M/E Roundtable: Cooling Data and Dot-Coms
As data centers, clean manufacturing and telecommunications come to dominate many an engineer's design work, unique issues are impacting heating, ventilation and air-conditioning (HVAC) systems. From air purity and cable management to thermal decay and fire suppression, mechanical designers must...
As data centers, clean manufacturing and telecommunications come to dominate many an engineer's design work, unique issues are impacting heating, ventilation and air-conditioning (HVAC) systems. From air purity and cable management to thermal decay and fire suppression, mechanical designers must smooth technical snags and hold down installed costs. In this month's M/E Roundtable, leading firms offer tips and approaches.
Marc Anderson , P.E., Senior Associate, Syska & Hennessy, Los Angeles
David Niemann, P. E., Senior Project Manager, Carter & Burgess, Inc., Dallas
Steven R. Rafferzeder , Senior Associate, CUH2A, Inc., Princeton, N.J.
Charles A. Rowland , P.E., Chief Engineer - Mechanical Services, Kling Lindquist, Philadelphia
Calvin L. Witt , P.E., Senior Mechanical Engineer, Hayes, Seay, Mattern & Mattern, Roanoke, Va.
CONSULTING-SPECIFYING ENGINEER (CSE) : What are the most demanding high-tech projects your firm is currently involved with?
ROWLAND : Data centers and the research and development market ... are driven by fast-track projects requiring a high degree of reliability and coordination between designers, owners and contractors. We are currently involved in a project with an owner to accommodate 200 watts per square foot of equipment load on a raised-floor space. Ten years ago, 40 to 50 watts per square foot would have been considered high.
ANDERSON : We are involved in ... telecommunication [point-of-presence] POP sites, financial and insurance data centers, studios and recording facilities, large colocation facilities, and a significant amount of the "dot-com" or e-commerce start-ups and expansions.
NIEMANN : The Department of Energy's "Strategic Computing Complex" at Los Alamos National Laboratories houses the fastest computer in the world while the "Nonproliferation and International Security Center" provides technology for threat detection and safeguards for our nation's safety. The integration of the M/E/P systems-especially the HVAC systems-with security measures, data/telecommunication systems and architecture has presented many issues for our engineers to coordinate and resolve.
RAFFERZEDER : Retrofit projects offer unique challenges, both from an equipment aspect and an existing building construction and layout aspect.
WITT : One major challenge with high-technology facilities is the rapid pace of change. The process housed in the facility and owner requirements can-and do-change several times during the design period. Challenges include-besides addressing the changed situation-keeping the design on schedule and the project within budget.
CSE : Do M/E/P engineers have to interface with information-technology (IT) personnel in design development?
ROWLAND : It's crucial that M/E/P engineers interface with the IT group to understand all of the nuances of the process equipment to be used on the project and how to provide flexibility to accommodate the next generation of equipment.
WITT : Our M/E/P engineers work with IT personnel in design charettes on high-tech projects, because IT requirements for cooling, temperature and humidity control, life safety, power and reliability frequently drive the design of the M/E/P systems. IT requirements also affect the architectural design, with requirements for pathways, telecommunication rooms, server farms and other IT equipment spaces.
ANDERSON : [It] helps us to accurately select the M/E/P support systems for the project and avoid making conservative estimates and unnecessary oversizing. While most IT personnel are a valuable resource ... higher-level management personnel are the best source to forecast future growth plans.
CSE : Have you noted any new or recurring needs for their HVAC systems?
NIEMANN : With equipment density increasing, cooling loads have increased from double-digit requirements to as high as triple-digit watts per square foot. Power quality and reliability have also placed load demands on the HVAC systems; [uninterruptible power-supply] UPS installation "on the floor" has shifted the loads from remote rooms to the conditioned space.
WITT : Recurring needs we see include rigorous temperature and humidity control, extra cooling capacity-to handle high heat dissipation from electronic equipment-and high system reliability. Most new and retrofit projects now include telecommunications closets, which create their own special set of design challenges.
ANDERSON : Due to the ever-increasing delivery-speed demands for data centers, especially in the e-commerce sector, a recent explosion in infrastructure equipment demand has occurred over the past few years. Items such as UPS, batteries, raised floor tiles and, of course, precision AC now have staggering lead times ranging from two months to nearly a year. This has led many smaller computer-room projects to employ comfort cooling or portable cooling as a means of primary cooling. These are usually temporary means, although we have seen the smaller dot-coms relying solely on these [systems].
Some of the larger nationwide data-dependent companies have leveraged their purchasing power to secure improved lead times with certain manufacturers.
RAFFERZEDER : We have seen an increasing demand for humidity control in addition to temperature control. Centralized process-cooling systems are another area of increased interest. IT areas are requiring "24/7" conditioning at lower temperatures, thereby increasing the demand for year-round cooling and causing many clients to rethink the issue of winter chiller-plant operation.
ANDERSON : New needs for precision HVAC systems include redundancy incorporated into the units, such as dual cooling-air-source and water-source-and reduced maintenance clearances for precision AC units, as many clients lease tenant space in high-rent districts where floorspace is at a premium. Increased-tonnage, above-ceiling units are also an increasing need, especially in the high-rise tenant projects, although ceiling-level air delivery may not be optimal.
CSE : Which high-tech clients worry the most about mechanical system redundancy?
ANDERSON : While most clients express a high level of interest in redundancy at the drawing board, only the larger companies, such as the financial and insurance firms, institute it as a strict rule. This can be attributed to the magnitude of their impacted business and their long-term experience ... Many of the e-commerce businesses lack this experience and, compounded with relatively less funding, we see many redundancy designs value-engineered out.
ROWLAND : Any client that has a high cost for their downtime is concerned about reliability. Data centers and research and development labs often have a considerable cost associated with downtime.
NIEMANN : Service providers in the telecommunication sector-data centers and call centers-as well as those in semiconductor production demand "24/7/365" operation.
WITT : In our experience, industrial clients are most concerned about redundancy. Their objective is to avoid single-point failures that can shut down processes. It is more economical to install redundant systems than suffer the normally enormous costs of an unintended, unplanned plant operations shutdown.
Military clients are also concerned about systems failures ... redundancy is a necessity in matters of national security. Museum clients fear failures that could harm irreplaceable collections, and a single failure could adversely affect a museum's reputation and the consequent ability to borrow pieces or collections from other museums.
ANDERSON : One of our more notable ongoing projects is an 80,000-square-foot financial-service data center located in China. The client insisted upon a water-free cooling system, [and] this meant utilizing multiple split-system, refrigerant-based, air-cooled systems throughout the raised-floor environment, despite the increased operating expense as compared to water-based systems.
CSE : What kinds of back-ups do high-tech clients employ?
ROWLAND : Typically, redundant equipment is provided for any parameter that could cause the loss of the process system: Chillers, pumps, cooling towers, fans, boilers, heat exchangers and diesel generators.
NIEMANN : Owners are embracing the n+1 strategy for critical air handlers and air-cooled condensers. Our central plant designs include standby chillers, boilers and pumps.
ANDERSON : The most common components made redundant are the fans, compressors and outdoor heat-rejection units. Typically, n+1 designs are incorporated. A few of the select projects invest in a 2n system-wide approach, which provides a completely redundant system down to the piping and controls.
RAFFERZEDER : We evaluate the specific functional criteria that must be maintained: temperature, humidity, pressurization and contamination control-or all of these. If there are multiple issues, they may be further prioritized-for example, pressurization must be maintained, but temperature may be allowed to fluctuate. Systems components as well as support system are then evaluated as to cost vs. risk, and appropriate levels of redundancy assigned.
WITT : Central equipment-chillers, cooling towers, boilers and pumps-is normally provided in n+1 or n+2 arrangements, because its loss affects the entire facility. Terminal equipment and air-handling units are not normally duplicated since their loss affects a limited area. Exceptions include computer rooms, server farms, command centers and telecommunication-equipment rooms, where redundant computer-room-type air-conditioning units are often employed in an n+1 or n+2 arrangement. In the case of individual telecommunication closets, it is often cost-effective to back up the primary cooling unit-which is often independent due to "24/7" operations-with a [variable-air-volume] VAV terminal unit from an adjacent office air-handling unit.
In some cases, it is cost-effective to connect adjacent air-handling systems together with coupling dampers. In the event of a unit failure, the coupling dampers open, isolation dampers close at the failed unit and terminal units serving noncritical spaces close off so that sufficient air will be available to serve critical functions.
We have not had a project where the client was willing to fund a completely redundant chilled-water piping system, although we have discussed that possibility. We regularly incorporate reliability-enhancing features into piping systems, such as adequate shut-off valves to limit the portion of the system that is down, and redundant feeds to piping loops to allow feeding from multiple directions.
Another approach to increasing reliability is to design systems with as much commonality as possible. For instance, using air-handling units of the same size allows the owner to stock a smaller supply of motors, bearings and similar items for quick replacement in the case of failure.
CSE : Does energy efficiency take a back seat in some of these high-technology projects?
RAFFERZEDER : I would say, yes, energy is secondary to maintaining the personnel or product environment and safety; however, it is always considered. There are always energy conservation and "green" design aspects that can be incorporated at all levels of cost, from simple temperature reduction to unique central plant designs.
NIEMANN : No, designs place energy efficiency, reliability, and maintainability in the total package provided to our clients. High-efficiency motors, pumps, fans and low-kW/ton chillers are always considered to lower operating costs for owners.
ANDERSON : Yes. Because energy-efficient designs typically increase first cost and can extend the project schedule, many of our e-commerce clients simply can't afford it. Their main goal is to get their revenue-producing businesses up and running as soon as possible. More experienced companies recognize the competitive edge gained by reducing annual operating costs.
ROWLAND: Ways to cost-effectively apply heat-recovery or free-cooling technology are always reviewed; variable-frequency drives [VFDs] are used more and more often so that the power used for fans or pumps matches the power required.
WITT : Energy efficiency is always a design consideration; however, most of our projects are cost driven. Reliability considerations and meeting functional requirements are typically more important factors. Higher-cost, energy-efficient features usually need to fight their way into the project.
Federal government projects normally have a long view, with 20- to 25-year life cycles. Industrial clients normally have a short payback-two to four years-or a requirement for high return on investment. Energy-saving features that normally "make the cut" include high-efficiency motors and chillers, direct digital controls [DDC], VFDs and economizers.
CSE : Have any projects incorporated thermal storage or "hybrid chiller plants" to improve system performance or to cut on-peak demand?
WITT : Thermal-storage projects are most cost-effective for facilities with distinct peak-load periods and low-load periods that allow recharging of the thermal storage unit. Many high-technology facilities operate 24 hours per day, seven days per week; or have the potential to add second and third shifts. For these reasons, we typically find limited application for thermal storage in high-tech projects.
ROWLAND : Add to that the redundancy and reliability requirements, and thermal storage is not always a cost-competitive solution. We've used hybrid chiller plants so that we could use a gas-fired chiller as a peak-shaving device when the site is at or near its peak electrical demand.
RAFFERZEDER : We've done a number of central plants with various combinations of electric-driven, steam-driven and steam-fired absorption machines. These plants have incorporated various methods of machine sizing, including providing "trim" chillers for the production of secondary low-temperature loops for process loads on a year-round basis or establishing a baseline winter load demand.
ANDERSON : The savings are magnified greatly for a 7 x 24 data center, [which] utilizes its peak full load continuously. These facilities will show the best payback periods for energy-efficient equipment and designs. Speed to market is the only drawback, however.
WITT : Many high-technology facilities will have backup generators for emergency standby power. This generator capacity may be used to shave peak power demand from a local utility and reduce the utility's associated demand surcharges.
CSE : What environmental variables are most critical to high-tech clients?
ROWLAND : Relative humidity, indoor-air quality, noise and vibration, lighting level and solar radiation all affect comfort or suitability of the space.
RAFFERZEDER : Airflow is the critical concern in high-tech facilities, and we incorporate various "active" or "passive" control strategies in all the engineering trades ... Construction methods that leak [sic] consume vast quantities of expensive conditioned air; fan energy is the largest operating cost parameter the owner has to bear in these facilities.
WITT : Pressurization is another environmental variable common in high-tech facilities. Negative pressures act to contain contaminants within isolation rooms and laboratories, while positive pressure is used to maintain clean conditions in operating rooms and environmentally sensitive production areas, and to protect command and operation centers from external threats.
NIEMANN : Indoor-air quality [IAQ] is a driver for controlling humidity and airflow in the administrative support areas at the high-tech facilities. Measuring outdoor air and CO2as well as utilizing high-efficiency and gas-phase filtration systems are common in today's control strategies.
WITT : Sixty- to eighty-percent-efficient filters are common in office and production areas. Higher-efficiency and HEPA [high-efficiency particle-arrestor] filters are often used for operating rooms and clean rooms. In some command and operation centers, charcoal and HEPA filters are used to filter the outside air stream to protect the facility and its personnel from external threats.
ANDERSON : While we aren't trying to have a clean-room class of cleanliness, we are looking to minimize the amount of dust that accumulates on the internal electronic components of the computer devices. A fine layer of dust will actually act as an insulator, inhibiting heat transfer from the [central processing unit] CPU and other heat-generating components.
NIEMANN : Maintaining humidity levels includes the building envelope as well as equipment and control strategy.
ANDERSON : Geographic location can be a major factor, and the focus on how well the space can be sealed to minimize water-vapor migration. A vapor barrier is necessary in all envelope components, as well as gasketed, tight-fitting doors and sealed pipe and conduit penetrations through the barrier.
WITT : Humidity control is essential ... Common strategies employ cooling plus reheat. Minimized reheat is typically obtained by reducing airflow to the minimum setting before applying reheat, or by using heat exchangers or runaround loops for precooling and "free" reheat. In addition, we have reduced outside-air-stream moisture levels by applying gas-fired desiccant technology. Common strategies for adding moisture to the air include steam humidifiers and fogging-type humidifiers; deionized water is often used where there is concern over contamination.
ANDERSON : Humidification is accomplished with typical internal humidifiers such as electrode-canister or infrared technology, although these are high energy users. These systems are commonly built into decentralized AC units on the floor; however, humidification can also be accomplished with a central ventilation unit serving the entire space. We are currently analyzing the viability of ultrasonic technology, [which offers] very low energy consumption.
CSE : What automation, control and monitoring systems are being used (see "Systems Integration, 2001," page 34)?
ROWLAND : Today's high-tech buildings generally use DDC controls. Campus situations often network individual buildings together through an open protocol, which allows equipment from various manufacturers to communicate.
ANDERSON : Use of standard protocols such as BACnet and LonWorks is increasing, although we still must educate the client as to their advantages.
NIEMANN : Our control engineers indicate that open protocols are becoming standard. The ability to easily integrate standard products from different vendors into a single control system provides the dependability and maintainability so important to our high-tech clients.
WITT : We have not seen any demand for standard/open protocols, although many clients ask about them. The HVAC control vendor is often tasked with integrating the HVAC system controls with other building-system controls, such as fire alarm and security ... [and] equipment with sophisticated, unit-mounted controls, such as chillers.
RAFFERZEDER : Most of the "big four" [controls manufacturers] are adapting Lonmark in lieu of BACnet. We don't see BACnet as being a truly viable means to allow true open connectivity for some time-five years.
ANDERSON : SNMP-standard network-management protocol-is more widely known in the computer networking industry than in the engineering world and may be more accepted by high-tech clients. [But] few product manufacturers are providing this protocol yet.
RAFFERZEDER : There seems to be a newer trend toward using third-party software packages-such as Intellution, Wonderware or Ionics-to permit a common operator workstation to serve as the interface to multiple vendor systems, [such as] building management, lighting and process controls, using OPC-Microsoft Object Linking and Embedding for Process Control-technology. This allows for open bidding with limited drawbacks-such as who maintains the software and graphics-and facility personnel must have a command of programming languages for all systems used.
CSE : Is the use of underfloor air plenums with raised floors increasing? What are some of the issues (see "Is There a Future for Underfloor Air?" on page 32)?
ANDERSON : Yes, the use of underfloor air plenums is increasing. One reason is that there are serious and growing risks of inadequate cooling to vital computer equipment if overhead air delivery is adhered to.
ROWLAND : Underfloor air plenums have been the standard for data centers for years. There are currently several underfloor distribution systems available for office type spaces but they are not being widely used in this country. There are concerns for the cleanliness of the air as well as comfort from a perceived draft.
NIEMANN : Underfloor air plenums and distribution is an emerging equipment application that has not seen wide use. The technology is primarily used for clients that are frequently reconfiguring spaces.
WITT : Raised access-floor systems are typically used where the expense is justified due to cabling and flexibility needs.
CSE : What specification criteria or code issues most impact the design of underfloor plenums?
ANDERSON : Plenum height is easily the number-one criterion. We are recommending a minimum of 24-inch-high raised floors, depending on the load density and the corresponding airflow needed per square foot. With plenum heights too low, the plenum is not consistently pressurized and results in inadequate airflow from perforated tiles near the fan discharge.
Underfloor fire safety
NIEMANN : National codes requiring noncombustible materials installed in plenums must be followed by suppliers.
WITT : Applicable codes normally require that materials in air plenums meet a flame-spread rating of 25 and a smoke-developed rating of 50. One concern is that materials that do not meet these ratings may be installed in the cavities.
ROWLAND : Plenum-rated cabling is required to minimize the risk of causing a smoke problem in the area. We have used addressable smoke detectors in underfloor areas to not only alarm upon sensing smoke, but to identify the origin.
RAFFERZEDER : One specification issue is the viability of the plenum as it relates to a sealed envelope that can be constructed cleanly and maintained for the life of the building. Additionally, wiring and piping trades that share this space must be controlled so that air distribution is not hampered.
CSE : What other advances in mechanical system technology are most critical to high-tech facilities?
ANDERSON : With the rising load densities of computer rooms and data centers, precision AC systems may need to provide additional rows of coils to provide a larger delta T. Additionally, fans have to provide more airflow while at the same time maintaining the same footprint dimensions.
NIEMANN : The increasing watt density of advanced electronic components may require that cooling fluids be pumped through the component. Small plate heat exchangers utilizing exotic fluids as well as thermionic devices may also become standard in high-tech facilities.
ANDERSON : Raised-floor height and the racks themselves become major players in how well computer equipment is cooled. Designs by rack manufacturers and architects will become more critical in applying new cooling solutions for these specialized and ever-changing facilities.