The air pressure in the various parts of an air handler are different–a lesson that the ninja experiences first-hand in this comic! On the air intake side, the air pressure is negative because the fan is sucking. On the other side of the fan, the air pressure is positive and blowing outward.

So why can’t the strong ninja handle the forces on these doors? Let’s do a little math.

A typical air pressure on a fan is about 4 inches of water. To understand what this means, think of a drinking straw. When you suck on the straw and the drink rises 4 inches up into the straw from the top of the drink, you are demonstrating 4 inches of static pressure. No big deal, right? Well, let’s figure out what the force on the door is.

First, let’s convert inches of water pressure to pounds per square inch. If your straw was 28 inches long, and you sucked all the way to the top, that would be 1 pound per square inch of pressure. Divide 4 by 28 to calculate a pressure of 0.14 pounds per square inch on the door.

If the door is 2 feet by 6 feet, that is 12 square feet, or 1,728 square inches.

Multiply 0.14 pounds per square inch by 1,728 square inches and you get 242 pounds of force on the door. Not all of that is directed at the ninja–some of it goes toward the hinge on the door. The actual force at the handle would be about 120 pounds . . . or enough to knock you over if you are not prepared!

Mark Robison, PE

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