The Dangers Of Compressed Air

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Re: The Dangers Of Compressed Air

Post by mistermack » Sat Aug 27, 2011 11:30 am

Woodbutcher wrote:
mistermack wrote:
Seth wrote:
Ian wrote:I accidentally blasted a small chunk of skin off my knee when I was power-washing my house a few weeks ago.

But that's a different kind of compressed air...
Er, isn't that compressed water?
You can't compress water.
You can, but it doesn't compress much.
How much? For practical purposes it's treated as an incompressible liquid.
I suppose at incredible pressures, you could measure some compression, but not for everyday situations.

I don't know how those power washers work. Do they use compressed air to pulse the flow, or just direct drive from the motor?
Last edited by mistermack on Sat Aug 27, 2011 11:57 am, edited 1 time in total.
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Re: The Dangers Of Compressed Air

Post by Gawdzilla Sama » Sat Aug 27, 2011 11:35 am

Damn little. The virtual non-compressibility of water is what makes depth charges work so well. You violently displace a volume of water and the shock wave generated travels through the water quite well.
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Re: The Dangers Of Compressed Air

Post by Seth » Sat Aug 27, 2011 8:22 pm

Okay, so "compressed water" was a technical error, but it was poetic license. It's not compressed air that blows chunks out of your flesh, it's water under high pressure, and no, pressure sprayers don't mix air, they just use a high-volume, high-pressure piston pump to boost the water pressure. And yes, the "incompressibility" of fluids like water and oil are what makes hydraulic systems work.

Sheesh. :blah:
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Re: The Dangers Of Compressed Air

Post by Gawdzilla Sama » Sat Aug 27, 2011 8:28 pm

Seth wrote:Okay :blah:
:fix:
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Re: The Dangers Of Compressed Air

Post by mistermack » Sun Aug 28, 2011 12:34 am

The reason I wondered if they used air was that I was couldn't see how they didn't wreck the pump, without any give in the system from the incompressible water. I imagine that the flexible hose must provide just enough elasticity to smooth out the forces.
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Re: The Dangers Of Compressed Air

Post by Seth » Sun Aug 28, 2011 12:39 am

mistermack wrote:The reason I wondered if they used air was that I was couldn't see how they didn't wreck the pump, without any give in the system from the incompressible water. I imagine that the flexible hose must provide just enough elasticity to smooth out the forces.
Actually, entraining air in the water stream is harmful to the pump, as it can cause cavitation and reduce lubrication, which the water provides.

The pulse issue is indeed dealt with by the hose. When the sprayer valve is off, there's a bypass circuit that recirculates water around the pump, but prolonged operation without flow can cause the water to heat up, boil, and cause cavitation.

Somebody predicted that collapsing air-bubbles in water can reach instantaneous temperatures exceeding 4000 degrees f. I'm not sure I understand or believe it, but they say that's what causes cavitation damage to speedboat propellers, if cavitation starts, the air bubbles actual melt the metal of the prop away, and can completely destroy a prop.
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Re: The Dangers Of Compressed Air

Post by Xamonas Chegwé » Sun Aug 28, 2011 12:44 am

Seth wrote:Okay, so "compressed water" was a technical error, but it was poetic license. It's not compressed air that blows chunks out of your flesh, it's water under high pressure, and no, pressure sprayers don't mix air, they just use a high-volume, high-pressure piston pump to boost the water pressure. And yes, the "incompressibility" of fluids like water and oil are what makes hydraulic systems work.

Sheesh. :blah:
Very true - and, as I understand it, water is not used in such systems only because it tends to vapourise too readily with changes of temperature, affecting the pressure of the system, not because liquid water is compressed. Hydraulic oil is far more stable within normal temperature ranges. :prof:
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Re: The Dangers Of Compressed Air

Post by Gawdzilla Sama » Sun Aug 28, 2011 12:51 am

Cavitation is an issue with submarine screws.
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Re: The Dangers Of Compressed Air

Post by mistermack » Sun Aug 28, 2011 12:54 am

I didn't think they would mix air with the water. I thought that they might use air to drive the plunger, instead of direct drive, to even out the pulses.
But the flexible hose does solve that.

The other reason I wondered about that was because they make a sound like an air pump, but I imagine that the sound comes from the rear of the plunger.
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Re: The Dangers Of Compressed Air

Post by mistermack » Sun Aug 28, 2011 1:06 am

Years ago I used to sell industrial control valves, and cavitation and flashing had to be taken into account all of the time.
They were for some pretty hairy applications like liquid oxygen etc. and I really didn't have a clue. Luckily my work was checked before it went through.
I can't even remember what's the difference between cavitation and flashing now.
It used to be a stock formula that you applied to avoid the problems, probably just a piece of software now.
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Re: The Dangers Of Compressed Air

Post by Gawdzilla Sama » Sun Aug 28, 2011 1:15 am

Cavitation, IIRC, is when pressure crushes a bubble. Flashing is when a liquid changes state to a gas when the pressure changes rapidly. vp/t
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Re: The Dangers Of Compressed Air

Post by mistermack » Sun Aug 28, 2011 1:25 am

Gawdzilla wrote:Cavitation, IIRC, is when pressure crushes a bubble. Flashing is when a liquid changes state to a gas when the pressure changes rapidly. vp/t
Makes sense. I suppose flashing comes before cavitation.
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Re: The Dangers Of Compressed Air

Post by Gawdzilla Sama » Sun Aug 28, 2011 1:38 am

mistermack wrote:
Gawdzilla wrote:Cavitation, IIRC, is when pressure crushes a bubble. Flashing is when a liquid changes state to a gas when the pressure changes rapidly. vp/t
Makes sense. I suppose flashing comes before cavitation.
I bluffed my way into that job, and managed to keep it up till I'd had enough. I don't think I killed anyone.
Put water under extreme pressure until it heats up near the boiling point for that pressure. Then remove the pressure quickly. The water flashes to steam. In cavitation the bubbles form in low pressure areas and then the pressure of the water crushes the bubble against the metal of the screw.
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Re: The Dangers Of Compressed Air

Post by mistermack » Sun Aug 28, 2011 2:05 am

Yeh. With highly volatile liquids like liquid oxygen, you can get flashing at virtually any temperature.
With control valves, you get a higher upstream pressure than downstream, and you can get flashing as the liquid passes through the variable restriction of the valve. Then you get cavitation erosion where the bubbles collapse, so everything has to be spot on with dangerous liquids.
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Re: The Dangers Of Compressed Air

Post by Gawdzilla Sama » Sun Aug 28, 2011 2:04 pm

mistermack wrote:Yeh. With highly volatile liquids like liquid oxygen, you can get flashing at virtually any temperature.
With control valves, you get a higher upstream pressure than downstream, and you can get flashing as the liquid passes through the variable restriction of the valve. Then you get cavitation erosion where the bubbles collapse, so everything has to be spot on with dangerous liquids.
I loved fluid dynamics classes. I would get totally lost the first five minutes and catch up on my sleep for the rest of the module.
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