that is an absolutely excellent point. i only had 1 chiller that was an open tank and that was a long time ago.
Originally Posted by Nuclrchiller
typical minimum flow rates are around 3 feet per second minimum but i don't know what you have for a chiller barrel.
Originally Posted by fxb80
this whole thread seems to me to center around the water molecules separating out or freezing at 32F instead of the SOLUTION freeze point temperature. this is not true in a flowing system. the water molecules are going to stay in solution and not freeze out separately and collect in the coldest part of the system (evaporator).
i do not have any chillers that have a design evaporating temperature colder than the solution freeze point...that would be dumb and counter productive. i have run across systems that were improperly maintained and had an evaporating temperature lower than the solution freeze point (due to low flow rates) and they did occasionally make slush...but i believe that it was a solution slush, not a water slush and the glycol still a fluid.
jayguy, I never said the water would freeze at 32F whether the solution is flowing or not flowing, or asked if it would. And we agree it's dumb to have a freeze point higher than the evaporator SST. But that's what I've got, a dumb situation. The system is about 120 gallons short on propylene (1800+ gallons system volume); the barrel was going to be replaced; I suggested then was a good time to not re-use the solution being drained; put straight propylene back in; it wasn't done. The man who made the decision said it would be easy enough to fix later. In the meantime, I think ice has formed on the evaporator.
Which really got me thinking. How do I prove there's ice? What will it take to thaw it? If it wasn't a glycol solution, or if it operated at normal HVAC temperature, it's easy enough to go by the SST if the refrigeration is idle and the evaporator is the coldest part. But, as long as the solution temp stays below 32F, and there's pure ice on the evaporator, the SST with the refrigeration idle would be less than or equal to the solution temperature. The owner won't tolerate letting the solution temperature rise to 32F if it can be helped. So I would turn off the pump, valve off one side of the barrel, and add heat from an external source (heat gun, etc.).
During this process, if the SST rises steadily, then plateaus at the freezing point of the solution, then rises again, that would mean the ice was returned to solution at the freezing point. If the SST plateaus at 32F, that would prove the ice must get to 32F to return to solution. So I can find out by experiment if there's ice and at what temperature it returns to solution.
Since the barrel is isolated, and if the evaporator had pure ice on it, and if the ice were to return to solution at the freezing point of the solution, the freezing point would rise during the thawing process due to the greater percentage of water in solution. Which adds another variable I might need to think through.
But if I could know with certainty that ice in a glycol solution must get to 32F+ to thaw, that's easier.
I read a paper about glycol solutions a few years back as I was taking care of a dew ice storage systems. It is my understanding that as your temp drops to the solution freeze point, it makes a slurry, seperating some water, and causing the solution to be a higher concentration, therefore lowering the solutions freeze point. Still, if you start glazing ice on your chiller tubes, your situation is going downhill.
Just read the Dow paper, thats exactly what it says.
i would not know how to go about proving your theory, although i would like to know. another factor to consider is the pressure inside the solution system. as the pressure goes up, the freeze point of water changes...so that is going to complicate things further.
two points that I have talked to chemists about. first glycol that is below 30% reacts to temps different then would be expected, meaning that the glycol does have the ability to chemicly seperate. second point is that as the solution starts to slush (in the barrel) the flow will decrease in some of the tubes causing problems with the chiller pressures/temps as now there is little heat transfer at those tubes. just my opinion but I never run my machines at less then 30%. another factor to consider is the inhibitors at less then 30%