## Archive for December, 2011

### Cell Tower

Wednesday, December 28th, 2011

While walking around on roofs doing field work, I have encountered lots of cell towers. The towers always have warning signs posted on them, telling you to not walk in front of them because of the huge amounts of energy at the face of the antenna. The radiation to the side and behind the antenna is significantly less than what comes out the front. When walking around on the roof, you must be careful to not get cooked.

In this comic, our ninjas are using the cell tower energy to roast their marshmallows. Their marshmallows will end up microwaved, and not toasted, but that’s the best they can do on a roof without starting a fire.

Merry Christmas from MEP Ninja!

Mark Robison, PE

### Fan Pressure

Wednesday, December 14th, 2011

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