How to determine Ice cooling rates 2

[abl33]

I have an application where I need to use an ice bath to store cold for an application requiring a constant feed of cold water just above freezing and while I can determine the mass of the ice required to prevent equipment short-cycling with a variable load, I'm not confident in my ability to determine how much surface area of ice I require in order to balance my incoming heat load and maintain a water temperature just above freezing.

The design will have an unknown length of copper pipe with refrigerant flowing through it so that ice builds up on the outside of the pipe with two ice sensors to ensure a minimum and maximum thickness of ice covering the pipes. There will be two water circuits pulled from this tank that both want water as close to freezing as possible and will each return water with an unknown rate and temperature anywhere from 3GPM to 20GPM with a temperature range between 38F and 70F.

There will be some water movement in the bath, but ideally it would rely on the suction and return of the feeds to draw the water across the ice. I do not know how to best quantify that effect on the thermal transfer between the ice and the water. All that being said, is there any general guideline for how to translate cooling capacity rate of ice as related to the surface area of ice? In this specific case, I am looking to make multiple 60kBTU circuits and need to know how much ice surface area of ice is required to cool the water at 60kBTU.

 

[abl33]

if this was a batch process, I'd consider crushed ice a viable solution, but this is a continuous process with a variable load up to 120,000 BTU.

Beyond the fact that crushing ice is more complicated to produce than ice rods and requires moving parts which would be more susceptible to breaking down, I don't know how crushed ice can be reliably controlled with the precision required to maintain the temperatures so close to freezing without potentially freezing the system solid if there is a lot of crushed ice in the tank and the heat load suddenly stops.

My concern with surface area is to the extend that i have no clue what the cooling capacity of ice is and my problem isn't a simple "I have 100# of water at 45F and want to cool it to 33F, how many pounds of ice do I need?"

My specific application question was about using ice to cool at a rate of 5T, but at its core, I'm generally curious about "how FAST does ice cool water" and that is something that apparently doesn't actually have a known formula or property.

 

[IRstuff]

Seems to me that you have this problem regardless of the form of the ice. Crushed ice has the highest possible surface area, so it'll have the best thermal transfer. How are you even getting the existing ice form into water; that requires mechanical or human work.

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[SwinnyGG]

If this is a continuous process, in my opinion, you need to throw this ice based concept in the garbage and find an alternate route.

 

[itsmoked]

Ditto! I can't think of a bigger waste of time and the company's money. This is a classic wild ass effort that will take months of resources before it all gets sh!t-canned and done reliably. People do this all the time reliably and efficiently without ice.

Keith Cress
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http://www.flaminsystems.com

 

[abl33]

so what you three are saying is that you don't know the answer, so you would rather avoid the question?

As I mentioned, this was originally direct refrigerant, but that solution is not capable of the turndown ratio and response time required. I won't get into all the reasons why refrigerants and anti-freeze are not ideal solutions for the process. I'm evaluating an alternative idea which involves ice and would be perfectly workable if the heat transfer of the ice was sufficient, the first attempt showed it wasn't. The question is straight forward and you can either help answer the question or not... "how much surface area of ice is required to remove 5T of energy from 34F water assuming the water remains at 34F and the ice remains at 32F?" With that information, I can evaluate if ice is a practical solution for us or not.

That being said, if you have an alternative solution that will provide cold water that is maintained between 32.1F and 35F when you have any heat load between 100 and 120,000BTU which can change >10,000BTU/min and involves return water temperatures between 35F and 70F and is capable of running 24/7, I'm all ears.

 

[TiCl4]

ABL33,

No, what they are saying is that this is an unexplored area of engineering FOR A REASON. They aren't trying to avoid the question - they are trying to suggest alternative means that are certain to work. There are simply easier, more consistent, and less expensive ways of doing what you want without needing to go to an ice slurry design.

That being said, if you have an alternative solution that will provide cold water that is maintained between 32.1F and 35F when you have any heat load between 100 and 120,000BTU which can change >10,000BTU/min and involves return water temperatures between 35F and 70F and is capable of running 24/7, I'm all ears.

Then lend me your ears, because there is solution that does not require ice. Itsmoked provided this with his "reasonably sized tank" comment. Your short-cycling compressor means that your tank is too small - the temperature is swinging too quickly when the compressor kicks on. The only way to solve this would be, as previously suggested, a multi-stage compressor, a larger tank, or ideally a combination of both. Each source can pull and return water up to 20 gpm and 70F EACH, by your OP. You have, as I understand it, a possibility of a Q = (40 gpm)(8.34 lb/gal)(1 BTU/lbF)*(70F-32F) = ~12,700 BTU/min heat load. BTW, please give heat loads in units of energy/time. Saying there is a 120,000 BTU requirement means NOTHING.

That heat load, when applied to your proposed 400 gallon tank with no cooling, results in a dT/min of ~3.8 F. I assume that your compressor is sized slightly larger than that heat load, so when it kicks on AND your heat load is low (i.e. 6 gpm return and lower temperature), your tank will hit setpoint in about a minute or less, short cycling the compressor. If your current tank is anywhere near 400 gallons, its no wonder you are having issues.

You DON'T NEED a chiller that has the same turndown as your heat load turndown. You simply need to size the chiller and reservoir tank volume such that the lowest turndown on the chiller will give it a full cycle time. This can be aided with correct setpoint selection and proper deadbands on the controls tuning.


If you must pursue the ice thing, here are a few things I found that may help you:
If you want to dig deeper on ice melting mechanics, I direct you the following paper (though it only explored natural convection melting of ice).

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=11570&context=rtd

If you really want to continue with the ice idea, you may want to look into systems like DeepChill, which generates very small particle size ice slurry. There are no existing models I know of to predict film coefficients for water-ice forced convection, so trial-and-error with smaller sized ice is the best you'll get.

https://www.sunwell.com/deepchill/

 

[SwinnyGG]

provide cold water that is maintained between 32.1F and 35F when you have any heat load between 100 and 120,000BTU which can change >10,000BTU/min and involves return water temperatures between 35F and 70F and is capable of running 24/7, I'm all ears.

What makes you think an ice slurry based solution can do all of that? You're going to need to come up with a whole additional system to provide ice.

You've said your thermal load is 60,000BTUh per stage, x 3, for a total of 180,000BTUh. If your system cools your process water at 100% efficiency, you're going to melt 1250lb per hour of ice- which means you need to make 1200 lb per hour of ice. For reference, a typical restaurant-grade ice machine will make roughly 10lb per hour of ice cubes.

Anyway, back to your main question- no one is providing a simple direct answer because the answer to your question depends on a pretty good list of factors that will be difficult or impossible for you to measure (i.e. mean water velocity over the ice surface). And even if you could measure them, the process you're laying out is unlikely to be energy efficient or cost effective - we're trying to help you.