Was hoping someone might be able to shed some light on why some stripping runs take much longer than others when variables appear to be the same? The takeoff rate can really vary and I'm not sure why. I use a steam-jacketed still that's cooled with municipal water. I mash in batches larger than my still so therefore do several stripping runs of the same mash meaning ABV is the same. (My spirit runs are also often very different durations but there are many more variables to control with three side-mounted dephlegmators to balance, etc so it makes more sense).
Any help is much appreciated!!
are you turning over the wort tank before you fill the still each time to harmonize the alcohol in the solution? with an ABV that low, i shouldnt think it would stratify so much to have an effect, but just trying to get a complete picture
Hey Classick, I'm not turning it over before charging the still but I do check ABV every time I fill the still and it's always about the same so I don't think stratification is the culprit.
maybe post a pic of your still? might give folks a better idea of the physics
I don't have a pic handy but it's a 20 plate hybrid with 4 plates on top of the pot and two side mounted columns with 8 plates each.
For my stripping runs I run it like a simple pot still: going from the pot, through the pot column with the four plates turned off, straight to the condenser (bypassing the two side columns).
The first thing that comes to mind is different ambient temp in the distillery and/or varying water temp for the condenser.
The ambient air temp doesn't ever fluctuate more than a few degrees and the municipal cooling water can fluctuate about 15 degrees between the seasons but not from day to day. Again, the different length runs are occurring from one day to the next.
Are you seeing the same type of fluctuation at the same time of day each day? Is it predictable?
If the size of the charge and the ABV are consistent from run to run, and you are not losing vapor to the atmosphere, then inconsistencies in batch time can only mean that your rate of heat input is varying. The rate of heat input is the product of three factors:
Heat rate = heat transfer coefficient x area x temperature difference
One of these 3 must be varying. Looking at them in turn:
Heat transfer coefficient. This is pretty much fixed by the geometry of your system and the most likely thing to change it would be fouling of the heat transfer surface. You would see this as a gradual decline in heat transfer and I doubt that it would improve by itself to cause inconsistencies.
Area. One might think that this cannot change, but if the condensate is not removed quickly enough the condensate builds up and masks some of the area. This method of varying the area is sometimes used to control heat input to the reboiler on large continuous columns. Check your steam trap and strainer for fouling or blockages, and maybe re-check the sizing of the steam trap. A steam trap should typically be sized for 3x the average flow rate.
Temperature difference. The temperature of your mash won't vary much between batches, so for the temperature difference to change your steam temperature (and therefore steam pressure) would have to change. Watch the pressure on the steam inlet to the jacket. The problem could also be with the steam trap. If it fails open and passes steam the pressure cannot build up in the jacket and the temperature will be lower than desired.
Of course with your spirit runs the rate of reflux will also impact on the batch time.
You mentioned that the plates were "off" - not sure if this means bypassed or that there was no dephlegmator water flow.
If there is no dephlegmator water flow, and there is minimal passive reflux, it must be heat input like Meerkat suggests.
We have a boiler feeding the brewery and distillery. There is a very noticeable difference in run times when the distillery is the only thing running. Plus, on a pretty regular basis throughout the day the stills will totally lose steam pressure when the water heater kicks on. Can make a 20 minute difference in takeoff time if the timing is bad. Are there other loads on your boiler?
Meerkat, what you're saying makes perfect sense so I'll investigate the things you've suggested. Thank you!
Classick, it's not at all predictable which makes it all the more frustrating!
Jamesbednar, by "off" I mean that the individual drain valves on each plate are in the open position and there is no water flowing through the dehplegmator.
RobertS, my steam boiler is only used for heating my mash tun and the still (which never happens simultansously) so there isn't any other load on the boiler. My steam system is a closed loop and I have to manually turn on the water to feed more into the system so I don't think it could be an overload caused by using water heaters at the same time.
Well at least thats a diagnostic... if it were predictable, you'd know something.. that it isnt... you know what it isnt.. and that something too haha. the absensce of evidence is not evidence of absence, as they say.
Only thing else i can offer is that when we notice our yields are not what we expect, and the settings at the still (flame, water etc) were according to SOP, then it means we had a leak in the still somewhere, i.e. we didnt latch down the top of the still properly, or one of the connections was loose.
Love to see the group troubleshooting though.
Reply:14 hours ago, StonesRyan said:
RobertS, I don't think you are grasping at straws at all - I believe you have nailed it. And it would be very easy to verify. If the boiler pressure decreases when the water is fed then that's it.
RobertS & Meerkat, I've only been manually feeding the water for about the last month. Before that, the boiler was overfilling and I had to drain a lot of water off after every use. The steam boiler mechanic suggested shutting off the water feed on the boiler feed pump and just keeping an eye on it and adding some when necessary (he thought that when the boiler is on and all of the water/ steam is in the steam line and steam jacket that it leaves the boiler empty so it would refill). Anyway, the run duration inconsistency was an issue before this change so I wouldn't think this would be the cause.
Ryan - I think your last post is the big clue - you shouldn't be pooling a significant amount of condensate in your jackets or lines, at least not so much that you would see a big overfill situation. You might want to make sure your steam trap is sized correctly and working properly. If you are getting a large amount of pooled condensate in your jacket - you are going to see significantly reduced heat transfer. Or, potentially, you are seeing a stall condition - http://www.tlv.com/global/US/steam-theory/stall-phenomenon-pt1.html
Is this something that happens at startup? If so - suspect undersized trap. If it happens sporadically, you might have a sticky trap - you can take these apart to inspect and clean out. We had a trap on our steam water heater clog up with junk - even though it was protected by a strainer.
Does your make up water system use a float in the condensate tank or on the boiler itself? (it should be the latter).
James, thank for the info. It makes perfect sense. It definitely seems to be sporadic. I had the boiler cleaned when the boiler mech was here a month ago and it was filthy with cutting oil from installation. They'd never cleaned it after installing it about a year ago so it's very possible the traps are sticky/ dirty.
I believe the float for the makeup water is in the boiler feed pump which seems to run almost constantly since they told me to shut off the water supply.
Thanks for the help!
UPDATE: I cleaned the strainers and steam traps. The strainers had a considerable amount of debris in them but the steam traps seemed just fine. Unsure if debris in the strainers will slow things down only sometimes and I haven't run the still yet but am hoping this helps.