Still Design: Why Air Resistance Matters!

24 June 2021


Air resistance is important when designing a car or parachute, but for a still? Really!?! Yes, it is. Distilling is all about managing and manipulating the vapors that rise from the boiler. The amount of resistance, that the rising vapors encounter, influences the speed of those vapors. And vapor speeds define flavor composition.

Okay, maybe we should call it vapor resistance, rather than air resistance, but it got your attention, right? And it is basically the same thing. Interested to learn more? You better be. Managing vapors successfully is what is required to become recognized as a skilled distiller, so let's dive in deeper.

Cuts for flavors

Have you ever noticed how a fast stripping run isn't ideal for making cuts? How, for your cuts to work out, a lower power setting usually works better? Cutting is about the master distiller deciding what flavors should be in the spirit and what flavors should be cut out.

Heads, hearts, and tails

As we saw in an earlier iStill Blog post, flavors associate with heads, hearts, and tails (for more reading, please see: Fruity flavors come over during the beginning of the run, followed by the hearts-associated substrate flavors. At the end of the run the earthy and rooty tails-associated flavors come over.

Since, during the distillation run, the original alcohol charge in the boiler is slowly depleted, the rising vapors will become less alcohol rich and more water dense. As a result the temperature of the gasses increases (water boils at higher temperatures than alcohol). The iStill takes advantage of this by having an extensive control and management system in place, that allows the distiller to cut consistently each and every time, given a certain power setting. It is the "given a certain power setting" part that I now want to draw your attention to.

Vapor speeds, separation, and smearing

Our stills are insulated. Therefore an increase in power results in the creation of more gasses and more product per minute or hour. Since the column and boiler are structural parts that do not all of a sudden change shape or size, that column - with a power increase - needs to process more gasses. A higher power setting will result in higher vapor speeds in the column.

Low boiling point alcohols (and their associated fruity flavors) come over during the early part of the run. They need relatively low energy settings to boil off. High boiling point alcohols (and their associated rooty, nutty, and earthy flavors) come over later in the run. These molecules need relatively high energy settings and higher vapor speeds to come over.

With the above in mind, now imagine that we do a slow run. The headsy factions boil off first, since they need the lowest amount of energy and the lowest vapor speeds to come over. When almost all the heads is gone, the transition towards hearts collection takes place. And when almost all the ethanol (and its associated substrate flavors) is gone, the tails slowly and gradually blend in.

Now imagine a fast run instead. A high power setting results in higher vapor speeds. The higher vapor speeds push hearts and tails into the first faction of the run. As a result heads aren't separated and depleted. During a very fast run, like a stripping run, the whole batch is smeared and tainted with heads, hearts, and tails. No separation, so not a great flavor. Quick and dirty. Do you see how vapor speed is essential to the amount of heads and tails we smear into hearts, and therefore how essential vapor speed is to flavor composition? If you do, let's continue with the next chapter!

Vapor resistance hampers run consistency

Consistent vapor speeds are essential for consistent flavor harvesting, which is essential for consistent spirit production. You want your whiskey, rum, or gin to taste the same, each and every production run, right? Resistance in the vapor path not only leads to inefficiencies, it also results in vapor speed fluctuation. And if you have read the above paragraphs carefully, you now understand that vapor speed fluctuation is basically the same as flavor fluctuation. Flavor fluctuation prevents the craft distiller to produce high quality spirits consistently. Hence our efforts to design stills with the lowest possible internal vapor resistance. The less vapor speeds fluctuate, the less flavor fluctuation you'll experience in the drinks you produce!

This is why air resistance matters. Resistance creates vapor speed fluctuation. Vapor speed fluctuation creates flavor inconsistency. Flavor inconsistency equals poor (or at least: suboptimal) quality. Poor quality impacts the craft distilling industry at large, and your personal business specifically. Poor quality destroys reputations as well as bottom-lines.

Do you want to become a better craft distiller? Do you want a better reputation and a better bottom-line? Control your vapor speeds. And ask your equipment supplier what he has done, design-wise, to give you that control. "What did you do to prevent vapor resistance?", "How does the technology you provide limit vapor resistance as much as possible?" are legitimate questions any still manufacturer should be able to answer. Only they don't and they can't. We do, and here are some examples of how iStill gives you total control over vapor speeds and associated flavors via vapor resistance reduction.

Wide boiler design

Given a certain boiler charge volume, a still designer can decide to make a tall and narrow or a lower and wider design. A tall and narrow boiler dissipates its energy over a limited surface area. As a result the boil-up will be higher, creating turbulence in the vapor bath above the liquids. This inner-boiler turbulence results in the column or riser being fed with vapors at speeds that continuously change. The constantly changing vapor speeds in the column result in more smearing of heads and tails into hearts, and in less control over the exact amount of smearing.

A wider and lower boiler gives a much larger surface area (square root function!) for vapors to boil off from. As a result, there is less boil-up, and less turbulence. The column can now draw the vapors from a more stable gas bed, at steady speeds. Steady speeds result in less smearing and more control over the amount of smearing! iStill has a wider and lower boiler design, specifically for that reason.


An uninsulated column creates a temperature difference between the outside and inside of that column. The colder environment cools the column. The cooler column liquifies rising gasses into (passive) reflux. This is more than just an inefficiency, because it also creates inconsistency.

An uninsulated column or riser results in passive reflux. Gasses that normally should have come over as product now fall back into the boiler. The colder the climate (or distilling hall) is, the more inefficiencies are introduced.

But an uninsulated column or riser is also a variable that boosts inconsistency. Cooler columns lead to more passive reflux and lower overall vapor speeds. Lower vapor speeds may make it difficult to create the right amount of smearing, for a certain product category. When the climate or temperature in the distilling hall changes, a bigger or smaller temperature differential establishes, resulting in lower or higher vapor speeds, and less control over flavor harvesting and reproducibility.

iStill insulates its columns to minimize the temperature difference outside and inside the column. A low to non-existent temperature difference leads to a more efficient run (less passive reflux), stable vapor speeds, and reproducible and consistent outcomes.

Inner column design

Let's compare iStill's perforated plate still with a traditional bubble-cap fruit brandy still. The traditional bubble-cap column has plates that hamper vapors, where our design allows vapors to pass through. Fruit brandy stills have sight glasses that stick out, generating vapor resistance in the column.

Traditional stills have dephlagmators that cool part of the gasses back to liquid state, depending on cooling management, coolant pressure, and coolant temperature, air pressure, and the differential between inner and outer column temperature. Their dephlags are designed with a limited number of through-tubes.

iStill designs its coolers with a maximized number of through-tubes. More surface area creates less turbulence. Less turbulence results in steady vapor speeds and consistent flavor profiles.

iStill doesn't use dephlagmators and cooling management, but instead invented the column cooler and liquid management. These innovations have taken variables that influence resistance and vapor speeds out of the equation. Coolant pressure, temperature, air pressure, and the difference between inner and outer column temperatures no longer screw things up.

If you didn't yet know why you wanted an iStill, now you do.

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