Making the Switch: Exploring CO2 Refrigeration System Designs
The United States along with 108 countries support a planned, orderly phase-down of hydrofluorocarbons (HFCs) on a global warming potential (GWP) weighted basis with the Montreal Protocol as the vehicle of choice to lead the phase-down. While some countries are still opposed to the phase-down, an increasing number of U.S. companies have recognized that the change is inevitable and are anticipating the refrigerant transition by testing environmentally friendly CO2 systems today. Plus, as more companies are being driven by the demand for corporate environmental responsibility, energy savings, and a need for resilient systems, CO2 is becoming an increasingly attractive option for refrigeration systems, and it is beginning to take shape as the next natural refrigerant to challenge the use of HFCs — especially since HFCs have 1,400 to 4,000 times the GWP of CO2.
While CO2 has been standard in certain industrial applications for some time, it is now being applied to a wider range of system applications, such as those used by the commercial and food retail industries. Although currently there is no involvement or regulations from the U.S. government driving CO2 into the industry as there is in Europe (via regulations, taxation, etc.), the benefits of CO2 make switching to it in system designs an attractive alternative. Implementing CO2 today can help anticipate and ensure the system is not using a refrigerant subject to the potential for phaseout or similar regulatory pressure in the future.
Using CO2 requires a different system design than the traditional HFC-based system. In fact, there are three standard design types that exist in the application of CO2 as a refrigerant in commercial food retail refrigeration systems:
• Subcritical (or cascade) systems; and
• Pumped (using CO2 as a heat transfer fluid as is commonly done with glycol).
Each type can have variations to meet different requirements or to increase benefits. Because CO2 is denser than HFC refrigerants, the overall refrigerant charge in these systems is less. This increased density, along with higher volumetric efficiency, allows for elevated coil temperatures, smaller pressure ratios (and, in turn, smaller compressors) and smaller liquid/suction line sizes, which result in initial and sustained energy and cost savings.
Due to the significantly higher pressures of a transcritical CO2 system, safety measures must be taken to ensure the integrity of the system. To date this has required steel piping for the high-pressure side of the system, and either steel or a higher-grade copper for the lower pressure portions of the system, as well as the use of specialized valves throughout the system that have been designed for CO2-based systems with higher pressure ratings. Additional safety parameters in the electronic control devices of the system also exist.
In the transcritical system design, CO2 is used as the only refrigerant. The basic design itself is very comparable to standard DX HFC systems that are commonplace today. One of the defining characteristics of this system is the use of the term “gas cooler” as opposed to condenser. The difference in nomenclature is the result of the phase state possibilities of the CO2.
In a transcritical design, higher ambient temperatures could result in the refrigerant existing in a supercritical state where liquid and gas are indistinguishable. In this state (when CO2 is above 88˚F), no condensing occurs through the condenser fans as typically seen in the standard system. By implementing a high-pressure gas cooler valve after the outlet of the gas cooler, expansion of the CO2 can occur, which will further aid in reducing pressure and temperature of the refrigerant. Pressure and temperature sensors at the outlet of the gas cooler are used to optimize the control of the valve, which is typically a stepper motor-type valve. Keep in mind, there also will be situations where the CO2 does not reach its critical temperature in the gas cooler and the CO2 does condense; in fact, it is more common to see the CO2 below its critical point than above it in a transcritical system.
Compressor and gas cooler fan control is typically executed through a rack (or pack) controller that contains logic to stage compressors and fans, along with options for other secondary features, such as oil control, depending on the specific device being used. These devices also typically incorporate variable-speed drives on either the compressor or gas cooler fans or both for additional energy savings from more precise control.
Evaporator control is also established through the use of specialized controllers. These devices will maintain proper feeding of the CO2 refrigerant into the coil, normally through the use of an electronic expansion valve that maintains a desired superheat. Past performance has shown that pulse width expansion valves show better results in coil performance compared to other valve types.
Depending on the application of the controller, auxiliary controls such as for defrosting, evaporator fans, and lighting also can be carried out through the device. For some medium-temperature applications, it is not uncommon to see a flooded coil design, where no superheat control takes place, but rather standard on-off control of a solenoid based on a predefined temperature (e.g., case temperature).
In this design, receiver pressure is crucial as it is normally the driving force behind temperature maintenance through the coil. This portion of the system is similar to a pumped CO2 system — the difference is that in a pumped CO2 system, the pump controls the flow of CO2.
One noticeable variation is the addition of a bypass line from the receiver to the suction line. This is to bypass any gas that exists in the receiver so that liquid feeding is ensured to the evaporator coils. It also allows for the piping from the receiver to evaporators to be made of higher-grade copper rather than steel. This bypass is controlled by another stepper motor valve, which can also maintain proper pressure in the receiver vessel. This functionality becomes particularly important in designs where receiver pressure is relied upon to drive the feeding of the CO2 to certain circuits.
Note: It is not uncommon for there to be different variations of the transcritical system design from one site to the next.
Subcritical (Cascade) Systems
The second system design, the subcritical system, has some similarities but also major differences to the transcritical system. One of the primary design functions of the subcritical system, by definition, is to maintain CO2 pressure and temperature below the critical point — in the subcritical range. Outside of system design, this is also important because the components selected for the subcritical system typically do not have pressure ratings as high as those used in the transcritical system. Depending on the exact design, the components could even include standard selections used on HFC systems.
To ensure the CO2 is maintained in the subcritical range, a heat exchanger is used to cool or condense the CO2 discharged from the compressors. In contrast to the transcritical system, the term “gas cooler” is not used with the subcritical system as the heat exchanger acts as the condenser for the CO2, and the CO2 never operates above the critical point. The refrigerant on the second side, or high side, of the heat exchanger used to cool the CO2 can vary, although ammonia and HFCs are more commonplace. The decision of which refrigerant to use can be influenced by a number of factors, including regulations, existing systems, and HFC reduction plans. A valve on the inlet of the high side of the heat exchanger is typically used to maintain a desired superheat throughout the heat exchanger coil. As with many of the valves on the high-pressure portions of the transcritical system, a stepper valve is common.
In applications of the subcritical design where direct expansion of the CO2 occurs in the evaporator coils, the installation will typically consist of two rack systems, each with a rack controller. One is found on the low-side CO2 portion of the system and a second on the high side, which controls the compressors for the secondary refrigerant. Because the two sides are linked by the heat exchanger, it is necessary to have compressors operating on the high side of the system before compressors start up on the low side, to ensure cooling of the discharged CO2. For this process to occur, the two rack controllers are sequentially interlocked (through wiring, programming, etc.).
Similar to the transcritical system, some coils can use a flooded design as opposed to direct expansion; however, in these designs on a subcritical system, a pump is used to feed the coils rather than relying on receiver pressure.
In a pumped CO2 system, there are no compressors on the CO2 side of the system, but rather a pump circulates the CO2 at a given temperature through the coils in the area to be cooled and back up to a heat exchanger where the heat is rejected into the high side that utilizes a different refrigerant.
Pumped CO2 systems are more comparable to glycol systems, and utilize less energy than glycol systems due to the higher heat transfer and lower viscosity of CO2 compared to glycol. This greatly reduces the energy consumption of the pump, and also greatly reduces the pipe sizes (although copper is required as opposed to PVC).
Choosing A System Design
When deciding which system to select — transcritical, subcritical or pumped — several factors can come into play. One of the primary concerns should be the ambient temperatures to which the system will be exposed. In cooler climates, where CO2 can be kept below its critical point for a large portion of the year, the energy savings of a transcritical system can be significant compared to a subcritical system. However, the costs of the higher-rated components and steel piping used in the transcritical system also must be taken into consideration when weighing initial costs and long-term savings.
In a transcritical system, CO2 would be above its critical point for longer portions of the year in warmer climates, so the cost of the energy needed to maintain this type of system makes it a less ideal option. In these situations, the subcritical system design becomes more common. Subcritical designs can also have reduced initial costs compared to a transcritical system for two reasons. Not only are the components generally less expensive, but there is also the possibility that a retrofit could utilize a system’s existing HFC-compatible components. There are, however, a greater number of components required in a subcritical system — two compressor controllers, more compressors, a heat exchanger controller, pump controller, etc.
Implications of CO2 for Contractors
The use of CO2 as a refrigerant does not require reinventing the wheel, though it is important that added safety precautions be taken to accommodate the system’s higher pressures. One of the biggest safety concerns is a CO2 leak. While leak detection is often used in HFC systems throughout the United States, leak detection for CO2 systems needs to be standard protocol. Immediate leak alerts are crucial, as is the response of the system. In most cases, the system will be designed to shut down to isolate the leak, as well as to notify someone on-site. The installed electronic components should have safety capabilities to shut down portions of the system when a problematic high pressure is encountered until it is corrected.
Service technicians should be knowledgeable about where these shutdowns may occur and how to correct them. Technicians must also be careful to fully evacuate the system when required and not to allow moisture into it. When water reacts with CO2, it produces carbonic acid which can corrode piping. Finally, technicians should be aware that many oils used in HFC systems are not miscible with CO2, thus requiring use of a nonstandard oil, which can vary from one design to the next.
Fueled by the emergence of new components, growing industry expertise and a wider range of applications for CO2 systems, the number of CO2 system designs existing in the United States is expected to increase significantly over the next few years. Inquiries about new or retrofit CO2 systems are increasing rapidly, causing the industry to quickly adapt and become more familiar with required CO2 system designs and product offerings. And while nothing has yet to be announced, the industry should anticipate the future emergence of state and federal regulations to improve carbon footprints and further encourage the deployment of environmentally friendly CO2 systems.
Publication date: 10/1/2012