First post! Woo!
After reading what feels like every post on this forum, I've noticed there's a lot of misconceptions about wort chilling and what's most efficient.
Thus I propose treating this concept like any other engineering problem.
Summary is at bottom.
What is heat transfer?
A complex study of removing energy from one substance and transferring it to another. At a basic level, we are transferring heat from our wort, to a fluid contained within copper, stainless steel, aluminum, or any other conductive material.
What affects heat transfer?
- Specific Heat of Wort (constant)
- Specific Heat of Cooling Fluid (constant)
- Surface Area of Chiller (constant)
- Gauge of Chiller Material (constant)
- Temperature Gradient
- Viscosity of Fluids (constant)
- Velocity of Wort
- Velocity of Cooling Fluid
- Kettle/Keggle Material (constant)
- Ambient Air Temperature
- Thermal Conductivity of Chiller (constant)
- Surface Area of Wort to Ambient Air (constant)
- Temperature of Chilling Fluid
*Constants based on your current equipment, or physical properties
We are not so much interested in solving for the specific amount of time it takes to chill 10 gallons of boiling water, but rather, what system provides optimal cooling abilities at the lowest cost.
We will assume a simple system (no loss to outside world):
1/(m0*A) = (1/(h1*A1))+dx/(k*A)+1/(h2*A2)
where m0 = Heat transfer coefficient
A=Surface Area of fluid to conductive material (note this is different on each side because materials have thickness)
h = Individual convection heat transfer coeffecient
k = Thermal Conductivity (sometimes denoted as sigma)
From this we can say:
1) More surface area = more heat transfer
2) Thinner conductive walls = more heat transfer (lighter gauge)
3) A higher thermal conductivity of the material = more heat transfer
Defining Individual convection heat transfer
This value changes based on the type of system used, i.e. counter flow, heat exchanger, immersion chiller. It also depends on length of pipe, resistance to water flow, laminar values, etc.
At an utmost simplistic view we will define h as:
h = (thermal conductivity)/(Dh)*(N)
where Dh = hydraulic diameter (we will assume this value to be at a maximum i.e. water fills the entire conductive piping.
N = Nusselt number based on Reynolds number, and Prandlt Number (we will not calculate these values, rather look at what properties of our system maximize heat transfer).
Reynolds number =
Prandlt number = (dynamic_viscosity*density*specific_heat)/(density*thermal conductivity)
What values maximize h?
Based on values we can control:
More velocity across conductive material = more heat transfer
More conductive tubing = more heat transfer
Laminar vs Turbulent Motion
For our purposes, we will assume chilling water is laminar.
Wort can either be move in a Laminar or Turbulent fashion.
To motivate this discussion, let's imagine we have water at 200 degrees F.
If you leave it sitting by itself on a stove, you will see steam coming off of the surface, and eventually it will come to equilibrium with the surrounding air.
Now, imagine the same water at 200 degrees F: If you start stirring the water, even slowly, what happens? More steam escapes from the surface, which means the water will cool quicker. Why is this?
If you were to look at the surface of the water at a microscopic level, you would see a very thin layer of air acting as an insulator to the outside air, and a very thin layer of water acting as an insulator to the water below it. And we know that insulation decreases speed of heat transfer. Thus, turbulence, or siring the pot, will remove that insulating layer, improving heat transfer to the outside world.
Great... How does this apply to Wort Chilling?
Let's assume we have a 20' immersion chiller and a 20' counter-flow chiller. We all know that the counter-flow chiller will cool the wort faster without agitating the wort surrounding the immersion chiller. This is because we reduce the laminar buildup on the outside of the chiller using a counter-flow chiller. We can express this quantity as v2 or the velocity of the wort against the conductive material. Let's assume that v1 (the velocity of the cooling fluid is constant for both chillers). The Reynolds number will be higher, thus making the h2 value for the counter-flow higher than the immersion chiller.
Immersion vs Counter-Flow vs Plate Heat Exchanger
The pros and cons list of each of these would go on forever, again, based on the values we have discussed above. I'm not going to derive optimal values for each of these, because you can probably determine, for yourself, the optimal values.
Values in your control: Make your cooling water as cold as possible, increase water pressure either by pump or faucet to maximum value possible, remove insulation from keg (if possible) while cooling.
Maximum Efficiency: Heat Exchanger. The surface Area and thickness of the plates provides maximum efficiency. Also, velocity of wort can easily be increased by using a higher pressure pump.
What I do
Immersion chiller all the way baby.
1) Simplicity simplicity simplicity
2) USE A PAINT STIRRER.
An immersion chiller gives me the best of all worlds. Using this paint stirrer, 1)I can oxygenate my wort the entire time it's spinning. If I wanted, I could have froth spilling over the top of the keggle.
2)At full speeds, the velocity of the wort over my coil is faster than that of a counter-flow chiller and heat exchanger.
3) Simplicity and Extra Low Cost.
4) The outside of the Keg acts as a heat exchanger to the outside air. So not only are you cooling the wort on the immersion chiller, you're cooling it against the keg walls.
5) Huge amounts of steam escape by destroying the laminar layer between the wort and outside air.
6) Cheap and effective way to BOTH oxygenate and cool wort at the same time.
7) Less parts to sanitize.
8) Keep it simple.
Using this technique, in the summer, where my hose water is about 63 degrees F. I can usually cool 5 gal of wort in less than 11 minutes. The best I've ever done, in the winter, was 6 minutes. I use a 20' coil of 3/8" copper tubing from home depot that i coiled myself. I use my hose water. I've also never had to oxygenate my wort other than letting it fall from the top of the carboy to the bottom because the paint stirrer does that for me along with helping cool the wort.
Thanks guys and I hope I didn't make too many errors.
Anyways, post your thoughts below and/or experiences.