The Battle for Comfort
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Rows and rows of high-performance equipment fill a dense trading environment to the rim. Heat generated from the exhaust of multiple computers in a single station roasts the trader, while the cool air flooding the space leaves other spots chilly in its attempt to meet the demands of the large load. None of the occupants is comfortable, and the equipment isn’t getting the relief it needs either.
The shift to 24/7/365 electronic trading has moved more of today’s financial services firms from open outcry exchange floors to private office suites, stoking a battle for comfort between the equipment and the occupants. Cooling hot equipment with air that is too low in temperature will induce stress on the electronics, causing excessive expansion and contraction in motherboards and challenging longevity. Moreover, adding large volumes of cool air will meet the computers’ needs, but will test the space’s acoustical properties and prove uncomfortable for occupants.
These competing needs are often compounded by the combination of existing building conditions and Moore’s Law. Add conflicting and slightly overlapping industry standards to the mix and pretty soon all odds are stacked against a workable solution.
The following ASHRAE standards define the parameters for the task at hand:
A SHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy , outlines combinations of indoor and personal factors needed to maintain human comfort, including temperature, thermal radiation, humidity, and air speed, while factoring in human activity and clothing.
ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality , sets minimum ventilation rates for commercial and institutional buildings.
ASHRAE Technical Committee (TC) 9.9, Mission Critical Facilities, Technology Spaces and Electronic Equipment, provides environmental guidelines for mission critical facilities through a series of documents, including Thermal Guidelines for Data Processing Environments (TGDPE, updated in 2008).
A successful design depends on the designer’s ability to work around existing furniture and building layouts and create flexibility to accommodate the growth of future cooling loads and multiple environments within a single space.
WRESTLING WITH EXISTING CONDITIONS
Designing office interiors to bear the significant loads brought on by the electronic trading environment is largely influenced by the space’s existing accommodations. What are the requirements? How is the furniture arranged? How are the building’s systems configured? Which equipment will be housed in the space?
Once a preliminary arrangement has been proposed, computational fluid dynamic (CFD) modeling can be used to evaluate the potential solution. Pre-programmed to aim for server inlet temperature ranges between 64.4 and 80.6 F and dew point limitations between 41.9 and 59 F as governed by TGDPE, modeling helps the designer identify both hot and cold spots, enabling him or her to tweak the floor layout and its support systems, thus optimizing the space.
Although today’s electronic trading environments now feature server and computer equipment in populated office spaces, owners still want power and cooling support systems to remain out of sight. Modeling is especially useful when configuring an aesthetically pleasing space that also can meet MEP demands.
For example, for the financial institution described in the first paragraph of this article, Environmental Systems Design Inc . (ESD) placed 10 traders in a linear arrangement (five in a row facing each other). With four workstations and eight monitors to a single trader, removing heat directly from its equipment source was the goal. Using CFD modeling, the ESD team determined that integrating an exhaust system to channel the heat from the computers directly through a chimney located at the center of the row of workstations (see Figure 2) would eliminate the unwanted warm air from the trader-occupied zone. Rising where people aren’t directly affected by it, the heated air mixes with the incoming cool overhead air supply and is subsequently recirculated into the space.
While this layout, which used an overhead air supply, was successful in improving trader comfort, as equipment loads increase, it may be difficult to provide sufficient cooling to the computers without overcooling the traders. In this case, because of the office’s physical limitations, the ESD team was not able to provide cool air directly to the equipment. Whenever possible, though, an underfloor air distribution (UFAD) system is advisable to allow the delivery of greater air quantities directly to the equipment without causing a drafty condition that would compromise comfort.
In cold climates, the humidity level recommended for computer areas also must be addressed. Most buildings are not built to contain the elevated levels of moisture specified in TGDPE. Building envelopes made of terra cotta, limestone, structural steel, and tie backs and clips, which are found in exterior walls, do not tolerate condensation and often succumb to the effects of corrosion or freeze/thaw after repetitive cycles of exposure.
When steel corrodes, the resulting scale will grow to occupy 10 times the volume of the original material, creating the potential for spalls in the exterior masonry and resulting in extensive damage to the building’s skin. In the worst case scenario, a falling piece of masonry or ice can be a life-safety hazard to pedestrians on the sidewalk below. A less detectable but equally problematic side effect of humidification can be the growth of mold within the exterior walls. If not properly accounted for in the design, this condition can occur in older masonry buildings as well as today’s modern glass high-rise towers.
One solution is to create a vapor control system with a vapor barrier at the perimeter of the building that can be used to control the migration of moisture through the wall. A vapor control system can remove leakage before it migrates into the exterior wall where it can condense and potentially freeze. We have had success creating a ventilated space between the vapor barrier and the exterior wall that is as small as a 2-in. wall cavity or as large as a full-sized corridor. It is important to supply dry air into the perimeter cavity. In this situation, it may be necessary to provide a supplemental set of windows in the interior wall to allow natural daylighting to reach the trading area.
For both new construction and existing structures, it is very difficult to design and build an exterior skin that is completely vapor-proof. While vapor barriers can slow moisture buildup at the building’s skin, there is no guarantee that moisture won’t penetrate. The ventilated cavity will ensure that moisture in the exterior wall will be maintained at a safe level, preventing damage to the building.
ASHRAE Standard 55 recommends a humidified environment for human comfort as well, but doesn’t require it, acknowledging the challenge it would pose to the average office building (see Figure 1). ASHRAE Standard 62.1, however, identifies minimum ventilation rates suitable for human occupants. Although computers do not require fresh air, it is important that the equipment reside in a dust- and corrosive-free environment as recommended by ASHRAE (Applications Handbook, Chapter 17). Also, there are initial and operating costs to condition outside air to meet the recommendations set forth in Standards 55 and TGDPE.
THE FLEXIBILITY FIGHT
Although a building’s MEP systems are supposed to last 25 to 30 years, rapid technological advancements have resulted in the need for greater flexibility in today’s electronic trading environments. The ability to reconfigure the air supply to match relocated or increased loads is crucial to the ongoing success of a trading firm. Not only does the technology change frequently, but the users are constantly trying new methods to increase their productivity as well. Support systems must be able to respond accordingly.
Existing buildings are typically designed to support an internal cooling load of 4 W/sq ft, and yet today’s trading spaces are introducing loads as high as 30 W/sq ft, with more bodies and less space. This steady increase, which shows no sign of slowing, is fueling the conflict between TGDPE and ASHRAE 55.
On one hand, TGDPE requires the circulation of large quantities of “cool” air to the equipment (64.4 F and higher), while ASHRAE 55 requires much less air to meet occupant comfort levels and prohibits high air velocities that can lead to an objectionable draft. Adding fuel to the fire, an increase in air supply ushers in higher noise levels, which impacts the perception of well-being in a space and can interfere with the work product. The solution to both challenges is to deliver cooling directly to the equipment. A high volume of cool air introduced into the equipment compartment will provide needed heat rejection without increasing sound levels in the occupied space.
High air velocities, delivered to the equipment between panels or walls, can meet equipment demands and simultaneously be shielded from occupants while buffering accompanying noise levels.
Again, take the article’s lead narrative as an example. By the time the air from the chimney is recirculated into the occupied space, its temperature is close to room temperature and its velocity is at 40 ft/min. In this case, loads were 12 to 15 W/sq ft, or just under the threshold of design where a UFAD system is recommended. Once beyond 15 W/sq ft, the increased flexibility of two air supplies in the space is highly recommended: one overhead system to satisfy ASHRAE 62.1 for air quality and ventilation, and another below the floor to support equipment and TGDPE requirements.
Another electronic trading firm recently asked ESD to design an office with “ultimate flexibility,” stemming from the owner’s desire to avoid future operational shutdowns for equipment and furniture changes. Here, ESD was able to use a raised floor, supplying air from below, while low-voltage and high-power cabling was coordinated in a way that prevented the cables from interfering with each other. Establishing the right-of-way for various services, including air and work flow supplies and power cabling, will help prevent interference. Using modeling, ESD established a new level of how many PCs and monitors a workstation can accommodate, giving the firm the flexibility to reach the heights of even higher-powered equipment at a later date.
Furthering flexibility by creating multiple environments within a single office space is often a necessary recipe for establishing human comfort in a trading environment.
Continuing with the article’s lead narrative, ESD specified separate VAV terminal units with exposed ductwork serving each row of workstations. Over each individual station is a double-deflection diffuser that supplies air to the trader. Because of the nature of the trading day (different traders are in the office at different times) and the variability of the cooling loads, one wireless thermostat supplies each row of 10 workstations. Designed to move around to the various stations in each row, the thermostat can adjust local air temperature, quantity, and air flow direction at each station.
Traders also have manual control over adjustable dampers in the ceiling. These provisions not only accommodate the traders’ varied schedules, but also helped provide comfort in the space as the firm grew from a skeleton crew to full occupancy in its expansion phase.
ASHRAE 55 permits elevated air speeds if the occupants can control it. This is a key feature of Standard 55 because it provides the flexibility to have higher space temperatures in localized areas.
THE NEXT FRONT
As electronic trading environments and their technologies evolve, finding the right cooling balance will remain a battle. TGDPE humidity requirements may advance or decline as building performance data from today’s compliant environments becomes available. In time, more advanced water-cooled devices and other more cost-effective and energy-efficient equipment will become available. Another significant evolution will be the partnership of furniture and systems designers who will collaborate to create environments that are aesthetically pleasing, flexible, comfortable, and energy efficient.
|Dykstra is a senior associate and Gupta is president of Environmental Systems Design Inc. (ESD).|
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