Quote:
Originally Posted by Old Mechanic
I always thought that high velocity air flowing through a restriction was cooler after the restriction.
Mech
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I'm gonna go around in circles, then hopefully bring it all together.
There is something fancy called the Ideal Gas Law, which has a formula of
PV=nRT
What it does is relate Pressure, Volume, Temperature, and basically the number of little molecules bouncing around.
It creates a simple way to figure out what happens to gases when you change one of the variables, and gives a simple formula for figuring out the end result.
So. Take an air compressor. It takes air floating around us at 14.7 psi. And the compressor changes the VOLUME. It does this by sucking air into the cylinder, and compressing it to a smaller volume.
Same number of molecules, but now the volume is much smaller then it used to be. The formula says the Temperature must rise, or the Pressure must rise, or both.
Does the top of the compressor get hot? Absolutely. It gets hot BECAUSE the air has been compressed.
If We cool the air by letting it sit in the tank, it will COOL OFF, which means because the volume hasn't changed, the pressure will drop.
Likewise. If we take 100 psi air out of our air tank, and squirt it on us, it feels COOLER then the air around us. that is becuase it is cooler - the Pressure went from 100 psi to 14.7 psi, the volume expands and the temperature drops.
So, the venturi itself doesn't cause the temperature change. In order for the air to go across the venturi, there MUST be a pressure difference. If there is not a pressure difference, no air moves.
The Venturi does indeed get very cold, but that is because the PRESSURE changes across the venturi because of the pressure differential.
Harley riders and old volkswagon drivers know this because when the engines are running at about 33 degrees on a rainy, sleety day, the carb will ice up and the engine will run like crap.
The carb ices up because the carb is getting cold because there is a pressure differential at the throttle plate.