Q.   

The next thing is to calculate is the so-called "system loss" curve that is the pressure drops under various flow rates thru the air intake duct up to and thru the exhaust duct up to the fan. Those values when plotted should approximate a second degree(parabolic) curve.  Note that if there are no ducting and wide openings thru  garage doors and walls, fan is mounted on the outside wall and there are no air flow restriction, you could base you fan flow rate on free air delivery.

Then select a fan whose characteristic curve provided by the manufacturer can then be plotted on the same graph sheet used in the "system loss" curve and where the two curves intersect will provide you a flow rate for  a particular pressure drop for that garage.  

Note that the fan static pressures will use the same graph sheet axis as that of the pressure drops mentioned above; units are to be consistent  

You should allow a 10% leeway in each side of the intersecting curves for any irregularities in the fan speed or partial obstruction of the air flow.

The fan horsepower should be provided by the fan manufacturer or distributor, however it can be easily calculated as you have the flow rate and static pressure value at the point of intersection. The formula can be referenced on the internet or engineering handbook.

Wow, to size the motor I would just select the fan from a manufacturer's catalogue.

If you give me the airflow and approximate static on the fan I'll run a selection for you.

Or you can go the first order differential equation route.

thank you guys...well i figured that out i didn't want it to get this complicated so i used a similar fan manufacturer's catalogue and compared the specs (since we already have the motor and fan running 24/7) also i am using another fan in a different building to compare it as well. my aim was to check first if we have an oversize case and to calculate the operating time for it in order to determine the consumption of electricity with and without CO sensors.so now i have the flow rate of the fan and now i am working CO sensors case.. so do you think the way i calculated it is right ???
thank you again for the help i really appreciate your help guys..