Upgrading SPD to full bore - question regarding joint size.

I’m in the process of upgrading my SPD to full bore from GL connectors. I noticed that lab society (and maybe the industry more generally?) seems to be moving away from 24/40 joint distillation heads. I was planning on going with a 24/40 head because the cold trap I’m looking at buying has 24/40 joints. My question(s) - Does full bore mean that every joint/connector is the same size? Would I notice an improvement with a distillation head with a 34/45 or 45/50 joint vs a 24/40 joint, even tho some components will still have a 24/40 joint? I’m assuming yes, but will the increase in speed going with a 45/50 head joint combined with a 24/40 cold trap joint be worth the extra money?

Also any recommendations on cold traps with larger joint sizes? I’m leaning towards the LS dual inline cold trap which has a 24/40 joint.

Thank you.

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Lots of them will be like that and will work just fine. Mine’s a 34/45 joint at the boiling flask for the head, but it’s 24/40 from there to the traps.

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Ok great! Did you ever use a boiling flask with a 24/40 joint? If so, did you notice a significant increase in throughput/speed from switching to 34/45? Just wondering if it will be with the investment as 24/40 heads seem a good bit cheaper and I’d also need to invest in a new boiling flask.

The general rule is the larger the joint size, specially at the bf, the faster the flow.


24/40 isn’t very good.

I’ve found cold trap joints around 34 or so work best. Too large of a joint and you scatter you vapor and bypass your trap.

The speed isn’t based on joint head size. The laminar flow pattern will determine the flow speeds.

Have you seen the inv cup cold traps from summit. They are the most powerful dry vacuum traps on the market.


The at to think about it is like this.

24/40 - 2l
34/40 - 5l
45/50 or 55/60 - 12/22l
71/60 -22/50l
Flange - any size basically.

The size of your bore will determine the reflux/dance you have. You don’t want a 24 or 34 head on a 12 liter flask as it will spit…


What do you mean by scatter your vapor? Wouldn’t the condenser determine what gets by into the cold trap and then the double trap stop anything that got past the first trap?

Also the size of the neck definitely determines the speed. 100%, as you confirmed in your own post. I will give you that the 34/45 joint should be the largest after that, but neck size matters for sure.

Not exactly. Alot of "full bore"systems fail to trap the vapors becusee it jumps past the trap. Generally nspeaking if you have too large of bores the vapors scatter through the traps and don’t allow proper traps to do thier Jobs.

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Ok, but in my experience the size matters. Where now I’m finally getting a 45/50 head from 34/45, it allowed me to go through my terps in less than 2 hours with 6l compared to 3-4 hours with a 2.5l (34/45). And my countless discussions with both you, when I saw you a couple of years ago, together with many others seem to be certain that the joint size increases speed.

Yes you are correct. A larger head has not functionality. I just wouldn’t want to throw that onna 24/40 hard pipe setup. I prefer 34

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The issue with high vacuum speeds causing the colt trap to be ineffective should only factor during initial pulldown or devol. This happens only when there is enough fluid that the pumping speed substantially dictates the particle velocity and you can suck the fluid through too quickly (ie shallow vacuum). However, cold traps are designed to function at deeper vacuum levels where the molecules bounce around and “find” their way through the piping (molecular flow). Practically, you shouldn’t have an issue with overrunning your cold trap when you are at deep vacuum like you should be (sub 100 mtorr) unless you are generating a ton of vapor that isn’t being condensed by your primary condenser (ie incomplete decarb or similar). You can save your pump by throttling the flow or introducing a large controlled leak (after the cold trap preferably) until you are at deep vacuum.

Once you get to molecular flow levels of vacuum, that bore size gets way more important because bernoulli’s effect doesn’t do fuck all to move vapor through the pipe, and the bigger the hole to bounce through, the more likely the molecules will find their way out. The cold trap is still limited by delta t and surface area like any other heat exchanger, but you’re really moving so little mass that it takes a lot for an overrun.

For big systems/pumps we use a soft-start valve that closes when there is high mass flow to keep this problem from happening. This is great with a pump that does 350 scfm and runs a couple diffusion pumps simultaneously on 2" piping, but manually throttling the valve works in a pinch.


Awesome post @SidViscous. Can I see some pics of some systems you run? I bet they’re sweet

Unfortunately it’s very difficult for me to take photos without catching something that I’m not allowed to show. I’m also in a very regulated market so I don’t want to show anything that could be “identifying”.

Now, I can describe our main vacuum system, which is an Alcatel ADP602LM multi-stage roots dry pump that is hard piped via DN100 (4") and KF50 (2") hard piped stainless. We use idealvac soft-start “valves” (really just a spring loaded flapper in the KF centering ring. This let’s us run everything from vac ovens to SPD gear on a distributed system. I have two Edwards Diffstak diffusion pumps that run off of it (I think one is a 160 and one is a 100). For vacuum regulation we use a combination of SMC ITV and IRV regulators combined with some ASCO and SMC solenoids for rapid pulldown bypass. I don’t have a high vacuum reg in house right now, but it hardly ever comes up. We almost never run SPD and the other high vac stuff is proprietary.

The cleverest thing we came up with aside from our giant Dewar style cold trap is a post heater after the cold trap and before the pump. Condensible vapor is the kiss of death for dry pumps (especially nasty vapors). What we do in case we overrun the trap, in addition to a big refrigerant dryer filled with adsorbent is to put a big water heater element in the piping between the trap and the pump. This superheats any vapor so that it won’t likely condense as it gets compressed. Because the unit is water cooled, this isn’t a big problem. So far it has kept a $20k+ (new, our isn’t) pump from having any trouble. The regulators and soft starts keep the whole room pretty minion-proof.

This is nothing compared to the systems we used to build when I worked in high vacuum (parallel turbos with cryo pumps for 10^-12 to 10^-14), but it’s pretty damn sweet compared to individual rotary vanes and diaphragm pumps. That and it was mostly eBay and stockroom stuff so we’re in for less than $25k.


We have a customer who asked to change the cold trap to a serpentine condenser. He may thought that it not only will have the function of cooling the vapor to water and can extend the vapor cooling time