DIYbanter - View Single Post

wrote on 5/28/2017 10:27 AM:
On Sunday, May 28, 2017 at 9:54:50 AM UTC-4, SbzWrš›† ï¼*ï½‰ï½‡ï½ˆï½”ï½™ •¬ ï¼·ï½ï½Žï½Žï½ï½‚ï½… †’š›AJFeU wrote:
wrote on 5/28/2017 9:23 AM:
On Sunday, May 28, 2017 at 2:17:58 AM UTC-4, HJARTš›† ï¼*ï½‰ï½‡ï½ˆï½”ï½™ •¬ ï¼·ï½ï½Žï½Žï½ï½‚ï½… †’š›lBYWJ wrote:
wrote on 5/27/2017 11:08 PM:
On Saturday, May 27, 2017 at 8:28:04 PM UTC-4, dWuVxš›† ï¼*ï½‰ï½‡ï½ˆï½”ï½™ •¬ ï¼·ï½ï½Žï½Žï½ï½‚ï½… †’š›DiDrO wrote:
wrote on 5/27/2017 3:20 PM:
On Saturday, May 27, 2017 at 2:26:40 PM UTC-4, Steve W. wrote:
wrote:
If a centrifugal pump with a maximum pressure of, say, 10 psi is
supplied with water at 80 psi will the water pressure coming out of
the pump be 90 psi? I think the pressure will be 90 psi. Am I wrong?
Thanks,
Eric

IF the volume of water remains constant the pressure coming out will be
at most 80 psi. if the pump is designed to produce 10 psi. It may be
lower depending on the size of the housing and the restriction the
impeller creates. Say your input side is 2" and the pump can produce 10
psi. at zero head pressure out of a 1.5" outlet.

Feed that pump with an 80 psi head pressure and the pump won't add any
pressure because it cannot pump faster than the water is already flowing
through it.

That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids.

Water is actually compressible. The compressed volume doesn't change as
much as gas would.

"The low compressibility of non-gases, and of water in particular, leads
to their often being assumed as incompressible. The low compressibility
of water means that even in the deep oceans at 4 km depth, where
pressures are 40 MPa, there is only a 1.8% decrease in volume."

https://en.wikipedia.org/wiki/Properties_of_water#Compressibility

Thanks. I suspect that most of the people here know that. The pressure/volume relationship, though, isn't in agreement with Boyle's law. Gases approximate it. It's easy to imagine a multi-stage non-positive-displacement compressor that keeps building pressure in a material that obeys Boyle's law. It's much harder to imagine it with liquids.

The idea is the same. You use pressure to reduce the volume of a fluid.
The gas/water restores it original volume after losing the pressure.

I don't think so. Centrifugal pumps are very lossy machines. They couldn't hold pressure that way.

From a physics point of view, I think the answer lies in sorting out the kinetic aspects of a turbo pump (velocity) and the potential aspects (pressure). A turbine pump that's pumping a liquid must be producing potential energy from kinetic energy.

I'd need to see a good, expert explanation to understand it. I see no way that an ordinary turbine pump can hold the pressure generated by a previous stage, unless the entire thing is kinetic, which we're then measuring as potential energy (pressure).

The graph in the link below shows that 200-bar of pressure (2900 psi)
will compress water by about 0.7% at 4Â°C (under STP, water is 1g/cc at 4Â°C):

http://www.engineeringtoolbox.com/fluid-density-temperature-pressure-d_309.html

No turbine pump could hold that 0.7% compression.

According to you, turbo fan aircraft engines are also very lossy
machines, but most turbo fan aircraft engines use two-stage air
compression.

They are very lossy machines. The part that's of interest here is compressor efficiency, which ranges from 0.70 to 0.85 in the best turbo machinery, including stationary and aircraft gas turbines. At the high end, 85% compressor efficiency, they're losing 15% to gas friction.

But again, you're talking about a gas turbine. Gas has a close ratio between pressure and volume (Boyle's law). Compressing gas with a machine, whether it's positive or non-positive displacement, like a turbo compressor, is not a problem. An example of positive displacement types is a vane-type supercharger. An example of non-positive-displacement types is a turbocharger. Or the compressor stage of a gas turbine engine, such as an aircraft jet engine.

Liquids don't behave according to Boyle's law.

https://www.youtube.com/watch?v=_LaKlE2h3Jw

Both water and air are fluids. I am sure you can modify the aircraft
turbo fan engine design to propel a submarine underwater (without using
fuel and combustion chamber, of course).

If a "very lossy" turbo fan engine can use two stages to compress air or
water...

How do you know that a turbo fan engine can compress water?

, then why wouldn't a two-stage centrifugal pump work?

Obviously, it *does* work. The question is "how."

I suggest you watch this centrifugal pump video to clear up your brain fog:

http://www.wermac.org/video/how-does-a-centrifugal-pump-work.html