The global shift to digital, integrated, smart technologies, and society’s reliance on technology, continue to rise, pushing engineers to develop more powerful chips and data technologies. One of the most significant barriers to technological advancement is managing the excess heat generated by the increase in data processing, speed and storage as well as power generation and supply. This is true across all major industries, from semiconductor and industrial process automation to enterprise, 5G and telecom, and to medical and transportation technologies.
Newer, higher power applications require more efficient cooling for high heat loads in compact volumes. This has led engineers to consider liquid solutions. Liquid has up to four times higher capacity to transfer heat than that of the same mass of forced air. Using liquid cooling enables improved heat transfer, higher thermal performance, design flexibility and scalability.
A liquid-cooled system is a hydraulic circuit. Typically, it consists of a cold plate that interfaces with the liquid cooling (the heat source and device, hoses and pumps that circulate fluid through the system) and a heat exchanger that rejects heat into the ambient environment. These integrated systems provide efficient cooling that is capable of keeping high power applications running reliably at peak performance; however, there are some limitations.
One of the most notable challenges when working with high power devices and facilities is maintaining precise temperature control within the ambient temperature. Another is high heat rejection into the ambient environment. Working within the given parameters, devices cannot be cooled below ambient or intake temperatures, limiting performance and increasing outbound temperatures.
Issues such as these are especially noticeable in facilities that have a large number of high power systems operating concurrently, or in applications that need to hold close temperature tolerances with a wide heat-load variation. In scenarios like these, utilizing a chiller often is a more effective solution.
Though chillers can be customized and integrated into myriad applications, most chiller designs rely on common components and form factors. The overall cost of adding a chiller to a liquid-cooled system is driven by the selection of key components, specifically the pump, compressor and heat exchangers.
A chiller is a device that removes heat from a liquid via vapor compression, adsorption refrigeration or absorption refrigeration cycles. They can be utilized as part of a more complex liquid system or as a stand-alone cooling solution. Chillers allow cooling to sub-ambient temperatures, which increases efficiency and enables tighter temperature tolerances.
Ultra-low temperature chillers can provide temperatures from -112 to-40°F (-80 to -40°C). To achieve these lower temperatures, cascade refrigeration technology often is employed, using two refrigerants in separate refrigeration loops that operate in a series.
Chiller designs range from standard modular systems and stand-alone products to heavily integrated, complex systems where each component is tailored to a specific application. Standard options are developed based on the most utilized customizations.
An example of common form factors for integrated chillers include standard 19" rack-mounted and compact door-mounted chillers. Typically, rack-mounted chillers are used in the server and semiconductor industries. Compact door-mounted chillers are used in industrial battery storage applications.
When choosing or developing a chiller for an application, it is important to optimize the components as well as the design environment. For example, choose a high efficiency compressor to reduce power consumption while in use. Another example is to incorporate enhancements such as a touch-screen human-machine interface (HMI) or continuous level sensors for fluid reservoirs. It is crucial to evaluate each component for maximum reliability and efficiency, especially for the pumps.
Stand-Alone Chillers. Stand-alone, standard chillers can operate on their own or as part of an integrated system. Typically, they are defined and categorized by their general-use specifications such as process temperature, capacity and environment. These recirculating chillers are designed for precise temperature control or cooling below ambient temperatures.
Standard chillers are fabricated with traditional components and use water or water/glycol as the process fluid. They operate at process temperatures from room temperature down to 32°F (0°C). Components are performance matched and include optimized thermal systems and engineered materials. They also include any necessary mechanicals such as pumps, compressors and valves.
Low Temperature and Cascade Chillers. Lower temperatures improve performance and efficiency, but they require variations to standard chiller systems. Standard low temperature chillers are developed for applications that require process temperatures ranging from -40 to 32°F (-40 to 0°C). Fluids used include water/glycol mixtures and specialty heat transfer fluids. The key difference is the process fluid/refrigerant utilized.
Some manufacturers offer ultra-low temperature chillers that enable process temperatures from -112 to -40°F (-80 to -40°C). In order to achieve these lower temperatures, cascade refrigeration technology is employed: using two refrigerants to two separate refrigeration loops that operate in a series.
High Capacity Chillers. High capacity chillers are designed to cool high heat loads while still maintaining tightly controlled output temperatures. These chillers can be designed with either centrifugal pumps or turbine pumps. Centrifugal pumps can be used for temperature control within 0.9°F (0.5°C). Turbine pumps have a higher pressure capacity that enables the highest level of cooling up to 50 kW. Pumps with dual-voltage options allow chillers to be switched easily from 60 Hz to 50 Hz.
Chiller designs range from standard modular systems and stand-alone products to heavily integrated, complex systems, where each component is tailored to a specific application.
Chiller Design Strategies
Define exact requirements: Before choosing a standard chiller or beginning the concept-and-design process for a custom chiller, it is vital to understand the minimum requirements to establish a base model (table 1).
Optimize Available Space. Chillers can be integrated into a number of form factors by adjusting the placement of the components and how the chiller is attached to the device or system. These decisions affect performance, volume, reliability and ease of maintenance.
When designing chillers for different form factors, it is important to provide the most cooling in the smallest footprint available. Layout needs to consider product performance, reliability and accessibility for servicing. It also needs to provide easy, error-proofed integration when the chiller is being installed into the application.
Some chillers provide cooling to sub-ambient temperatures and provide tight temperature control.
Ensure Long-Term Reliability. One of the limiting factors of utilizing chillers is the addition of more moving or mechanical components. When designing a system that includes a chiller, it is crucial to have testing, reliability and maintenance protocols in place. These include considerations for:
- Remote monitoring.
- Energy metering.
- Leak detection.
- “Hot swap” designs to avoid downtime.
- Maintenance contracts.
Pumps are the most significant components to evaluate in order to ensure a long operating lifetime and system reliability. Take the time to do comparisons and testing to select pumps that use less power and provide the optimal pump flow with the required pressure head for the application. Selection of the rest of the plumbing materials and flow components can further enhance pump performance and system efficiency.
Process Fluid. The process fluid selected has a significant effect on chiller performance, usage and efficiency. Most liquid chillers utilize water or a water/glycol solution. The addition of corrosion inhibitors and biocides is critical to prevent long-term corrosion and algae growth that can bring down a chiller system by leaking or blocking flow. Understanding the wetted chemistry of all of the surfaces that the process fluid touches is critical in selecting the correct corrosion inhibitors. The use of glycol, which offers freeze tolerance, also will likely require the use of a biocide because glycol is food source for algae. There are many premade process coolants available off the shelf.
Additionally, it is important to remember that corrosion inhibitors need to be replenished over time. They are consumed over months of operation while they serve to passivate the surfaces as the process fluid flows. Either changing out the process coolant or replenishing the corrosion-inhibitor chemicals should be performed as preventive maintenance.
Refrigerants. Refrigerants are used in chillers as a closed-loop heat transfer from the process coolant. The two do not come in direct contact with each other.
Refrigerant selection can have a significant effect on energy efficiency. In operation, the refrigerant flow consists of both vapor and liquid. Vapor quality is the percentage of saturated mixture that is vapor; in other words, saturated vapor has a “quality” of 100 percent and saturated liquid has a “quality” of 0 percent. Vapor quality is a function of operating pressure and temperature. Each refrigerant is rated for either low, medium or high pressures. This significantly affects selection of the compressor motor. Poor refrigerant choice could offset any efficiency gains from using an energy-efficient compressor.
Green Refrigerants. New, more environmentally friendly refrigerants are gaining popularity. Global-warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere. It is expressed as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (GWP of CO₂ = 1). Older standard refrigerants such as R22 (Freon) have been replaced by a range of more environmentally friendly refrigerants with a lower GWP. This is an important consideration when choosing process fluids as global environmental regulations become stricter.
|Heat Load (Watts or BTU/hr)||What is the heat load? Is it steady or fluctuating?|
|Access/Access Point to Chilled Facility Water||A chiller must have access to chilled water despite the coolant used. The access point and integration with the rest of the liquid system affect form factor and mounting.|
|Coolant Fluid||If not employing water or water/glycol, be sure to have precise fluid details, including any corrosive attributes. (See section on process fluid.)|
|Required Coolant Temperature (Setpoint)|
|Ambient Temperature (Intake Temperature)|
|Flow Rate of Process Fluid (gpm or lpm)|
|Required Fluid Pressure||Be aware of fluid pressure and pressure drops.|
|Power Input||Electrical power available (volts, phase and Hz).|
|Environmental Conditions||Environmental conditions impact chiller design and engineered materials utilized. Examples include humidity, dust and particles and vibration or shock.|
Communication Options. Advancements in chiller design include improved connectivity, communication and HMI. Conventional communication protocols integrate with end-user systems to monitor process variables such as temperature, flow and pressure as well as system health. With newer interfaces, users also can control chillers remotely. Some custom chiller solutions will utilize made-for-use protocols to monitor and report based on application needs. Being able to monitor and control variables allows for peak efficiency and tighter control over the system.
Cost Considerations. The overall cost of adding a chiller to a liquid-cooled system is driven by the selection of key components, specifically the pump, compressor and heat exchangers. This initial cost combined with the cost of running the chiller is its overall cost. Water-cooled systems are energy-efficient systems and, therefore, have the most efficient usage costs. Maintenance and certification costs must be considered along with these usage costs for the total running cost the chiller.
The internal components of a standard chiller are shown in cutaway view. Chillers can be integrated into many form factors based on the placement of components.
Next Steps in Choosing a Chiller
Chiller adoption will continue to rise as high power electronics advance in capacity and heat load. Cost will likely be the most significant barrier to adoption. The best ways to mitigate these costs will be:
- Manage initial costs.
- Maximize energy efficiency.
- Select a system that requires less or simplified maintenance.
- Engage a solution partner early in the process.
Manage Initial Costs. Cutting costs by using subpar or low quality parts ultimately will incur higher long-term maintenance and repair costs, leading to a higher overall cost. A better choice would be to choose a standard base chiller (at a lower cost) rather than a custom model. From there, customization can be added to the standard model as needed. Choosing a standard model from a well-vetted manufacturing partner also can cut initial costs through streamlined production.
Maximize Energy Efficiency. By examining energy efficiency while determining design choices, chillers can run at lower energy usage costs, driving the operating costs down.
Limit Maintenance. Partner with a solution manufacturer that provides reliability testing and reliable components to lessen the need for repairs and maintenance. Be sure that warranties are in place and that the solutions partner has internal processes for streamlining the maintenance process.
Engage a Partner. Because there are so many factors in optimizing your chiller and liquid system to ensure performance, reliability and costs, it is crucial to engage a design and manufacturing partner as early in the process as possible. This will ensure that you are utilizing a chiller optimized for your exact application requirements.