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myv65 · 02-26-2003, 05:27 PM

Average velocity is nothing more than flowrate divided by cross sectional area. A handy conversion is that 1 US gallon = 231 in^3.

1/2" tubing really has an inner diameter of 1/2", so an area of .196 in^2. 200 gph in a 1/2" tube is 200 * 231 / 3600 / .193 = 65 inches per second or ~ 5.4 feet per second. That's a little on the high side and means you would have rather high friction losses from tubing. 3/4" tubing is 2.25 times larger in area, so would have a velocity of 29 inches per second, or 2.4 feet per second.

The other factor to bear in mind is that you lose head when you change velocity. If you have a long run of tubing you'll always do better with a larger diameter. If you have a short distance between two smaller fittings, however, a larger tube may actually result in higher net losses. ie, if you had to connect two half inch fittings with a run of tubing six inches long, you'd probably have lower net losses with 1/2" tubing than 3/4" tubing simply because of the head lost in changing water velocity. Every time the water speed changes you tend to lose head to the tune of delta-V^2/2. The amount lost can't exceed V^2/2 and the percentage lost is a function of how the change is handled. Gradual changes ("funnels") have lower loss for a given area change than abrupt, sharp-cornered adapters.

02-26-2003, 05:27 PM	#8
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	Average velocity is nothing more than flowrate divided by cross sectional area. A handy conversion is that 1 US gallon = 231 in^3. 1/2" tubing really has an inner diameter of 1/2", so an area of .196 in^2. 200 gph in a 1/2" tube is 200 * 231 / 3600 / .193 = 65 inches per second or ~ 5.4 feet per second. That's a little on the high side and means you would have rather high friction losses from tubing. 3/4" tubing is 2.25 times larger in area, so would have a velocity of 29 inches per second, or 2.4 feet per second. The other factor to bear in mind is that you lose head when you change velocity. If you have a long run of tubing you'll always do better with a larger diameter. If you have a short distance between two smaller fittings, however, a larger tube may actually result in higher net losses. ie, if you had to connect two half inch fittings with a run of tubing six inches long, you'd probably have lower net losses with 1/2" tubing than 3/4" tubing simply because of the head lost in changing water velocity. Every time the water speed changes you tend to lose head to the tune of delta-V^2/2. The amount lost can't exceed V^2/2 and the percentage lost is a function of how the change is handled. Gradual changes ("funnels") have lower loss for a given area change than abrupt, sharp-cornered adapters.