Condenser Capacity based on Vapor Volume

[Twisted Brick]

I tried to answer this question and realized it's over my head technically. It also may be splitting hairs, but hopefully someone can chime in here.

On a pot still, given the relative volumes and vapor speeds produced by a 2" and a 3" riser/head respectively, which of the two would require the larger condenser capacity, say in a 2" shotgun?

Or, asked another way: Would a 2" riser require more condensing capacity because of its vapor speed, or would a 3" riser require more because of it's increased volume?

Also puzzling is the fact that each riser either feeds or is reduced into the same 2" diameter of the shotgun.

[Chauncey]

Subbed

Keep answering the question to myself but when I type it out I think it seems wrong. Waiting for the gurus


Higher vapor speed should require more length to prevent blowby. But that's in the condenser itself. the vapor speed should go to the same rate thru the shotgun for the 3" as with the 2" though right? So ample coolant flow should still knock it down

[Saltbush Bill]

I think your splitting hairs , provided the condensers in question were not ridiculously small the condenser required would be the same, as both are extracting the same amount of heat.
The amount of vapor leaving the boiler and entering the column which in turn has to be cooled by the condenser is dependent on heat , not column size.

Or, asked another way: Would a 2" riser require more condensing capacity because of its vapor speed, or would a 3" riser require more because of it's increased volume?

Short answer No
Might be wrong but that's my take on it.

[Yummyrum]

I agree Salty , it's more about matching the condenser to the boiler power ......certainly the difference between a 2" and 3" is splitting hairs ...I'd go as far as to say it would not make any noticeable difference .

[still_stirrin]

Cooling power should equal heating power. Since vapor production (mass per minute) is a function of heat input rate, in order to condense the produced vapor, you must remove as much heat as you put in (minus any loss to the ambient, which you should neglect).

But vapor speed does affect efficiency of the heat transfer inside the condenser. So, regardless of the size of the riser, the size of the condenser does matter. The vapor speed should be below 20 feet per second to efficiently transfer its heat to the walls of the condenser. This speed or more appropriately “dwell time” allows the vapors to condense on the surface of the vapor tube.

If the speed is too fast through the bank of vapor tubes, it doesn’t transfer its heat in time and simply puffs out of the end of the tube. So, a longer vapor tube is needed.

More tubes provide more surface area for a given length but the diameter of the tubes determines how many tubes you can fit into a specific diameter of water jacket (shell). Also, consider that as the tube diameter is reduced beyond a certain point, it starts to act with capillary action (L/D ratio). So, vapor tubes smaller than 3/8” ID start to behave as capillary tubes.

In summary, the size of the riser doesn’t affect the performance of the process significantly, especially if you’re using the same shotgun condenser as the PC. The design of the PC does affect its condensing power however and that should be designed to manage the boiler’s heat input.

Sure, if you have a massive product condenser (6”x 36” with 20 vapor tubes) and tons of heat input power (10kW), the riser tube could become a choking point due to excessive vapor speed. But that would be an extremely mis-designed system. Yours isn’t like that.

For what it’s worth, 2” is cheaper than 3” and the 3” doesn’t add any advantage over the 2” given the size of the product condenser. So why not just keep the riser as 2”?
ss

[Twisted Brick]

Thanks, ss. I have always valued your insight and this is no exception.

Now I'm always gonna wonder if the 2" shotgun might be restricting my pot any and if I should've built a 3" shotty. I always just assumed it was more than adequate for the job.

[bluefish_dist]

I ran a 2” shotgun over 12,000w on a large boiler. I Usually ran 3 in series, but it would function with 2. I later added a 3” to replace the first 2” one and while it added some increased capacity for cooling it was less than adding another length of 2”. I have found a single 2” shotgun handles my vm at 5500w no problem.

[Twisted Brick]

Thanks, Bluefish. My NG burner's output is rated at 40,000btu's (11.7kw) but I suspect its probably closer to 35-38,000. With my little 1/25hp pump at 170gpm, my shotgun shell is only hot for the first 2", and runs cold the remaining 19".

I'm curious what your still specs were, assuming it was for stripping. And, what were the lengths of your shotgun 'segments' and output rate?

[Oldvine Zin]

I have both a 3" and a 2" shotgun and have used them both with little difference between the two, the 3" is nice for a no brainier fast strip run. Building a tiny 1.5" one for my mini still


OVZ

[bluefish_dist]

My big still was 120 gallons and I ran it at 12,000w, this was for DSP co-20051. Configured as a pot/stripping still with a 4” riser I could strip in 6.5 hrs start to finish. About 3 hrs to warm up. I could get about 30 gallons of low wines in 3.5-4hours stripping. When configured as a 6” vm with sieve plates I could pull 1/2 gallon every 6-10 min at the start depending on the wash/wort abv. That works out to about 3-5 gallons per hour at the start. It slowed down as the abv dropped and I usually stopped when it was down to less than 1/2 gallon per hour.

With the vm head I ran two condensers. I used the longer 2” shotguns from stilldragon. I later added a 3” that was about 2 ft long.

[Twisted Brick]

Damn, bluefish. I'm amazed one can stuff 6" of vapor into 2" (or 3") tube set. I guess I've always felt there should be some kind of bottleneck. Granted, vapor contracts as much as it originally expanded, but 3-5 gal/hr is just crazy...

Thanks for the specs.

[Twisted Brick]

I have both a 3" and a 2" shotgun and have used them both with little difference between the two, the 3" is nice for a no brainier fast strip run. Building a tiny 1.5" one for my mini still


OVZ

Nice. I think NcHooch stuffed 5 x 3/8" tubes in his 1.5".

I would love to build a 5-6 gal all-copper scottish pot for the stove top...

[Oldvine Zin]

I'm going three 3/8 tubes leaving more room for the cooling water

OVZ

[Oldvine Zin]

Oh and the mini is only 1000W

OVZ

[Twisted Brick]

I'm going three 3/8 tubes leaving more room for the cooling water

OVZ

I like it. A lot of condensing power in a tidy package.