24 votes
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How do suction cups work?

TL;DR: Its not the air that's enclosed inside that creates the force, but the lack of air - or more precisely pressure. First of all some nomenclature (this is for the closed type, there is also the ...
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  • 22.6k
18 votes
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Why was Freon used in spray cans before it was declared a threat to ozone?

Spray cans work by containing a propellant which has a boiling point a small amount below room temperature. When the can is sealed the liquid and vapour reach equilibrium at a relatively modest ...
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17 votes
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If aircraft are pressurised, why do our ears pop during liftoff and landing?

Because the cabin isn't pressurized to sea level pressure instead it to about 8k ft equivalent. (while the plane is 4.5 times higher) This means there is less differential pressure than if the cabin ...
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15 votes
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Is a circle the strongest 2D shape for containing internal pressure?

Despite being difficult to manufacture, a sphere is the best shape for a pressure vessel, but due to the manufacturing difficulties are more costly to make. From the American Society of Mechanical ...
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  • 9,462
14 votes

What is the difference between pressure and stress?

While some of these answers are close, they are (at the time this answer is written) all incorrect to some degree. Pressure and stress are very closely related -- in fact, one could argue that ...
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  • 330
13 votes
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How to calculate the pressure a pipe can withstand?

According to the ASME Process Piping Code (B31.3) $$p = \frac{2 * t * S * E}{D - 2 * t * Y}$$ where $p$ = internal pressure $t$ = wall thickness $S$ = material's tensile strength $D$ = outer ...
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13 votes

How to calculate flow rate of water through a pipe?

Laminar Flow: If the flow in the pipe is laminar, you can use the Poiseuille Equation to calculate the flow rate: $$ Q=\frac{\pi D^4 \Delta P}{128 \mu \Delta x} $$ Where $Q$ is the flow rate, $D$ ...
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  • 2,989
12 votes
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Is it useful to stuff my glasses with newspaper to protect them?

In itself putting paper inside a glass will make very little difference. Glass is a brittle material and tends to fail by shock and point loading. Its static tensile strength is actually pretty good. ...
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10 votes
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How do I calculate the weight needed on a press board to deliver a certain amount of pressure?

The obvious place to start is "what does psi mean?" It's an abbreviation for "pounds per square inch". If we have a 24" x 24" board, then we have $$24~in\times24~in=576~in²$$ To get 40psi, i.e. 40 ...
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  • 3,135
10 votes

Would engineering bricks explode if used to build an oven?

I'd be less worried about the shards and more about the structural integrity of the oven. When the bricks crack they lose most of their structural integrity. This may lead to the oven collapsing. It'...
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10 votes
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How does pressure change with depth in earth?

I guess there is no similar law for pressure in earth as it is to different, depending on where you are. But is there a rule of thumb? What do engineers who build tunnels / underground stations do? I ...
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  • 3,587
9 votes
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How to seal a pressure vessel

I have done this before so would like to share some of what I know. Super critical CO2 has some unique properties, for one it is extremely difficult to keep in a closed container since it has near ...
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9 votes
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Would engineering bricks explode if used to build an oven?

The pressure rise is easy enough to calculate from the ideal gas law $$ PV=nRT. $$ In your case $V$, $n$, and $R$ are fixed so you can convert this to a simple ratio $$ P_2=P_1\frac{T_2}{T_1}. $$ Be ...
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8 votes

How does pressure change with depth in earth?

As someone who has been involved with underground infrastructure to depths of at least 1400 metres, there are no rules of thumb. It all comes down to geology and the local conditions. Soils behave ...
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  • 9,462
8 votes

Is it useful to stuff my glasses with newspaper to protect them?

Filling inside the cup with paper is just to lock in the wrapping of the same paper around the glass. It also offers coverage and protection of the rim of the glass. Because of the integrated, all-...
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  • 21k
7 votes

Why is my Venturi pump much weaker than expected?

The problem is that the airspeed drops as soon as the airflow leaves the nozzle. It is not the same as a water-jet expelling into air (in that case the density of the water is much greater than that ...
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  • 5,930
7 votes

Can I seal the inside of a water well to use it as water reservoir when the well is dry?

Well liners are generally made to order. One of your issues will be to find a company that can make what you want. You will need to ensure the liner is the correct size and that it does not tear ...
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  • 9,462
7 votes

How do I calculate the max flow rate for a pump with no power restrictions?

This phenomenon is known as cavitation for pumping liquids - i.e. for anything that can change phase. If you are pumping gas, then the scenario really does come from choked flow. Let's say you've ...
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  • 5,303
7 votes

Water pressure in a filled sphere

$$p = p_0 + \rho g \frac{d_{sphere}}{2}$$ $p_0$ would be 1 Bar I assume, you are not specific under what conditions you fill the sphere and I assume the spheres border are made from a solid material. ...
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  • 1,704
7 votes

How metal fittings seal connection without rubber gaskets?

The common pipe threads that are used in buildings for water and gas are tapered threads. The thread is cut on the cone rather than cylinder. In the US, these threads are called National Pipe Thread ...
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  • 1,045
7 votes
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Why can soda bottles withstand so much pressure?

Is it due to the PET material that it is made of or is it more the shape and structure of the bottle? Short answer: Partially because of both. With that I mean it would be possible to produce weaker ...
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7 votes
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Units to use in this formula?

You can use any consistent set of units. That includes SI. But if you try to use a pressure in ATM, a sphere radius in feet, a wall thickness in mils, and want the stress in tons per square inch, you ...
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  • 12.5k
6 votes
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For a small valve in a fluid system, what is the relationship between pressure and flow rate?

I think the best (and simplest) way to describe something like this is Bernoulli's equation. $$P+\rho gh+\frac12 \rho v^2=constant$$ To use this, we're looking only at instantaneous velocity, ...
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6 votes

What is this pressure gauge on the London Underground measuring?

This appears to be the pressure gauge measuring the brake pressure of the brake cylinders. According to Transport For London, The gauges that the requestor is describing are the Brake Cylinder ...
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  • 1,158
6 votes

How an airfoil works

It is true that the air on the "top" (suction side) is faster than on the "bottom" (pressure side). The interesting thing is that the air on the suction side is so fast, it overtakes the air on the ...
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  • 1,202
6 votes

Turning an engine over with engine compression

There is another reason for this in multi-cylinder engines. While one cylinder is compressing the air, another is decompressing it. The net result is that the torque as a function of position gets ...
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  • 11.2k
6 votes
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Are all pressure-volume processes polytropic?

No, all thermodynamic processes are not polytropic. As an example, consider a real engine cycle. Any undergraduate engineering thermodynamics text should have a plot showing the true P-V diagram. You ...
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6 votes

Water pump at top floor or bottom?

Think about the pressures. With the pump above the supply line, the pressure at the input of the pump will be lower than the supply. At roughly 2 PSI per foot, you're down about 18 PSI. Since air ...
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  • 11.2k
6 votes
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Making a radial (only) seal inside a small tube

The idea that I was describing in the comments earlier is pretty similar to that described by Mart - I imagined the tube (red) flaring out underneath the O ring, and the opposing taper being either ...
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6 votes
Accepted

Fan performance curve

The yellow curve corresponds to a fan that will provide mostly a set pressure differential, say, keeping fumes from escaping a tank through elsewhere than dedicated vents. If there is a circumstance ...
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