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Dehumidification of outdoor air in datacom environments for air-side economizer operation.

Article Excerpt
DISCUSSION

Datacom facilities have the potential to save significant quantities of energy by using air-side economizers. One of the main reasons for this is that the supply air condition of a datacom facility can be significantly higher than that in an office facility. Assume an office discharge supply air temperature of 55[degrees]F (13[degrees]C) versus a datacom facility discharge air temperature of 68[degrees]F (20[degrees]). As seen in Figure 1, in Los Angeles (LA), the outdoor air dry bulb is below 55[degrees]F (13[degrees]C) for 1500 h/yr, while it is below 68[degrees]F for an additional 5500 h/yr. The number of hours with full economizer use can, thus (in theory, ignoring humidity for the time being), increase by 5500 hours by Increasing the supply setpoint from 55[degrees]F to 68[degrees]F (13[degrees]C to 20[degrees]C). This increase of 5500 hours represents 63% of the hours in a year. It is important to note that even more hours are available for partial economizer use in conjunction with mechanical refrigeration for the final cooling. The term integrated economizer is used to describe this condition, where the enthalpy of the outdoor air is less than the return air, but some mechanical or process cooling is necessary to bring the outdoor air temperature to the desired supply air condition. For LA, full or integrated economizer operation is a possibility for about 90% of the hours in a year in a datacom environment with typical operating conditions.

[FIGURE 1 OMITTED]

There is, unfortunately, a catch to this significant increase in economizer hours; datacom facilities also have a humidity requirement. Based on ASHRAE's Thermal Guidelines for Data Processing Environments (ASHRAE 2004a), the recommended Class 1 and Class 2 temperature range for supply air into datacom equipment is 68[degree]-77[degree]F (20[degree]-25[degrees]C), and the relative humidity (RH) range is 40%-55%. This operating environment is shown as the shaded area in Figure 2.

[FIGURE 2 OMITTED]

For the purposes of this study, it was assumed that all supply air, whether obtained through the use of an economizer or by cooling of return air, is conditioned to 68[degrees]F (20[degrees]C) and 53% RH. Figure 2 also shows why this point is chosen; recirculation of sensible air to rack supplies can increase the supply air temperature. Air at a temperature of 68[degrees]F (20[degrees]C) in the range of 53%-55% RH has the best chance of meeting the recommended supply air condition to all racks. Even if there is a 9[degrees]F (5[degrees]C) increase in the sensible temperature due to local recirculation, the inlet condition to the IT equipment (77[degrees]F or 25[degrees]C, 40% RH) still falls within the recommended supply air range for the ASHRAE class.

Looking back at outdoor air temperature/humidity bin data for LA, it turns out that of the 7000 hours (80% of the year) that outdoor air has a dry bulb temperature potentially suitable for 100% economizer use, for 4870 h/yr (70% of the 7000 hours) this outdoor air would need to be dehumidified. Figure 3 shows where this range of conditions exists on the psychrometric chart. The range (the shaded area) is similar for all locales (tweaked only by adjustments in the supply air condition and the return air condition), but the number of hours per year that the ambient conditions fall into this shaded area varies significantly with climate. For LA, 56% of the hours in a year fall into this shaded area, so analysis is clearly indicated to determine whether economizer operation makes sense or not. LA is admittedly somewhat extreme in the percentage of hours per year in this portion of the psychrometric chart. For the dryer Denver climate, for instance, there are only 660 h/yr where the ambient conditions fall into the same shaded area, so analysis is less critical. The number of hours for various other cities can be obtained through use of the ASHRAE Weather Data Viewer (ASHRAE 2005) or in Table 4.1 of ASHRAE's Best Practices for Datacom Facility Energy Efficiency (ASHRAE 2008).

[FIGURE 3 OMITTED]

To better appreciate the problem of using outdoor air with high ambient humidity in an air-side economizer for a data center, we need to look at a psychrometric chart and at the differences between the standard office cooling process and a data center cooling process. First, let's look at a standard airside economizer. Figure 4 shows what is often termed a dual enthalpy economizer. With this type of economizer, the enthalpy of the return air stream is actively measured and compared to the enthalpy of the outdoor air. If the enthalpy of the outdoor air is less than that of the return air, outdoor air is introduced into the building. It may need to be cooled by mechanical refrigeration to reach the desired supply air temperature to the space (integrated economizer), but the...

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