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CRACs with underfloor fan section

Air conditioning unit conventional and with underfloor fan section

In recent years the market for closed-circuit air conditioning units is increasingly coming up with models where the fan unit is housed underneath the A/C unit in a raised floor. What is behind this trend? Installation is undoubtedly more complex, and the units are generally taller – and probably more costly as well. So there must be benefits that prompt customers to opt for such equipment. What are they then?

There are two main benefits here: Such systems offer a marked improvement in efficiency, in addition to a higher cooling capacity in relation to their footprint. So customers not only get a unit that is more efficient, but also one that requires less space to attain a specific cooling capacity.

That's easier said than done. See below for an explanation how these two benefits are achieved. A conventional precision air conditioning unit is generally approx. 2 m tall and stands on a raised floor. The raised floor under the unit does not normally contain anything more than cold air. In other words, wasted space. The designers have now taken advantage of this fact, removing the fans from the conventional precision air conditioning unit, installing them in a separate fan unit and positioning them under in a raised floor. The space gained this way in the A/C unit has been used to install a more powerful heat exchanger and larger filters. Above the raised floor the A/C units don't look any different and are still around 2 m in height. The fan unit contained inside the underfloor section then makes up the overall height to 2.5 m or so.

Positioning the fan unit under the A/C unit in an underfloor section means that the flow of air from the fans to the raised floor is now directly horizontal. In conventional precision air conditioning units with integrated fans – i.e. where the fans are above the raised floor – the air exiting the fan has to change direction twice before entering the raised floor horizontally (see figure). The associated turbulence and impact losses affecting efficiency are eliminated by positioning the fans under the raised floor. Fan power consumption falls, and efficiency improves.

Left: Conventional precision air conditioning unit
Right: Air conditioning unit with underfloor fan section

The space gained in the A/C unit is used to provide a more powerful heat exchanger and larger filters. The airflow speed over the heat exchanger and filter is reduced. This lowers the airside resistance of the heat exchanger and filter, so reducing the power consumption of the fans and so further improving efficiency. Cooling capacity is also increased thanks to the larger heat exchanger: Customers thus get greater cooling capacity with the same footprint. They may even be able to use a smaller unit, and large data centers can sometimes then manage with fewer units.

Here are two examples:

1. Chilled water system, 2.5 m in width, approx. 150 kW cooling capacity with an airflow of 30,000 m³/h and return air conditions of 30 °C and relative humidity of 30%.
Benefits of units with an underfloor fan section:
Fan power input: approx. 40% lower
Useful cooling capacity: approx. 5% more

2. Precision air conditioning unit with compressor cycle and Indirect Free Cooling, 1.8 m in width, approx. 50 kW cooling capacity with an airflow of 12,000 m³/h and return air conditions of 33 °C and 30% relative humidity.
Benefits of units with an underfloor fan section:
Fan power input: approx. 18% lower
Useful cooling capacity: approx. 5% more



The latest generation of precision air conditioning units with an underfloor fan section are more efficient while offering a higher cooling capacity per footprint. The result: a significant improvement in the Airflow Efficiency Ratio (AER), the indicator for airflow efficiency. The savings in operating costs achieved thanks to the improvement in efficiency very quickly make up for the slight increase in investment costs, and there is also a marked reduction in the total cost of ownership (TCO).

About the author

Benjamin Petschke was born in 1969 in Germany. After studying physics he joined STULZ in 1996 and worked since then in the R&D, Export and Marketing department on different positions. With 19+ years' experience in the Data Center cooling industry, Mr Petschke is specialised in Data Center cooling design, energy saving and acoustic issues.

He works closely with the Joint Research Centre of the European Commission for the Code of Conduct on Data Centres on the Best Practice section and recently with the German DKE in development of the DIN EN 50600, Information technology – Data Centre facilities and infrastructures.

Mr Petschke authored White Papers on subjects like Best Practice for Data Center Cooling and Indirect Free Cooling with Dynamic Control Logic.