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TCSIC (Nuclear)
27 May 06 23:49
In all my reading I have read that Ethylene (EG) has a higher specific heat capacity than Propylene Glycol (PG).  Yet when I study the specifications, it seems that the PG actually has a higher specific heat capacity. Which has a higher specific heat capacity?

What is the relationship between heat capacity and thermal conductivity?  What is it's effect on the overall thermal performance?

How much of an effect will the increased viscosity of PG have on the heat transfer performance of a solar domestic hot water system?

Excluding the issue of toxicity, which would provide better thermal performance in a solar domestic hot water system, propylene or ethylene glycol?

Yes, I know that EG has a higher toxicity than PG.  I am using a double walled heat exchanger with leak detection and my potable water system is maintained at a greater pressure than the glycol. It seems that from a practical standpoint PG is more readily available than properly inhibited EG.
IRstuff (Aerospace)
28 May 06 1:04
PG has a lot of pluses, but its operational temperature on the cold is more limited than EGs.  Its overall viscosity is higher at cold temperatures than EG.

TTFN



willard3 (Mechanical)
28 May 06 10:30
I think the thermal/viscous property differences between pg or eg are not that significant, but their toxicity is.

You should also pay attemtion to additive/inhibitor stuff in either eg or pg and how they will affect system materials...there are some nasty surprises there.
IRstuff (Aerospace)
28 May 06 14:38
All depends on what temperature range you're using.  PG mixtures at -40ÂșC are quite sluggish compared to EG.

TTFN



TCSIC (Nuclear)
28 May 06 15:03
Thanks to all. The link (Brines.pdf) provided by 25362 was especially helpful.  Although, when I approximate the values for Cp from the graphs, it again shows that PG has a higher Cp than EG.  I still don't see the evidence to support the statement that EG has better heat transfer capabilities than PG.  What am I missing?  I believe that the second formula in the above link may take into account the other variables needed to support the original premise, but I don't have sufficient background to apply the formula.

The other issue is the high temperature stability of the corrosion inhibitors.  The inhibitor degradation when the system is operating close to the maximum temperature is a potential problem.  The collectors are already 23 years old and I am refurbishing the rest of the system.  It has also been widely stated that PG has to be replaced more frequently than EG due to inhibitor breakdown.

I know that the fluid should be checked for pH and inhibitors annually.  But, how often does PG and EG have to be replaced based on oprating experience.

I am inclined to use EG, but the data doesn't seem to justify the use of the less available EG.

I have included a table of the various fluids on which I have gathered data.  I would like to use the most thermodynamic fluid with the highest maximum temperature and the longest potential useful life.
 
Thanks again.

        Mix    Cs    Cs Temp    Viscocity    Boil/Max    Freeze
Hercules Cryo-tek Original    PG    Undiluted    0.908    160        220    -22
Hercules Cryo-tek 100    PG    Undiluted    0.843    160        230    -70
Hercules Cryo-tek 100    PG    75%                    -17
Hercules Cryo-tek Artic Grade    PG    40%                    -8
DowTherm SR-1 EG    EG    50%    0.842    180    0.94    250    -34
DowTherm 4000 EG    EG    50%    0.835    180    0.94    350    -34
DowCal 10 EG    EG    50%    0.835    180    0.94    350    -34
DowFrost    PG    50%    0.902    180    1.10    250    -28
DowCal N    PG    50%    0.902    180    1.10    250    -34
ThermalSafe    PG    50%    0.901    180    1.07    220    -27
ThermalCool EG    EG    50%    0.842    180    0.95    226    -34
                            
Mathamatical Model EG (apprx)    EG    50%    0.86    158    1.30        -35
Mathamatical Model PG (apprx)    PG    50%    0.88    158    1.10        -32
        




TCSIC (Nuclear)
28 May 06 17:10
The reference temperature that I am using is 180 degrees F.  The overall operating temperature range is 70 degrees to 200 degrees.

While some fluid in the system could be at sub-freezing or even sub-zero temperatures at system startup, the lowest operating temperature once the system is operating for several minutes would be above 60 degrees F.  I presume that the viscosity differences of PG and EG at these temperatures would be insignificant.
Yorkman (Mechanical)
29 May 06 12:30
   When looking at the specific heat values of a substance remember: It's a ratio of the heat capacity of a substance compared to that of water. Water having a value of 1. If a substance like glycol (ethylene or propylene) is mixed with water it in effect lowers the heat capacity of the liquid (.97-.93) depending on the concentration. To be able to transfer the same number of B.T.U.'s, as water with a heat capacity of 1, the mass flow rate will need to increase.
   The other consideration is that the specific gravity of the liquid also increases when you add a soulution of glycol to water (1.05 - 1.08) again depending on concentration. In effect this increases the wieght per gallon of the liquid. This may not be noticed while running at the original G.P.M.. But if you increase the flow rated to meet the heat transfer needs of the process you may find that the pump/motor is too undersized to handle the change in specific gravity of the liquid.
   I've seen this happen when a plant operator puts glycol into his/her chilled water system, has not made any pump modifications and can't figure out why the evaporator approach has gone to heck and the chiller is dropping out on low suction pressure. Hope this helps in putting something tangible to the subject matter.
                            A.J. Gest, York International  

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