IC: Flow rate and efficiency

I was brewing last weekend and got into an argument with my dad about my immersion chiller that I hook up to a garden hose. He thought that it would work just as well if not better with the hose at a lower flow rate, and turning it up high would just waste water. I on the other hand thought it seemed pretty intuitive that a higher flow rate would mean greater chilling efficiency. What do you guys think about whether a higher flow rate would yield a higher chilling efficiency?

I adjust the flow rate as the wort cools. I have it running a little higher at the start and slow the flow rate down so to get the heat transfer maximized. I also frequently stir the wort which helps a lot. I guess I agree with your dad.

Efficiency = how well you accomplish a goal. If the goal is to chill as fast as possible, run it as fast as it will flow. The tradeoff is more water usage.

If you want to chill but use as little water as possible, run the water at a speed such that the output water is very close to the temp of the wort. The tradeoff is very slow chilling.

There is no "best" or most efficient unless you state your goal. ;-)

if wasting water bothers you, try my solution. I have a small pond pump (lowes hardware store, in the garden section, one of their inexpensive pond pumps) I have it hooked to the 'inlet' side of the wort chiller (I use a copper coil, but thats irrelevant) with a hose, the pump sits in the bottom of the wifes' mop bucket, ice in the bucket with just enough water to get the pump going, and the return end of the chiller empties into the bucket -by the time the ice is melted (it doesn't take long) the wort is chilled. I usually just freeze a couple of 1gal milk jugs full of water for the ice, but in a pinch a 10lb bag works perfectly.

I think you're both right in some way. I would define efficiency as the amount of heat removed per unit volume of cooling water. By my definition, I think your view is wrong unless you replace the word 'efficiency' with 'speed'. I think your dad's opinion is close to the truth - lower flow rates will not work as well in terms of raw chilling speed, but they will work better in terms of efficiency (by my definition).


Of course higher flow rates will chill the fastest. Think about it from the wort's perspective. If the flow rate is low, only part of the chiller is cold - then it grows gradually warmer further along the tube. If the flow rate is very high, then the whole chiller is cold.
Heat transfer occurs proportional to the temperature differential, so the cooler the chiller is as a whole, the higher the heat transfer, period.

But, with an immersion chiller your biggest problem by far is keeping the wort evenly mixed so you don't have a cold spot around the chiller. At high temperatures, the huge temperature differential keeps the chilling efficiency pretty good even if you aren't stirring the wort, but as it cools down, the efficiency plummets if you're not stirring - you get a cold spot around the chiller and then you're removing VERY little heat. Even if you do stir, the efficiency is naturally still going to drop since the temperature differential is much lower.

Chilling water flow is a case of diminishing returns. When the wort is relatively cool, the temperature differential is low and running the water full blast will not keep the chiller all that much colder overall than if you only ran it at, say, 50%, so using more water is more or less a waste. So, most of us pay attention to the outlet water temp and throttle the water flow accordingly. If you throttle the flow down as chilling progresses so that it is always at least warm, you keep your efficiency up to an acceptable level, saving water.

IMHO, chilling quickly is the ultimate goal. I run the coolant as fast as possible and try to collect and reuse all the output water. This is stuff I'd spend the water on anyway so it's not a waste at all.

Constant stirring (gentle while hot) will make a dramatic increase in chilling effeciency. You didn't mention whether or not you stir during the chilling. I set my flow to about .75 gallons per minute and collect my first 7 gallons (to fill my smaller kettle) for washing up. I also stir pretty much continuously during the chilling. Gently until the temp is below about 110, then vigorously after that, which has the added benefit of starting the wort aeration process.

In the winter I'm down to pitching temp (avg 65F) in about 12-13 minutes. Summer it's closer to 25.

You can only transfer heat so quickly through the copper and into the water. Running water faster when hotter will cool faster, but as the wort cools, running water faster just wastes water.

I know this post has sat a while, but I had the same argument in my head today when using the wort chiller I just made this morning for the first time. As Funkenjaeger said, if you define efficiency as temperature reduction per gallon water used, slower is better. The water coming out is almost as hot as the wort, which means it's taking the most heat out possible. But since it is a lower flow rate, it will take longer. Hence the fact that the more tubing in your chiller the more efficient it is, because it grabs more heat.

Of course running the water full blast should be faster, the differential between the coolant and the wort is greater which makes it more like sticking a perpetually cold element into the wort.

In the end there is probably a perfect intersection where there is enough length of tubing to keep the water running full blast and still coming out nearly the same temp of the wort, but that would require me to try and dig up my old thermodynamics textbooks, and I think I sold those for $5 worth of beer money in college...

Holy necropost, Batman!

Haha, yeah it's from back in the archives, isnt it. Searched around to see if anyone else had the same curiosity I did, not a surprise someone did.

Actually, there will be a flow rate for each IC that is optimum for a given wort temp. If the flow gets beyond the heat transfer capability of the system, "cooling" actually declines.

Ask a 1/2 mile track racer what happens when they remove the thermostat on their car. It will literally burn up from the inefficient heat transfer of the higher flow rate. Thats why they always have a washer with a large hole in the middle installed to replace the thermostat. The washer restricts the flow and allows the heat transfer to be optimized for the cooling system. It's not uncommon for them to diddle with this hole diameter to get things just right.

So in the end, it's all about the copper surface area and an optimal flow rate. Keeping the wort moving is just as important since it behaves in a manner similar to the cooling water, its just on the other side of the copper. The heat transfer rules apply equally on both sides.

I've been using a common approach to optimizing the IC by using tap water initially to chill to about 100 - 90 degrees. I then add ice and water to an old IC chiller in a bucket which is now used as a pre-chiller for the cooling water. This gets me to 65 - 70 in about 25 minutes total on 8 gal using summer tap water temps. Initially the system is total loss for cooling water (good for the lawn) but I have recently gone to circulating the pre-chilled water without a great hit on the ice loss. Key to this is a continual stirring of the wort to assist the wort to chiller heat transfer. This is one area I plan to incorporate a powered stirrer since continual stirring isn't on my list of fun things to do in brewing.

There you have it... one man's approach to a problem with a thousand solutions.

Ya old post, but if he posted it as a new topic he would have gotten a use the search button comment. So 6 of one or half a dozen of another.

I am glad this came back because I think about this every time I use the chiller.

At my apt. I am lucky enough to have a hose to use. The water pressure is very high to the point of being more than 5 gal a min. Now I am thinking that is higher than I need as the water comes out as cold as it went in. So my process is to turn the pressure down until the water coming out is just warm. I wonder though if I sometime let it go full blast and see how fast it can go. I think my next time I will turn the pressure down until it is warm then turn it up until it is just cold again. then adjust this as needed. I am not that worried about wasting water as it will still be uncontaminated water and I can use it to water my garden, but it is hard to deal with 100 gallons of water if it were to take 15 min or so.

The more I thought about this the more I think that there is a final answer to what is more efficient, but it includes all factors. The most efficient way to cool the wort would be using the least water possible in the quickest time possible. That would simply be a matter of determining the maximum heat transfer through the copper into the cooling water using the maximum flow rate possible and needing enough surface area to transfer all the heat. Problem is, this will vary depending on your wort starting temperature, water temperature, final gravity, etc.

In reality, that's why a counterflow chiller works so much quicker. It's just like having an immersion chiller long enough to have just a thin layer of wort all the way around.

jgilbreth said:

The more I thought about this the more I think that there is a final answer to what is more efficient, but it includes all factors. The most efficient way to cool the wort would be using the least water possible in the quickest time possible.

Click to expand...

I don't think there is a single final answer. You are not really describing efficiency, but rather are describing an optimization over several parameters. Such an optimization depends on assigning some (subjective) weighting to the costs/value of the different parameters, in this case time vs water usage. Any final answer depends on the relative weights/values assigned to the parameters.

FWIW, in my last brew, I chilled 12 gallons to 78 degrees using ~60 deg water in 24 minutes, using about 30 gallons of water, with no ice and no recirculation of chilling water. You can see that I had the flow choked way down. The real key, as others have stated, is stirring or agitating the wort aggressively.