Dry Coolers for Data Center Free Cooling

Dry Coolers for Data Center Free Cooling

IT equipment (servers, switches, etc.) in a data center generates a large amount of heat continuously during operation (up to 5-50kW per cabinet) and requires a cooling system to maintain the ambient temperature of the server room (typically 18-27°C). Traditional air conditioning relies on compressor refrigeration, energy consumption accounted for 30% -50% of the total energy consumption of the data center; free cooling technology is the use of dry cooler, in the lower ambient temperature (such as winter, spring and autumn) directly with the outdoor cold air to cool the circulating fluid, instead of mechanical refrigeration, cooling energy consumption can be reduced by more than 50%, especially suitable for cooler climates or high latitude areas.

The free cooling system in data centers usually adopts the indirect design of “liquid circulation + dry cooler heat exchange” (to avoid humidity and dust problems caused by direct entry of outdoor air into the server room), and the process is as follows:

1. Heat collection in the server room

Heat generated by IT equipment is collected by the cold plate radiators in the server room (attached to the back panel/top of the servers) or the air conditioner at the end (e.g., column air conditioner), and is then transferred to the server room. The heat generated by IT equipment is absorbed by cold plate radiators (attached to the back/top of servers) or air-conditioning terminals (e.g., column air-conditioners) in the server room: the circulating coolant (usually water or glycol solution to prevent freezing at low temperatures) in the cold plate/terminal is exchanged with the equipment and the temperature is raised (e.g., from 20 to 35°C).

The warmed up coolant is transferred to the dry cooler outside the room by a circulation pump. 2.

2. Apparent heat exchange between the dry cooler and outdoor air

The dry cooler consists of a finned tube bundle (usually copper or aluminum fins, with steel or plastic tubes as carriers) and an axial fan (forced ventilation), which is mounted on the outside wall or roof of the equipment room:

When the temperature of the outdoor air is lower than the temperature of the circulating fluid (e.g. 10°C outside and 35°C in the circulating fluid):

The fan is started and draws in cold outdoor air, which flows over the surface of the finned tube bundle of the dry cooler.

The circulating fluid flows inside the tube and the heat is transferred to the air through the tube wall and fins: the temperature of the circulating fluid decreases (e.g., from 35℃ to 20℃), the air absorbs the heat and then increases in temperature (e.g., from 10℃ to 25℃), and is discharged by the fan to the outdoor area.

3. Return of cooled circulating fluid

The cooled coolant (around 20℃) returns to the cold plate or air conditioning end in the machine room through the pipeline to absorb the heat of IT equipment again, forming a closed loop. At this time, the data center's mechanical cooling system (such as chiller) can be completely shut down to achieve “100% free cooling”.

4. Hybrid mode in transition season

When the outdoor air temperature is slightly higher than the target temperature of the circulating fluid (e.g. 22℃ outside, the circulating fluid needs to be cooled down to 20℃), but the threshold of needing mechanical cooling has not been reached, the system can be adjusted by the following ways:

Reducing the fan speed of the dry cooler, reducing the air flow and controlling the heat transfer efficiency;

Partially turning on the chiller to work with the dry cooler to cool the circulating fluid to the target temperature (e.g., the dry cooler reduces the 35°C to 25°C, and the chiller reduces the temperature to 20°C) to avoid running the chiller at full load.

Key design and advantages of dry coolers for free cooling in data centers

Adaptable climatic conditions:

Ideal for regions with low average annual temperatures (e.g., Northern Europe, Northeast/Northwest China), where free cooling can be achieved for up to 8-12 months a year;

To cope with short-term high temperatures, dry coolers are often paired with a glycol solution (10%-30%) as the circulating fluid, which prevents freezing in the winter (the freezing point can be as low as -20°C), and also prevents freezing in the summer through a more efficient cooling process. To cope with short-term high temperatures, the dry cooler is often combined with a glycol solution (10%30% concentration) as the circulating fluid, both to prevent freezing in winter (freezing point can be as low as -20°C) and to increase the efficiency in summer through a higher temperature difference between the heat transfer.

Synergistic control with mechanical refrigeration:

The system automatically switches modes by means of temperature sensors (detecting outdoor air temperature, circulating fluid inlet and outlet temperatures):

Outdoor temperature＜target temperature of the circulating fluid: the dry cooler operates independently (free cooling);

Outdoor temperature≥target temperature of the circulating fluid: mechanical refrigeration is activated (the dry cooler stops or operates as an auxiliary).

Energy consumption and PUE optimization:

dry cooler's energy consumption only comes from the fan (power is much lower than the compressor of the chiller), so the PUE (energy use efficiency) of the data center can be reduced to 1.1-1.3 under the free-cooling mode (the PUE of traditional air conditioning system is mostly 1.5-2.0), which is especially suitable for large-scale data centers (such as cloud computing centers) that are sensitive to energy consumption.

Maintenance and Reliability:

dry cooler has no cooling water evaporation (different from wet cooling tower), no need for frequent water replenishment or scale treatment, low maintenance cost; however, the fin surface needs to be cleaned regularly of dust and willow (to avoid clogging and affecting the heat exchange), especially in dusty areas need to be equipped with automatic flushing device.