Air Circulation in Data Centers: Rethinking Your Design

Michiel de Jong is an engineer at Low Speed Ventilation Datacenters.

A number of data center issues are related to the circulation of air – local pressure differences, short cuts in air circulation and subsequent hotspots, among others. Most of these issues, if not all of them, stem from the basic design of the data center.

Air Velocity and Hot Spots, Combating the Problem

Transferring heat from the data room calls for substantial volumes of air – some 1.2 million gallons per minute for each megawatt IT-load. During its recirculation, this air has to pass through air coolers. Therefore, it is no surprise that air velocity is high within some areas of the data center, especially close to the air coolers.

Due to a phenomenon called the Venturi-effect, relatively high air velocity will produce local pressure differences within the data room. This effect is a major cause of hot spots. Spots with low pressure may hold back the flow of air through the server or suck air back from locations where it has already been used and is therefore hot. Both instances lead to high temperatures in the servers.

This problem is usually addressed by applying overpressure and by strict separation of hot and cold areas in hot/cold containment. Blanking panels, adjustable floor tiles, a tight control of temperatures and pressures within the entire data center – the efforts to “cure the symptoms” are well-known and multiple. Whether or not the problem of hot spots is solved may vary, but in any case a lot of resistance is built into the air circulation pattern.

The Not-So-Cool Side of Powering the Fans

Seven to 9 percent of the total energy cost in a data center comes from fan power. Consequently, 7 to 9 percent of the total heat generated within the data center is caused by the fan motors. Therefore, the fans require 7 to 9 percent of the cooling capacity for themselves. Moreover, the pressure they upload may lead to air leakage. Due to these two factors, significantly more air is circulating within the data center than is required for cooling the servers.

Is this an inevitable consequence of cooling by means of air? Not necessarily. An alternative approach to air circulation in data centers can eliminate hotspots, reduce fan energy costs and significantly trim down air cooler maintenance.

Rethinking Data Center Equipment and Design

An alternative approach to air circulation in data centers is a system designed for low speed ventilation. Using this method would lead to coolers with a relatively large cross-sectional air-flow area – a design that would require minor adjustments to the layout of the data room.

Instead of a row of air coolers mounted along the wall of the data room, these thin but large coolers would themselves serve as a wall. Air would flow at a low speed from a corridor between the outside wall and the air coolers, through this wall of air coolers, into the data room.

After the air has absorbed the heat from the servers, the hot air is channelled back into this corridor via a plenum. The low speed ventilation thus created, eliminates local pressure variations, makes pressure control unnecessary and brings down the need for fan power. The coolers are large and uncomplicated, with minimal fan wear and modest maintenance requirements.

In the absence of pressure differences, climate control is also simplified. Instead of controlling the volatile pressure and temperature situation in the data center, climate control in a low speed ventilation approach sets focus on the idea of “cool air availability.” By measuring the air flow through a tube between the hot and cold compartment in the data room, a direct shortage/surplus-measurement is received about the amount of air demanded by the servers and supplied by the air coolers. The fan speed is adjusted in order to balance demand and supply.

An additional advantage of this approach is the easy application of free-cooling by the use of outside air. The corridor between the outer wall and the coolers provides an ideal pre-treatment space for letting in the outside air, mixing it with return air, filtering the air and raising the humidity if necessary. The temperature range for free-cooling can be extended from 54 F to 75 F – and even above these temperatures the reduction of cooling costs are significant. For a moderate climate a PUE of 1.07 is possible.

Deciding What’s Right for Your Data Center

The main reason for the operational costs to be lower is obvious. A cut of approximately 6 percent on energy consumption implies a proportional reduction of the energy bill. Likewise, the considerably reduced load on the fans in the air coolers leads to lower maintenance costs.

The reason why the investment level of a low speed ventilation data center will be lower (compared to a conventional data center) is less obvious. The aforementioned separation wall will certainly bring additional cost. However, the capacity of the power supply equipment can be reduced by 7 percent due to the structural reduction of the energy consumption. Since less heat is generated by the fans, the cooling equipment can be scaled down as well.

When compared to investment in a conventional solution, the example above will bring a saving of approximately 4 to 5 percent of the total capital engaged (excluding IT-equipment). It may not sound like much, but a 6 percent savings on the energy bill combined with a reduction of the investment by approximately 4 percent makes a huge difference in overall financial performance.

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