Using Wet-Bulb Economizers. Data Center Cooling

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1 The following article was published in ASHRAE Journal, August Copyright 2008 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. Using Wet-Bulb Economizers Data Center Cooling By C. Mike Scofield, P.E., Fellow ASHRAE; and Thomas S. Weaver, P.E. D ata processing environments were estimated in 2006 to consume approximately 1.5 of the total electricity in the United States. Data center power consumption has roughly doubled in the last five years and is expected to double again in the next five years to more than 100 billion kwh.1 Some data centers require 40 to 50 times more power than comparably sized office space. Of the electrical energy consumed by these so-called Internet hotels, often only 30 to 40 of that power is used to operate the computers, servers, and the other electronics in the data center. Most of the rest goes to keeping the hardware cool and humidified. It is estimated that future electrical energy demand could double for these facilities from 2006 to In Northern California, the renewal of data center construction is causing the local utility some concern. The state hosts five 52 ASHRAE Journal to 7.5 million ft2 ( m2) of data center construction. In Northern California, the electricity consumed by data centers amounts to 400 MW to 500 MW or 2.5 of the total electricity supplied by Pacific Gas and Electric (PG&E) Company.3 Mark Bramfitt, PG&E high tech market manager, has proposed the following actions by his company to stem the electrical demand increase. On a case-by-case basis, PG&E will waive the $500,000 incentive cap for data centers using air-side and water-side economizers for new data center construction. Raise the incentive rate from the current eight cents per kwh to 14 cents for air-side and water-side economizers. In March 2007, PG&E, along with Lawrence Berkeley National Laboratory (LBNL) issued a report entitled Data Center Economizer Contamination and Humidity Study, which published the results of monitoring eight existing data centers in Northern California.4 The purpose of this study was to investigate the effect of air-side economizers on energy savings and their impact on indoor particle concentrations and humidity control. Although the data centers using recirculation and minimum outdoor air had lower particle concentration, the data centers using air-side economizers About the Authors C. Mike Scofield, P.E., is president of Conservation Mechanical Systems in Sebastopol, Calif. Thomas S. Weaver, P.E., is a partner at Conservation Mechanical Systems. a s h r a e. o r g August 2008

2 had annual average concentrations that still meet the ASHRAE requirements. By increasing the filtration effectiveness from ASHRAE 40 to 85, particle concentrations in data centers using air-side economizers may be reduced to levels associated with the minimum outdoor air design. Because of California s mild climate, the humidity levels in data centers served by air-side economizers were within ASHRAE recommended levels. During cold ambient temperatures, if the mixed air delivery temperature to the cold aisle results in a lower than 40 relative humidity entering the electronics, humidification may be required to achieve the desired room dew point. One data center using air-side economizers was found to have a 30 mechanical cooling power reduction when the economizer was active. Annual savings at this center were estimated to be within the range of 60 MWh to 80 MWh per year. With such compelling evidence that air-side economizers save energy in data center cooling systems, why is the design community reluctant to specify this design solution? With air economizers, people perceive that the operation of the data center hardware will be compromised by the introduction of increased levels of pollutants, both particulate and gaseous, and Outdoor Air Final Filter VAV Plenum Fan Pre-Filter Mixing Box Dampers Return Air 75 F at 55 F Dew Point by reduced humidity levels in cold weather. A wet-bulb economizer (WBE) may be used to provide both stable room humidity (dew point) control and reduced room particulate and gaseous concentrations. The latter is true, since the direct evaporative cooling device scrubs the air passing through the wetted pad. The Wet-Bulb Economizer Figure 1 shows a rooftop central station air-handling unit layout, which uses a wetted rigid media direct evaporative cooler (DEC) section to extend data center economizer cooling hours and provide humidity (dew-point) control. Unlike air-side and water-side economizers, the WBE uses the heat generated by the electronics inside the space to evaporate water off a wetted rigid media pad during cold ambient conditions. The 12 in. (305 mm) deep rigid media pad should be selected at 400 ft/min (2.03 m/s) face velocity which yields a 90 saturation efficiency (sea level) with only a 0.l4 in. w.g. (35 Pa) static pressure loss. The sump water recirculation pump would be sized to circulate 1.5 gpm/ft 2 (1.0 L/[s m 2 ]) of horizontal media pad area. When the DEC water recirculation pump is activated, the mixing box dampers blend outdoor air with data center return air to achieve a wet-bulb condition which, after the addition of fan heat, results in the setpoint supply air temperature and dew point required to cool the electronics. Figure 2 shows a typical winter outdoor air condition of 40 F (4 C) dry bulb (DB) at 60 relative humidity (RH) mixing with a room return at 75 F (24 C) DB and 50 RH. To simplify this illustration, the saturation efficiency of the rigid media pad is assumed to be 100. Therefore, the supply condition at 55 F (13 C) DB would also yield a 55 F (13 C) WB and 55 F (13 C) dew point (DP) supply condition to the space. Refrigeration is not required. Since data centers add heat but no moisture to the space, the room load line is shown to be horizontal. Direct Evaporative Cooling Section, Rigid Media Type With 100 Efficiency and With Water Recirculation Pump Refrigeration Cooling Coil Supply Air 55 F DB/55 F WB 55 F Dew Point VAV Relief Air Fan Relief Air Figure 1: A roof mounted air-handling unit with a wet-bulb economizer blends outdoor air with return air to produce the required supply air dry-bulb temperature to the space. For simplicity, the figure shows performance based on a 100 saturation efficiency for the adiabatic device. Fan heat is not included. Space positive pressure is maintained by the variable air volume relief air fan. By blending 31 outdoor air with 69 return air (Figure 2) we are able to deliver 64 F (18 C) DB and 55 F (13 C) WB to the wetted pad which then produces the setpoint 55 F (13 C) DB, 55 F (13 C) DP required for the space sensible cooling load. Note that different room supply air setpoint conditions may be generated by this system simply by changing the proportions of outdoor air and return air delivered to the mixing box (Figure 1). Current design criteria for cooling Class I and II data centers allows dry bulb delivery temperatures as high as 68 F to 77 F (20 C to 25 C), which would greatly expand the free cooling hours of this design. Dew-point (absolute humidity) maximums of 63 F (17 C) for Class I and 70 F (21 C) for Class II data centers would set the upper limit for the delivery condition off the adiabatic cooler/humidifier system. An examination of Figure 2 shows that an air side economizer, on the same winter day, would also produce the required 55 F (13 C) DB room setpoint, but at a 40 F (4 C) DP. To achieve the 55 F (13 C) DP absolute humidity condition in the data center, 26 grains of moisture per pound of air (3.72 g/kg) would be required to be added by a separate humidification system. Heat August 2008 ASHRAE Journal 53 Sump Roof

3 recovery and humidification provided by the WBE system is most significant in colder climates such as Boston, Chicago, Atlanta and Denver (Table 1). In cold climates, an air blender may be needed in the mixing box section (Figure 1) to ensure proper winter mixing of the warm return air with frigid outdoor air. Figure 3 uses San Francisco, bin weather data to point out additional benefits of the WBE in more arid climates. Zone 1 is the area where the WBE uses heat recovery for free humidification while keeping refrigeration off. Zone 2 in Figure 3 is the ambient conditions in San Francisco when an air side economizer, in arid climates, introduces 100 outdoor air with a requirement for both refrigeration and humidification. During these hours, a WBE provides both free cooling and humidification and keeps refrigeration off. Zone 3 contains the hours per year where the WBE controls the room delivery DP (humidity) with reduced refrigeration by introducing 100 outdoor air to the adiabatic device. Refrigeration is required to develop the setpoint supply air temperature. Location of the refrigeration coil last in the direction of airflow provides three benefits: 40 F/35 F Typical Winter Outdoor Temperature Mixed Air 64 F/55 F With 31.4 Outdoor Air Space Supply Air 55 F/55 F Location Cold condensate from the coil drains into the DEC sump reducing makeup water requirements and recovering some of the latent cooling effect. Leaving air off the coil is effectively saturated so that room DP temperature (absolute humidity) is the same as the DB temperature. In arid climates where ambient conditions are often below the room DP, refrigeration requirements may be reduced by 15 to 17 compared to a system where the cooling coil is located upstream of the rigid media DEC. In Zone 4 of Figure 3, outdoor air dampers close to their minimum position required to maintain data center positive pressure control. The variable air volume (VAV) relief fan shown in Figure 1 is off during the hours shown in Zone 4. Refrigeration is required, but the DEC recirculation pump is off. The static pressure loss and dirt loading of the rigid media are reduced in this mode. Saturation Temperature F Zone 1 Room 75 F at 50 RH 55 F Wet Bulb F DB 64 F DB 75 F DB Zone 2 90 RH 70 RH 50 RH 80 RH 60 RH 55 F Dew Point Zone 3 Zone 4 Boston Chicago Atlanta Denver Sunnyvale, Calif Table 1: Percentage of annual hours that reside in each zone of Figure 3 for five North American cities based on bin weather data and a 24/7 duty cycle. With a better understanding of how the WBE works, let s examine its application in meeting current mission critical facility best practices design requirements. Best Practices for Datacom Facilities Now that the data center room dew point has been established with the WBE, the abundance of heat generated by the electronics may now be used to add sensible reheat to our supply air system. The recently published ASHRAE design guide, Best Practices for Datacom Facility Energy Efficiency, provides Table 2.1 on Page 19, which lists recommended cold aisle inlet dry-bulb temperatures and relative humidity ranges. Class I and II inlet conditions from the cold aisle into the equipment may range from 68 F to 77 F (20 C to 25 C) DB and between 40 to 55 RH as recommended in ASHRAE s Thermal Guidelines for Data Processing Equipment. 54 ASHRAE Journal ashrae.org August RH 30 RH 20 RH 10 RH Figure 2: The wet-bulb economizer process is illustrated at a typical 40 F ambient outdoor winter condition. Both room dew-point (humidity) and setpoint supply air temperature are generated with the heat recovered from the data center return air by the rigid media cooler/humidifier. Fan heat is not included Grains Moisture Per Pound of Dry Air

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5 Figure 4 shows a rooftop air-handling unit schematic arranged to allow excess heat available from the data center hot aisle to sensibly reheat the air leaving the WBE at the required room dew point. Figure 5 shows that, while maintaining the required room 55 F (13 C) DP with humidified outdoor air, this air-handling unit configuration would provide sensible reheat to a 75 F (24 C) cold aisle inlet temperature at a relative humidity of 50. This condition now meets the Table 2.1, Best Practices, cold aisle inlet requirements for Class I and II data centers. From Figure 5, it is apparent that the WBE may now offer tonhour reductions until ambient conditions exceed the hot aisle enthalpy of 33 Btu/lb (76.8 kj/kg) (95 F [35 C] DB and 69 F [21 C] WB). Let s revisit Table 1, Zone 3, where refrigeration load may be reduced and humidification eliminated. Now, based on Figure 5, the hours per year in Zone 3 may be greatly expanded in Boston, Chicago, and Atlanta. The annual hour percentage for Zone 3 in Boston increases from 16.5 to 30.5, in Chicago from 17 to 34 and in Atlanta from 21 to 44. An additional advantage of the unit configuration shown in Figure 4 is the location of the final filter downstream of the supply fan. This position of the filter ensures that both the outdoor air and recirculation air are cleaned before entry into the data center. Zone 1 3,261 hr/yr Saturation Temperature F 55 F Wet Bulb F DB Zone 2 2,530 hr/yr 90 RH 70 RH 50 RH 80 RH 60 RH Zone F Wet Bulb 55 F Dew Point 40 RH 20 RH Zone 4 90 hrs/yr Zone 3 2,865 hr/yr 10 RH 30 RH Zone 1 Has 3,261 hours per year where mechanical cooling and humidification energy may be eliminated. Zone 2 Has 2,530 hours per year above the 55 F supply air setpoint, when an air-side economizer would require refrigeration operation. In this zone the wet-bulb economizer provides all cooling and humidification requirements for the data center. Zone 3 Has 2,865 hours per year where the outdoor enthalpy is lower than the return air enthalpy and refrigeration energy costs may be reduced by the introduction of 100 outdoor air. Zone 4 Has 90 hours per year where the wet-bulb economizer will position dampers for minimum outdoor air as required for space pressurization. The direct evaporative cooling device will be off. Figure 3: San Francisco wet-bulb economizer zones where free cooling and humidification are available based on bin weather data and a 24/7 duty cycle. Relief Air Fan With BDD EA 5 CD 5 Return Air 1 OA 2 DEC CWC 3 Data Center VAV Supply Fan Filter Supply Air DEC = Direct Evaporative Cooler/Humidifier CWC = Chilled Water Coil BDD = Back Draft Damper CD = Control Damper Figure 4: A schematic of a roof mounted central station air-handling unit designed to use a portion of the data center return air for reheat, at the required room dew point, to meet the Table 2.1 ASHRAE design guide cold aisle inlet air conditions specified for Class I and II data centers. CD 4 Moisture Grains Per Pound of Dry Air Refrigeration Redundancy For Arid Climates Murphy s Law dictates that refrigeration cooling systems will fail on the hottest day of the year, so Table 2 shows the data center hot aisle return air temperature and relative humidity with and without a WBE backup in five western U.S. cities. Cold aisle inlet conditions would be ambient dry-bulb temperature and relative humidity, ignoring fan heat, for the refrigeration only design. The adiabatic evaporative cooling/humidification 56 ASHRAE Journal ashrae.org August 2008

6 system, with a 90 saturation efficiency, would provide cold aisle inlet conditions of 65 F (18 C) DB and 90 RH for San Francisco, 64.4 F (18 C) DB and 85 RH for Reno, 65.4 F (19 C) DB and 85 RH for Salt Lake City, 63.3 F (18 C) DB and 85 RH for Denver and 63.6 F (18 C) DB and 84 RH for Albuquerque. Although these cold aisle inlet conditions are not within the Table 2.1 recommended levels for Class I and II data centers, they would allow continuous operation of the electronics during a refrigeration failure and prevent a costly data center shut down. The hot aisle resultant return air conditions produced by the electronics, assuming a 20 F (7 C) temperature rise, are all within Class I through IV operating limits, and the room dew point does not exceed the 63 F (17 C) maximum limit either at sea level or at 5,000 ft (1524 m) elevation for the five cities listed. By submitting the internal cooling load directly to an adiabatic WBE, the space may be held within acceptable limits using 100 outdoor air. This would not be true with either an air-side economizer or a water-side economizer. Location Filtration Redundancy Air contamination in the data center is a valid concern which needs to be addressed. Consistent with the LBNL study 4 recommendations, air side economizers which introduce large amounts of outdoor air need to be equipped with better filtration than data center systems that use 100 recirculation. With 100 recirculation systems, filters with a MERV rating of 8 or 9 (ANSI/ASHRAE Standard , Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size; equivalent to 40 efficient based on the older dust spot efficiency rating of ANSI/ASHRAE Standard , Gravimetric and Dust-Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter) are used. When outdoor air is introduced, it is necessary to increase the MERV rating to 10 or 11 (equivalent to 85 efficiency based on the duct spot method). 5 The WBE uses a 12 in. (305 mm) deep wetted media pad that acts as an air scrubber that will further reduce gaseous and particulate contaminants from the airstream when the economizer * The 12 in. wetted media demonstrates a 90 removal effectiveness for some gaseous contaminants such as ammonia, urea, oxides of nitrogen, formaldehyde, radon, and chlorine, as well as 60 removal of ozone. 7, F WB 40 F 55 F 68.5 F 75 F 95 F 1. Outdoor air at 40 F DB and 35F WB. 3. Dew-point supply air at 55 F DP and 55 F DB. 2. Mixed air at 68.5 F DB and 55 F WB. 4. Reheated air to cold aisle at 75 F DB and 55 F DP. 5. Hot aisle return air at 95 F DB, 69 F WB and 55 F DP. ASHRAE 0.4 Summer Design F DB/WB Resultant Room Condition With Ventilation Only F DB/ RH Resultant Room Condition With Evaporative Cooling F DB/ RH San Francisco 83/ F at F at 45 Reno, Nev. 95/ F at F at 43 Salt Lake City 96/ F at F at 44 Denver 93/ F at F at 43 Albuquerque, N.M. 96/ F at F at 42 Table 2: Direct evaporative cooling as refrigeration backup. Best Practices Cold Aisle Inlet Conditions dampers are open to outdoor air. Tests run in 1988 and 1991 at the Air Filter Testing Laboratories in Louisville, Ky., 6 show a dust spot efficiency of 16 at 500 ft/min (2.5 L/s) face velocity with 1.5 gpm/ft 2 (1.0 L/[s m 2 ]) water recirculation through the media.* Although a direct evaporative cooler/humidifier should not be used as the primary source for data center contamination removal, it provides a valuable redundancy to the principal filtration system in an air-handling unit. Makeup water and recirculation water furnished to the rigid media pad should be treated to reduce the risk of airborne microbial or particulate contamination of the supply air. A new, nonchemical, water treatment system that uses a pulse-power technology is recommended for the DEC sump recirculation water flow. 8 August 2008 ASHRAE Journal F DP F WB Figure 5: The psychrometric conditions are shown for each state point at the locations indicated in Figure 4. Fan heat not included. 5

7 Originally developed for cold pasteurization in the food industry, this system encapsulates water hardness minerals and particulates into a nonadherent powder that is harmlessly deposited in the bottom of the sump. The device controls scaling of the wetted media pad and biological growth in the sump water. Under proper operation, the pulse-power component will maintain clean sump water with low bacteria counts free of biofilm and will eliminate the breeding ground for the amplification of Legionella and other waterborne pathogens. 9 Conclusions For years, computer room air-conditioning (CRAC) units have been the design standard for data center cooling. Increasing heat densities produced by the modern electronics inside these facilities has now forced designers to reevaluate central station air-handling units as a design solution. In addition to their ability to incorporate the WBE, large air-handling units offer the following advantages over CRAC system: Recapture of valuable data center floor space for computer hardware; Increased computer-center security through the elimination of in-room maintenance of CRAC units; Advertisement formerly in this space. Simplified room dew-point control for the data center with less risk of humidifiers and dehumidification cooling coils fighting one another; Filtration redundancy; and Refrigeration redundancy. With an installed cost in the range of $0.75 to $0.90 per cfm ($1.60 to $1.90 per L/s), the payback for an adiabatic cooler/ humidifier component in a central station air-handling unit should be less than one year for the 24/7 duty cycle of a data center. With credit given for the elimination of cooling towers, water-to-water heat exchangers, unit mounted coils, pumps, and piping associated with water side economizers, the owner first cost of a WBE may be more than offset. Since we are addressing the data center cooling load directly with the ambient wet bulb, we eliminate the heat transfer losses of a water-side economizer at the tower, the water-to-water heat exchanger, and the unit mounted cooling coil. Sensible cooling Energy Efficiency Ratios (EER) for the WBE are a function of the air side static pressure loss and the recirculation water pump penalty. EERs for direct evaporative cooling systems often exceed 100 Btus converted per watt expended ( kj/w) compared to refrigeration systems EERs, which range between six and 10 Btu/W (6.33 and kj/w). 10 Data center cooling and the slang phrase swamp cooler have never before been coupled in an article on green computer center design. Perhaps at long last, it is time to take another look at a technology that has been around since the time of the pharaohs. Wet-bulb economizers provide a sustainable solution to a growing computer cooling problem. References 1. EPA Report to Congress on Server and Data Center Energy Efficiency. Washington, D.C.: U.S. Environmental Protection Agency. 2. Associated Press Tech Companies Tout Greener Data Centers. B. Bergstein. The Press Democrat, August 9, p. E Fok, S Improving Industrial Competitiveness by using the Environment for Free Cooling in Data Centers: Eliminating the Fear Factor. Seminar 45, ASHRAE Winter Meeting. 4. Tschudi, W Data Center Economizer Contamination and Humidity Study. LBNL/Pacific Gas & Electric. com/3y4es7 (or EconomizerDemoReportMarch13.pdf). 5. Sorell, V OA economizers for data centers. ASHRAE Journal 49(12): Munters Evaporative Cooling Division Engineering Bulletin. EB-006-PR. 7. Sohr, R.T The most precise and clean mode for humidification of space. ASHRAE Transactions 103(2): ASHRAE ASHRAE GreenGuideThe Design, Construction, and Operation of Sustainable Buildings, 2nd ed. ASHRAE Green Tip 14, p Puckorius, P.R., et al Why evaporative coolers have not caused Legionnaires= disease. ASHRAE Journal 37(1): Watt, J.R., W.K. Brown. Evaporative Air Conditioning Handbook, 3rd ed. Lilburn, Ga.: Fairmont Press pp.182 and ASHRAE Journal ashrae.org August 2008