How Rack Density and Delta T Impact Your Airflow Management Strategy8 min read
Most are well aware that there is a direct link between conditioned airflow volumes and the amount of IT equipment (or kilowatts) that can be cooled. Less understood is how Delta T (ΔT)¹ through IT equipment affects airflow considerations. IT equipment ΔTs vary by:
- Design – IT manufacturers are designing to higher ΔTs in an effort to reduce the power consumption of IT equipment cooling fans. Historically, ΔTs are rising.
- Configuration – Firmware configurations for energy-saving features affect airflow volumes and ΔTs through IT equipment.
- Server workload – higher the workload generally the higher the ΔT.
Airflow management considerations must be made at both the row and cabinet levels:
- AFM considers the flow required by each cabinet, and the row as a whole.→ If there is no containment, then it becomes more important to match airflow volumes of tiles to the cabinets that are immediately adjacent.→ If there is containment in place, then balancing the total volume of conditioned air supplied into the cold aisle is sufficient.
- Volume required by IT equipment can vary due to server activity. The volume supplied should be slightly more than what’s required by servers at full load.
A few ways that you can measure airflow CFM include the following:
- Rotating Vane Anemometer – Unfortunately, they don’t work well for data center environments! A rotating vane anemometer senses air movement and then calculates its velocity. They do work well when the cross section of the measured airflow has uniform velocity. They don’t work so well for measuring perforated tiles and grates velocity or airflow volume since perforated tiles and grate structures create great variations in velocity. High-velocity areas can cause inaccurate high-velocity and volume readings.
- Capture Hoods – Those with back-pressure compensation work well and are the most accurate and easiest to use.
- Static Pressure Gauges – Can also be used to determine volumetric flow rates from tiles and grates. Using static pressure, you can read the volume of flow from the manufactures’ pressure-vs.-flow rate flow curve.
Advanced AFM Technologies
There are times when you may want to employ some advanced technologies into your AFM strategy. These can include:
- Fan-assisted tiles – Can be fix-fan speed, on:off, or controlled by temperature or static pressure.
- Dampers – Can be manual, or auto controlled by temperature or static pressure.
- Cooling units with variable frequency drives (VFD) – Adjusts to server fluctuations by varying the amount of supplied conditioned air, so energy is not wasted moving more air than is required. You can control VFDs by monitoring the static pressure or temperature.
Calculating the Relationship between Heat, Airflow Volume, and ΔT:
- Heat Load In Watts: Q (Watts) = 0.318 x CFM x ΔT (°F)
- Required Airflow Volume in CFM: CFM = Q (Watts) / 0.318 x ΔT (°F)→ 157 CFM are required to cool 1 kW, if the ΔT through IT equipment is 20°F→ 125 CFM are required to cool 1 kW, if the ΔT through IT equipment is 25°F→105 CFM are required to cool 1 kW, if the ΔT through IT equipment is 30°F
- If a cabinet contains 5kW of IT equipment with a ΔT of 25°F, then the cabinet will consume 625 CFM of conditioned air.
¹Defined as the change in temperature. For cooling units, it is the temperature change from return air to supply air. For IT equipment, it is the temperature change from inlet air to exhaust air. The higher the delta, the lower the required airflow volume.
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