Engineered Systems - April 2008 - (Page 36)

This Time, Water Cooled Was All Wet Condition Temperature control range Maximum temperature rate of change Relative humidity control range ASHRAE Guide allowable level 59˚F to 90˚ 10˚/hr 20% to 80% 63˚ max dewpoint 40% to 55% ASHRAE Guide recommended level 68˚ to 77˚ LCCA study level 70˚ to 74˚ 10˚/hr 40% to 60% TABLE 1. Data center design conditions. Utility Energy use (MWh/yr) Makeup water (Mgal/yr) Sewage rate (Mgal/yr) Pennsylvania water cooled 5,400 38 6 Pennsylvania air cooled 5,600 North Carolina water cooled 5,500 38 23 North Carolina air cooled 6,700 less of how I feel about those terms, the reality was that I was in that room using them. Now the concept guy was crazy — like a fox. He knew better than I that it made no sense to fall on his sword for a concept that the owner had clearly squelched, but he was principled enough to stick by his guns. Especially because he knew deep down that he was right about the water issue. So together we schemed. And then, what I came to call the “elegant solution” revealed itself. THE ELEGANT SOLUTION So let’s recap the marching orders. We had to limit water use but keep simplicity integral to the design. We had to do this while using as little energy as possible and keeping operating costs in line. And we had to do all this at a reasonable cost in a redundant and reliable configuration that could operate 24/7/365 with absolutely no down time. My first suggestion was to use air cooled chillers (how remarkably courageous). But while this addressed water and simplicity, it was a letdown in the energy and operating cost arena. Further, it was hopeless due to the low ambient operating limit. So what if we added a dry cooler sub loop to provide cooling when the temperatures required the chillers to be shut down at low ambient? Basically, since dry coolers are simply coils with fans on them, if the ambient outdoor air provided a sufficient drybulb approach, then you could chill the water using air only, eliminating the chillers from the equation. This would be a kind of low ambient economizer. Economizer? Did I just say economizer? That’s when it hit me like a ton of air cooled bricks. If we could provide sufficient capacity with dry coolers to handle the entire load at 0°, couldn’t we get partial economizer operation at say 15°? And, if 15° worked, couldn’t we go higher? Heck, if my chilled water return temperature is between 50° and 55°, couldn’t I, theoretically, start getting some free precooling at a 5° approach when the outdoor temperature was in the range of 45°? Eureka! In spite of myself, I (actually we) had found it. PROOF AND THE PROVERBIAL PUDDING So at first, this looked like an interesting idea that would satisfy this particular client and the design team’s preferences and concerns. It was a compromise that I thought would probably cost more to operate, but we could feel better about ourselves because we had struck a bargain with each other and Mother TABLE 2. Energy and utility use matrix. Utility First cost Life cycle costs Pennsylvania water cooled $3,700,000 $11,200,000 Pennsylvania air cooled $2,700,000 $8,800,000 North Carolina water cooled $3,700,000 $11,900,000 North Carolina air cooled $2,700,000 $10,300,000 TABLE 3. Life-cycle cost matrix. On the other hand, the concept designer made a very eloquent case for the dry coolers based on their simplicity and the fact that the significant water use associated with evaporative towers would be eliminated. As a guy who considers himself pretty savvy when it comes to sustainable design, I admit my curiosity was piqued at the possibility of eliminating all of that water evaporating or going down the drain, along with its associated chemicals and cost. But the idea of compressorized equipment throughout the building was a non-starter. AIR AND WATER So what about using air cooled chillers? You eliminate the compressors and the wasted water, but again, our conventional wisdom tells us that air cooled chillers are significantly less efficient than water cooled, especially if your water-based system has a waterside economizer. On top of that, there was a strong possibility that this project might be sited in the American Northeast, and air cooled chillers won’t operate at temperatures at or near 0°F. So it would seem that the only option was traditional water cooled chillers and towers. But the issue of water conservation wouldn’t go away. Not for the client, because they had a corporate mandate to use the fewest natural resources possible. Not for the concept designer, because he was steadfast in his insistence that water conservation and equipment simplicity out weighed the possibility of lower operating costs. And not for me, because I was in the unique position, as the engineer of record, of having to carry a design forward that would satisfy both parties while still meeting my own professional responsibility to do no harm to man or the earth. So the concept engineer and I were sequestered in a room and given a whole hour to come up with a design that met the various demands. And it should go without saying, but cost had to stay within the project budget. So there we were: The big idea concept guy with his dry cooler paradigm and yours truly with my own mechanical blinders on, advocating for the traditional plant. By the way, I hate the term “traditional.” It’s right up there with “industry standard” and “standard practice” in the lexicon of phrases that convey sound design, but belie the fact that they provide cover for the unimaginative designer who will sacrifice innovation for convenience. Regard- 36 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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