Case Study: Cabinet Cooling Using a Water and Refrigeration System Escalating computing capabilities and srinking cip packages are leading to corresponding increases in server and rack poer densities. Cooling te ne iger-poered racks is a serious callenge to system designers and data center managers. In te past, designers tried suc metods as cabinets it better airflo configurations, adding central air conditioning systems, raising floors and using coldaisle and ot-aisle layouts. Today, engineers are turning teir attention to ater-cooled tecniques and refrigeration tecnology to solve cabinet overeating problems. Many liquid-cooled cabinets are available on te market, based on different cooling metods. IBM as a patented rear door eat excanger design. Emerson manufactures bot ater-cooled and refrigeration-cooled cabinets. HP s Modular Cooling System uses cilled ater to dissipate eat. Tis paper ill not discuss ic company as te best tecnique for cabinet cooling; rater, it addresses te concept of a liquid-cooled cabinet by simple calculation and comparison. 1. A Cabinet it an Integrated Air-Water Heat Excanger Tis configuration sets te baseline of liquid cooling and includes te folloing cases: 1)A rear-door, air-ater eat excanger ic cools te ot air from te cabinet to 0 C (Figure 1(a)). )A sealed cabinet it an air-ater eat excanger inside (Figure 1(b)). Figure 1 sos te simplest ater-cooled cabinet. Te rear door eat excanger design as developed by IBM Corporation. Te Figure 1(b) represents te ater-cooled cabinet it internal air circulation. Consider a cabinet, it dimensions of 00 x 800 x 100 mm, containing multiple servers ic generate 30 kw of aste eat. Te cabinet as a fan tray ic delivers 5000 m 3 / (940 CFM) of airflo to te servers and dissipates 1 kw of eat to move te air. Te air temperature at te cabinet inlet is 0 C (68 F). Te cabinet is supplied it cilled ater ose temperature is 1 C (54 F) at te inlet. To cool tis cabinet, at are te requirements for te eat excanger and ater supply system? If te refrigeration system is used, at are its advantages? Figure 1. Scematics of a Cabinet it Different Integrated Air-Water Heat Excangers. Te total eat dissipation of te cabinet (from servers and fan tray) is 31 kw. Te average air temperature at te servers exit is q 31000 T a _ o Ta _ i 0 38.5 C m C 1. 5000 / 3600 1005 18
Assuming te ater temperature at te outlet is 18 C (64.5 F), te ater flo rate needed to cool te cabinet is, 000 m 0.0013m 3 / s(19.5g PM) C(T _ o ) 1000 400 (18 1) Te efficiency of te air-ater eat excanger depends on bot air and ater properties and on te temperature. For air: For ater: C 1. (5000 / 3600) 1005 1675 W / C C 1000 0.0013 400 5166 W / C So te maximum eat te eat excanger can dissipate is: (T a _ i ) 1675 (38.5 1) 44.4kW Te eat excanger s effectiveness is: Wit 000 0.70 44400 1675 0.3 5166 Te NTU (number of transfer units) of te eat excanger (according to [1]) is: A a a N TU A 1.7 Assuming te average eat transfer coefficient beteen air and eat excanger fin is: Te eat transfer coefficient of tube is: N uk 11 0.6 1660 W / m.c D 0.01 Te total surface area needed for te tubes is: N TU 5166 1.7 A 0.69m 1660 Te lengt of eac tube is: A / 4 0.69 / 4 L 5.49m D 3.14 0.01 Te friction factor of te tube is: f 0.184 R e 1 / 5 0.184 3500 0. 0.03 Te estimated pressure drop for eac straigt tube is: 1 L 1 5.49 P f 1000 3.9 0.03 96000Pa(13.9PSI) D 0.01 From te calculation e find tat a total of 0.0013 m 3 /s (19.5GPM) of ater is needed to cool tis cabinet. Te estimated ater pressure drop across te eat excanger is 96000 Pa (13.9 PSI). Tis cooling solution as minimum impact on te CRAC (Computer Room Air Condition) system because, in teory, tere is no eat dissipation from cabinet to te ambient. a 50 W / m.c Te total surface area needed for te fins is: 0.0013 3.9m / s 4 3.14 0.005 For te ater, assume four parallel tubes it internal diameter of 10 mm are used for te air-ater eat excanger. Te average ater velocity inside tube is: m 0.0013 3.9m / s 4 3.14 0.005 Te Reynolds number is: D 3.9 0.01 R e 3500 6 1. 10 Te Nusselt number for fully developed turbulent flo is: N u 0.03 R e 4 / 5 Pr 0.4 0.03 3500 4 / 5 7.6 0.4 11 Figure. Scematic of a Cabinet it a Front Integrated Air-Water Heat Excanger. In general, it is not advisable to put an air-ater eat excanger in te front of a cabinet, as son in Figure. Tis configuration as very lo eat excanger efficiency because te temperature difference beteen te inlet air and inlet ater is very small. In tis case, it is only 8 C. In te best scenario, te air can only be cooled don to about 1 fter passing troug te eat excanger. February 010 Qpedia 19
. A Cabinet it an Integrated Refrigeration System Using a refrigeration system to cool electronic components as been applied in te field for long time. Today, tis tecnique is used by companies suc as Emerson Corporation for cabinet cooling. It gives te cabinet more flexibility and more potential to reac iger poer densities. It also makes te cabinet more complex and adds cost. In tis study, te integrated refrigeration system calculation covers te folloing cases: 1) A rear-door air-refrigerant eat excanger ic cools te ot air from te cabinet to 0 C (Figure 3(a)). ) A sealed cabinet it an air-refrigerant eat excanger and an refrigerant-ater eat excanger inside (Figure 3(b)). Assume R134a is used as refrigerant. Te refrigeration system can be simplified as a eat transfer system it to eat excangers, as son in Figure 4. For tis system, assume te coefficient of performance (COP) of refrigeration is 4: q C O P 4 W c So te poer of compressor is: W c 7.8kW C O P 4 Te total poer generated by cabinet and compressor is 38.8kW. Figure 4, Illustration of a Cabinet it a Refrigeration System. Te flo rate of refrigerant R-134a needed is: m r 0.191kg / s 16.6 For R134a in te air-refrigerant eat excanger: m C 0.191198600 37930 W / C So maximum eat te eat excanger can dissipate is: (T a _ i T r _ i ) 1675 (38.5 0) 64.5kW Te effectiveness of air-refrigerant eat excanger is: 0.48 64.5 Wit 1675 0.044 37930 Te NTU (number of transfer units) of te air-refrigerant eat excanger (according to [1]) is: A A N TU a a r r 1.7 Assume te average fin eat transfer coefficient is: a 50 W / m.c Figure 3. Scematics of Cabinets it Integrated Air-Refrigerant Heat Excangers. Te total surface area needed for te fins is: C N TU 1675 0.7 A a a 3.5m a 50 Compared to te air-ater eat excanger, te required fin surface area of te air-refrigerant eat excanger is reduced by more tan alf. A more compact eat excanger can be used for tis configuration. 0
For te refrigerant-ater eat excanger, te total eat transferred from R134a to ater is 38.8 kw. Assume tat te temperature of te ater leaving te tube-to-tube eat excanger is 5 C. Te ater flo rate needed is: q 38800 7.110 4 m 3 / s(11.3g PM) C(T _ o ) 1000 400 (5 1) And: C 1000 0.00071 400 98W / C For R134a at condenser: m C 0.191 883 158W / C So te maximum eat te refrigerant-ater eat excanger can dissipate is: (T r _ i ) 158 (50 1) 60kW Heat sink effectiveness is: q 38.8 0.65 60 Wit: 158 0.53 98 Te NTU (number of transfer units) of te refrigerant-ater eat excanger (according to [1]) is: Te lengt of eac tube is: A / 4 0.61/ 4 L 4.86m D 3.14 0.01 Te friction factor is: f 0.184 R e 1 / 5 0.184 19170 0. 0.06 Te estimated pressure drop for straigt tube is: 1 L 1 4. 86 P f 1000.3 0.06 33400Pa(4.8PSI) D 0.01 In tis configuration, 7.1 x 10-4 m 3 /s (11.3 GPM) of ater are needed to cool tis cabinet. Te estimated pressure drop for ater is 33400 Pa (4.8 PSI). Compared to te cooling system illustrated in Figure 1, tis system requires less ater and pumping poer. But, it requires a bigger ciller at te data center s cooling system. One possible configuration for te refrigeration system is to put te air-refrigerant eat excanger in front of te servers, as son in Figure 5. Tis design ill enable te cabinet to provide sub-ambient temperature airflo to te servers. Because te air-refrigerant eat excanger orks as an evaporator for R134a, te air temperature after te eat excanger can be very uniform irrespective of location. Te air-refrigerant can be quite efficient because te refrigerant evaporating temperature is regulated by a compressor. A N TU A r r 1.7 For ater, assume four parallel tubes it an internal diameter of 10 mm are used for tube-to-tube refrigerantater eat excanger. Te average ater velocity is: 0.00071.3m / s 4 3.14 0.005 Te Reynolds number is: D.3 0.01 R e 19170 1. 10 6 Te Nusselt number for fully developed turbulent flo is: N u 0.03 R e 4 / 5 Pr 0.4 0.03 19170 4 / 5 7.6 0.4 138 Te eat transfer coefficient of tube is, N uk 138 0.6 880 W / m.c D 0.01 Te total surface area needed for te tubes is: N TU 98 1.7 A 0.61m 880 Figure 5. Scematic of a Cabinet it a Front-Integrated Air-Refrigerant Heat Excanger. February 010 Qpedia 1
Compared it oter liquid-cooled cabinets, te cabinet it a simple air-ater eat excanger is te most energy efficient. It transfers te aste eat from te cabinet to a central ciller, it no additional ork required for cooling. A cabinet it a refrigeration system adds an extra eat load to central ciller, but it also brings advantages of better cabinet cooling, a less sopisticated eat excanger design and less requirements of te ater delivery system. Coosing a liquid-cooled cabinet it te rigt cooling tecnique and configuration depends on te customer s requirements, including te rack poer density, ater supply system, CRAC system and budget. Te configuration must be selected on an individual basis. Wit te continuing increase of poer dissipation e are sure to see more liquid-cooled cabinets in data centers. Reference 1. Incropera F. and DeWitt D., Fundamentals of Heat and Mass Transfer, 5t Edition, Jon Wiley & Sons, 00. First olume of Qpedia Termal emagazine Articles No Available in Hardcover Book! ATS as publised Qpedia Termal emagazine, olume 1, Issues 1-1, a ardbound, full-color book tat compiles 46 tecnically compreensive articles offering expert-level coverage on a ide range of eat management issues tat impact virtually all of today s electronic devices. Qpedia Termal emagazine, olume 1, Issues 1-1, ISBN- 978-0-615-3660-5, te ardbound, full color book can be ordered for $94.95 (USD) by calling 781-769-800 or by email Qpedia@qats.com. Contact ATS or visit.qats. com/qpedia.asp for details. Candlestick Sensor TM ATS Automatic Temperature and elocity Measurement Systems deliver unmatced accurate, stable and versatile performance for all aspects of termal analysis. Fully automated, tese researc-quality instruments take accurate single- or multi-point measurements of air temperature, velocity and surface temperature in complex environments, suc as PCBs and electronics enclosures. Flexible, robust, base-and-stem design allos continuous repositioning and reading Measures temperature and velocity Narro and lo profile minimizes te flo disturbance Temperature range from -30 C to 150 C (±1 C) TEMPERATURE AND ELOCITY MEASUREMENT SYSTEMS 89-7 Access Road, Norood, MA 060 USA T: 781.769.800 F: 781.769.9979.qats.com