While it might not be the pressure
of needing to hit the game winning shot with 0 seconds left in the championship
game, our exhaust fans are under a different pressure to perform – Static
Pressure.
The dictionary defines it as: “The
force per unit area that is exerted by a fluid upon a surface at rest relative
to the fluid”. If you are like most
people, that doesn’t really clear it up –
especially when it comes to ventilation. I thought we are moving air not fluid?
The very basic definition is how much pressure is exerted to move a given
amount of fluid a defined distance. For airflow (ventilation measured in Cubic
Feet per Minute – CFM) that means how much CFM is needed. For instance a fan
might move 100 CFM at 0”
of static pressure. When the static pressure is increased to 0.1” the same fan might only move 90 CFM.
Okay, the advanced
physics lesson is over, lets take a look at what all this means. It can be
summed up as, the higher the static pressure, the harder the fan needs to work.
When it comes to exhaust fans, there are many contributing factors that can
negatively affect the static pressure of the system. As you are determining
what type/size exhaust fan you will need, these are some of the factors that
need to be taken into account:
Duct size: This is a
case of bigger is better. The wider the duct, the easier it is for air to flow
through it. Think of one of those small coffee-stirring straws. Try to blow as
much air as you can through it. Have you turned red yet? Now take one of those
large milkshake straws and exert the same amount of energy blowing air through
it. Not even a comparison. This is exactly what is happening in your system
when you use smaller diameter ducting. The smaller the duct size, the more force
you will need to overcome the static pressure created. This is multiplied if
you are reducing the size of the duct coming from the fan to match the rest of
the system. For instance going from 6”
to 4” will have a big impact.
Duct Length: Lets stay
with our straw illustration. Take 20 straws and put them together. You will
notice you can still get some air through them, but once again, it takes a lot
more effort. A shorter duct length will create less static pressure.
Twists and Turns: Each
time you add an elbow or turn in the ductwork, you add more static pressure. Its
been working so far, so lets stay with the straw. Now take the straw and bend
it into 10 different twists and turns and try to blow air trough it. Each time
you add a twist, it will get harder to move air through it.
Great, we now know about Static
Pressure, but what’s the big deal. As we
learn more about indoor air quality and the benefits of proper ventilation, it
is vital that the systems are working as expected. As we saw in the above
example if the home calls for 100 CFM of ventilation but because of the static
pressure in the system, we are only getting 90 CFM or worse, we have an issue.
In extreme cases you might only be getting a fraction of the airflow you
thought. This can lead to moisture issues like mold and mildew.
To learn more
about ventilation, visit www.airkinglimited.com
