Dedicated Herms Guide, Problems And Solution Thread

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Not sure I posted this so...
Trialed my 3kw PVC HX on a 50L brew, I did get better ramp times (the aim) but i did get deformation of the PVC HX.
I forgot to put a vent hole in the top so this may have caused the problem. Any part that had water in it was fine but where there was air it deformed.
Still works fine but looks munted.
Nev
 
TheWiggman said:
This is correct. For a given length of pipe, assume 100mm long, imagine the two scenerios -

1. Very skinny like a pen ink tube
2. 10m in diamater

With heat applied to the outside, which of these two options would heat the entire volume of liquid in the pipe faster? The skinny one of course. This is the basic principle why having a smaller pipe will allow more rapid heat transfer. There is obviously a lot more to it than this, and as QldKev says you'll suffer from flow restictions if the pipe is too narrow/long. Fine for positive displacement pumps with enough power, no good for centrifugal pumps.

Your comment about the pipe length QldKev is partly right. Minimising water volume is the goal, but the longer the pipe with heat applied to it the more opportunity for the heat to transfer. The ratio of pipe volume : water volume is the critical factor I believe. Generally with a larger pot the ratio will drop due to diamater and affect your ramp times due to the fact the element needs to heat the additional water. The HERMS coil seems to be an ideal ratio in reality.
In an extreme case, if you put say 10kW of energy into a single HERMS in a little PVC pipe, you will probably find the temp in the water will greatly overshoot your HERMS output temp as there is not enough heat transfer able to occur over a 2.5m pipe length to equalise energy in = energy out. The only way to overcome this would be to add more length, and increase flow up to a point.
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.
 
Not sure I posted this so...
Trialed my 3kw PVC HX on a 50L brew, I did get better ramp times (the aim) but i did get deformation of the PVC HX.
I forgot to put a vent hole in the top so this may have caused the problem. Any part that had water in it was fine but where there was air it deformed.
Still works fine but looks munted.
Nev
Nev, how much better were your ramp-times compared to the original 2Kw element?
 
TheWiggman said:
. The ratio of pipe volume : water volume is the critical factor I believe.

...

In an extreme case, if you put say 10kW of energy into a single HERMS in a little PVC pipe, you will probably find the temp in the water will greatly overshoot your HERMS output temp as there is not enough heat transfer able to occur over a 2.5m pipe length to equalise energy in = energy out. The only way to overcome this would be to add more length, and increase flow up to a point.
The surface area of the coil is more the point. A circular profile coil is actually the least ideal from a heat transfer perspective - look at the shape of the plates inside a plate chiller for something more appropriate. Obviously a round tube is handy for other reasons. The volume inside the coil isn't a primary factor IMO.

I wouldn't be surprised if the regular small stainless HERM-IT coil could indeed handle 10kW of input power - with enough flow. The real question is, can the heating element keep the HX boiling when the pump is pushing wort through the coil at full flow?
 
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.
 
Wiggman, thank you. Really appreciate your thoughtful response. The most obvious factor that was not included in my thinking is the volume in the pipe vs. the surface area of the pipe. Definitely agree with you on the observational comments, hence me asking the question.

Once again, thank you for the reply.

EDIT: I think from a practicality point of view and also to prevent flow restrictions, I will keep my design at 12mm, but it does raise the question of coil length, displaced volume and heating output.
 
MartinOC said:
Nev, how much better were your ramp-times compared to the original 2Kw element?
Martin
I didnt actually do the ramp times as it was a late afternoon spare of the moment brew.
I can just say signification, at a guess 1c per min in 50L brew but need to be verified.
Nev
 
I've had some time to go over the text books and want to confirm one thing. For a given:

Flow rate
Pipe length

The larger the pipe diameter, the greater the heat transfer. I can supply equations if boringly required.

This might sound like a fallback on the previous post. The principle still remains that if you have a certain amount of space to play with, use the smallest diameter pipe and cram as much of it in there as possible (provided your pump can tolerate the additional restriction).
 
Thanks Wiggman - that makes sense and kind of makes you guys both right :D Once again, appreciate the input.
 
Is the flow rate in the equations a distance/time or a volume/time ?
 
Well I have just tested my HERMS last weekend. I have a 5L SS vessel with a 4Kw element and a 4m long 1/2" SS coil.

I have tested it with water in the HX vessel and had 1.98°c per minute rise with 60L of water and also with canola oil in the HX vessel and had 1.35°c per minute with 60L of water.

On Sunday I did a triple batch of British Extra Special so 13Kg of grain with 53L of water in the MLT. Using water in the HX vessel I was achieving 1.4°c per min rise through all my steps (35°c>42°c>56°>64°c>76°c). One thing that I did notice was the flow rate as it was my first time testing the MLT. I could have the pump running at 8L per minute. When I mashed in I was around 4L per minute on the pump and I was getting about .7°c per minute rise.
 
dent said:
Is the flow rate in the equations a distance/time or a volume/time ?
Volume/time. Obviously as the diameter increases, velocity slows in ye olde squared relationship. Essentially this means more contact time with the pipe.
 
I was testing with oil in the HX to see if it would hold heat better and also it would not need to be topped up as often. It was just too slow to get the heat in to the oil so it was ditched for good old water.
 
I've never had to top up my HX during a brew. It never really boils unless I turn the pump off and leave the element on. YMMV.
 
It wasn't a brew by brew top up I was thinking. As you can see in these pics my element is vertical in the HX and is close to the top and also it is not the easiest lid to get off because of the plumbing.
gallery_25427_1158_119379.jpg

gallery_25427_1158_65555.jpg

I have solved the problem with a float guage so I know when the water is low and needs topping up.
 
Nice insulation! What's your HX vessel made from?
 
I made it out of some SS from the scrap metal dealer. It was a massive heat exchange unit. The outer is what I made HX out of and if you look at the pipe in the middle of my MLT (pic below) they were the inner lines. There were about 15-20 inner lines fitted inside the 125mm diameter pipe. It must have been massive as it was all cut in to 1.5m lengths and dumped in the bin and it was crammed full. Best $25 spent.
gallery_25427_1158_1490794.jpg
 
I had my PID enclosure finished and set up over the weekend (parameters for now - including both offsets for the PT100's - and will tune the PID's this weekend). I just have the single supply for now, but designed it for two 25A supplies for up to 6kW (definitely overkill, but like to have the option) if I ever wanted for when we move into our next place, which will hopefully be at the start of next year. I decided for two separate supplies as opposed to one 50A supply due to easier access to suitable hardware and smaller cables. Plus I liked the idea of having the enclosure split for HLT on the left and HERMS on the right.

I've just got a 15A outlet and cable for now which feeds both RCD's, so will only be able to use 1 element at a time until we move, but that's all I've been doing for now with my STC1000 controllers so not really an issue. The PID's are able to be switched on and off via the red switches, similarly with the pumps and the green switches. Not sure if I'll worry about labels, although they probably would be a nice touch. I've left room at the bottom of the enclosure just in case I decide to add anything else down the track, like multi-function meters..

The two flush mount outlets are for either pump, and the other black cables for the supplies to the HLT and HERMS elements. I'll just need to upgrade the element supplies and glands if I decide to go above 2.4kW.


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