water bain-marie question

[Birzzz]

Setup: electric 11kw water jacketed bain marie 26 gallon with a 5psi PRV on the jacket.


I understand that it is the concentration of alcohol that dictates the boiling point of the wash. If you have direct heating and you put more power once boiling, I know the evaporation rate will get faster. But with a bain-marie, how does putting more power can increase the vapor speed of the wash that boils at let's say 93c. If the water is boiling in the jacket, if you crank the power up, the water will evaporate faster, but does it affect anything on the wash? Sorry if it's a dumb question..

Thanks,

[still_stirrin]

Same thermodynamics applies...heat input affects vapor production rate, not the temperarure of the water jacket. The fact you have a water jacket just ensures the temperature will not exceed the boiling point of the fluid in the jacket for the given pressure in the jacketed system.

Adjusting the heat input into your bain-marie will correspond to the heat transfer through the jacket into the wash inside your boiler. Nothings different here...only how you apply the heat.

However, because of the multiple walls the heat tranfer must conduct through, the rate of change of the heat input adjustments will be more “reactive”, that is, lag behind the adjustment to the jacket heaters.
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[Birzzz]

thanks, it almost makes sense in my head now. I need to take thermodynamics classes I guess...

[zapata]

hmmm, I use a pressure jacket boiler, not a bain marie, but I think the temperature very much matters. I haven't taken a thermodynamics class ever, but isn't energy flow dependent on the temperature differential? Meaning that the lower the temp difference, the slower the energy is transferred and vice versa.

I really don't know, but always assumed that this was a limit to water filled bain maries. And that pumping more energy into the jacket would not transfer into the wash at a 1:1 rate. The pressure jacket "gets around" this both by harnessing the energy of phase change, steam gives up it's energy to the wall it condenses on which happens constantly in a steam jacket, as well as the higher temperature differentials. Example, in a 15 psi steam jacket, the temperature is 250*C, assume a 4 kwh energy input, won't there be more energy transferred to the wash than there would be in a 4kw bain marie where the temp is a 212*F max?

[Birzzz]

hmmm, I use a pressure jacket boiler, not a bain marie, but I think the temperature very much matters. I haven't taken a thermodynamics class ever, but isn't energy flow dependent on the temperature differential? Meaning that the lower the temp difference, the slower the energy is transferred and vice versa.

I really don't know, but always assumed that this was a limit to water filled bain maries. And that pumping more energy into the jacket would not transfer into the wash at a 1:1 rate. The pressure jacket "gets around" this both by harnessing the energy of phase change, steam gives up it's energy to the wall it condenses on which happens constantly in a steam jacket, as well as the higher temperature differentials. Example, in a 15 psi steam jacket, the temperature is 250*C, assume a 4 kwh energy input, won't there be more energy transferred to the wash than there would be in a 4kw bain marie where the temp is a 212*F max?

This is what I imagined at first, not that well explained though. there must be some differences regarding heat transfer from a water bain-marie, pressure jacket or a oil bath bain-marie.

[still_stirrin]

The saturation temperature of water at 5 psig is 227*F. If you add heat enough to boil the water in the jacket, it will be somewhere between 212*F and the pressure relief vent setting (5 psig) for a maximum of 227*F, as it starts producing steam and you add more and more heat, until the pressure from the steam pushes the PRV open.

But you’re not really trying to boil out the water jacket fluid. You just want the water jacket to transfer the heat from the element to the inner wall of the boiler. As Zapata said, temperature differential is the “motive force” for the heat transfer. So getting the jacket temperature above the wash temperature will transfer heat to the wash, the greater the differential the greater the transfer rate.

But remember, some of the energy you put into the system will go to raising the jacket temperature, and if you’re getting a phase change, it will consume a lot of that energy. Saturated steam carries a lot of energy, especially at elevated pressures.

The advantage of the bain-marie is that the temperature of the heat transfer medium (water jacket) is less likely to cause a scorch inside the boiler. If the wash was direct fired via an element, localized temperatures on the element could far exceed the water jacket (and wash) temperatures.

But if you look at the system (in steady state operation), the heat into the system is equal to the heat taken out (in the product condenser) plus any heat loss to the ambient. And some of that loss would be in the “waste” steam to vent out the PRV, if it does.

So, ultimately, the heat transfered into the water jacket will be transfered to to boiler for production of vapors. And remember, your boiler is essentially an atmospheric vessel, ie - not pressurized. So the temperature of the wash will still depend on the makeup of the wash, while the vapor production rate will be determined by the heat input. The bain-marie is simply a “thermodynamic circuit” for the heat transfer, although it will control the temperature to which the energy transfer mechanism follows.
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[Birzzz]

Thanks a lot for that thorough explanation. It is getting clearer and clearer in my head

[Represto]

I posted this in a separate thread but thought it might provide some added clarity in this conversation as well:

http://distillique.co.za/distilling_sho ... ket-boiler" onclick="window.open(this.href);return false;" rel="nofollow