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Cooling capacity - How to compare apples with apples

Cooling capacity

Manufacturers often provide different kinds of information about cooling capacity in their documentation. This means that in many cases, customers are comparing apples with oranges. Below, we explain how the different cooling capacity figures differ, and what you therefore have to watch out for when comparing them.

Gross cooling capacity is produced by the air conditioning unit via the heat exchanger. Fans are used to move the air through the air conditioning unit. These consume energy, which is ultimately converted into heat. This heat, also produced in the air conditioning unit, lowers the gross cooling capacity. The result is the net cooling capacity.

Modern precision air conditioning units cool the air without dehumidifying it. All the cooling capacity generated is therefore used precisely for what is actually needed: cooling the air. In older air conditioning units and those with components not of an ideal size, or with a bad choice of return air conditions, it can happen that some of the generated cooling capacity is inadvertently used to dehumidify the air during the cooling process. Valuable cooling capacity is lost and the air conditioning unit works less efficiently. The entire sum of cooling capacity generated is known as the total cooling capacity. The proportion that is used for purposely cooling the air is called sensible cooling capacity. Any proportion of the cooling capacity inadvertently used to dehumidify the air is called latent cooling capacity. In an ideal situation with no unwanted dehumidification, sensible cooling capacity is the same as the total cooling capacity. The ratio of sensible cooling capacity to total cooling capacity is referred to as the "sensible heat ratio", or SHR for short. In ideal conditions without dehumidification, the SHR is generally 1.0.

So as we can see, it is vital to take care to compare like with like when comparing technical data. If you are unsure whether the manufacturer documentation is talking about total gross cooling capacity or the effective, usable sensible net cooling capacity, it makes sense to ask before comparing data from different manufacturers.

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 Centre Cooling and Indirect Free Cooling with Dynamic Control Logic.