How to size a pressure tank

A pressure tank is used to prevent a pump from cycling (switching on and off frequently) for a system with frequent demand. Such systems are common in small residental or commercial applications for example laundary water supply. A pressure tank simply uses air pressure to keep the pressure in the storage tank at a desirable level for the demand. So, the supply pump will only have to run when the tank is at a minimum level. For the most of the time after the initial pump-up, the pump remains off and the charged (pressurized) air pocket (which is now at a higher pressure than the pre-charge pressure) does the work. I tried to illustrate the idea in the following diagrams:

Now compare the above system to the system with a pressure tank installed between the supply and demand:

By drawing the graph I wanted to show how air pressure maintains water supply preventing the pump to have to cut in frequently. To illustrate how this works, consider the following diagram:

Since this post is titled "how to size a pressure tank", I also illustrated, in the note below, the fundamentals behind the commonly used "acceptance factor" when sizing a commercial pressure tank:



How to design a weld

Fillet weld is a common type of weld. In this post I will calculate the length of longitudinal weld of a fillet weld used to connect a tension member to a gusset plate:

Step 1: Analyse the transverse weld

Step 2: Analyse the longitudinal weld

Step 3: Draw and label the weld

How to solve the three reservoir problem

The three reservoir problem is a useful example to solve for common problems faced in a flow system e.g. determining flow diversion from a critical pipeline to a drain line.

How to design a pipe for waterhammer

Just a simple calculation to determine peak head due to waterhammer in a pipeline downstream of a reservoir. The calculation is for a sudden valve closure downstream of the pipeline.

The file can be downloaded for free:

http://www.scribd.com/doc/129330866/design-for-waterhammer

Helicopter needs that tail!

Why is it that a helicopter has a tail with a propeller on ONE side? Humans like symmetry, and helicopter's odd tail does not make sense.

Humans are emotional beings and tend to forget the laws of nature, more specifically the laws of physics. A quick refresher on Newton's Third Law of Motion:

"For every action there is an equal and opposite reaction" 

A helicopter flies up with a huge propeller mounted on top it that is powered by a huge motor. When this huge motor rotates (clockwise) it creates an equal and opposite rotating force (counterclockwise) that is called torque. Keeping this thought, lets now consider the small propeller that is oddly installed on one side of Helicopter's tail. Naturally, this propeller creates force in the opposite direction due to the wind draft it creates. 

Without the tail propeller the helicopter would spin due to the counter torque of the motor! the tail controls this spin, keeps the helicopter pointed in the same direction. The tail propeller is also used to turn the helicopter by adjusting the speed of propeller. 

Taken from http://www.start-flying.com

 

How a bicycle works

Do you know how to ride a bicycle and do not know why you can do it? A widely accepted theory behind bicycle stability is the gyroscopic effect. David E. H. Jones went on a journey to find out the truth. In his paper “The stability of the bicycle” published in 1970, he proved that gyroscopic effect is not the only ingredient to the magic. Here he is on the experimental bike with a counter rotating wheel: 

 

He added an extra counter-rotating front wheel to cancel the gyroscopic effect and according to him the bicycle stayed stable even at lower feels. The only difference was that it felt a bit strange riding it due to a large moment of inertia about the front forks.

So what makes a bicycle going without falling over? “On the stability of a bicycle on rollers” by P.A.Cleary and P.Mohazzabi does an excellent job of summarizing the “Trail” effect that contributes to the stability of a bicycle. I drew free body diagram of the tire patch to demonstrate the key idea:

Imagine you are riding a bicycle with its front wheel configured for positive trail. Now imagine leaning to your right. Can you picture what would happen to the wheel? Remember that the wheel can only rotate about the steering axis. So if you were leaning on the right, the ground will push your wheel to the right about the steering axis i.e. the wheel turns with the right lean of the bicycle. How does this help in stability? It lowers the center of gravity which adds stability. If you also add centrifugal force acting on you when making a right turn/lean, the bike is being pushed to left while the ground forces the wheel into your turn or lean. This is what gives you the confidence when going around a corner on your bicycle, centrifugal force and the trail.

Now imagine you are riding a bicycle with its front wheel configured for negative trail. Imagine leaning to your right. And as we know the wheel can only rotate about its steering axis, the point of contact with the ground will push the wheel to the left when you want to lean to the right; ladies and gentleman, this is a recipe of a face plant. 

I hope now you know what it takes to ride a bike: a positive trail, centrifugal force from god, and a helmet.

How a train goes around a corner

Of course trains can go around a corner! we will find out how in a bit. Previously I posted a video about car differentials. Why do we need a car differential? to allow the car to go around a corner i.e. during a turn let the outer wheel rotate more than the inside wheel. 

BUT how about trains? how do they go around a corner? The answer is NOT a differential, ever look closely at a train axle? Its a solid shaft going through the two wheels. There is no differential in between like your car. The simple yet not so intuitive secret is the slightly coned shaped wheels.Watch the Richard Feynman interview and you will know why coned shaped wheels work.

If we love cars how come we didn't know this?

Majority of students in my 3rd year mechanical engineering class asked this question, we all started doubting our love for cars. Prof. Cleghorn showed this video in class and this was the best tutorial a class had ever seen. If you drive a car or have a slightest hint of love for it you owe it to yourself to watch this video

The Daily Engineer

Did you learn how to ride a bike and don't know how you did it? I asked this question a few years ago. Now as a growing engineer I see answers to this and many other questions. 

In The Daily Engineer, I will post interesting article on not just how things works but also why they work. Through out, you will learn some physics and general knowledge of machines. 


Many of us fail to understand that nature drives everything, even machines. To understand nature, her laws and her acts of wonder is the satisfaction we all sometimes seek and indeed “the noblest pleasure is the joy of understanding”, Leonardo da Vinci. So if you are interested in getting to know about my hunt for answers and perhaps sharing yours, stay tuned!