Motivair Chilled Door High Density Rack Cooling
By removing heat at its source before it enters the server room, active rear door coolers or a Chilled Door System can monitor and adjust cooling performance by varying fan speed, chilled water temperature and flow. The energy savings from using chilled water up to 68F instead of the traditional 44F decreases chiller size and energy consumption by 35%. By pairing active rear door coolers with a Free Cooling Chiller (or alternative cooling source) the operating cost savings are increased to 93% depending on the site location.
The efficiency of all active rear door high density rack cooling systems can be greatly improved by utilizing chilled water or refrigerant at the highest possible temperature. While the industry standard (ARI and ASHRAE) chilled water supply temperature is 44-45F, active rear door rack coolers are best designed for 60-65F entering chilled water. This insures 100% sensible cooling at all times and avoids any accidental de-humidification and subsequent re-humidification, both of which consume power unnecessarily. The greatest savings however are achieved by operating any type or brand of chiller at 60-65F leaving water temperature. The kW per ton required by any chiller is reduced by 30-35% compared to operating the same chiller with 45F leaving chilled water. Operating passive rear door coolers with 60-65F chilled water severely limits their cooling capacity because of inadequate (server) airflow. CRAC units and custom air handling systems are not designed or suitable for 60-65F chilled water.
Even greater energy savings can be achieved when the data center chiller is fitted with an integrated free cooling chiller system that allows reduced or eliminated compressor operation during periods of lower outside air temperature. When supplying chilled water (entering the rack cooler) at 65F, a free cooling chiller can provide 100% cooling capacity without compressor operation at 45F outside ambient temperature. During this period the chiller operating cost is reduced by about 85%, using only the required VFD fan power in the chiller. In the Central & Northern United States, Canada, Central & Northern Europe the annual energy savings using 65F supply chilled water and a free cooling chiller can reach 93% compared to a traditional CRAC chilled water cooling system. Air-cooled free cooling chillers are available to 350 tons and have the added advantage of eliminating water use and water treatment for cooling towers.
Best practice for a state-of-the-art high density rack cooling system is a data center without a raised floor and all overhead mechanical & structured electrical services. Active rear door coolers should be rated for the full server load plus a reasonable anticipated growth factor. The rear door coolers should each be fitted with independent PLC controls that allow them to control each rack plus the entire data center space temperature without the use of hot or cold aisles, CRAC units or other air handlers.
The active rear door coolers should be supplied with chilled water at 65F from packaged air-cooled free cooling chillers located outside the building and designed to provide 100% free cooling at an outside ambient temperature of 50-55F. The result is a lower cost installation with dramatically reduced electrical operating cost. Data center reliability is increased with the use of an independent active rear door cooler per rack. Chiller redundancy should be a minimum of N +1.
