Posted by **Matt** on Sunday, September 10, 2006 at 2:30pm.

Your green house irrigation sytem is plumbed with 3/4" pvc pipe from which 1/2" branch lines are routed to each row of plants. if the city supplies water that flows at 3ft/sec what is the velocity of the water at plants level? WEhat is the flow rate in gallons per min at plant level?

To determine the rate we need to know the volume/time for a given surface area. The city supplies 3ft^3/sec for 3/4" diameter pipe.

We could use a proportion

r1:A1=r2:A2 where

r1=3, A1= pi*(.75)^2, r2 is what we want, A2= pi*(.5)^2 Thus

r2=r1*A2/A1 = (3*(.75)^2)/(.5)^2 = 3*2.25=6.75 ft^3/sec

You'll need to determine the units yourself. This needs to be converted to gal/min.

Check the arithmetic and the units.

Using the continuity equation assumes all the flow is in the same line (ie, there is one supply line and one branch line). If there are n branch lines, then it becomes..

r1:A1=n*r2:A2 Normally, one would have several branch lines off of the supply line.

I wondered about that after I did it Bob. It seemed to me that branching off should increase the surface area and decrease the flow rate. I am glad that I remembered the basic equation for this kind of problem though.

Thanks for checking this.

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