idzy said:
Wiggman, your information differs significantly to Boerderij_Kabouter at the following thread:
http://www.homebrewtalk.com/f51/stainless-steel-copper-herms-coil-138139/
I would be interested in hearing the science. For example, what you are saying makes sense, but so does what he is saying.
EDIT: Again - I am not an engineer and discounting issues with pumps. A quick look on a tube calculator tells me a 12mm tube 3m long has more surface area than a 3mm tube 13m long.
Convection is the interaction that is is allowing the heat transfer. If there is a difference in termperature between the pipe and the fluid, thermal transfer will occur. There are three primary factors at play - flow at the interface between the pipe surface and the fluid, the difference in termperature between the pipe surface and the fluid, and the coefficient of conductivity of the surface.
Let's only look at the inside of the pipe. Because this transfer occurs at the boundary layer between the fluid and pipe, there will be a heat gradient between the boundary layer and the centre of the fluid (i.e. slice a pipe in half so you have a circle. In the case we're ramping up our mash, the liquid touching the pipe wall will be hottest and the liquid in the middle will be coolest).
The fluid will be cooler than the pipe until they are equal at the boundary layer. It will then take time for the heat to transfer through the liquid. Hence, the smaller the diameter, the quicker this transfer will occur and all of your liquid will be at the same temp of the pipe.
Where I'd challenge Boer...blah is the relationship between the volume of liquid and the exposure to the convective surface.
Volume = cross section area x length.
Pipe exposure area = diameter x length
Volume is a squared relationship (Pi x r^2), diameter is proportional. As the pipe diameter increases, the volume to surface area ratio decreases. If this doesn't make sense, look at the numbers -
1m long Ø12 pipe
Volume = 1.1309 E-4 m^3
Surface area = 0.0377 m^2
Ratio = 333.36 area:voume
1m long Ø100 pipe
Volume = 7.854 E-3 m^3
Surface area = 0.3141 m^2
Ratio = 39.99 area:volume
Like my example earlier, the smaller the pipe the better for a given flow velocity. I'll stand by this to my grave.
So, back to my first 3 factors-
- The higher the velocity, the more effective the convection
- The greater the difference in temp, the faster the rate of change
- The better coefficent of thermal conductivity, the more effective the convection
In reality, you can observe a two-way process going on. The wort is cooling the HERMS water, and the HERMS water in turn is heating your wort. Let's ignore this but it's worth considering to understand how the interaction works.
Regarding pipe volume vs. water volume ratio, look at an ideal system. To me, an ideal system is one that will make wort come out of the HERMS at the temp we want it to come out. In at 56°C, out at 67°C for our sacc rest.
If a Ø12mm HERMS pipe was infinitely long and the HERMS water was 67°C, the wort would come out at 67°C. If the pipe was 50mm long, you need to address it with any of the three factors above (slow the wort down or increase HERMS temp). Alternatively, if the water was only 1mm thick between the pipe and the heating element then the heat will be applied almost directly and the rate of change will be huge. So either minimise your water or maximise the length of tube, but fundamentally this could be treated as a ratio between the water volume and pipe volume.
Don't misunderstand the technicality as arrogance, I've tried to stay away from long words, but more clarity is required because there's a lot of observational comments going on. That said, with systems like this I look at the extremes - if the pipe was 1mm long vs. 1km, which would work better? This will almost always lead you in the right direction.
Once again, appreciate the input.
