Engineered Systems - April 2008 - (Page 58)

It’s The Humidity Hi gh 1960 - 1970+ 1970 - 1992+ 1993 - 1999 2000 - 2005+ FIGURE 1. History of heat dissipation techniques (A. Kaveh, Electronics Cooling Magazine, January 2000). Coefficient of Performance (COP) = Qi / Wnet Decreased condensing temperature (TC) 2 3 TC ABSOLUTE TEMPERATURE T St ab le er pow y igh log H hno tec Low Low Adv anc pow em er ate rial s Low capacity cooling technologynatural convection sp te ee ch d no lo gy Hi gh er High capacity cooling technologyfor compact systems and components Reduce the area under the curve = less work/power Wnet TE 1 Ql Increased evaporating temperature (TE) 0 ENTROPY S La rg e High capacity cooling technologyno spatial constraint po w er di ss ip at io n sp ee d Low capacity cooling technologyforced convection perature and humidity, as well as dust and other particulates would really help shed light on how to solve these design questions. This data is necessary when looking at the first cost of the computer equipment as compared to the ongoing expense of operating a very tightly controlled facility. Certainly placing computers in an environment that can cause unexpected failures is not acceptable. However, if the computers are envisioned to have a three-year in-service life and relaxing stringent environmental requirements will not cause a reduction in this service life, a data center owner may opt to save ongoing operating expense stemming from strict control of temperature and humidity levels. This seems easier said than done, however. There is published data on failure mechanisms of electronic equipment, such as MILHDBK-217, “Reliability Prediction of Electronic Equipment,” but much more research is required to determine the influence of the many interdependent factors, such as thermomechanical, EMC, vibration, humidity, and temperature4. Also, the rates of change of each of these factors, not just the steady state conditions, will have an impact on the failure mode. Finally, a majority of failures occur at “interface points” and not necessarily of a component itself. Translated, this means contact points such as soldering often cause failures. So it becomes quite the difficult task for a computer manufacturer to accurately predict distinct failure mechanisms, since the computer itself is made up of many sub-systems developed and tested by other manufacturers. POINT 3 A SAVINGS EXAMPLE Design of HVAC systems for data centers requires a close look at the psychrometric processes, fine-tuned by using simulation tools to evaluate inherent transient conditions. There is much discussion taking place in the data center industry on the optimization of the temperature and moisture content of the air entering into computers, storage devices, networking gear, and other equipment. The current ASHRAE guidelines provide data on the range of acceptable inlet conditions. These conditions are summarized in Table 1. An important point of this information is that the recommended conditions of the air are at the inlet to the computer. There are a number of legacy data centers (and many still in design) that produce air much colder than what is required by the computers. Also, the air will FIGURE 2. Increasing evaporating temperature (supply air temperature) reduces the amount of power needed by the compressors in central cooling equipment. Power density and cooling technology transient conditions). Most computer servers, storage devices, networking gear, etc., will come with an operating manual that stipulates environmental conditions with a range something like 20% to 80% non-condensing rh and a recommended operation range of 40% to 55% rh. What is the difference between maximum and recommended? It has to do with prolonging the life of the equipment and avoiding failures due to electrostatic discharge (ESD) and corrosion failure that can come from out-of-range humidity levels in the facility. However, there is little, if any, industry- accepted data on what the projected service life reduction would be based on varying humidity levels. In conjunction with this, the use of outside air for cooling will reduce the power consumption of the cooling system, but with outside air comes dust, dirt, and wide swings in moisture content during the course of a year. These particles can accumulate on and in between electronic components, resulting in electrical short circuits. Also, accumulation of particulate matter can alter airflow distribution and thus adversely affect thermal performance3. Actual field-collected data on failures related to tem- 58 En gi neer ed S y stem s April 2008

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Engineered Systems - April 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips This Time, Water Cooled Was All Wet Cleanroom Design In 10 Easy Steps It's Not The Heat, It's The Humidity Issues & Events Computers and Software Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - April 2008

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