Chiller Selection: Balancing Precision and Cost
Knowing your application’s threshold for temperature stability is key in selecting a suitable process chiller.
Specifying a chiller for a given application requires understanding the process to be cooled and its temperature stability demands. Many processes can be adequately supported with ±3°F temperature precision. Others benefit from more precise temperature control.
Increased precision often comes at an added cost, so determining whether the process can benefit from such precision should be considered. Balancing cost and temperature stability leads to a lower total cost of ownership (TCO) and improved reliability for the end user.
Chillers Evolve with Modern Industrial Processes
As industrial processes and equipment have advanced to support higher productivity in small packages with increased thermal density, chiller designs also have evolved to meet the demand for reduced variation in fluid temperatures, especially in more complex and sensitive applications. In some processes, performance advantages are enabled by more precise temperature control. MRI, laser and semiconductor equipment are examples in which technological advances require close regulation of process temperature. Such close regulation is enabled by appropriately featured chillers.
In MRI equipment, tight temperature control helps to produce higher resolution scans. Lasers are another example that benefit from close temperature control to deliver consistent results. In each of these applications and many others, precise temperature stability is required to achieve optimal results. Table 1 lists example applications, cooled machine elements, temperature stability advantages and the associated tolerance window.
Dynamic vs. Static Performance Requires Consideration
A chiller’s published specification for precision is usually based on steady-state performance. These figures typically represent the precision available once the system is at a stable temperature and experiencing a constant heat load.
OEM and end users must consider dynamic-state precision as well: cases of use when the heat generated by the process undergoes rapid and significant rates of change. Variation in MRI scan intervals or continuous versus intermittent cutting with a laser are examples of dynamic-state process variability.
In some applications, however, dynamic changes in load can occur by design and require a responsive cooling system to achieve the intended process result. In these situations, a well-designed control strategy that may include responsive, low thermal mass sensors and intelligent logic can provide the desired performance.
While dynamic states can be accommodated, they can increase chiller system cost and complexity. These systems require close collaboration between OEM and chiller manufacturer to define and tune the system. Early discussions with manufacturers regarding expected operating cases and conditions are key to achieving performance as well as overall impact on the total cost of ownership.
Design Methods Support Temperature Precision
As mentioned, some applications require more precision than others (table 1). By understanding your stability requirements, you can evaluate solutions that best meet your needs and provide the lowest TCO. In many cases, increased temperature precision adds little to no benefit to the application being cooled. In those situations, good design practices can provide robust and reliable performance without added investment.
For example, simply turning the compressor on and off provides the most basic control of the system. While low cost, this approach places an upper limit on system precision. It also creates a potential for premature compressor failure. Frequent start-stop cycles lead to significant wear and tear from increased exposure to motor inrush currents and inadequate distribution of compressor lubricant. For these reasons, additional design practices are needed to mitigate risk.
One way to address this hazard — and to simultaneously improve temperature stability — is to create a thermal flywheel effect by adding thermal mass. Typically, this is accomplished by adding a holding tank in the hydraulic circuit. In distributed systems, additional thermal mass also is created when there is significant fluid volume in the system piping. While a larger tank offers greater temperature stability, it comes at an additional cost and a larger machine footprint.
Using multiple compressors is another alternative that can improve temperature regulation, particularly in situations where part-load conditions occur. Modulating cooling capacity by staging compressors in a multi-compressor system to match the rate of change in the process heat loads improves overall control system response. In addition, this approach can increase system reliability by balancing run hours amongst the available compressors. In general, to offset the additional costs, this strategy can be viable for capacities above about 30 kW.
Techniques Offer Finer Control
When additional precision or stability is needed, there are various methods that can be employed. These are accompanied by cost differences and mixed energy, footprint and other considerations, so be sure to weigh all of impacts on TCO. Examples include hot-gas bypass (HGBP), variable-speed compressors and pumps, and control-based solutions. Table 2 highlights a sampling of methods and the associated operational benefits, cost impact, temperature stability and design considerations.
For applications that could benefit from improved temperature precision, hot-gas bypass (HGBP) offers an additional method to modulate capacity. This approach enables the compressor to run for longer periods by essentially bypassing the condenser portion of the refrigeration circuit to reduce capacity.
Capacity and temperature control depend upon the accuracy of the hot-gas bypass valve and other system sensors and controls. Solenoids that are modulated using a pulse-width modulation (PWM) signal are lower in cost but can introduce reliability concerns. Stepper-motor-driven valves also are available, and they offer increased precision. As expected, improved performance and reliability come at an increased cost.
While hot-gas bypass helps to protect compressors from overcycling and to improve temperature regulation, this is accomplished through capacity modulation only. Power consumption is not reduced, leading to decreased energy efficiency and increased total cost of ownership. Despite those shortcomings, this ability to modulate compressor capacity enables many of the advantages of variable-speed technologies — including improved temperature regulation — but at a higher first cost.
Pump, fan and compressor motor-speed control using variable-frequency drives (VFDs) are the next frontier in process chillers. By adjusting compressor output instead of switching components on and off, system efficiency and reliability can be improved. This technology is already common in building comfort or HVAC applications, where continuously changing heat loads and around-the-clock operation offer significant opportunities to recoup higher system costs through energy savings.
As the cost for VFD controls become more competitive with other technologies — and as demands for energy efficiency in process chillers increase — the application of this technology is poised to grow significantly.
Partners Provide Chiller Insight
The designs for process chillers are as varied as the applications they support. Understanding your application’s needs is a great first step. Partnering with an experienced process chiller manufacturer is the second. With expert assistance, both OEM and end users will arrive at a solution that provides the best fit from a total cost and performance perspective. An OEM partner can help to evaluate whether your process could benefit from precise temperature control and identify which technologies to leverage to achieve the right balance between performance and total cost of ownership.