Understanding Delta T in Data Centre Cooling

Understanding Delta T in Data Centre Cooling What is Delta T? Understanding what Delta T is, where its sources can be found, and how it effects Data Centre cooling, can help mitigate cooling and inefficiency problems. Δ (Delta) is the capital of the fourth letter of the Greek alphabet and is often used as a mathematical symbol defining a “difference” of any changeable quantity. Delta T (ΔT) is a term used in science that defines the difference in temperature between two measured points. This difference varies either in position or over a period of time. Understanding where these ΔTs occur within the Data Centre environment can help identify issues and inefficiencies, which once remediated improves operational efficiency, restores a Data Centre to mechanical health and can help reduce operational costs. Recently, Upsite Technologies published two blog articles by their Senior Engineer Lars Strong, a thought leader and recognised expert on Data Centre cooling optimisation. In this article we summarise these two blogs to highlight why interpreting this metric is vital in understanding how ΔT effects Data Centre cooling. The More Commonly Know Delta Ts in Data Centre Cooling In Upsite’s first blog “The More Commonly Know Delta Ts in Data Centre Cooling” it highlighted that there are in fact four ΔTs which should be considered and that contribute to the health of the Data Centre. Two of these more common ΔTs are often discussed within the Data Centre industry as a single metric – “The ΔT” and depending on who is discussing it, it could be affecting two totally independent factors which should really be considered and monitored separately. ΔT is commonly considered to be the rise in temperature of the air as it passes through IT equipment, where it picks up heat produced by that equipment and removes it and the difference in temperature across the cooling equipment’s cooling coils, or the difference between supply and return air temperatures. Monitoring these metrics separately gives a much better understanding of the health of a Data Centre’s cooling. Through IT Equipment When considered as a single metric, ΔT is often based on the temperature drop across the cooling equipment or the temperature rise across the IT equipment. However, there can be several reasons why these values can be different, but will generally fall into two categories: The return air to the cooling units is being cooled by bypass airflow (air failing to go through equipment) or is being warmed because of hot air re-circulation. The original blog then looks at the science needed to calculate the relationship between airflow, temperature differential and heat which can vary based on altitude and units of measurement (°F / °C – CFM, LPS or CMH). In actual applications the ΔT across IT equipment typically ranges from around 20°F (11.1°C) up to around 35°F (19.4°C) depending on the type of equipment. Through Cooling Units Through Cooling Units, the ΔT is the drop in temperature across the cooling coils, or across whatever is removing the heat from the Data Centre air. Ideally, the delta should be the same as the delta across the IT equipment, meaning that the cooling resource is perfectly matched to the heat load being served. However, this is rarely the case. It’s important to understand some design factors that can account for some of these differences. In legacy DX CRAC units, the temperature drop was fixed typically in the region of 18°F (10°C) and there was not much allowance for ΔT variance from that baseline. For example, where there was an increase in the return air temperature it resulted in an associated increase in the supply air temperature e.g., a 5°F (2.7°C) increase in return air temperature could result in a 3 or 4°F (1.7 or 2.2°C) increase in the supply temperature. Most modern water-cooled CRAH units can remove heat equivalent to anywhere from 45°F (7.2°C) up to 65°F (18.3°C) temperature drop across coiling coils. The ΔT across the coiling source is also affected by setpoints. For example, if there is a lot of bypass airflow it is possible that the return air can actually be below the setpoint and therefore returned into the Data Centre without any heat being removed.

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