Jump to content

Glycol (& other heat fluids) vs. Steam

Jedd Haas

Recommended Posts

I am considering different ideas for a stripping still in the range of 55 to 200 gallons. I've looked into steam, and the evidence suggests it's a good option. But maybe heat transfer fluids are more cost-effective?

Propylene glycol goes up to 370 F, but here is a heat transfer fluid http://www.therminol.com/pages/products/xp.asp that goes up to 600 F. (It's mineral oil.) So here the question is whether having a larger heat differential will speed up operation (boiling of the wash) enough to justify its use. The glycol costs around $700 per drum, while the Therminol is $1,150. Of course, it's probably best to not heat either fluid up close to the max temperature, both for longevity, and for safety. But let's say you took either one up to 300 F, how would it perform, relative to steam?

While the temperature differential is one point, I've seen some postings that suggest that the heat-transfer function is still inferior to steam for various other reasons.

I visited a facility today that uses glycol, (for chilling, not heating) and they mentioned that they have slight losses in their system. They weren't sure if the problem was a leak, or maybe the stuff sublimates, very slowly? Or maybe the fluid breaking down causes it to shrink? Bottom line, you would probably need to consider it a consumable.

Now the question of direct heat vs. running thermal fluid in a loop. This seems to be the area where the fluid has an advantage, as you can likely use smaller pipe sizes. Maybe 1" black steel for a system the size I mentioned, which is a lot more fun (haha) than 3" black steel for steam.

I considered one design for direct heat. This would be a 200 gallon still with an outer shell holding 20-50 gallons of glycol, depending on fill level. The contact area would be at least 10 square feet, and could go as high as 35 square feet, depending on the fill level. Let's say you heated that with 8x 5.5kW immersion heaters, going around the outside, equidistant. That sounds pretty good until you calculate that at 110v, you'd be pulling 400 amps at max power! (Of course, you could have a variety of series-parallel selections to vary the power by steps. Now it could be I'm overestimating the power requirement (or worse, underestimating it). Either way, 400 amps, or even 200, doesn't come cheap. So gas makes more sense.

It seems that direct firing a jacketed setup like this, I'd have to build my own firebox. Not sure if that would pass inspection, so the glycol loop seems more attractive. Definitely more modular, for adding more equipment; I also like the safety factor of removing an ignition source from the immediate vicinity of the still.

Does anyone have experience with a system like this?

Link to comment
Share on other sites

Actually, steam is often a BEST choice for heating to near boiling. Why? Because the most efficient means of transferring energy to the boiler is by condensing the steam near boiling temperature onto the boiler wall. You can't overheat, and there is far more thermal energy with changing phase from gas to liquid than with the convective flow of a heating fluid.

Link to comment
Share on other sites

Blue star, I'm familiar with the point you mentioned regarding the phase change. However, I'm not so sure that's the final word, in capital letters, as to what's best. As I mentioned, there are several other factors I am considering. Do you have some hands-on experience with both types of systems you can share?

Link to comment
Share on other sites

I'm not much of an engineer, but I'm going to take a crack at this one. Assuming that there is some benefit to the quality of your distillate with higher temperature heat-exchange, as our friends from Cognac might, you should remember that what we are talking about is a transfer of heat, not a transfer of temperature.

The specific heat of water is 1 btu/lb (more or less). So, if you wanted to move 100,000 btu/hour from your boiler to your kettle, you would need to move 100,000 lbs of water (which I believe is something like 12,500 gals).

On the other hand, the latent heat of evaporation of water is 542 btu/lb. In order to achieve the same heat exchange, I believe that means that you would need to move 542 times less water; i.e. 23 gallons.

Pretty big difference. I wonder if you could physically move 12,500 gals of water through your jacket in a reasonable amount of time without having to install the big pipes that you are trying to avoid which are necessitated by steam.

I believe that the specific heat of glycol is even lower than water's too. This would only make the problem worse.

You should double-check my numbers though. I've never actually done these calculations before so there might be some errors.

But I'm pretty sure that there is a reason everyone uses steam or direct fire which is along these lines, even if the above numbers are not exact.


Link to comment
Share on other sites

Steam is the best option for heating your still, but high pressure steam brings with is many issues.

a steam heating system isn't something you can put together yourself, you need to have it engineered and installed by professionals.

operation requires a steam engineers ticket.

Low pressure steam has more forgiveness, and does not require the same certification to run. Low pressure steam is not as efficient at transferring heat, but your equipment still needs to be certified for use.

this is where heat transfer fluids are good. they do not require pressure vessel rating and are nearly as efficient as steam, the shortcomings can be overcome by running at higher temperatures. Both steam and heat transfer fluids have their inefficiencies and their disadvantages, but in proper application Heat transfer fluids can be a better option especially as you first begin.

Once I get my micro licence, I Intend to use glycol heating with an external boiler and circulation.

Link to comment
Share on other sites

VB, that touches on a lot of the points that have come to mind. HP steam is not an option, while LP steam, maybe. Oil or glycol are "no pressure" as the system is vented. So now the question, how high does the temperature need to be for it to make sense?

You say "heat transfer fluids...are nearly as efficient as steam." Obviously, a couple comments above differ. Do you have any data or hands-on experience to elaborate on this point?

Also, have you made any initial selection of external boilers? Regarding the circulation, I am familiar with the Grundfos pumps and they seem to be pretty good. What are your thoughts on the pipe size required vs. still size?

Link to comment
Share on other sites

First a quick lesson is Heat transfer-

When calculating heat up times engineers typically draw a diagram that looks like a couple resistors in series. Each resistor is a heat transfer rate. For a basic still there would be a transfer rate from steam (or HX fluid) to the boiler wall, transfer through the metal in the boiler wall (which for our case is negligible) and finally transfer from the boiler wall to the wash.

Heat transfer rates vary based on the fluid, but also velocity/turbulence, and temperature delta. If you blow on your coffee it cools much faster than if you just leave it on the counter, it also cools much faster if you stick it outside on a cool day.

Point is knowing these parameters one could design a gylcol or some other HX fluid system to heat as well, or better than a steam system (or at least save some money over a comparable steam system, as I will explain below, steam is the best HX medium- but there are some trade offs)

For example, you are limited to much lower temperature delta with saturated steam (not pressurized). Yes the steam can hold more energy, but, you are limited by the amount of energy the boiler wall can transfer into your wash. You could have a HX fluid at a higher temperature than the steam transfer more heat since the rate between wall and wash would increase due to the greater temperature delta. This could also burn your wash. One thing for sure is you want turbulence in your HX fluid to transfer the most heat.

You are going to want to look at a steam table http://www.efunda.com/materials/water/steamtable_sat.cfm

and http://www.x-eng.com/XSteam_Excel.htm

Blue star is correct- its pretty much impossible to beat plain old water for heating and cooling, water can hold and transfer more heat than any other heat transfer fluid.

The other really cool property of water is the enormous amount of energy it can hold within a very narrow temperature range from just below boiling to boiling. We used to use boil water in a vacuum to cool super computers in my graduate lab since it did such a good job of pulling heat away while maintaining a very steady temperature.

So, the advantages of steam should be pretty clear now-

The disadvantage of course is pressure. You will have to do some reading, but I am confident you could make a HX fluid system work as well for less money than steam on smaller stills.

Link to comment
Share on other sites

Loss of Glycol and not knowing where it went? Red flag there!

I know this and several threads before this are concerned with Glycol/oil/steam/open flame/etc for heating of the still. I imagine we are all here to be in or about to be in the business of craft distilling. Business meaning return on investment. If you are going to build out a facility, you should be aware of all the uses of heating and cooling in your plant. Let your facility and cost of energy contribute to your decision on how to heat your still. Are you going to have a stream boiler anyway?

We do use Glycol, to a proper dilution by the way, for managing temps of Fermentation. I am greatful each time we turn the circulation pumps on that I do not end up with a piping crack allowing 400 gallons on the floor. We use steam to raise temp. We use steam throughout the plant. The Still, Mashing, pre heat H2O for several uses. For us, the boiler and piping also contribute to heat the plant for most of the year. For "Scrooge McRyan" it is cheaper than those electric overhead blowers, again, for us. Just consider the whole of your operation and also what can go wrong, how long it will take you to recover from a fail point and how expensive it would be to fix.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...