High-Rise HVAC

Green design, underfloor air delivery and commissioning are key trends driving HVAC design in today's high-rise buildings

10/01/2001


Forty years ago, when energy was plentiful and relatively inexpensive, the mechanical systems in high-rise commercial buildings were, in many ways, admirable by today's standards. They could utilize 100% outside air, taking advantage of the economy of free cooling whenever possible; they could completely purge the building with outside air; filtration was superb and noise problems were uncommon.

Of course, these systems weren't perfect. Energy consumption was astronomical and the easily alterable pneumatic controls could throw the overall system out of whack with any occupant adjustments. But, in general, those systems created interior environments that were clean, pleasant and healthy—one of the chief aims of today's sustainable or green approach to HVAC design.

Then, something happened. Or rather, a number of things happened in tandem to alter how high-rise office buildings—and their mechanical systems—were designed. The energy crisis resulting from the OPEC oil embargo of the early 1970s not only sent energy costs spiraling, it also made Americans realize that fossil fuels were not inexhaustible. Next, the revolution in office electronics in the 1980s and 1990s enormously increased a typical office's electrical requirements. Office space became ever more expensive to build and lease, which meant that less room could be allotted for mechanical-system components. Furthermore, the economy went global, lengthening the hours most office buildings operated.

The OPEC crisis spawned a horde of building codes designed to curb energy use sharply. New office buildings were sealed tight to restrict infiltration. During air-conditioning season, increases in space temperatures were mandated and the use of outside air was severely limited. The upshot was an entire generation of office buildings that were prone to the symptoms of what was soon dubbed sick building syndrome.

But there were other problems, too. As space increased in value, the size of ductwork shrank, creating noise problems that somehow had to be ameliorated. Cheaper, disposable filters came into widespread use, and when not maintained properly, contributed to indoor environmental difficulties such as bacteria build-up in ductwork and coils. Correcting the mistakes of yesteryear

All approaches to HVAC system design developed over the past decade are, in part, responses to the well-intentioned errors of the '70s and '80s. While the goal of reducing energy consumption has remained prominent, experience has demonstrated that energy-saving strategies must always be accompanied by equal concerns to enhance indoor ecology.

In addition to cutting energy use and creating the healthiest indoor environment possible, a successful high-rise HVAC system design should also do the following:

  • Anticipate and minimize operating and maintenance costs.

  • Permit, as much as possible, individual control over comfort conditions.

  • Maximize flexibility for varying heat-load requirements; for rapid, inexpensive reconfiguration of furniture and equipment, especially in corporate offices; and for varying hours of operation among tenants or departments within a single facility.

  • Control first costs by taking advantage of the full range of government- and utility-sponsored programs to offset the costs of high-efficiency equipment and renewable energy strategies.

  • Anticipate—and try to minimize—payback times.

  • Ensure reliability.

Accomplishing these goals while working within the constraints of real-world budgets is a pretty tall order. Three general concepts—green design, underfloor air delivery and commissioning—drive contemporary HVAC engineering, always with an eye toward balancing overall goals with affordability. Great green engineering

A decade ago, engineers were constantly talking about intelligent buildings, but today, the buzzword is green. Actually, the two concepts have a lot in common, since a "smart" building's intelligence is largely a function of its building automation system (BAS), and sophisticated BAS are likewise a prime component of green commercial buildings.

Buzzwords, however, cover a lot of territory and can lead to misconceptions. To clarify, it is important to understand that green design is a process that has at least three aspects:

  • Intensive collaboration among all members of the project team—owners and managers; architects, engineers and other design consultants; contractors, suppliers and manufacturers'technicians.

  • A holistic approach in which the interaction of all systems, components, building materials and construction techniques are meticulously evaluated—in many cases using Department of Energy modeling software—before the final design is reached.

  • Ongoing evaluation, in which there is a commitment to commissioning and post-occupancy testing to ensure that design intent is maintained far into the life of the building.

There's no such thing as a "green building" per se. Instead, there are levels or degrees of greenness, something that's implicitly recognized in the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) rating system.

Although green is often thought of as synonymous with expensive, there are lots of things that can be done to make a greener design without adding appreciably to the cost. While it is true that, in most cases, incorporating renewable energy sources into the design of a high-rise is costly and payback can be so lengthy that it becomes uneconomical, it doesn't mean that such measures shouldn't be taken. For instance, placing systems-integrated solar collectors on a building sends an important symbolic message and lessens the facility's dependence on the electric grid.

Some may argue that green design is easier to achieve in small buildings, but environmental and sustainable principles are successfully being followed in the design of some large, multi-story buildings. Two good examples are 4 Times Square—the 48-story headquarters of Conde Nast Publications—and a soon-to-be-constructed, 27-story residential tower at Battery Park City Site 18A, in the southwestern corner of Manhattan.

Both these buildings are pioneers in the application of green design to high-rises. Upon its opening in late 1999, 4 Times Square was the largest green building in the world, and perhaps the first speculative office tower ever to be designed and constructed along environmental/sustainable lines. The Battery Park City project, to be completed in 2002, is also a first: the first green high-rise apartment building in the country.

With both of these buildings, the HVAC systems are integrally related to all the other architectural and engineering systems and components of the building. For instance, HVAC design for the Battery Park City tower includes fresh-air supply to each of the building's 262 units—something that's required in office-building HVAC design but highly unusual in residential installations. Why is this being done here? First, to enable filtered air to enter each apartment. Also, among the energy-conservation measures being applied at the tower are special construction techniques that will also minimize infiltration of outside air into the building. The point to remember is that green design is holistic, and every design decision must be understood in the context of all the others.

The tower's HVAC system also includes gas-absorption chillers for producing chilled and hot water. Employing these units not only limits the building's dependence on utility-produced power, but the chillers produce less air pollution. Moreover, to help tenants reduce HVAC-related energy use, all apartments are equipped with programmable thermostats that residents can adjust.

At 4 Times Square, the HVAC system incorporates numerous components—a sophisticated BAS, together and in conjunction with renewable energy measures, carefully selected building materials, trash chutes for automated recycling, low-E exterior glazing and so on—that raise the building's energy efficiency, protect its interior ecology and moderate the building's negative impact on the environment.

Specific HVAC-related components of 4 Times Square include:

  • High-efficiency—85%—filters, with fresh air supplied at a rate 50% higher than mandated by current code.

  • CFC/HCFC-free natural gas-fired absorption chillers.

  • Variable-speed drives on all fans and motors.

  • Individual air-handling units on each floor, with local control to ensure minimum use of the system.

  • A dedicated closed condenser water system, operating 24 hours a day, that allows tenants to install their own water cooled units for off-hour specialty operations. This saves energy during low-load periods because the central chiller plant does not have to remain in operation to serve individual tenants'varying off-hour needs.

HVAC design for both of these high-rises also includes measures for ongoing monitoring and evaluation of indoor environmental quality. At the Battery Park City tower, the indoor air quality of individual apartments will be tested, using portable units, whenever those apartments become vacant. At 4 Times Square, long-term control of contaminants is ensured by permanent sensors, installed in mechanical rooms and occupied spaces on each floor, that can monitor particulates and volatile organic compounds. Utilizing underfloor air

Another extremely promising trend in HVAC design is the widening use of underfloor air delivery. The technology, in essence, relies on the simple principle of convection: when cool air is delivered to the occupied space via an underfloor plenum, it rises as it warms, removing airborne contaminants along with it, until it is exhausted through return-air vents placed at or near the ceiling. Supply-air grilles are set directly in the floor tiles, and because there is no ductwork, the location of these adjustable grilles can be changed at will, greatly facilitating office reconfigurations and permitting pinpoint individual control of comfort conditions. Because it works passively, by displacement, underfloor air requires a lower static supply pressure—less fan horsepower—and delivers air at warmer temperatures, thereby requiring less refrigeration than conventional systems. Traditional VAV systems deliver air at 55

Underfloor air has been in use in Europe and Canada for quite some time, but until recently its application in the United States has been limited. It was only five years ago that Cosentini Associates designed the underfloor air system for Owens Corning's new headquarters in Toledo, Ohio—to our knowledge the first use of underfloor air in a large-scale, commercial application in this country. That system began demonstrating its value immediately. After the first year of operation, the facility manager reported that the system had dramatically cut costs associated with churn—reconfiguration of office space—already saving the company a half a million dollars that would have been spent on HVAC contractor fees if Owens Corning had gone with a traditional, ducted air-supply system.

But the Owens Corning headquarters, though large at 450,000 sq. ft., was only three stories tall. Moreover, it was designed for a single occupant, with multiple rooftop air-handling units serving the building. The question remained whether underfloor air would prove advantageous in high-rises—especially office buildings intended for multiple tenants, where loads might vary widely from space to space and where the need to firewall tenants' spaces from one another posed a challenge to underfloor air, which seemed to require an open, undivided plenum.

It turns out that the answer was yes, as demonstrated by the new Woodfield Preserve complex in the Chicago suburb of Schaumburg, Ill. Developed by the Hines organization, Woodfield Preserve consists of two 300,000-sq.-ft. office buildings, each six stories high, that are cooled and ventilated entirely by underfloor air. Though the Woodfield Preserve buildings are hardly skyscrapers, lessons learned in their design show that underfloor air can be used in any commercial building where underfloor power and voice/data cable distribution and a need to cut down on the cost of churn justify the added initial expense of a raised floor. Moreover, the HVAC designers' experience with the project proves that the "problems" of applying underfloor air to multi-tenant facilities can be solved relatively easily.

The first building opened in 2000 and the second opened this past summer. Just how beneficial the underfloor air system is can be gauged from the first building's energy-use data for the initial year of operation. The owners report that annual energy costs that averaged about $0.93/sq. ft., compared with an of average $1.35/sq. ft. for similar buildings cooled and ventilated by conventional means. Prioritizing commissioning

In addition to green design and underfloor air, commissioning is the third ingredient commonly being incorporated into HVAC high-rise design.

Commissioning is no longer an afterthought, nor a process merely performed, just before occupancy budget permitting. In an era when energy efficiency and long-term reliability are of paramount concern, commissioning assumes an integral role that starts during the design process . Commissioning measures how closely all building systems in operation match design intent and makes corrections and adjustments when performance falls short. Today's buildings are complex, and since efficiency and reliability are results of the interaction of many different systems, full-scale commissioning begins to look more cost-effective.

Why is this? Because operational costs over the life cycle of a typical building will average five times the initial mechanical construction cost, according to ASHRAE data. Performance, of course, is the key to controlling operations costs, and commissioning augments performance in three ways:

  • By fine-tuning all systems as they interact with one another. By generating a library of documentation—records, as-built drawings and operations and maintenance manuals—that can be used to evaluate and rectify system performance over the life of the building.

  • By training operations personnel in the management and maintenance of today's complex systems.

That last point is especially important. It is increasingly crucial that the commissioning process begin during the early stages of design and that operating personnel interact with the design team and give input regarding their expectations of how the systems should operate; that at least some senior-level operations staff participate in commissioning during the functional testing period; and that commissioning continue after full occupancy to ensure that systems continue to work at peak efficiency. Participation by O&M personnel early in the project allows time for training in equipment operation, identification of problems, corrective procedures and record-keeping.

Though commissioning scrutinizes the performance of all systems as they interact in actual operation, it may be useful to list some of the ways in which specifically HVAC-related functions are evaluated by system or component:

  • Supply-air and exhaust systems. Are the right amounts of outside air being introduced into the building to ensure that proper indoor air quality is always maintained?

  • Boilers and refrigeration units. Is the right combination of equipment being selected to provide the highest level of efficiency under actual conditions (system optimization)?

  • Instrumentation. Are sensors and meters properly calibrated and providing exact readings?

  • Modes of operation. Do systems perform per design intent under all modes of operation—normal, emergency, failure? For example, is the stair pressurization and/or smoke purge properly integrated with the fire-alarm system?

  • Energy efficiency. Is the system set up to meet tenant requirements while still operating at peak efficiency? Factors here would include maximized use of free cooling, summer/winter modes, occupied/unoccupied modes, optimized start/stop and so on.

Today's high-rise buildings are much more technically sophisticated than those of decades past. Consequently, it takes a higher level of skill and expertise to operate and maintain them. Commissioning ensures that efficiencies built into HVAC systems are continually realized over the life of a building.

By utilizing commissioning, sustainable design strategies and developing technologies, engineers continue to fine-tune their ability to walk the fine line of optimizing both energy efficiency and indoor air quality in the commercial buildings that decorate the skylines of U.S. cities.

  • A central rooftop unit generating a 48

  • Fan-powered boxes at the perimeter, which receive the 48

  • A series of air columns, which are essentially vertical fan-powered boxes. The columns mix the 48


Acknowledgements

Marvin Lewin, Vice President of Cosentini Associates; Igor Bienstock, Senior Vice President; and Bud Spiewak, Director of Cosentini Associates' Chicago office, all contributed to this article.



BAS Plays into Sustainable Design

A good building automation system (BAS) can also play a key role in achieving and maintaining sustainability. Because some equipment operates most efficiently at less than peak load, a sophisticated BAS now includes software that can evaluate manufacturers' information on a machine's performance under various part-loads and distribute load among two or more components, as necessary, to achieve maximum efficiency.

Studies of the HVAC system planned for a new Manhattan high-rise, the AOL Time Warner Center—soon to rise at Columbus Circle—show that this system part-load value (SPLV) approach, using manufacturers' data, will improve the overall efficiency of that building's system by as much as 10% to 15%.

Widely available, but currently underutilized, the SPLV strategy is potentially a very great asset in keeping green buildings green.

Underfloor Air for Multi-Tenant Facilities

One of the challenges with designing an underfloor air system for a facility involves how to deal with a multi-tenant floor where fire zones must be created. This was an issue in designing the Woodfield Preserve project in Schaumburg, Ill. The problem was resolved by devising the walls between zones to extend through the floor air plenum, down to the slab, and inserting openings equipped with fire dampers wherever it was necessary for air to cross between zones.

To address the slightly differing temperature needs from floor to floor and the more widely differing volume needs from one tenanted space to another, the air-handling system consists of three primary components:

A central rooftop unit generating a 48

Fan-powered boxes at the perimeter, which receive the 48

A series of air columns, which are essentially vertical fan-powered boxes. The columns mix the 48



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