Vessel diagram
Fill level shown as a proportion of each vessel's capacity; colour reflects ABV.
Run charts
Updated live while running.
How a thumper (retort) works
A thumper, also called a retort, is a sealed intermediate vessel connected between the boiler and the condenser. Vapor from the boiler bubbles up through liquid sitting in the thumper before continuing on to the condenser (or a second thumper). This does two things at once: it transfers heat into the thumper's liquid, and it strips some ethanol out of the passing vapor while picking up ethanol from whatever is already in the thumper. Effectively adding a genuine extra distillation stage without a second heating element, run, or fire.
The catch is that a thumper has no direct temperature control of its own (unless you choose to heat it directly). While it's still cold, all the incoming vapor condenses into it, meaning it fills. Once it reaches its own bubble point, it starts producing vapor of its own and can begin to drain: but only if its own vapor output is faster than the rate the boiler is condensing into it. Whether a thumper ultimately fills, empties, or overflows partway through depends on the boiler's vapor rate, the thumper's size, its starting charge, and whether it's heated. This simulator models that balance directly rather than assuming a fixed outcome.
A note on scope: this simulator uses a single lumped heat-loss coefficient per vessel (same simplified model as the ambient/insulation setting in the pot still simulator) rather than modelling heat transfer through each specific surface (hotplate, vapor-filled headspace, liquid-filled walls) individually. It captures the real fill/drain/overflow dynamics correctly, but is not a claim to that level of surface-by-surface detail.
Detailed calculation model
1. Boiler
Identical physics to the pot still simulator: Van Laar VLE, Antoine vapour pressures, a bisection bubble-point solve against your local atmospheric pressure, and an energy-balance vapour rate once at rolling boil.
2. Thumper mass and energy input
Vapour arriving from upstream condenses into the thumper's liquid, adding its volume and releasing its latent heat (ethanol 841 kJ/kg, water 2260 kJ/kg, weighted by composition) as the thumper's primary heat source. An optional direct heating element adds further power on top.
3. Two-phase behaviour
Below its own bubble point, all available energy raises the thumper's temperature (thermal-mass heat-up, same equation form as the boiler) while its liquid volume only grows. At or above its own bubble point, temperature tracks the equilibrium value and any energy surplus vaporises the thumper's own liquid via the same VLE and energy-balance approach as the boiler, a genuine second equilibrium stage, not a fixed multiplier.
4. Overflow and dry-boil detection
Each vessel has a set capacity. If accumulated liquid would exceed it, the run flags an overflow. If a directly-heated vessel's liquid volume approaches zero while still receiving heating power, the run flags a dry-boil risk. Both are checked every step, not estimated after the fact.
Chaining is sequential: the boiler's output becomes thumper 1's input; thumper 1's output becomes thumper 2's input (if enabled) or the condenser's input otherwise. Ambient temperature, insulation, thermometer error and atmospheric pressure apply to every vessel the same way they do in the pot still simulator.
Frequently Asked Questions
A thumper (also called a retort) is a sealed intermediate vessel plumbed between the boiler and the condenser. Vapour from the boiler passes through liquid sitting in the thumper on its way to the condenser, transferring heat and exchanging ethanol with it. Effectively adding another distillation stage without a second heating element or run.
While a thumper is still below its own boiling point, all the vapour arriving from the boiler condenses into it rather than passing through, so its liquid volume only grows. If the boiler is producing vapour faster than the thumper can heat up, or the thumper is undersized for the amount of vapour it will absorb before reaching its own bubble point, it can overflow before it ever starts producing its own vapour. This simulator flags that risk directly rather than assuming it away.
A directly heated thumper can run dry if its own vapour production outpaces the rate of incoming condensing vapour, draining its liquid faster than it's replenished. Running a heating element against an empty or near-empty vessel risks scorching. This simulator flags a dry-boil risk whenever a heated vessel's volume approaches zero.
Many distillers pre-charge a thumper with feints, tails, or heads from a previous run rather than starting it empty. This gives the thumper a head start on ABV and thermal mass, generally shortening the heat-up phase before it starts producing its own vapour. If the pre-charge ABV is lower than the boiler's incoming vapour, the thumper will tend to fill; if it's already quite strong, it's more likely to reach its own boil and start draining sooner.
Yes: a direct heating element on the thumper adds power independent of the vapour arriving from the boiler, helping it reach its own bubble point faster and reducing overflow risk. It also means the thumper can continue producing vapour even if the boiler's own output slows down later in the run.
Yes: enable Thumper 2 after Thumper 1. Vapour flows boiler → Thumper 1 → Thumper 2 → condenser, with each vessel receiving the previous vessel's output as its own vapour input. This models a genuine three-stage distillation in a single run, common in some rum production setups.
Yes. Every vessel's bubble point is solved from the same local atmospheric pressure, set in the Advanced panel. Lower pressure at altitude lowers the boiling point of the boiler and every thumper together, without changing the achievable ABV much.
Each vessel uses the same Van Laar activity coefficients and Antoine vapour pressure equations as the pot still simulator, with a real energy balance driving heat-up and vapour production. Real thumpers differ due to vapour distribution inside the vessel, splashing, incomplete mixing and heat losses specific to your equipment's geometry. This tool is for education and planning, not a guarantee of a specific outcome.
Still parrot with alcoholmeter: The simulator predicts the ABV curve; a parrot on the condenser outlet is how you read the real one jar by jar without stopping the run.