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Conversion formula needed

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Member Posts: 232
So,
If I have a 7 psi pressure differential across my pump, I can multiply that by 2.31 to get feet of head and then go to pump curve and figure flow? No matter what temp and density my fluid is? I guess I just somehow figured glycol concentration and temp of fluid would somehow change that "2.31" number.
Thanks for the info,
Rocky

• Member Posts: 232
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Where can I find info for figuring out how many feet of head psi differentials equate to for fluids of varying temps and glycol levels? I know 2.31ft/psi for pure water at 60 degrees, or something like that. But if I have a system with , say, 7 psi diference across a circuit setter, with the fluid at 180 degrees with 45% glycol, how do i figure ft of head with those variables? Any info or a place to look would be great, or the formula for figuring.

thanks alot,
Rocky
• Member Posts: 1,790
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Unit conversion

Glycol and temperature affect the fluid properties, but if you are reading 7 psi on a gauge it makes no difference if the fluid is water, gasoline, or molasses, which I believe may be the key ingredients in Jaegermeister.

If you are calculating the pressure drop during design, the fluid properties do make a difference. The Grundfos HVAC Guide has tables for different temperatures and glycol concentrations. The slickest way to go is to use Siggy's HDS.
• Member Posts: 53
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Actually the weight of the fluid does matter

Feet of head to PSI using the 2.31 feet per PSI or 0.433 PSI per foot applies to water. If molasses it weighs more, as would gasoline, or for more contrast, mercury; any other substance of different density would impose a different weight for a given column height.

I sort of see your point and take it you are half-kidding.
• Member Posts: 1,790
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closed or open?

I was thinking of closed loop head loss, rather than an open loop based on the circuit setter comment.
• Member Posts: 53
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Got it

I see you are talking dynamic losses, not static head, which you will agree does differ based on the fluid used.

Still, the PD (and HP) will vary with temperature, fluid density and viscosity across different fluids aside from water in a given system of the same height. This is another reason why I like Siggy's software.

Cheers!

• Member Posts: 1,790
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Yep

Psi is psi, no matter the fluid.

If you had equal columns of glycol and water with a gauge at the bottom of each, they would read different pressures. The conversion of each of those pressures to feet is still the reading in psi * 2.31.

• Member Posts: 1,405
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Nope

You have to adjust the factor to convert from psid to feet of head for any fluid other than water at 60 F. The ASHRAE Applications Handbook chapter on testing, adjusting and balancing covers this topic very well. I scanned and attached a table of conversion factors from the 1999 edition.
• Member Posts: 1,790
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I give up ;-)
• Member Posts: 1,405
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Full explanation

After I posted the table I realized that I have the whole handbook on CD so I pdf'd the three pages that provide a full explanation.
• Member Posts: 1,405
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Oops

I forgot to check it before I posted it. This one is good.
• Member Posts: 232
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Thanks to all who replied

I downloaded the PFD file and printed it for future reference. Thanks again to all who took time to respond.
Regards,
Rocky
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