How To Control A Chiller (February 2001)
It hasn't been easy. Especially the last point. With numerous bas out there, and different protocols at every turn, it's been difficult to make a chiller that will meet everyone's needs. But there have been profound changes lately in the chiller controls world. These changes will make it easier for engineers to specify a chiller for just about any situation, and they will also help improve utility bills.
Mark Tozzi, controls business manager, Carrier Corp. (Syracuse, NY), says that with proper control of the entire chiller plant, including the towers, secondary pumps, and chiller staging, the controls can significantly reduce chiller energy costs. "For example, lower pressure drops through chillers can add up to significant reductions in pumping costs. The ability to run the chiller at lower condensing temperatures at times can vastly improve some chiller manufacturers' kW/ton performance while not improving the performance of another manufacturer."
Interoperability May Soon Be HereEngineers have complained for many years that controls manufacturers have exaggerated the ability to achieve total interoperability of systems within a building. That being said, it is true that the controls industry is moving - although many say too slowly - toward open protocols and industry standards. The standard protocols are allowing the heavy-equipment manufacturers, including chiller manufacturers, to finally be able move away from "gateways" and start incorporating those standard languages as native protocols. The problem with gateways has been that over time, they have a tendency to become out of sync with the information in the chiller unit controller itself; for example, if new software is downloaded to a chiller unit controller that is connected to a bas by means of a gateway, the software in the gateway must often be upgraded as well in order to pass the new information on to the bas.
"There is a trend towards providing -from the factory - multiple native communications capabilities without incurring, and passing on to the user, the cost of multiple controller platforms. With native communications, the new information is immediately available to the building automation system," says Jeff Hamilton, director, McQuay Controls, McQuay International (Minneapolis).
Indeed, some chillers now allow the user to select from among multiple standard protocols for communications with current bas that utilize, or can be upgraded to utilize, standard protocols. Offering choices for protocols allows integration into a broader spectrum of bas.
Eliminating the gateways and embracing open protocols will be helpful for all concerned, says Jack Gornik, building automation systems director, York International (York, PA). "This means a much more seamless communications path, more and faster data, and reduced equipment and commissioning costs. These communications breakthroughs are made all the more important by the expanded functionality of the chiller controls themselves," he said.
This expanded functionality is due to fact that chillers now rely on microprocessor controls rather than electromechanical controls. Indeed, today's chiller control panels include full-featured, industrial-grade, direct digital controllers, complete with on-board graphical human-machine-interface screens and more computing power than the typical building automation components to which they connect.
"This means that not only do the chillers operate better in every sense, but that there is a lot of data available to the operator and the building automation system," says Gornik. "This data, accessed and used correctly, can provide operating efficiencies not dreamt of even five years ago. A decade ago, if you got five pieces of information from a chiller in real-time you were doing well; today some of our chillers provide two to three hundred pieces of live data per second."
Conflicts Can Still AriseEven though this new breed of chiller may be able to communicate more effectively with bas, it doesn't mean that conflicts don't arise. When conflicts do occur, the problem may lie in the varying interpretations of the specification. For this reason, it is important that the controls section and the chiller section of the specification both address connectivity, and that the verbiage be very precise.
"An engineer who tries to cover all the bases by overlapping the requirements in different specification sections often does a disservice to the project and the contractors. A savvy estimator knows what work is covered elsewhere in the spec and will avoid estimating those dollars. If both overlapping contractors assume the other guy is doing the work, conflict results," says Gornik.
Another area of conflict that sometimes occurs between chiller unit controllers and various bas is in the area of the protocol used to communicate information and the media over which the protocol is used. Protocol conflicts are resolved with gateways, while media conflicts are resolved with gateways, routers, and repeaters.
"A major problem occurs when the chiller manufacturer provides the unit controller, the building automation system manufacturer provides the system controls, and the connectivity products are overlooked. As the responsibilities of 'systems integrators' become better defined, and the question of who is acting in the role of systems integrator is clearly answered for each job, the protocol and media issues will be less likely to cause conflict," says Hamilton.
Backward and forward compatibility can also cause problems; that is, a newer chiller may not communicate with an older bas or an older chiller may not communicate with a newer bas. This can be a source of conflict between generations of controls systems by a single manufacturer, as well as between equipment from multiple manufacturers. And any of these scenarios can adversely affect economies.
For example, a bas provider introduces a new, and significantly more expensive, system based on an industry standard protocol, while continuing to provide their existing, or legacy, system. At the same time, the chiller manufacturer introduces unit controllers based on the same industry standard protocol and discontinues production of equipment with controllers using their legacy, or proprietary, protocol. A major conflict arises when a user wants to expand an existing bas, or put in a new bas based on a provider's legacy system, but the solution-specific methods of achieving interoperability with the chiller manufacturer's legacy controls are no longer available.
"Compatability problems are caused, in many such situations, by a shortening duration of the technology lifecycle for direct digital controls in the hvac industry, which is tracking closely behind that of the computer and information technology industry," says Hamilton. "Short technological lifecycles are somewhat at odds with equipment (chillers) typically amortized over 20 to 30 years."
Tozzi adds that older chillers cannot provide the same amount of management data or control information to the bas because the controllers are electromechanical instead of ddc. "The bas will have to install additional sensors and controls to reach the same level of control that is available with today's factory-mounted ddc controllers. And even then, there are some control points, such as motor-bearing temperatures and system pressures, that are almost impossible to field install."
What About Choosing A Protocol?So what, if any, impact, does the choice of protocol have on the choice of a chiller? Not much, says Gornik. "No current protocol will keep a properly designed system from functioning well, nor will any of the available protocols improve system performance in and of themselves. The protocols should allow connectivity and the development of a system, and they will all allow a competent provider to do that. Where problems arise, they arise from naive acceptance of the hype that has surrounded the 'BACnet' and 'LonMark' camps.
These are communications protocols, first and foremost, and neither one knows the right way to control a chiller in a given facility," says Gornik. Hamilton takes a different approach, noting that the choice of protocol can significantly affect the chiller/bas relationship. "If the user's existing building automation system is older, either based on a proprietary protocol or custom, and the exchange of information with additional equipment is required, users will tend to select new equipment that is compatible with the building automation system."
He does note, however, that compatibility, or more appropriately interoperability, can be obtained by solution-specific methods (e.g., special hardware, software, and tools). And users are now using standardized protocols in order to provide future flexibility in the selection of equipment, systems, tools, and providers. Once a specific protocol such as BACnet or LonMark is implemented, this concept can significantly influence a user's selection decisions, notes Hamilton.
Another way in which a protocol may influence a user's equipment selection decision pertains to facility management. Owners of commercial, institutional, and industrial buildings are trending toward facility management, multiple facility management, and remote facility management, either with in-house resources or under [performance] contract, through the use of emerging information technology (e.g., the Internet).
"As those performing the facility management services begin to rely on emerging information technology to monitor and control facilities, the need for seamless equipment interoperability with existing or specified building automation systems that have advanced communications capabilities becomes paramount. The building automation systems having such advanced communications capabilities are those employing industry standard protocols," says Hamilton.
An Engineer's ResponsibilityWhen it comes to designing chiller systems, many engineers rely on what they've done in the past, and that may be a mistake. According to Gornik, engineers sometimes regurgitate 10- or 20-year-old designs and sequences that may have worked well before but do not take advantage of fully proven technology improvements.
"Engineers tend to be conservative, and for good reason. But, some of the designs fail to recognize the vast opportunities the microchip has opened up in the central plant," says Gornik.
Hamilton believes that engineers typically make the following mistakes when designing a chiller controls system:
- They overlook the capabilities "built in" to unit and system level chiller controls (missed opportunities for diagnostics, performance enhancements, savings, etc.);
- They design "custom" solutions or utilize unique control approaches when existing methods and products meet the requirements (They reinvent the wheel.);
- They fail to coordinate unit controller software with integration equipment (Gateways cannot translate information properly.); and
- They overlook seemingly trivial components. (Connectivity equipment, such as routers or terminators, is often missing.)
He would like to see engineers specify factory-mounted chiller unit control systems, and develop a detailed chiller plant sequence of operations. In addition, Hamilton believes that engineers should use applied ("canned") chiller-system-level controls available from equipment manufacturers where possible and migrate towards standardized protocols.
Gornik's final advice is to involve the equipment manufacturers very early in the process and get their input. "Not just about what the footprint is for a given tonnage, but about what the best equipment mix and control strategy is for a specific application. If the equipment salesman doesn't seem exceptionally adept in the control discussion, ask him to involve his local controls specialist. The results may change your entire approach to the project design." ES
New Technology - Properly Applied - Can Save An Owner MoneyA chiller and its controls are intricately linked, and how those systems are put together - and how new technology links those systems - can have a dramatic effect on an owner's costs. Jeff Hamilton of McQuay International illustrates these points: For years, high-cost integrated circuits made centralized control necessary in order to amortize controller costs over a variety of control requirements (compressors, fans, solenoid valves, starters, etc.) for a chiller. This was necessary despite the high costs associated with additional power supplies, point-to-point wiring, miscellaneous connectors, control boxes, and increased labor.
Reductions in the cost of silicon and advancements in manufacturing of integrated circuits have reduced the cost of distributed controllers relative to the overall cost of the components (compressors, starters, etc.) of a chiller on which they are factory mounted. Meanwhile, the costs associated with peripheral equipment and labor needed to support centralized control have remained somewhat constant, thus becoming a greater percentage of the overall cost of a chiller control system. Empirical data suggests that transitions from centralized to distributed control can yield overall cost reductions in the range of 25% to 55% of the complete equipment (chiller) control system cost.
Chiller controls that are designed and applied utilizing advancements in computing technology can have significant performance advantages as well. Look at the case for distributed control within the confines of a single piece of equipment, keeping in mind that from a distributed control perspective, peer-to-peer communications is the most logical communications scheme and a fundamental element of the distributed control performance advantage.
In master/slave communication schemes, device actions and reactions may be totally unrelated, however, individual device performance can be limited by the performance (cycle, or scan time) of a centralized, or master controller. Master controllers can easily become communications "bottlenecks" or single points of failure. Furthermore, adding sequences of operation is usually complicated and often limited by the capacity of the master controller. The complexity disadvantage of master/slave communications grows with the number of components requiring control.
In distributed control systems using peer-to-peer communications, intelligent devices communicate directly with each other, eliminating unnecessary processing through "master" controllers. Compared to master/slave systems, for equivalent control functionality, such communications require less than half the bandwidth for unacknowledged transmission. For acknowledged transmission (the kind required to ensure commands from a keypad controller to a compressor control module, for example, are received), less than one-quarter of the bandwidth is required. Minimal bandwidth congestion and the resulting reductions in sequence of operation (loop) times provide superior system performance. ES