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propylene glycol correction

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mgmasc

mgmasc

Mechanical
Aug 31, 2008
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Our testing found the the freezing point of the prop/water mixture was -15F. From the chart found on the net below the mixture is approximately 46% prop/ 54% water

Freezing Point
Propylene Glycol Solution
(% by mass) 30 40 50 60
Temperature 7 -8 -29 -55


The operating temperature of the mixture is 45 deg F. The application is an air cooled chiller. The system as installed did not have a flow measuring device installed in the piping. During a recent shutdown a nexus twin-tube pitot
was installed in a nice straight run of 3" pipe.

Our testing indicated that the pressure drop across the twin tube was 8" wc. In the flow chart provided in the link below the flow for a 3" twin tube w/ an 8" delta P is 100 GPM based on water at 60 deg F.

Question:
1)What is the correction factor for the flow rate of mixture based on the data given above?

2) If there is a "layman's version" of the criteria that are used to develop the correction factor, I'd be interesting in hearing it.

Thanks

Mark
 
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FYI, the chart that I have for PG/W show roughly a 50% increase in viscosity between 60ºF and 45ºF.

It's from Lyondell, labelled "Viscosity of Aqueous Propylene Glycol Solutions."

TTFN

FAQ731-376
 
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I'd look at density, not viscosity. The pitot tube equation only has density as a factor:

V = (2 * DP / rho)^.5

V is velocity pressure, rho is density.

It looks like your mix is around ~43-44% range. Density of water at 60°F is 1000 kg/m3; density of your mix is about 1045 kg/m3.

So your density is about 4½ percent higher making the velocity pressure you read 2.1% lower. So correct UP your 100 gpm flow rate by 2.1%, or 2.1 gpm (this difference would probably be too fine to do on the chart).

For mass flow, I believe it would then be compounded because it is rho * area * velocity. Taking density into account again here would make the mass flow an additional 4.5% higher.

I believe this is correct without over thinking it... hopefully it helps.

-CB
 
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Did anyone have any tip for the percentage increase in friction loss if I use 50% Poprylene glycol instead of water. This is for engine jacket water, so I figure that at the cold start the temperature would be around 32 def F and it would rise up to 210 deg F. I search through lots of article, and i can only find that they refer back to darcy equation which is way too much. Help is greatly appreciated. Thanks.
 
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