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

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 process efficiency is essentially the same as that of cooling the return air. In an office environment, there is typically a significant internal latent load, which requires cooling the return air to saturation to wring out the moisture gain in the space.

[FIGURE 4 OMITTED]

Next, let's look at the psychrometrics of conditioning a datacom space. There are two important differences. First, there is, in a relative sense, almost no internal humidity gain in a datacom environment (computers don't reject moisture like humans do!). This allows for a supply air temperature off the cooling coil that is not at saturation, as long as the facility is under positive pressure and the outdoor air is pretreated to remove excess humidity. Second, the recommended supply air condition to datacom equipment is between 40% and 55% RH. This condition is nowhere near saturation. So, one can simply sensibly cool the return air from the datacom space back to the desired supply air condition. Figure 5 illustrates this very simple psychrometric process. Mechanical refrigeration cooling of this process can be quite efficient, with chiller efficiencies of less than 0.50 kW/ton and COP >7.0.

[FIGURE 5 OMITTED]

Now comes the crux of the problem with an air-side economizer for a data center: how can we efficiently bring a high-dew point ambient condition to the desired supply air condition? If it can be done more efficiently (or less costly) than cooling the return air, it makes sense to operate in economizer mode; otherwise it does not. Figure 6 shows two possible paths to achieve this conditioning, along with mechanical refrigeration cooling of the return air for comparison.

[FIGURE 6 OMITTED]

The first path utilizes a combination of mechanical refrigeration and reheat to achieve the desired result. It can be readily calculated by looking at the difference in enthalpy of the two cooling processes that more energy will be expended to adjust outdoor air (at this specific condition) to the desired condition than to simply cool the return air. In the case of the return air, the difference in enthalpy is 28.91 - 24.76 = 4.15 Btu/ lb. In the case of mechanical refrigeration of the outdoor air, the enthalpy difference for the cooling component of the conditioning process is 26.39 - 20.30 = 6.09 Btu/ lb or 47% more than using the return air. A reheat process is also required, which may require additional energy, although reheat with low-temp waste heat is also a possibility (ASHRAE 2004c). The relative efficiency of this path is easy to calculate and does not appear to be an efficient way to dehumidify the outdoor air. It is not treated further in this paper, though integrated economizer use of a noncondensing mechanical refrigeration system for cases where the dew point of the outdoor air is less than the dew point of the space should be quite efficient.

The second path uses a desiccant dehumidifier. A desiccant dehumidifier uses the process of adsorption or absorption to draw moisture from the air stream (ASHRAE 2004b). After being loaded with moisture in the process air stream (in this case, dehumidifying the outdoor air is the process), the desiccant is reactivated (dried out) by heating with an independent stream of reactivation air. The reactivation air temperature is usually relatively high (180[degrees]F-300[degrees]F [82[degrees]C-149[degrees]C), which can make the drying process quite inefficient as compared to mechanical cooling and dehumidification. In many cases, however, there may be a source of waste heat that can drive the reactivation process, in which case there is little or no cost to the reactivation process. Unfortunately, there is also usually significant heat gain of the process air stream during the desiccant drying process, because the thermal mass in the desiccant wheel or other structure used to hold the desiccant in place transfers some of the heat from the reactivation air stream to the process air stream. Depending on the exact outdoor air condition, mechanical refrigeration may still be required to achieve the desired supply air condition.

The next step in the analysis is to extend the type of comparison shown for a single ambient condition in Figure 6 to the entire set of dry bulb/wet bulb conditions in the area of interest. For this purpose, the shaded area in Figure 3 was divided into joint frequency bins of 2[degrees]F (1.2[degrees]C) dry bulb x 2[degrees]F (1.2[degrees]C) wet bulb. This corresponds to the way that the data is provided in ASHRAE's Weather Data Viewer (ASHRAE 2005). For the case of a 68[degrees]F/53% RH (20[degrees]C/53% RH) supply air temperature and 85[degrees]F (29[degrees]C) return air temperature, there are a total of 39 bins that need to be examined. These bins are shown in Figure 7.

[FIGURE 7 OMITTED]

The next step in the process is to perform energy use calculations. In practice, there are at least five components of a desiccant dehumidifier's energy use:

1. The energy consumption of the reactivation process (heated air, possibly some percentage of free waste heat)

2. The energy required to cool the process air from the heated and dehumidified condition to the desired supply air condition (if the dry bulb temperature of the process air leaving the desiccant wheel is greater than the desired supply air temperature).

3. A credit for mixing a certain percentage of the return air with the outdoor supply air if the temperature of the outdoor air after the desiccant dehumidification process is still less than the desired supply air temperature.

4. The fan energy required for blowing air through a desiccant wheel (process side and reactivation side fan energy)

5. The energy consumption of the wheel rotation motor (typically small)

Figure 8 depicts a desiccant wheel dehumidifier showing these five energy components.

[FIGURE 8 OMITTED]

In the case of desiccant dehumidification, there is a wide range in the COP of the regeneration stream heating process depending on the quantity and quality of free heat energy available. This study looked at desiccant system reactivation heat COPs in the range of 0.5 to 7.0 and also added the energy required to cool the process air back to the desired supply air temperature. A mixing credit was also taken if less than 100% outdoor air was needed to achieve the dry bulb temperature, but the energy consumption of the wheel rotation motor and increased fan energy were not included. For a full energy comparison and cost analysis, these should be included, and the COP of the heating, cooling, and electrical inputs should be calculated separately, as there are different costs to each of these components (Yong 2006).

RESULTS

Based on the results of this analysis, it doesn't pay to install a desiccant dehumidification system unless there is at least some waste heat (or very inexpensive regeneration heat) available. The COP of the desiccant dehumidifier is quite low without any free heat (close to 0.5), and there would not be enough hours of the year with an energy/cost payback to justify the expense of the system. It would be better to simply ignore the use of the economizer during these high ambient humidity hours of the year.

If waste heat is available for regeneration, however, it's a different story. Table 1 shows an example comparison for the case with a desiccant dehumidification process COP = 3.0. This would correspond to a case where substantial waste heat is available for reactivation. The exact temperature of waste heat needed to obtain a COP = 3.0 is dependent on several variables related to dehumidifier design (ASHRAE 2004b) but would probably have to be greater than 100[degrees]F (38[degrees]C). In this case, 12 of the 39 joint frequency bins of interest yield lower energy for the economizer mode than for mechanical cooling of the return air.

It is also possible to show the trend of the joint frequency bins for which economizer use makes sense with the increasing COP of the desiccant reactivation process. This trend, shown on Figure 9, indicates that modest amounts of free waste heat start to increase the number of joint frequency bins where economizer operation makes sense. If the reactivation heat is completely free, it makes sense to use the outdoor air whenever it has an enthalpy less than the return air, as long as the sensible heat gain in the process stream does not cause the outdoor air to exceed the return air temperature from the data center.
Table 1. Comparison of Relative Energy Use of Air-Side Economizer with
Desiccant Dehumidification (with Reactivation Process COP = 3.0) to
Return Air with Mechanical Refrigeration (COP = 7.03 to 9.26)

                  DB          DP        Enthalpy

Supply Air      68.00       50.26        24.76
Return Air      85.00       50.26        29.91

                                                        Desiccant
                                                     Dehumidification
                                                       (Economizer
                                                          Cycle)

WB,               DB,         DP,      OA Enthalpy,       Desired
[degrees] F  [degrees] F  [degrees] F     Btu/lb     Dehumidification
                                                        Enthalpy at
                                                        Constant Dry
                                                        Bulb, Btu/lb

51.5            51.5        51.6         21.11             20.71
53.5            53.5        53.6         22.27             21.20
53.5            55.5        52.1         22.26             21.68
55.5            55.5        55.6         23.49             21.68
55.5            57.5        54.2         23.48             22.17
55.5            59.5        52.8         23.46             22.66
55.5            61.5        51.3         23.45             23.14
57.5            57.5        57.6         24.75             22.17
57.5            59.5        56.3         24.74             22.66
57.5            61.5        55.0         24.73             23.14
57.5            63.5        53.6         24.71             23.63
57.5            65.5        52.1         24.70             24.12
59.5            59.5        59.6         26.07             22.66
59.5            61.5        58.4         26.06             23.14
59.5            63.5        57.1         26.04             23.63
59.5            65.5        55.8         26.03             24.12
59.5            67.5        54.5         26.02             24.60
59.5            69.5        53.1         26.00             25.09
59.5            71.5        51.6         25.99             25.58
61.5            61.5        61.6         27.44             23.14
61.5            63.5        60.4         27.43             23.63
61.5            65.5        59.3         27.42             24.12
61.5            67.5        58.1         27.40             24.60
61.5            69.5        56.8         27.39             25.09
61.5            71.5        55.5         27.38             25.58
61.5            73.5        54.2         27.36             26.06
61.5            75.5        52.7         27.35             26.55
61.5            77.5        51.2         27.33             27.04
63.5            63.5        63.5         28.88             23.63
63.5            65.5        62.5         28.86             24.12
63.5            67.5        61.4         28.85             24.60
63.5            69.5        60.3         28.84             25.09
63.5            71.5        59.1         28.82             25.58
63.5            73.5        57.9         28.81             26.06
63.5            75.5        56.7         28.79             26.55
63.5            77.5        55.3         28.78             27.04
63.5            79.5        54.0         28.76             27.53
63.5            81.5        52.6         28.75             28.01
63.5            83.5        51.1         28.73             28.50

                DB             DP         Enthalpy
Supply Air     68.00         50.26          24.76
Return Air     85.00         50.26          29.91

                                                          Desiccant
                                                      Dehumidification
                                                        (Economizer
                                                            Cycle)

WB,             DB,     DP, [degrees]F  OA Enthalpy,      Desiccant
[degrees]F  [degrees]F                     Btu/lb     Dehumidification
                                                      Enthalpy Change,
                                                           Btu/lb

51.5           51.5          51.6           21.11           0.40
53.5           53.5          53.6           22.27           1.08
53.5           55.5          52.1           22.26           0.58
55.5           55.5          55.6           23.49           1.80
55.5           57.5          54.2           23.48           1.31
55.5           59.5          52.8           23.46           0.81
55.5           61.5          51.3           23.45           0.31
57.5           57.5          57.6           24.75           2.58
57.5           59.5          56.3           24.74           2.08
57.5           61.5          55.0           24.73           1.58
57.5           63.5          53.6           24.71           1.08
57.5           65.5          52.1           24.70           0.59
59.5           59.5          59.6           26.07           3.41
59.5           61.5          58.4           26.06           2.91
59.5           63.5          57.1           26.04           2.41
59.5           65.5          55.8           26.03           1.91
59.5           67.5          54.5           26.02           1.41
59.5           69.5          53.1           26.00           0.91
59.5           71.5          51.6           25.99           0.41
61.5           61.5          61.6           27.44           4.30
61.5           63.5          60.4           27.43           3.80
61.5           65.5          59.3           27.42           3.30
61.5           67.5          58.1           27.40           2.80
61.5           69.5          56.8           27.39           2.30
61.5           71.5          55.5           27.38           1.80
61.5           73.5          54.2           27.36           1.30
61.5           75.5          52.7           27.35           0.80
61.5           77.5          51.2           27.33           0.30
63.5           63.5          63.5           28.88           5.25
63.5           65.5          62.5           28.86           4.75
63.5           67.5          61.4           28.85           4.25
63.5           69.5          60.3           28.84           3.74
63.5           71.5          59.1           28.82           3.24
63.5           73.5          57.9           28.81           2.74
63.5           75.5          56.7           28.79           2.24
63.5           77.5          55.3           28.78           1.74
63.5           79.5          54.0           28.76           1.24
63.5           81.5          52.6           28.75           0.74
63.5           83.5          51.1           28.73           0.24

               DB         DP         Enthalpy

Supply        68.00      50.26      24.76
Air

Return        85.00      50.26      29.91
Air

                                            Desiccant Dehumidification
                                                (Economizer Cycle)

WB,          DB,        DP,        OA         Desiccant       Leaving
[degrees]  [degrees]  [degrees]  Enthalpy,  Dehumidification  wheel Dr
F              F          F       Btu/lb         Process        Bulb,
                                                 Cop (1)      [degrees]
                                                                F (2)

51.5          51.5       51.6      21.11          3.00           53.0
53.5          53.5       53.6      22.27          3.00           57.7
53.5          55.5       52.1      22.26          3.00           57.7
55.5          55.5       55.6      23.49          3.00           62.7
55.5          57.5       54.2      23.48          3.00           62.7
55.5          59.5       52.8      23.46          3.00           62.6
55.5          61.5       51.3      23.45          3.00           62.6
57.5          57.5       57.6      24.75          3.00           67.9
57.5          59.5       56.3      24.74          3.00           67.9
57.5          61.5       55.0      24.73          3.00           67.8
57.5          63.5       53.6      24.71          3.00           67.7
57.5          65.5       52.1      24.70          3.00           67.7
59.5          59.5       59.6      26.07          3.00           73.3
59.5          61.5       58.4      26.06          3.00           73.3
59.5          63.5       57.1      26.04          3.00           73.2
59.5          65.5       55.8      26.03          3.00           73.1
59.5          67.5       54.5      26.02          3.00           73.1
59.5          69.5       53.1      26.00          3.00           73.0
59.5          71.5       51.6      25.99          3.00           73.0
61.5          61.5       61.6      27.44          3.00           78.9
61.5          63.5       60.4      27.43          3.00           78.9
61.5          65.5       59.3      27.42          3.00           78.8
61.5          67.5       58.1      27.40          3.00           78.8
61.5          69.5       56.8      27.39          3.00           78.7
61.5          71.5       55.5      27.38          3.00           78.7
61.5          73.5       54.2      27.36          3.00           78.6
61.5          75.5       52.7      27.35          3.00           78.6
61.5          77.5       51.2      27.33          3.00           78.5
63.5          63.5       63.5      28.88          3.00           84.8
63.5          65.5       62.5      28.86          3.00           84.7
63.5          67.5       61.4      28.85          3.00           84.7
63.5          69.5       60.3      28.84          3.00           84.7
63.5          71.5       59.1      28.82          3.00           84.6
63.5          73.5       57.9      28.81          3.00           84.5
63.5          75.5       56.7      28.79          3.00           84.4
63.5          77.5       55.3      28.78          3.00           84.4
63.5          79.5       54.0      28.76          3.00           84.3
63.5          81.5       52.6      28.75          3.00           84.3
63.5          83.5       51.1      28.73          3.00           84.2

                DB          DP       Enthalpy

Supply Air    68.00       50.26       24.76
Return Air    85.00       50.26       29.91

                                                      Desiccant
                                                  Dehumidification
                                                 (Economizer Cycle)

WB,            DB,         DP,        OA       Mixed Air   Economizer
[degrees]F  [degrees]F  [degrees]F  Enthalpy,    Radio    Reactivation
                                     Btu/lb    Reduction   Energy Use,
                                                 Credit   dimensionless

51.5           51.5        51.6       21.11       0.53        0.07
53.5           53.5        53.6       22.27       0.62        0.22
53.5           55.5        52.1       22.26       0.62        0.12
55.5           55.5        55.6       23.49       0.77        0.46
55.5           57.5        54.2       23.48       0.76        0.33
55.5           59.5        52.8       23.46       0.76        0.20
55.5           61.5        51.3       23.45       0.76        0.08
57.5           57.5        57.6       24.75       1.00        0.86
57.5           59.5        56.3       24.74       0.99        0.69
57.5           61.5        55.0       24.73       0.99        0.52
57.5           63.5        53.6       24.71       0.99        0.36
57.5           65.5        52.1       24.70       0.98        0.19
59.5           59.5        59.6       26.07       1.00        1.14
59.5           61.5        58.4       26.06       1.00        0.97
59.5           63.5        57.1       26.04       1.00        0.80
59.5           65.5        55.8       26.03       1.00        0.64
59.5           67.5        54.5       26.02       1.00        0.47
59.5           69.5        53.1       26.00       1.00        0.30
59.5           71.5        51.6       25.99       1.00        0.14
61.5           61.5        61.6       27.44       1.00        1.43
61.5           63.5        60.4       27.43       1.00        1.27
61.5           65.5        59.3       27.42       1.00        1.10
61.5           67.5        58.1       27.40       1.00        0.93
61.5           69.5        56.8       27.39       1.00        0.77
61.5           71.5        55.5       27.38       1.00        0.60
61.5           73.5        54.2       27.36       1.00        0.43
61.5           75.5        52.7       27.35       1.00        0.27
61.5           77.5        51.2       27.33       1.00        0.10
63.5           63.5        63.5       28.88       1.00        1.75
63.5           65.5        62.5       28.86       1.00        1.58
63.5           67.5        61.4       28.85       1.00        1.42
63.5           69.5        60.3       28.84       1.00        1.25
63.5           71.5        59.1       28.82       1.00        1.08
63.5           73.5        57.9       28.81       1.00        0.91
63.5           75.5        56.7       28.79       1.00        0.75
63.5           77.5        55.3       28.78       1.00        0.58
63.5           79.5        54.0       28.76       1.00        0.41
63.5           81.5        52.6       28.75       1.00        0.25
63.5           83.5        51.1       28.73       1.00        0.08

                DB          DP      Enthalpy

Supply Air     68.00       50.26     24.76
Return Air     85.00       50.26     29.91

                                                       Desiccant
                                                   Dehumidification
                                                  (Economizer Cycle)

WB,            DB,         DP,        OA       Economizer  Economizer
[degrees]F  [degrees]F  [degrees]F  Enthalpy,   Sensible    Sensible
                                     Btu/lb     Cooling     Cooling
                                                Enthalpy      COP
                                                 Change,
                                                 Btu/lb

51.5           51.5        51.6       21.11       0.00       9.26
53.5           53.5        53.6       22.27       0.00       8.79
53.5           55.5        52.1       22.26       0.00       8.79
55.5           55.5        55.6       23.49       0.00       8.37
55.5           57.5        54.2       23.48       0.00       8.37
55.5           59.5        52.8       23.46       0.00       8.37
55.5           61.5        51.3       23.45       0.00       8.37
57.5           57.5        57.6       24.75       0.00       7.99
57.5           59.5        56.3       24.74       0.00       7.99
57.5           61.5        55.0       24.73       0.00       7.99
57.5           63.5        53.6       24.71       0.00       7.99
57.5           65.5        52.1       24.70       0.00       7.99
59.5           59.5        59.6       26.07       1.31       7.65
59.5           61.5        58.4       26.06       1.30       7.65
59.5           63.5        57.1       26.04       1.28       7.65
59.5           65.5        55.8       26.03       1.27       7.65
59.5           67.5        54.5       26.02       1.26       7.65
59.5           69.5        53.1       26.00       1.24       7.65
59.5           71.5        51.6       25.99       1.23       7.65
61.5           61.5        61.6       27.44       2.68       7.33
61.5           63.5        60.4       27.43       2.67       7.33
61.5           65.5        59.3       27.42       2.66       7.33
61.5           67.5        58.1       27.40       2.64       7.33
61.5           69.5        56.8       27.39       2.63       7.33
61.5           71.5        55.5       27.38       2.62       7.33
61.5           73.5        54.2       27.36       2.60       7.33
61.5           75.5        52.7       27.35       2.59       7.33
61.5           77.5        51.2       27.33       2.57       7.33
63.5           63.5        63.5       28.88       4.12       7.03
63.5           65.5        62.5       28.86       4.10       7.03
63.5           67.5        61.4       28.85       4.09       7.03
63.5           69.5        60.3       28.84       4.08       7.03
63.5           71.5        59.1       28.82       4.06       7.03
63.5           73.5        57.9       28.81       4.05       7.03
63.5           75.5        56.7       28.79       4.03       7.03
63.5           77.5        55.3       28.78       4.02       7.03
63.5           79.5        54.0       28.76       4.00       7.03
63.5           81.5        52.6       28.75       3.99       7.03
63.5           83.5        51.1       28.73       3.97       7.03

               DB        DP      Enthalpy

Supply       68.00     50.26     24.76
Air

Return       85.00     50.26     29.91
Air

                                            Desiccant Dehumidification
                                                (Economizer Cycle)

WB,          DB,        DP,       OA         Economizer    Relative OA
[degrees]  [degrees]  [degree]  Enthalpy,     Sensible     Economizer
F              F          F      Btu/lb    cooling Energy  Energy Use,
                                                 Use,      dimensionles
                                           dimensionless

51.5          51.5      51.6     21.11          0.00           0.07
53.5          53.5      53.6     22.27          0.00           0.22
53.5          55.5      52.1     22.26          0.00           0.12
55.5          55.5      55.6     23.49          0.00           0.46
55.5          57.5      54.2     23.48          0.00           0.33
55.5          59.5      52.8     23.46          0.00           0.20
55.5          61.5      51.3     23.45          0.00           0.08
57.5          57.5      57.6     24.75          0.00           0.86
57.5          59.5      56.3     24.74          0.00           0.69
57.5          61.5      55.0     24.73          0.00           0.52
57.5          63.5      53.6     24.71          0.00           0.36
57.5          65.5      52.1     24.70          0.00           0.19
59.5          59.5      59.6     26.07          0.17           1.31
59.5          61.5      58.4     26.06          0.17           1.14
59.5          63.5      57.1     26.04          0.17           0.97
59.5          65.5      55.8     26.03          0.17           0.80
59.5          67.5      54.5     26.02          0.16           0.64
59.5          69.5      53.1     26.00          0.16           0.47
59.5          71.5      51.6     25.99          0.16           0.30
61.5          61.5      61.6     27.44          0.37           1.80
61.5          63.5      60.4     27.43          0.36           1.63
61.5          65.5      59.3     27.42          0.36           1.46
61.5          67.5      58.1     27.40          0.36           1.29
61.5          69.5      56.8     27.39          0.36           1.12
61.5          71.5      55.5     27.38          0.36           0.96
61.5          73.5      54.2     27.36          0.36           0.79
61.5          75.5      52.7     27.35          0.35           0.62
61.5          77.5      51.2     27.33          0.35           0.45
63.5          63.5      63.5     28.88          0.59           2.34
63.5          65.5      62.5     28.86          0.58           2.17
63.5          67.5      61.4     28.85          0.58           2.00
63.5          69.5      60.3     28.84          0.58           1.83
63.5          71.5      59.1     28.82          0.58           1.66
63.5          73.5      57.9     28.81          0.58           1.49
63.5          75.5      56.7     28.79          0.57           1.32
63.5          77.5      55.3     28.78          0.57           1.15
63.5          79.5      54.0     28.76          0.57           0.98
63.5          81.5      52.6     28.75          0.57           0.81
63.5          83.5      51.1     28.73          0.56           0.64

                  DB           DP       Enthalpy

Supply Air       68.00        50.26      24.76
Return Air       85.00        50.26      29.91

                                                   RA Cooling with Dry
                                                          Coils

WB,         DB, [degrees]F      DP,        OA         RA        RA-SA
[degrees]F                  [degrees]F  Enthalpy,  Enthalpy,  Enthalpy,
                                         Btu/lb     Btu/lb     Btu/lb

51.5             51.5          51.6       21.11      28.91      4.15
53.5             53.5          53.6       22.27      28.91      4.15
53.5             55.5          52.1       22.26      28.91      4.15
55.5             55.5          55.6       23.49      28.91      4.15
55.5             57.5          54.2       23.48      28.91      4.15
55.5             59.5          52.8       23.46      28.91      4.15
55.5             61.5          51.3       23.45      28.91      4.15
57.5             57.5          57.6       24.75      28.91      4.15
57.5             59.5          56.3       24.74      28.91      4.15
57.5             61.5          55.0       24.73      28.91      4.15
57.5             63.5          53.6       24.71      28.91      4.15
57.5             65.5          52.1       24.70      28.91      4.15
59.5             59.5          59.6       26.07      28.91      4.15
59.5             61.5          58.4       26.06      28.91      4.15
59.5             63.5          57.1       26.04      28.91      4.15
59.5             65.5          55.8       26.03      28.91      4.15
59.5             67.5          54.5       26.02      28.91      4.15
59.5             69.5          53.1       26.00      28.91      4.15
59.5             71.5          51.6       25.99      28.91      4.15
61.5             61.5          61.6       27.44      28.91      4.15
61.5             63.5          60.4       27.43      28.91      4.15
61.5             65.5          59.3       27.42      28.91      4.15
61.5             67.5          58.1       27.40      28.91      4.15
61.5             69.5          56.8       27.39      28.91      4.15
61.5             71.5          55.5       27.38      28.91      4.15
61.5             73.5          54.2       27.36      28.91      4.15
61.5             75.5          52.7       27.35      28.91      4.15
61.5             77.5          51.2       27.33      28.91      4.15
63.5             63.5          63.5       28.88      28.91      4.15
63.5             65.5          62.5       28.86      28.91      4.15
63.5             67.5          61.4       28.85      28.91      4.15
63.5             69.5          60.3       28.84      28.91      4.15
63.5             71.5          59.1       28.82      28.91      4.15
63.5             73.5          57.9       28.81      28.91      4.15
63.5             75.5          56.7       28.79      28.91      4.15
63.5             77.5          55.3       28.78      28.91      4.15
63.5             79.5          54.0       28.76      28.91      4.15
63.5             81.5          52.6       28.75      28.91      4.15
63.5             83.5          51.1       28.73      28.91      4.15

              DB         DP       Enthalpy

Supply       68.00     50.26       24.76
Air

Return       85.00     50.26       29.91
Air

                                             RA Cooling with Dry Coils

WB,          DB,        DP,        OA           RA          RA Energy
[degrees]  [degrees]  [degrees]  Enthalpy,  Conditioning       Use,
F              F          F       Btu/lb     Process COP  dimensionless

51.5          51.5       51.6      21.11        9.26           0.45
53.5          53.5       53.6      22.27        8.79           0.47
53.5          55.5       52.1      22.26        8.79           0.47
55.5          55.5       55.6      23.49        8.37           0.50
55.5          57.5       54.2      23.48        8.37           0.50
55.5          59.5       52.8      23.46        8.37           0.50
55.5          61.5       51.3      23.45        8.37           0.50
57.5          57.5       57.6      24.75        7.99           0.52
57.5          59.5       56.3      24.74        7.99           0.52
57.5          61.5       55.0      24.73        7.99           0.52
57.5          63.5       53.6      24.71        7.99           0.52
57.5          65.5       52.1      24.70        7.99           0.52
59.5          59.5       59.6      26.07        7.65           0.54
59.5          61.5       58.4      26.06        7.65           0.54
59.5          63.5       57.1      26.04        7.65           0.54
59.5          65.5       55.8      26.03        7.65           0.54
59.5          67.5       54.5      26.02        7.65           0.54
59.5          69.5       53.1      26.00        7.65           0.54
59.5          71.5       51.6      25.99        7.65           0.54
61.5          61.5       61.6      27.44        7.33           0.57
61.5          63.5       60.4      27.43        7.33           0.57
61.5          65.5       59.3      27.42        7.33           0.57
61.5          67.5       58.1      27.40        7.33           0.57
61.5          69.5       56.8      27.39        7.33           0.57
61.5          71.5       55.5      27.38        7.33           0.57
61.5          73.5       54.2      27.36        7.33           0.57
61.5          75.5       52.7      27.35        7.33           0.57
61.5          77.5       51.2      27.33        7.33           0.57
63.5          63.5       63.5      28.88        7.03           0.59
63.5          65.5       62.5      28.86        7.03           0.59
63.5          67.5       61.4      28.85        7.03           0.59
63.5          69.5       60.3      28.84        7.03           0.59
63.5          71.5       59.1      28.82        7.03           0.59
63.5          73.5       57.9      28.81        7.03           0.59
63.5          75.5       56.7      28.79        7.03           0.59
63.5          77.5       55.3      28.78        7.03           0.59
63.5          79.5       54.0      28.76        7.03           0.59
63.5          81.5       52.6      28.75        7.03           0.59
63.5          83.5       51.1      28.73        7.03           0.59

                      DB           DP        Enthalpy
Supply Air          68.00        50.26        24.76
Return Air          85.00        50.26        29.91

                                                         RA Cooling
                                                          with Dry
                                                            Coils

WB, [degrees] F       DB,          DP,     OA Enthalpy,  Economizer
                 [degrees] F  [degrees] F     Btu/lb     Usage Flag

51.5                 51.5         51.6         21.11         1
53.5                 53.5         53.6         22.27         1
53.5                 55.5         52.1         22.26         1
55.5                 55.5         55.6         23.49         1
55.5                 57.5         54.2         23.48         1
55.5                 59.5         52.8         23.46         1
55.5                 61.5         51.3         23.45         1
57.5                 57.5         57.6         24.75         0
57.5                 59.5         56.3         24.74         0
57.5                 61.5         55.0         24.73         0
57.5                 63.5         53.6         24.71         1
57.5                 65.5         52.1         24.70         1
59.5                 59.5         59.6         26.07         0
59.5                 61.5         58.4         26.06         0
59.5                 63.5         57.1         26.04         0
59.5                 65.5         55.8         26.03         0
59.5                 67.5         54.5         26.02         0
59.5                 69.5         53.1         26.00         1
59.5                 71.5         51.6         25.99         1
61.5                 61.5         61.6         27.44         0
61.5                 63.5         60.4         27.43         0
61.5                 65.5         59.3         27.42         0
61.5                 67.5         58.1         27.40         0
61.5                 69.5         56.8         27.39         0
61.5                 71.5         55.5         27.38         0
61.5                 73.5         54.2         27.36         0
61.5                 75.5         52.7         27.35         0
61.5                 77.5         51.2         27.33         1
63.5                 63.5         63.5         28.88         0
63.5                 65.5         62.5         28.86         0
63.5                 67.5         61.4         28.85         0
63.5                 69.5         60.3         28.84         0
63.5                 71.5         59.1         28.82         0
63.5                 73.5         57.9         28.81         0
63.5                 75.5         56.7         28.79         0
63.5                 77.5         55.3         28.78         0
63.5                 79.5         54.0         28.76         0
63.5                 81.5         52.6         28.75         0
63.5                 83.5         51.1         28.73         0

(1.) Reactivation heat COP is defined as the ratio of the
dehumidification enthalpy change (assuming constant dry bulb) to the
reactivation input energy. High COPs correspond to substantial free
heat.
(2.) The temperature of the process air leaving the desiccant wheel
will be a function of many parameters. For this study, it is assumed
that the process air is dehumidified and heated along a constant
enthalpy line.
(3.) The COP of the RA conditioning is assumed to be 0.50 kW/ton with a
wet bulb temperature of 63.5[degrees]F, and to increase with a
reduction in wet bulb temperature.


[FIGURE 9 OMITTED]

SUMMARY AND CONCLUSIONS

This study examined the opportunity for using air-side economizers for data centers in cases where the supply air temperature falls within ASHRAE's thermal guidelines (ASHRAE 2004a). This is a substantial opportunity for greatly increased economizer operation relative to a traditional office system with a 55[degrees]F (13[degrees]C) discharge temperature, but with this opportunity comes the difficulty of efficiently dehumidifying outdoor air to the level recommended by ASHRAE's thermal guidelines. In at least one locale (Los Angeles) this need for dehumidification occurs for over 50% of the year.

Dehumidification purely by mechanical refrigeration was briefly examined, but it does not appear to be efficient due to the need to cool the outdoor air to saturation to dehumidify the air. Another opportunity is desiccant dehumidification, which was examined in this paper. Based on the analysis, desiccant dehumidification will be economical if there is a source of free heat for reactivation, if the system is well designed so that the outdoor air is not heated above the return air temperature of the data center during the dehumidification process, and if the increased pressure drop energy penalty due to the added dehumidification process is small relative to savings in other areas. If it is only possible to provide a percentage of the waste heat for the reactivation, the number of joint frequency bins for which economizer use is economical drops off. It does not appear to pay to install a desiccant dehumidification system if there is not any waste (or free) heat available.

FUTURE STUDY

Other studies of desiccant dehumidification looked at various combinations of desiccant dehumidification and dehumidification with mechanical refrigeration (e.g., Yong 2006). Examination of such hybrid systems may yield an increased number of joint frequency bins for which an economizer could yield savings with high ambient dew point conditions. The relatively small magnitude of dehumidification (lb moisture/ lb dry air) required for datacom applications also deserves analysis of possible alternatives to the traditional desiccant wheel for efficiently meeting this need.

Note: ASHRAE is in the process of adjusting the recommended Class 1 and Class 2 Operating Environment (see Figure 2). Any quantitative conclusions drawn from this paper should be revisited by those performing similar analysis in light of the new conditions.

REFERENCES AND BIBLIOGRAPHY

ASHRAE. 2004a. Thermal Guidelines for Data Processing Environments. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 2004b. 2004 ASHRAE Handbook--Systems and Equipment. Chapter 22, Desiccant dehumidification and pressure-drying equipment. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE 2004c. 2004 ASHRAE Handbook--Systems and Equipment. Chapter 47, Mechanical dehumidifiers and heat pipes. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASHRAE 2005.

ASHRAE Weather Data Viewer v3.0. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE 2008. Best Practices for Energy Efficiency in Datacom Facilities. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Henderson, H., and J. Sand. 2003. An hourly simulation tool to evaluate hybrid desiccant system configuration options. ASHRAE Transactions 109(2):551-64.

Jalalzadeh-Azar, S., S. Slayzak, R. Judkoff, T. Schaffhauser, and R. DeBlasio. 2005. Performance assessment of a desiccant cooling system in a CHP application incorporating an IC engine. International Journal of Distributed Energy Resources 1(2):163-84.

Yong, L., K. Sumathy, Y.J. Dai, J.H. Zhong, and R.Z. Wang. 2006. Experimental study on a hybrid desiccant dehumidification and air conditioning system. Journal of Solar Energy Engineering 128(1):77-81.

Thomas A. Davidson, PE

Member ASHRAE

Thomas A. Davidson is a senior mechanical engineer at DLB Associates Consulting Engineers in Eatontown, NJ.
COPYRIGHT 2009 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2009 Gale, Cengage Learning. All rights reserved.

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Author:Davidson, Thomas A
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2009
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