Sizing a dry cooler – the calculation for 5, 10 and 50 kW

A note up front: This article shows the calculation method and orders of magnitude. It does not replace sizing by the dry cooler's manufacturer or a specialist planner. The capacity of a unit depends on its design, the site and the coolant; the cooler's datasheet and the miner manufacturer's specifications are binding. We accept no liability for damage resulting from your own sizing based on these example values.

A dry cooler releases the heat of the cooling circuit to the outdoor air via fins and fans. Whether it is sufficient is decided on the hottest day of the year.

Step 1: the heat load

The heat load is the electrical power of all connected miners. On top of that comes a 20% reserve for fouling of the fins and hot days.

• 5 kW of miners: 6 kW design load
• 10 kW of miners: 12 kW design load
• 50 kW of miners: 60 kW design load

Step 2: the temperatures

A dry cooler cannot cool the coolant below the outdoor temperature. The coolant always leaves it a few degrees warmer than the air. This gap is the most important variable in the sizing. 5 to 10 kelvin is common; less than about 4 kelvin is hardly achievable technically. The smaller the gap, the larger and more expensive the cooler.

Against this stands the miner's maximum inlet temperature. Bitmain allows 40 or 50 °C depending on the model.

An example for a site with 35 °C on the hottest day:

• Miner with a 50 °C limit: 15 kelvin of headroom. A cooler with a 10 kelvin gap delivers 45 °C and is sufficient.
• Miner with a 40 °C limit: 5 kelvin of headroom. The cooler must be built for a 5 kelvin gap and turns out considerably larger.

So the same heat load can call for two very different coolers depending on the miner and the site. That is why the question to the supplier is always: what capacity at my outdoor temperature and my inlet temperature?

Step 3: the flow rate

How much coolant has to circulate follows from the heat load and the temperature difference between flow and return:

Flow rate in litres per minute = 14.3 × power in kW ÷ temperature difference in kelvin

The formula applies to water. With 30% glycol you need around 5 to 10% more, depending on the glycol and the temperature.

At a difference of 10 kelvin:

• 6 kW: around 9 litres per minute
• 12 kW: around 17 litres per minute
• 60 kW: around 86 litres per minute

However, the miner sets its own flow rate. Bitmain requires 8 to 10 litres per minute per unit for the S21 XP Hyd. So run the formula backwards as well: at 9 litres per minute and 5.4 kW, the coolant heats up by just under 9 kelvin in the unit.

Step 4: the air volume

The air has to absorb the same heat:

Air volume in m³ per hour = 3,000 × power in kW ÷ temperature rise of the air in kelvin

At a temperature rise of the air of 10 kelvin:

• 6 kW: around 1,800 m³ per hour
• 12 kW: around 3,600 m³ per hour
• 60 kW: around 18,000 m³ per hour

Fan size, the fans' power consumption and noise all depend on this. A generously sized cooler moves the same air volume with slower fans and is quieter.

The material

The tubes of many dry coolers are made of copper. In the circuit of a miner with aluminium cold plates that is a problem. There are two solutions: a cooler with stainless steel tubes, or a stainless steel plate heat exchanger between the miner circuit and the cooler circuit. The heat exchanger costs a few kelvin of temperature gap, which you have to deduct in step 2.

Winter

In winter the dry cooler cools too well. Bitmain units do not start below an inlet temperature of 20 °C. You therefore need:

• Speed-controlled fans that run according to the coolant temperature and stop when it is cold.
• A mixing valve or a bypass that routes warm coolant past the cooler until the inlet temperature is right.
• Antifreeze in the coolant for the lowest temperature at the site.

The installation location

• Unobstructed intake and exhaust air. Warm exhaust air must not be drawn in again.
• Shade lowers the intake temperature.
• Distance from neighbours. Ask for the noise level at night speed, not at full load.
• Access for cleaning the fins.

What you tell the supplier

1. Heat load with reserve in kW
2. Maximum outdoor temperature at the site
3. Maximum permitted inlet temperature of the miner
4. Flow rate and coolant with glycol content
5. Material of the tubes
6. Permitted noise level

Sources

• Bitmain: S21 XP Hyd, specifications with inlet temperature and flow rate
• Bitmain: notes on hydro miners, no start below 20 °C
• Temperature gap of dry coolers: Suez Water Handbook, Thermax
• Dow, physical properties of glycol mixtures (PDF): ethylene glycol, propylene glycol

As of October 2026.