Cooling question

Toad

Apologies that this is a bit off topic, but my sense is that the expertise in this group will be all over this.

How much greater (a lot, a fair bit, just a little more or none) would the energy usage/run time be for a water chiller to maintain a volume of water with a starting temperature of 45 degrees at 45 degrees (F) versus an otherwise identical setup where trying to maintain a starting water temperature of 55 degrees at 55 degrees.

May be easiest to think of as 80 gallons of water in a somewhat insulated and covered vessel being circulated through an aquarium chiller through insulated tubing.

Ideally wanting to target the lower 45F temp but trying to understand extra energy and run-time of equipment once that temp is achieved.

Thanks for taking a scenic detour on this one!

DCContrarian

This is in an environment that is at room temperature, say 70F? Heat flow is always going to be proportional to temperature difference. At 55F your difference is 15F, at 45F it's 25F. So 25/15=1.67, or 67% greater.

Toad

Apologies, had meant to include that - room temp will vary but figure 75 degrees as average. Wasn’t sure how much the fact the vessel was insulated might mitigate that. (50 degree water rose to about 55 degrees over 8-10 hours without the chiller running.) Thanks.

DCContrarian

If it's the same vessel it doesn't matter how it's constructed, all that matters is the temperature.

mattmia2

Depends on the temp differential, the surface area of the vessel exposed to ambient and the r/u value of the insulation on the vessel.

Toad

Thank you both. Much appreciated!

EBEBRATT-Ed

The colder the chiller runs the more energy you will use. How much? I don't know.

hot_rod

Are you a number cruncher? :)

Toad

Ha! This is actually great. Just need to try to figure out my R-value.

I was also thinking of getting a sense of things by flipping things around a bit. Was going to get water to 45F, turn off chiller and then see how much temp rises after, say, 2, 4 and 8 hours. Then repeat starting at 55F (or just pick up those measurements once the 45F water gets to 55F). Thinking is, if insulation were “perfect” which it’s obviously not, energy required would be same in both cases. But if the 45F warms more rapidly than the 55F, wouldn’t that relative relationship give a sense of how much extra energy would be required to keep water at 45 versus at 55? Or is that logic flawed? (And importantly, not trying to safely launch a satellite here, just trying to get a rough sense of things - but I will still definitely take a crack at using that formula so thanks again!)

mattmia2

The delta t between 45 degree water and ambient is 10 degrees greater than the delta t between 55 degree water and ambient. Heat will move faster from the air to the 45 degree water than it will to the 55 degree water.

hot_rod

It gets a bit more complicated for tank losses as stratification comes into play. Called thermocline. So the entire tank is not the same temperature from top to bottom. So the heat loss varies.

The height of the tank influences that also, taller tanks, more stratification.

I have seen test tanks that have a sensor every 6" on the side, under the insulation to nail down tighter loss numbers.

Figure R-5 per 1" of foam insulation.

Indirect water heaters storing 120f claim less than 1/2 degree per hour heat loss for an example. But without knowing the test data, like the ambient air temperature around the tank, that is more sales talk :) The hotter the room the better that performance number. If the tank is at 85 and the room is at 85° the loss numbers are impressive.

As @mattmia mentioned, the higher the ∆T, temperature difference, the faster the energy transfers form hot to cold.

Screenshot 2024-07-26 at 11.45.02 AM.png

mattmia2

I knew the 1/2 degree per hour numbers were bogus when I saw the same spec for the 20 gallon and the 120 gallon tank.

Toad

I just wanted to offer a last broad-based thank you to this group who’s knowledge and generous willingness to share that knowledge never disappoints! Have a great weekend and thanks again!