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Old 01-21-2013, 11:46 AM   #187
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Join Date: Sep 2012
Location: Louisville, KY
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My Ride: 2004 325Ci vert
Originally Posted by mkodama View Post
False. You talk about and explain NPSHA(strangely similar to how engineering toolbox and wikipedia do), but you don't calculate it or even estimate it in the case of an E46, or any car for that matter, so I'm not sure how you come to that conclusion. Further disproving your claim, there are many examples of water cavitating at room temperature. Just look on youtube...
My source was actually Cameron Hydraulic Data, published by Ingersoll Rand in 1981, which pre-dates both engineering tooxbox and wikipedia, although I use those resources as well.

I don't do any calculations because I don't have enough information. I don't have the pump curves for the water pump, which would tell me what the pump suction head requirement is at different speeds and flow rates. I don't know the vapor pressure of a 50/50 mix of coolant and water at different temperatures. I don't know the pressure loss through the cooling system at different flow rates. I'm speaking in generalities.

I'm not saying that pumps cannot cavitate at room temperature. As you point out, they can. I'm just pointing out that the head available to the pump is dependent on the system pressure and the vapor pressure of the fluid being pumped in a closed system such as an engine cooling system. The other factors, lift and line losses also have an impact, and these can cause a pump to cavitate at room temperature and pressure if the wrong pump is selected for the application.

That being said, I don't think that the coolant in a closed system would ever boil. As the temperature of the water goes up, it's the vapor pressure of the water at that temperature that raises the pressure. As long as everything stays together, the pressure will continue to rise with temperature, and the water will remain water. It's when the relief valve opens, allowing the system pressure to drop, that the water will boil and steam will be created. This is because the system pressure is now below the vapor pressure of the water.

My thinking, simple as it may be, is that when you fill a cooling system at 70F and close it up, the system pressure is 14.7 psia vs. the vapor pressure of 0.36 psia. This is a 14.3 psi difference (33 feet of head, which is a lot for a pump) that will be maintained between the system pressure and the vapor pressure as the water heats up. The air space in the expansion tank allows the air to compress to maintain that pressure differential.

So as long as the pump's suction head requirements are met by this differential (after subtracting the flow losses), the pump will not cavitate.

As always, I'm open to feedback on this.
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I just don't know what I'd do with 560 hp that doesn't involve getting arrested.
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