Question, Chilled Water Tank Sizing

[Pete_K]

Hi All,

First, if this question should be somewhere else, please let me know...i am new to this forum.

I'm doing work for a customer on a piping system upgrade. The system supplies chilled water (~60 F) from an open tank to molding machines, and returns the water to a partitioned side of the same tank it came from. Water in that partitioned area either goes thru a chiller or a heat exchanger to bring the temp down. The temps of the streams into the "cold side" are probably within 10 degrees F of that 60 degrees outlet temp.

My problem is, the customer is looking to us to provide direction on sizing the tank. The output flow is ~900 gpm. All they have offered is that the chiller requires to have 1000 gallons of water in the tank. This kind of work is not in my background - I'm more of a machine designer. The closest experience i have to this is sizing large reservoirs for hydraulic oil for oil bearings. In those cases typically there is a "retention time" that you are aiming for, so if my desired retention time in this case was 3 minutes, i'd need 2700 gallons.

Is there some "rule of thumb" or other sizing rule in this case? My typical efforts of researching on Google have not been fruitful, so any help i could get would be much appreciated. I have a feeling that the tank will end up around 3,000 gallons, but I would like to be able to back that up with something.

Thanks in advance.

p.s. i am going to post this question as well in the Storage Tank forum.

 

[chicopee]

So are you trying to size the volume within the partitioned section before the 70 dF water enter the heat exchanger or is there a second partition of the tank holding the cooling coil?

 

[chicopee]

Also get the specs on the cooling coil or heat exchanger since that information will be required when determining the size of the tank

 

[Drazen]

you should find out firstly what your tank is for. if tank cold temp. were higher than room temp, i would say having it open helps in additional cooling, but cooling water and than letting it lay in open, uninsulated tank, looks like terrible waste of energy.

what do you store actually? what is chiller flow compared to required mold cooling flow? does mold cooling cycle in few minutes of much longer?

rules of thumb help in very typical systems, and yours is very un-typical.

 

[John2025]

It sounds like you're talking about a reservoir feeding a cooling tower/chiller with one pump and the process water with another. I'd say you need enough capacity to drain the tower and the piping (including process piping) without dumping a bunch of water down the drain plus maybe another 500 gallons to safely feed the pumps without running low on water in the reservoir at startup. The reservoir is probably going to be only half full when running, so double the number you get.We purchased a small pre-packaged system based on their recommendation, but the reservoir is a little smaller than I would have liked (250 gallons for about 125 GPM total flow and 100 gallons in the tower). YMMV

 

[cry22]

Just like your car, you wouldn't have one bucket of water and expect heat to be removed from your engine. You need to fill your radiator to remove the heat adequately.

All cooling systems require what is called Minimum system volume. Your chiller manufacturer has the minimum required system volume.
System volume is the water volume in the piping, coils, chiller, etc..
And you are in the right direction, because most chillers require a minimum of 3 GPM per ton. since you are at 900 GPM, the 2700 GPM you suggest is in line with the tank size that should be sued. add some safety factor.

Injection molding machines are not critical operations, if they were, you'd need to size the tank, assuming that you chiller is down, in which case, you'd use a buffer tank sized for the number of minutes you need it without chiller operation.

 

[Pete_K]

Hi again all, sorry I haven't responded sooner, and thanks for all the responses.

In the end, the tank was designed with a partitioned section, which could be considered a separate tank for the purpose of describing its function. Water returning from the mold machines goes to this "return tank". From there it goes either to a chiller or a heat exchanger, whose cooling capacity comes from well water. Chilled water is deposited in the chilled water tank. We ended up baffling the tank and using an educator to promote mixing so that by the time the water made it's way to the distribution pump nozzles, the water was pretty well mixed with regard to temperature. The chilled-water side has about 2 minutes of retention.

The machines mold wax (into shapes with which one might draw on paper), and require hot and chilled water, as can be imagined.

Thanks again for the responses, this forum seems like a great community.