01 April 2004
Smart, small, easy to install
Knowledge of technology trends helps users select control for process applications.
By Dennis Schneider
The ability to accurately control, display, and record temperature values is critical to many industrial applications such as chemical and pharmaceutical processing, plastic injection molding, commercial ovens, and kilns. With the wide variety of temperature controls available—each with specific benefits and application advantages—engineers involved in selecting these devices should make sure they are up to speed on the latest technology trends.
Understanding selection criteria, cost requirements, and the operational and performance characteristics of each unique process can lead to reduced installed and life-cycle costs and help users employ the right amount of control for their application requirements. Improper selection and installation of temperature controllers, such as the single-loop style, can have a dramatic affect on product quality and usefulness.
A matter of size
Semiconductor technology has advanced to the point of building controllers with fewer components. That means greater reliability, smaller sizes, and lower costs. As a result, it is now possible to achieve a high degree of control accuracy and functionality with a relatively small investment. The trend toward the miniaturization of single-loop controllers has evolved during the past several years. Recent introductions include the smallest device yet seen on the marketplace: a 1/32 DIN controller with a front-panel measurement of only 48 by 24 millimeters. Currently, the most popular controller size is the 1/16 DIN, which measures 48 by 48 milli-meters. As the trend continues toward building temperature controllers inside machines and control panels, there may be a shift toward the 1/32 DIN device.
In general, users should look for temperature controls that are easy to install and to replace and that offer easy-to-set-up temperature and loop tuning parameters. Users should first determine what panel size they want as the base for the panel space and consider the minimum controller application feature set they need for the task at hand. Smaller controller sizes are typically less expensive, but sacrifice some functionality. However, smaller units may allow a smaller control panel, which will reduce overall cost.
Obviously, on a larger display it is easier to read process values from greater distances, while a light-emitting diode display offers better visibility under various levels of ambient light. If you need a small-sized controller to meet a very limited panel space, you have to weigh the display size and range of capabilities against the panel size requirements. If users select the panel size that meets the functional requirements of the application in the first step, they can make the most appropriate and cost-effective purchase.
For larger users or original equipment manufacturers who have multiple applications with different degrees of functionality, the most cost-effective solution is not always the best one. This is due to the time required to install and configure the controllers in the different systems, which adds to the overall cost. If end users with a multitude of systems have a variety of controllers from different vendors, their engineering and installation staff will not install or, more importantly, configure the systems as efficiently or cost effectively as they would if all the controllers came from one vendor.
You should also consider the availability of PC-based programming software for the temperature controllers you selected. This capability can improve installation efficiency, because you can store controller programs in PC memory and copy them to multiple controllers. This provides significant time and cost savings. Instead of spending a lot of time punching in parameters from a keypad, users can use the software to more easily configure the controller. This allows you to download identical programs into identical applications. It also allows you to more easily modify similar programs for slightly different applications and helps you monitor process values remotely.
Another key capability the software provides is data recording and trending analysis—standard protocol in most processes. This capability allows users to review and analyze potential process failures or performance faults. You can also use the information to predict process trends, improve quality, and reduce waste.
Digital control systems with built-in intelligence are desirable because of their ability to provide optimum process and temperature control efficiency and maximum product yield. This feature is typically in the form of proportional, integral, derivative (PID) autotuning or a self-tuning (fuzzy logic) function that enables the controller to program itself to match process specifications. Manual PID loop tuning is more of an art than a science. In the past, many technical papers explained how to tune PID loops. However, these did not truly resolve the problem because every loop is different—even ones that are physically identical—and no two components (such as steam valves) or losses within loops are the same.
In addition, PID parameters change over time as the components in a loop age. Therefore, what is needed is a temperature control system that constantly looks for changes in a loop and makes PID parameter alterations based on the changes. Built-in intelligence with autotuning gives users the ability to self-tune process parameters that previously were extremely difficult to program. As a result, manufacturers no longer need to have process knowledge as in-depth as in the past. Moreover, built-in controller process knowledge helps reduce the potential for human error. It is no surprise these small, low-cost, single-loop controllers are more often replacing more sophisticated products in a wider range of applications.
Different applications require different levels of intelligence, usually dictated by process economies. For example, package heat sealing applications probably do not require the same level of control sophistication as chemical or pharmaceutical applications. In between are applications such as injection molding and food process applications.
Controlling multiple zones
The need to control multiple zones and provide quick updates via a single controller depends on the application and the user's preference. Quite a few users prefer to use a single-loop controller to control each zone of a process and link the temperature controllers and zones to a supervisory controller for coordination and remote single-point monitoring. In addition to lower cost, the advantage of a single-loop controller over a multiloop controller is it provides the fastest response and immediate display of each zone's temperature and set point. (Because of display size limitations, multiloop controllers typically only display one loop at a time). Of course, communications from the single-loop controller to a PC or programmable logic controller is required as either a standard or optional function.
The need to communicate to the temperature controller is application dependent. For example, in an application that stations an operator at or near the application to view the temperature controller and the process, there is typically no need to include a network. However, if an application requires several machines, each using a single-loop controller, and users want to monitor them from a location that is not in close proximity to the controller itself, then a network becomes important.
Because not all applications require communications, there is no need to purchase network capabilities if you don't need them. However, if you think you are going to need them at some point, you should consider whether a network link is an option and factor this into your purchasing decision.
Although the trend toward packing more functionality into smaller-sized controllers will certainly continue, it is unlikely they will get much smaller in the future. Anything smaller would make the displays nearly impossible to read and the devices difficult to program and wire. One likely trend with single-loop controllers is the move toward connecting them to high-function networks such as DeviceNet or even Ethernet. Many of today's large multiloop controllers already have this capability. Once the data links to Ethernet, it will easily connect to the Internet, which opens up a range of communication possibilities.
Control manufacturers have taken the technology to the next level of performance and continue to develop more reliable, more accurate, and less expensive devices to control temperature and other process variables. As new trends emerge, users should take each one into consideration and determine which, if any, may affect their process. This will help ensure the controller they select delivers maximum value and benefit over its service life.
Behind the byline
Dennis Schneider is a product manager at Rockwell Automation in Milwaukee, Wis.
Chilled water plant upgrades in semiconductor clean room
Researching, developing, and manufacturing wafer processing tools for the semiconductor industry is the goal of Applied Materials, a 30-building facility headquartered in Santa Clara, Calif. Building 2 includes a large clean room research facility on the lower level and offices on the upper level. The space between the levels is used to provide facilities services to the clean rooms.
The building's first chilled water plant used one 500-ton York chiller. In 1994, the plant installed two new 750-ton York chillers, because it was expanding clean room operations on the first floor. Current plant operation reserves one of the 750-ton chillers as a backup and uses the other with the 500-ton chiller to supply 40°F chilled water—meeting cooling and dehumidification loads for the building. The chilled water plant also includes three open-loop cooling towers (each sized to match the three chillers) with a common sump.
Since the mid-1990s the facility has installed a variable speed drive (VSD) on the 500-ton chiller and condenser water supply temperature optimization. With the VSD, the 500-ton chiller performs better at partial loads (25% to 75%), where chillers operate much of the time, than at a full load. The physical explanation for this efficiency improvement is the VSD allows reduction of chiller capacity by reducing compressor speed. It doesn't close inlet guide vanes, which throttle back on the refrigerant flow by increasing pressure drop. Inlet guide vanes reduce the total energy the compressor requires, but at a rate slower than the rate of reduction in cooling output. This explains the lower efficiency at lower loads. Because the VSD consumes little power, the full load efficiency for the VSD chiller is slightly poorer than for the non-VSD chiller.
The VSD chiller allows more efficient operation at almost all loads. Before the facility installed the VSD, if cooling loads in building 2 reached, say, 1,000 tons, operations would require a combination of a 750-ton and the 500-ton chiller. And at least one of them would need to run at a partial load. With the VSD, the 750-ton chiller can run at full load while the 500-ton chiller covers the remaining load efficiently. Likewise, if the total cooling load is low, the 500-ton chiller can cover the load alone with much better performance.
Condenser water reset is one of the most cost-effective ways to improve chilled water plant performance. It typically requires changing the control logic at a low cost and can improve chiller performance dramatically. The reason is compressor power is proportional to pressure the compressor develops. This in turn depends on the desired refrigerant temperatures at the compressor's inlet and exit. These two temperatures combine into a number known as the refrigerant lift. The lower of these temperatures is determined by the chilled water supply temperature (CHWST), and the higher temperature is dependent upon the condenser water supply temperature (CWST). Therefore, if the CWST is reduced for a constant CHWST, the refrigerant lift, pressure developed by the compressor, and compressor power are all reduced.
The normal method for reducing CWST is to increase cooling tower capacity by either running additional tower fans, or speeding up tower fans with VSDs if installed. The only limits to the CWST set point are the capacity of the cooling towers and the lower temperature limit the chiller can safely handle. Very cold condenser water can affect the oil that lubricates the compressor and can cause rubber seals to leak. Both can result in maintenance problems. Most chilled water plants tend to have excess cooling tower capacity, especially plants for clean room facilities, which typically have backup chillers with dedicated cooling towers. Proper piping and control logic easily allow access to the excess tower capacity—even when the backup chiller is not in use.
The York chillers operating at Applied allow condenser water temperatures down to 55°F or lower. The facility has implemented controls to maintain 55°F at all loads. This required some control programming to stage the three cooling towers to maintain the new set point. Both inadequate tower capacity for this low CWST and prevailing weather may be significant factors in this difficulty.
Applied also allowed water to run over the fill in all three towers, regardless of the tower fans being on or off. This optimized the cooling towers and allowed for a small but useful amount of evaporative cooling within the towers without using any fan energy.
In addition, the facility installed a new DDC control system to allow optimization of staging for both the chillers and the cooling towers. The cost was nearly $201,000, the annual cost savings—about $74,000 a year.
Source: Lawrence Berkley National Laboratory, CIEE Clean Room Case Studies (www.lbl.gov).