Originally Posted by macleod319
*Crazy scientific discussion ahead*
Alright, a little science from a power plant mechanic. Heat transfer in a condenser/cooler/wort chiller/etc. is boiled down to the basic following equation:
Q = mc(T2-T1)
Q=overall heat transfer in BTU/time
m= the mass flow rate of your cooling medium in vol/time
c= the specific heat transfer capability of your chilling surface (i.e. the copper surface of the wort chiller in this case)
T2= wort temp (higher temp substance)
T1= water temp (lower temp substance)
The only way to increase the heat transfer capability of your cooler (without structural modification) is to either:
INCREASE the mass flow rate of your cooling medium (not decrease) by turning the water flow up
INCREASE the difference in the two temperatures, by using a pre-chiller or some other way of getting colder water inside the chiller.
This can be proven with further explanation if required. Anyone that disputes this fact is temping the laws of thermodynamics, and I would like to hear an arguement against. Discuss.
You're absolutely right that increasing the flow rate will increase heat transfer. However, your use of that particular equation is absolutely wrong. This equation can be used to measure the heat transfer of the water only
The proper use of the equation's variable is this:
Q = heat transfer rate
m = mass flow rate of water
c = specific heat capacity of the water
T2 = the leaving temperature of the water
T1 = the entering temperature of the water
You can calculate your heat transfer rate if you know all of your variables. In an immersion chiller scenario, increasing mass flow rate will tend to decrease T2. Decreasing mass flow rate will tend to increase T2. However, knowing that still doesn't tell you what conditions will maximize your heat transfer rate. The best way to determine your ideal conditions is by experiment.
Obviously, your calculated heat transfer rate of the water (Q from above) will also be the amount of heat removed from the wort by the water. Again though, the only way to properly calculate the conditions at which the heat transfer rate will be highest is by experiment (or by modeling with FEA/CFD software).
The being said, I intrinsically know that your heat transfer will be at its maximum when the flow rate is at its maximum. For a moment ignoring any knowledge of the water in the chiller, the way in which you'd calculate heat transfer from the chiller to the wort is very complicated (and nearly impossible by hand). But, I can tell you that the heat transfer rate will largely be proportional to two things:
- The surface area of the chiller
- The temperature difference between the chiller and the wort
Since the temperature of the chiller is a gradient throughout the length of the chiller, any method of calculation involves calculus (and a lot of assumptions and Reynolds numbers and Prandtl numbers and blah blah blah). But think of it this way: you want a maximum surface area of the chiller to be as cool as possible.
A lower flow rate will cause the latter half of the chiller to become warmer, thereby decreasing the heat transfer from the chiller to the wort. A higher flow rate will keep a larger portion of the chiller at a lower temperature, thereby increasing heat transfer from the chiller to the wort.
Also, efficiency in this context is very wishy-washy term. If your goal of "efficiency" is to decrease water consumption
, then there will be an ideal flow rate that balances heat transfer and flow rate. On the other hand, if your goal of "efficiency" is to decrease cooling time
, then you want the flow rate to be the maximum possible.
(Edited for clarity)