An Azita Token Mill is a ten-kilowatt machine that has to live in a room that was never designed for one. Every watt it draws becomes heat, all of it, continuously, in a space with a door and probably a window. This note works through the thermal arithmetic that fixed the mill's envelope, and explains why we chose air over liquid when liquid would have let us pack in more.
Ten kilowatts, felt
Start with scale. A large domestic room air conditioner is rated at 1.5 tonnes of refrigeration, which is about 5.3 kW of cooling. A mill at full load produces heat equivalent to two of those rooms running flat out, in one cabinet, around the clock. Nothing else in an office is like it. A photocopier is a kilowatt for a minute. A pantry is two. The mill is a small industrial process, and the room has to be treated as one.
Ten kilowatts is also the number that lets the machine take the room's existing circuit rather than a new one. The mill's intake is 415 V three-phase at twenty amps a phase, off the board that already feeds the floor, with no new connection. At the meter it draws 14.0 to 14.4 kW, not ten: the fans, the supplies and the losses are real, and the room has to reject that heat, which is why the airflow below is worked at the ten-kilowatt load and the room unit is sized with margin above it. That is the whole reason the envelope was frozen where it was. A bigger machine is a better machine right up to the moment it needs a feed the building does not already have, at which point it stops being a machine you can install and becomes a project you have to permit.
Moving the heat
Heat leaves a cabinet in air, and the amount of air required is fixed by physics, not by preference. The relation is Q = ṁ · cp · ΔT: heat carried equals mass flow, times the specific heat of air, times the temperature rise across the cabinet.
Q = 10 kW
cp (air) ≈ 1.006 kJ per kg·K
ΔT = 10 °C
ṁ = 10 ÷ (1.006 × 10) ≈ 0.99 kg/s
ρ (air, room conditions) ≈ 1.2 kg/m³
volume flow ≈ 0.99 ÷ 1.2 ≈ 0.83 m³/s ≈ 1,755 CFMSo a ten-kilowatt mill at a 10 °C rise moves about 1,755 cubic feet of air a minute. That is the number the enclosure's fans, filters and outlet geometry are designed around.
Why 10 °C? A smaller rise means more air for the same heat: at 5 °C the flow doubles, the fans get louder and hungrier, and the room becomes a wind tunnel. A larger rise means hotter exhaust: at 20 °C the outlet air on a 35 °C day leaves at 55 °C, unpleasant to stand near and hard to mix back to room temperature before the inlet sees it again. Ten degrees is a rise the boards inside the mill are comfortable with, and it keeps the exhaust cool enough to blend back into the room.
Rejecting the heat: one 5 TR unit per mill
Moving the air only relocates the heat to the room. Something has to take it out of the building. The mill carries no cooling plant of its own; the room does the rejecting, and the sizing rule is simple.
1 TR = 3.517 kW
5 TR = 17.6 kW
14.0 to 14.4 kW at the meter + the room's own gains (lights, walls, solar) fits under 17.6 kWOne 5 TR room unit covers one mill with margin for the room's own gains. It is a standard commercial split or cassette unit, installed by the trade the building already uses, running on the building's HVAC circuit, outside the mill's own feed. Where a room already has central air handling rated for the load and running when the mill runs, the unit may not be needed at all; the scan decides.
Note the boundary. The mill's electrical intake is 415 V three-phase, twenty amps a phase, and it reads 14.0 to 14.4 kW at the meter. The room unit is not inside that intake. It is the building's equipment, on the building's circuit, and its consumption is metered and reimbursed like everything else the mill causes the building to draw.
Efficiency, stated rather than hidden
Power usage effectiveness is total power divided by the power that reaches the machines doing the work. A purpose-built hall with hot-aisle containment and free cooling does far better than a cabinet in a store room, and we are not going to pretend otherwise. The shape of our arithmetic is this: the mill's own draw, plus the electrical draw of the room unit rejecting that heat against a hot-day coefficient of performance, over the load actually serving requests. On a hot afternoon the room unit works hard and the ratio is poor; on a mild night the same unit coasts and it is much better. The honest number to publish is the bad-day number.
One line of that arithmetic is an assumption rather than a constant: the room unit's own draw. The unit sits on the building's circuit, not the mill's, so how it is metered and how its cost is allocated between us and the host is set in each site's hosting terms. Where the unit's draw is measured, the measured figure replaces the coefficient of performance we assume.
40 °C at the face: ASHRAE A3
ASHRAE's TC 9.9 thermal guidelines define equipment classes by the inlet air temperature the hardware is allowed to see. Class A3 allows an inlet of up to 40 °C. We design to A3 at the mill's face, which is a deliberate statement about where the mill lives: a plant room or store room with the building's central plant switched off can reach 40 °C at the inlet, and the mill has to keep running when it does.
Two consequences follow. The silicon is chosen for its rated inlet envelope as much as for its memory bandwidth, and the fan curve is set so the boards see recommended temperatures most of the time and allowable temperatures in excursions. And the figure is a design point. Bench validation measures a mill against it, sensor by sensor.
The real constraint: after hours
The electrical margin is rarely the binding constraint in a room. Cooling after hours is.
Where time-of-day tariffs apply, electricity is cheapest overnight, roughly ten at night to six in the morning. That is exactly the window in which a commercial building's central air handling is switched off. A mill that can only run when the building's chillers run is a business-hours mill, and its economics are the economics of the expensive hours.
This is why the room unit is a fixed part of the design and not an optional extra, why it sits on a circuit the building does not switch off at night, and why the scan carries an explicit after-hours screen: what runs, what does not, and what the room does at two in the morning in May. Every duty assumption in our model is labelled business-hours or 24/7 on that basis, and the label is checked at bench validation rather than assumed.
What liquid cooling would have cost
Direct liquid cooling would have let us put several times the compute in the same footprint. We considered it and rejected it, for reasons that have nothing to do with the chips and everything to do with the room.
Liquid needs a coolant distribution unit, a secondary loop, leak detection, and a way to reject the heat: a dry cooler on a roof the owner may not let us reach, or a tie-in to a chilled-water loop the building may not have. It needs plumbing in a room with no drain, and a plumber alongside the electrician. Each of those is a trade beyond the one the mill already requires. Each is a question the building's fire NOC and its insurer will ask. And a coolant leak in a room above someone's office is the kind of event that ends a hosting relationship.
There is a second cost, and it is the one that fixed the decision. The mill is designed as a type, so that a design proven once can be built many times without re-qualification. Under our change-control rules, a card swap inside the envelope is a patch; a new card class is a minor change with a delta of qualification; and anything that touches the intake or the envelope is a major change, on the order of $180,000 to $420,000 and six to twelve months of work, decided at board level. A liquid loop touches the envelope. It would have made every site a re-qualification, which is precisely the nine-month problem the company exists to remove.
So the mill moves air, the room rejects heat, and the bill of materials is sheet metal, fans, filters and a breaker on a circuit that was already there. We gave up density to get a machine that can be repeated. The envelope is the product.