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Been reading and reading and reading. Can't find a definitive answer.

I have a mixed diameter column setup.

2" x 90 cm with 70cm of tightly packed copper
The a 20cm open air gap during which it scales up to 3"
Then 60 cm of tightly packed copper mesh bore it scales back to 2" and a crossflow reflux condenser.

This setup will produce 94.5% at 1.25 lph

Question is, what is the hetp of this?
Can't find a calculator for this.

You won't find a calculator for exactly that. You can calculate your HEPT following this thread. Considering you have certain variables, the results will be unique to your exact setup compared to a specific packing that is uniform and with a column that is consistent in its diameter.

Take 10 guys and they will roll their copper mesh differently. Not all copper mesh is identical. They'll get different results. That is why I prefer structured packing of similar size. I use lava rock of similar size. Some use SPP. I get the exact same results using structured packing. I have seen different results with stainless scrubbers depending on how dense I pack them in the column. How you pack them is different from run to run because how I pack them is not always the same. If I pack it too tight I get flooding and low ABV. Too loose I get lower ABV. Structured packing is VERY reliable in my experience.

Thanks. Will read that thread

There is no such thing as "HETP for copper mesh". You could have a "minimum HETP" or a "typical HETP", but will be dependent on the still and how it's run.
If you want to calculate your HETP you can calculate it from your boiler and product ABV just by marking off the steps on a Mccabe-Thiele diagram. But if you are doing an entire run at 94.5% distillate, your HETP is not constant through the run because you are changing the reflux rate.
Is there something practical you are trying answer by calculating HETP? That may be easier to answer directly.

I do have to ask, what's up with the diameters? Is it just a matter of what you had available, or are you trying to accomplish something specific? It seems to the the 3" is totally wasted on this setup and likely offers slightly less performance than if it were 2" throughout.

Zapata

Curiosity - reading leads to questions!

As to why the setup the way it is, I started out with a 2" x 90cm packed riser. (60cm packed tightly)
Was happy with that, sort of - then a special came on for a 3" x 80cm riser (60cm packed tightly) and i started using that. I found that the heads were more compact (coming out at 88 - 90%) and the final product was smoother. So i stayed with that riser.

Up until then, never did a proper reflux - tried once on a short column, max i got was 90 - 91% - not enough (made a decent vodka though)

But more reading, higher columns, reflux (that's where the HETP questions came from) and i decided to put the 2" at the bottom of the stack and the 3" above it. (need a step ladder to assemble it). Thinking was that it would move through the 2" and the packing, then hit an open spot before hitting the 3" with the packing through which it would move more slowly which should help with separating 'stuff'

I have not yet built a CCVM and only have a small cross-flow (2 pipes) reflux condenser. It works to a degree, can't stop the output completely, but i can slow it down to a drop or 2 per second with quite a bit of flame under it.

So, as it is an inadequate condenser, and i am still getting 94.5 to 95%, was curious as to what the HETP would be - it obviously has some impact as it must be more than 1 (for the pot).

As to staying constant at that ABV, it stays there or thereabouts for about 35 to 40% of run, then starts to drop as the temp goes up. I don't change water flow, nor power until i am past the hearts and into the tails. When i do increase the power, the ABV drops quickly and the temps go up quickly.

Curiosity Curiosity

Now, to find the parts (difficult to find 3" copper stuff here) or give in an build a 2" CCVM or get a tubed dephlag and just go CM

Just an observation - when i run a spirit run in Pot mode (most of the time), using the 3" riser, once the head of the boiler gets to 60°C, i slow down the flame a lot, and then it takes another 45 minutes before i get my 1st drip of fores - and that stuff is nasty strong. I keep at that power level until the temp suddenly jumps about 2 - 3 °C, toss that bit, and start collecting heads, hearts, tails
There must be some passive reflux happening in the riser as it takes a long time for the heat to crawl up to the head.