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"Parallel" Pump Head Calculation

"Parallel" Pump Head Calculation

"Parallel" Pump Head Calculation

(OP)
This is kind of hard to put into words.  I wish I could attach a sketch, but i couldn't figure out how.  

I have a long primary chilled water loop (maybe 1000' total) with a 400 ton chiller as the source.  The primary pump is at one end of the loop and the chilelr at at the other.  The secondary loops draw off the primary near the pump.    

Out near the chiller, I want to draw a small amount of water (30 gpm) from the loop.  Although relatively small, I hesitate to make this a secondary loop, because the return from this new loop would discharge back into the supply of the existing secondary loop.

Instead, I want to put my new pump essentially in parallel with the existing primary.  So now, my new pump's suction is on the suction side of the existing pump and it's discharge is on the discharge side of the existing - albiet way down the line no where near the existing primary pump.

So, in my head calculation, Do I figure the entire run through the chiller and my new loop as though it were it's own separate primary?  Or do I just figure my new piping and not the chiller?  Or, do I need to figure enough head to "fight" the existing pump that I am in parallel with?  

    

   

RE: "Parallel" Pump Head Calculation

You're right.  I am a bit unclear on the arrangement.

Do like this,

I post images to the host site called Image Shack at,
www.imageshack.us

Just browse your computer for your diagram and upload it.

Image Shack will return a number of ways to link the picture you just uploaded, so it gets posted to other sites, such as this option here,

[url=http://imageshack.us][img=http://img70.imageshack.us/img70/5969/waterfallne9.jpg][/url]

Unfortunately that doesn't work on Eng-Tips, so you have to edit that link by removing EVERYTHING I have indicated in red here,


[url htp://imageshack.us]

[img=http://img70.imageshack.us/img70/5969/waterfallne9.jpg][/url]

Once you do that, preview your intended post to make sure you got it right, then you will get an autolink to the drawing like this one,

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: "Parallel" Pump Head Calculation

You must include the whole new circuit, from the delivery of your new pump, all the way around the new circuit plus the chiller and any portion of the existing primary circuit that is part of your new circuit, back to the suction of the new pump.

The complication is that in calculating the pressure drop through the common part of the existing circuit and the chiller you need to take the combined flowrate from the existing pump plus your new pump into account.

Luckily for you your new circuit will probably add very little to the existing flowrate so you can simply stick in a couple of pressure gauges where you will make the suction and delivery tie-ins and measure the actual pressure drop. I guess your existing flowrate is in the order of 700 gpm so adding 30 gpm will make a small difference. If you assume the pressure drop through the common part of the circuit will increase with the square of the flowrate then your final pressure drop will just be (730/700)2 times the measured pressure drop. Add this to the calculated pressure drop through your new piping with the 30 gpm and you have the required head for your new pump

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

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