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Published by on Monday 14th July 2008, 19:18 | Related | Filed Under

 

Dry coolers are used in many air cooled refrigeration systems. In case of low outdoor temperatures, outdoor air can be applied for cooling applications. In dry coolers, large fans create a forced airflow along air-to-water heat exchangers. Subsequently, the water circuit can be used for cooling of data centers, office buildings and industrial processes. 

Dry coolers play an important role in Data Center refrigeration systems. External air is used to cool the liquid being pumped around the CRAC units in data centers without resorting to a refrigeration process. In simple terms the hot liquid from the CRAC units is pumped through the dry coolers into cooling vanes, electric fans blow air at ambient temperature over these. We still use energy to pump the liquid and power the fans but much less than if we need to refrigerate the liquid.
By raising the return air set point on our CRAC units we can make use of dry coolers for more of the year, not just winter months. As an example raising the return air set point to 28 °C can increase the efficiency of a cooling system from 16% (free air cooling with return set point at 22 °C) to 26%.
From an aesthetic point of view, dry coolers are not the crowning glory of the architect’s work. Therefore, dry coolers are often hidden from sight by extended façades or higher parts of the building. Unfortunately, in many cases this will result in a short-circuit in the air flow. A short-circuit in the air flow leads to a significant reduction of the cooling capacity of the dry coolers.

For a guaranteed cooling capacity of the dry coolers, the temperature of the forced airflow must be as low as possible. It is often assumed that the air temperature at the inlet of the dry cooler corresponds to the free field outdoor temperature. User manuals of dry cooler systems give only rough guidelines to achieve this:

  1. A sufficient free area at the air inlet, where the free space around the dry cooler must be at least 1 meter.
  2. The exhaust air side must be free of obstructions.
  3. No short-circuits in the air circulation.

Practical experience has shown that following the first two guidelines is not sufficient. The question is how the third guideline can be guaranteed. Apart from temperature measurements and smoke tests afterwards, Computational Fluid Dynamics (CFD) can be used to analyse the performance of dry cooler systems.

Practical advice is avoid locating dry coolers behind walls where airflow can be diverted from the output back into the input airflow. Even where the first two installation rules are followed, temperature increases of up to 1.7 °C have been observed when units are installed adjacent to walls or other airflow obstructions.

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