24
votes
Accepted
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 ...
- 24.2k
19
votes
Accepted
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 ...
- 5,751
18
votes
Accepted
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 ...
- 15.1k
14
votes
Steel expanding in a tight space
The cylinder would expand until constrained by the cylindrical hole. After that it depends on the relative strengths. If the block has thin walls it may be stretched by the expanding cylinder and ...
- 10k
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, $D$ ...
- 3,044
12
votes
Accepted
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.
...
- 15.1k
10
votes
Accepted
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 ...
- 3,587
10
votes
Do I need a pressure relief/safety valve if my pump can't exceed the rated system pressure?
Not a plumbing engineer, but:
What about thermal expansion?
You reach a dead end, pump stops. Pumps often have a checkvalve that prevents backflow, so you have now created a pressurized container.
...
- 109
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 ...
- 9,582
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-...
- 21.9k
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.
...
- 1,734
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 ...
- 1,035
7
votes
Accepted
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 ...
- 236
7
votes
Accepted
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 ...
- 12.5k
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 ...
- 1,192
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 ...
- 11.4k
6
votes
Accepted
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 ...
- 986
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 ...
- 11.4k
6
votes
Accepted
How do gas flow regulators avoid choking?
The short answer is that they don't. Choking is not a bad thing. For a gas flow regulator, it's actually a good thing, because under choking conditions, the mass-flow rate is no longer dependent on ...
6
votes
Accepted
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 ...
- 7,662
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 ...
- 6,125
6
votes
Accepted
Steel expanding in a tight space
Assuming your hypotethical question as what it would be for a difference of 200 degrees celsius and assuming the hole as rigid.
The volumetric expansion index of steel is
$$ \text{ $9*10^{-6}$ - per 1°...
- 21.9k
5
votes
Accepted
Calculating maximum internal pressure from maximum rated external pressure?
Vacuum pressure on a vessel is quite different than positive pressure.
Under positive pressure, the shell of the vessel is essentially under a uniform membrane tension due to the shell wanting to ...
- 3,587
5
votes
Would engineering bricks explode if used to build an oven?
The bricks themselves should be fine, at least for one cycle, IF they are heated reasonably slowly. Plenty of people have put bricks in barbecues and bonfires. Remember they were already fired in a ...
- 426
5
votes
How metal fittings seal connection without rubber gaskets?
In addition to tapered threads for creating seals, there are Compression Fittings which use a matching male and female mating low angle taper to create a sealing surface. When properly torqued, they ...
- 1,578
5
votes
Accepted
How do temperature and pressure influence the rate of evaporation of water in a closed vessel?
$\frac{1}{22.4}\frac{\mathrm{mol}}{\mathrm{L}}$ is only valid for standard conditions. Use the ideal gas law to calculate the concentration for your conditions.
$s \equiv$ standard conditions
$2 \...
- 1,734
5
votes
Pipe-network pressure - mass flow relation
Actually the continuity equation means that at steady state, the mass flowrate in a pipe must be constant (as mass cannot be created or destroyed not volume) but the volume can vary.
so:
...
- 163
5
votes
Small bubbles on a pump discharge
I have noticed that on the discharge port of the pump, a short row of bubbles comes out at high flows and low pressures.
This does indeed sound like cavitation to me.
Cavitation occurs usually in ...
- 3,587
5
votes
Accepted
Small bubbles on a pump discharge
As well as 'proper' cavitation which is a fairly high energy phenomena and certainly a concern in terms of wear on the working surfaces of the pump it is possible that pressure changes are causing ...
- 15.1k
5
votes
Accepted
A latch that’s easy to open under lateral pressure
How about a levered latch like this (very rough sketch) showing the catching tooth above the self latching spring and the pivot point.
- 15.3k
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