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Originally Posted by sidewayz_control
One point that has not been taken into acount is the flow rate of the water running threw the chiller, beer fridge, freezer, whatever... If your water is running full blast threw the piping, it will never cool down. It's like removing the thermostat on a car (which is actually a restrictor). Most cars will overheat because the coollant is going to fast threw the rad and not getting coolled down enough.
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Yes and no. The slower the flow the lower the exit temp of the water will be, but that does not mean that more energy was removed overall. The driving force for heat transfer is the difference between the temp of the water and the temp of the fridge. As the water temp approaches the temp of the fridge the rate at which energy can be removed decreases. You do not want slow flow but you also don't want so much that the pump you are using adds more heat into the system then being removed. You also have to take into account the energy used by the pump if you are trying to figure out the most "economical" solution.
Below is a basic equation for the heat transfer in a heat exchanger (I am a chemical engineer so bare with me as I get semi technical).
Q = U * A * Delta T
Q: Amount of energy moved
U: Overall heat transfer coefficient of the system (constant for said system)
A: Area of heat transfer (this is why more coils give you a bigger A, thus more Q)
Delta T: This is the temp difference between Tcold and Thot, or in our case the fridge and tank water. As Delta T approaches zero you can also see that Q approaches zero. Think about it this way, if your pump used to circulate the water through the fridge breaks, how much overall energy will be removed from the system. Answer is, none, because the water never makes it to the system. Another example, would you rather cool 1 gallon of water by 10 degrees or 15 gallons of water by 1 degree? Remember, the Q = M * Cp * Delta T so the total amount of energy removed is basically the mass times the temp difference.
And just because you home thermometer does not read a temp drop on the tank water after the chiller does not mean heat wasn't removed. The thermometers most of us use at home are not that accurate really. It might say that the exit temp is the same as the inlet temp, but really, the inlet temp might be 82.4 C and the exit may be 81.5 C but on our basic therms it reads 82 C and 82 C due.
Now I am not saying blast 2000 gph through your system, but slow flow is not the answer. One more point I should point out, if your flow is slow enough to be laminar this will greatly reduce the efficiency of your chiller and it's ability to cool your tank. Laminar and turbulent flow is determined by the Reynold's number that is based on the tubing diameter, viscosity, flow rate. We want turbulent flow (also called "well mixed") so there are no temperature gradients inside of the tubing in the chiller.
