2 stage glycol cooler

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cke11y

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Hi everyone. Wasn't sure about location of this post. Hopefully this appropriate.

So glycol chillers made from old air cons are a bit crap. There's freezing up of coils to worry about, longevity of compressors and such. A lot of the drama is because when used, generally some pretty warm liquid is dumped back into the reservoir.

I've been thinking I'll return glycol to my reservoir via a plate chiller with rain water flowing across it.

Or!!!!.....?????

What about somehow sending the return glycol directly across the coils of a second air con unit???? This unit would be made to be permanently 'on' when pumping cooling liquid, and would certainly be more likely to bigger up, but couldn't this work?

I'm no fridge but I like to theorise.

Any thoughts? And "you're bloody nuts, just stick in a plastic container for a week" isn't what I want to hear. I like complex as much as you like simple
 
I'm not quite sure what you're suggesting, but here's my 2c (I do industrial refrige stuff).

The problem with using air con units is that theytypically do not have a very low evaporation temperature. Imagine a room with 1000 air con units in it. It will only ever get to around 12 degrees (assuming normal outside conditions) as that is the temperature that the refrigerant used evaporates at the pressure compressed to. in general, the bigger the difference between the evaporation temperature and the target temperature, the less efficient air cons are.

The whole idea of using glycol is to reduce the freezing temperature of the water to make feasible for brewery refrigeration. Chilled water (usually 5 degrees) is used in HVAC. Direct-expansion air con units, since the evaporator coils directly contact the air, can get away with ~12 degrees as there is only one heat transfer interface. Furthermore, the refrigeration capacity of the A/C unit approaches zero the closer the temperature approaches the evaporation temperature (12 in this case). So, in essence, using air con units for refrigeration in breweries is a bad idea as they are woefully inefficient at their lower temperature range AND generally cannot cool low enough.

That said, if you're up for some exotic work you could set up a cascade refrigeration system. The first air con unit blows cool air over the condsenser coils of a second unit. This will lower the evaporation temperature of the second unit. It will be horridly inefficient due to all the heat transfer interfaces and pumping losses.
 
So the more ideal system (in the interests of direct comparisons) is a fridge?

Removes heat relatively quickly and designed to go to lower temps? How would the guts of a theoretical fridge go, with cooling coils sat in a glycol reservoir? Is this more attune to commercial glycol chillers except they have customised coils to fit small reservoirs?

I'm not sold on any particular method of cooling down my coolant source, but I'm sold on the benefit of reducing the temperature of liquid returned to the reservoir. That must help! My thought was simply to have very close contact to a second set of cooling coils on the way back to my reservoir to achieve this?
 
Hey klangers if you water cooled the condensor say with a water spray or submerge it in water and then circulate that water to a cooling tower or tank that will increase the efficiency of the system and drop overall temps?
 
cke11y said:
So the more ideal system (in the interests of direct comparisons) is a fridge?

Removes heat relatively quickly and designed to go to lower temps? How would the guts of a theoretical fridge go, with cooling coils sat in a glycol reservoir? Is this more attune to commercial glycol chillers except they have customised coils to fit small reservoirs?

I'm not sold on any particular method of cooling down my coolant source, but I'm sold on the benefit of reducing the temperature of liquid returned to the reservoir. That must help! My thought was simply to have very close contact to a second set of cooling coils on the way back to my reservoir to achieve this?
Yes, a fridge is more ideal if you get the evaporator (cooling) coils into the glycol. Bear in mind though that fridges have rather low capacity (heat power they can remove) and this usually gets eaten up by the pumping losses from the glycol circulation pump at this scale.

All you are effectively doing with cooling the glycol return as well as the glycol reservoir is adding another A/C unit in parallel. As I mentioned in my previous post, this will only help if your glycol is returning at a temperature substantially above the evaporation temperature AND one unit is already running full boar. Otherwise, all that you will do is make 2 units run at eg 50% rather than 1 at 100%.

As an analogy, imagine that you have a drink and you want to make it cooler by adding ice which comes out of a fridge at a set temperature. No matter how much ice you put in your drink (assuming you have a very large cup), it will only ever reach the temperature of the ice. This is what is happening with your proposed system - you are adding more ice, which will only cool more if "1 cube" of ice is not enough to already bring it to temperature.

Hey klangers if you water cooled the condensor say with a water spray or submerge it in water and then circulate that water to a cooling tower or tank that will increase the efficiency of the system and drop overall temps?
Yep that's certainly possible and is what happens on big ol systems. I would caution you on a couple things:
  • The condenser coils can get easily corroded if you aren't careful with water spraying/immersion.
  • Any energy that's used to run a pump and/or cooling tower adds to the electricity consumption of the system and at this scale usually worsens efficiency because of this, but adds capacity. Sort of like a hotted up car. More powerful, but less fuel efficient
 
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