ASHRAE Journal - March 2011 - 46

(5.6°C). Such a water-side economizer requires approximately 5.5 gpm (0.35 L/s) per 1,000 cfm (471.9 L/s) at sea level and may require 100 ft (30.5 m) of pump head, or more, depending on the installation. When integrated with scavenger air evaporative cooling, refrigeration capacity may be significantly reduced on IASE systems in virtually all climates, which is not true of conventional direct air-side economizers or wet-bulb economizer installations that require supply air dew point to be maintained below the current allowable value for Class I environments of 63°F (17.2°C). IASE units have a rapid restart after power outage with little delay reaching full heat rejection potential.

Scavenger Air Entering HX Dry-Bulb
(°F)

Scavenger Air Leaving HX
Dry Bulb (°F)

Supply Air Leaving HX*
(Dry Bulb [°F])

Wet-Bulb
(°F)

Flow
(cfm)

HX Pressure Drop
(In. w.c.)

90 80 70 65 60 50 40 30 20 10 0 –10 –20 –30

85 78 69 64 59 49 39 29 19 9 –1 –11 –19 –30.5

28,000 28,000 28,000 22,500 18,100 13,350 10,665 8,875 7,580 6,585 5,800 5,160 4,625 4,175

98.5 95.7 92.8 94.4 96.3 98.3 99.3 99.9 100.3 100.6 100.8 100.9 101.0 101.1

0.61 0.63 0.65 0.44 0.30 0.18 0.12 0.09 0.07 0.06 0.05 0.04 0.03 0.03

92.4 84.4 76.3 75 75 75 75 75 75 75 75 75 75 75

Dry HX IASE System

Referring back to Figure 1, when a HX = 26,760 acfm; supply-side HX pressure drop = 0.56 in. w.c. HX with an effectiveness of 80% is se- * Scavenger airflow varies to maintain target supply air dry bulb = 75°F lected and a 95% efficient direct evapo- Table 1: Dry HX performance at various ambient conditions. rative pre-cooler is used, most of the ICT load from a data center may be rejected to ambient air City Hours Percent of Annual Hours through the HX, even during a hot and humid ambient condiAlbuquerque, N.M. 8,759 99.99% tion as indicated on the figure. Given a 100°F (37.8°C) hotaisle temperature and 75°F (23.9°C) target cold-aisle temperaAtlanta 6,943 79.26% ture, whenever the outdoor air dry-bulb temperature is 68.4°F Baltimore, Md. 7,416 84.66% (20.2°C) or lower, the IASE cycle can reject 100% of data Boise, Idaho 8,760 100.00% center heat (assumes 1.5°F [0.83°C] rise across supply fan). Boston 8,302 94.77% Similarly, when a 95% efficient scavenger air evaporative preChicago 8,085 92.29% cooler is included, 100% of data center heat may be rejected by the IASE cycle if the ambient wet-bulb temperature is 65°F Boulder, Colo. 8,759 99.99% (18.3°C) or lower, regardless of how hot the corresponding Fort Worth, Texas 5,937 67.77% ambient dry-bulb temperature may be. Las Vegas 8,474 96.74% Table 1 shows how the counterflow plate-type HX IASE Los Angeles 8,697 99.28% system performs at various ambient conditions. Note that the required scavenger airflow falls rapidly with decreasing Minneapolis 8,104 92.51% ambient dry-bulb entering temperature. At an ambient dryPhoenix 7,391 84.37% bulb temperature of only 50°F (10°C), the scavenger flow has Portland, Ore. 8,742 99.79% dropped by more than 50% of its design flow, and correspondSalt Lake City 8,758 99.98% ing scavenger-fan motor power is reduced to less than oneSan Francisco 8,755 99.94% eighth of the power consumed at design condition. Table 2 shows the annual hours in a typical year (as deSeattle 8,755 99.94% fined by TMY2 weather data) where an 80% effective HX from TMY2 augmented with a 95% efficient scavenger air evaporative Note: Computed to 75°F coldweather data. 101.5°F (allows for 1.5°F of fan heat) hot aisle cooling aisle. pre-cooler can cool hot-aisle air from 101.5°F (38.6°C), including supply fan heat, to 75°F (23.9°C). Note that the Table 2: Annual hours where an IASE using an 80% efficient IASE cycle rejects 100% of data center heat for almost 80% dry HX with a 95% efficient DEC scavenger air pre-cooler of annual hours in Atlanta. In Salt Lake City, the IASE cycle rejects 100% of data center heat. rejects 100% of data center heat for 99.98% of annual hours. If server inlet temperatures are allowed to occasionally rise may be eliminated from data center heat rejection systems in to the upper end of the recommended range, refrigeration many cities around the globe.
46 ASHRAE Journal ashrae.org March 2011

Note: Hot-aisle entering (including fan heat) = 101.5°F; 200 kW ICT load heat rejection; supply airflow entering

ASHRAE Journal - March 2011

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