When NOT to use economizer mode for data center cooling

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As explained in the last blog, ASHRAE guidelines define a baseline data center cooling system to establish the minimum performance requirement – typically a chilled water system with the chiller bypass via fluid heat exchanger economizer mode. https://www.ashrae.org/standards-research–technology/advanced-energy-design-guides 

While ASHRAE 90.1 does not stipulate that this exact system be used, it must meet or exceed the performance of this baseline system. Because of the fouling effects of water flowing through piping, cooling systems that use water to transport heat energy generally have a shorter life expectancy than those that use air. Cooling systems that use evaporative assist are limited by the surfaces subjected to the water. Overall, the life expectancy of any cooling is significantly affected by the amount of maintenance performed over its life.

The footprint of the different cooling systems is normalised to the maximum IT load the data center can support, with the air conditioner bypass via direct fresh air cooling system having the smallest footprint. The footprint of air conditioner bypass via air heat exchanger cooling is only slightly higher due to the addition of an air-to-air heat exchanger. The air conditioner bypass via air heat wheel cooling system has the largest footprint of all the air-based economizer modes and is nearly as large as a chilled water plant with cooling tower.

Economizer mode can increases the risk of downtime and is not recommended for high availability production data centers, where refrigerant-based cooling might be better suited. Furthermore, very few locations around the world provide a cold climate year-round. In most cases, an economizer mode will need at least a partially rated refrigerant-based backup mode to assist on the hottest days of the year.

Air conditioner bypass via direct fresh air economizer mode does not have any heat exchanges since the outdoor air is supplied directly into the data center. This means it can operate all year, in dryer climates, with a partially rated mechanical cooling system. However, due to the air quality risk and the need for humidity control, a fully rated mechanical system is required. Although the air conditioner bypass via air heat exchanger economizer mode has one heat exchange, it avoids the air quality risk and humidity control issues and thus the capital and operational expense of a fully rated mechanical system.

In a future where virtual machines allow critical processes to fail over to other data centers, it is realistic to expect some data centers to operate entirely on economizer mode with no refrigerant backup at all. IT equipment inlet temperature thresholds are expected to increase in the future making full-time economizer mode operation even more probable. In the meantime, it is best not to use economizer mode.

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