Re: air flow

Jeff -

> In my kitchen, I have an exhaust hood over my cooktop. There
> is an 8 inch diameter duct running from the hood to the
> outside wall. Does it make any difference in air flow to
> place the fan in the hood (pushing the air though the duct)
> or on the outside wall (pulling the air through the duct)?

It would make a little difference.

Fans are usually installed in the hood for at least a few good
reasons:

-       The air moves faster the closer it is to the fan.  Since
        the primary reason for fan and hood is to pull airbourne
        material away from the stove top quickly, this can be
        accomplished more efficiently if the fan is closer to
        the stove top.

-       The hood filter is more effective when it is next to the
        fan.  If the filter were in the hood and the fan on the
        outside wall, the material in the air (grease, etc.)
        would not make it up to the filter and would fall back
        down on the stove top... at best this is a mess; at worst,
        it invites a grease fire.  (Keep that baking soda near by!)

-       Outdoor airflow (wind blowing by) passing the outside
        vent to the duct will provide a natural pull on the
        warm air inside the duct.  Since there is already a
        "pull" on the air in the duct, the fan in the hood will
        provide an additional "push", which is also more efficient.

This whole business with air flow was studied heavily by a man
named Daniel Bernoulli, a Swiss scientist who lived from 1700 to
1782.  Basically, the concept is that there are two types of
pressure: static (stationary) pressure, and dynamic (moving)
pressure.  If one is high, the other is typically low, and air
(and other stuff) tends to move in the direction of the pressure,
that is, from high to low.

An example of this is when air blows across the opening of the top
of a soda straw, the air in the straw will be pulled up, and might
even pull the soda in the straw up with it!

I hope this helps you out. :)

Thanks for your question!