CO2 Refrigeration: Debunking 3 Common Myths

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Stellar is gearing up for ATMOsphere America 2017, the leading forum for discussion about the business case for natural refrigerants in North America. The three-day conference next month will host more than 400 industry stakeholders in San Diego, California, and will feature discussions about the latest in refrigeration technology and regulation. Among the hot-button issues in the industry: the diminishing role of hydrochlorofluorocarbons (HCFCS).

With HCFCs being phased out worldwide by 2030 due to their harmful environmental impact, plant owners are searching for sustainable refrigerant alternatives. However, there is no one-size-fits-all solution.

Carbon dioxide (CO2) is commonly used as a cryogenic refrigerant, but it has also been used as the working fluid in mechanical refrigeration systems as far back as the 1800s. Transcritical or subcritical CO2 refrigeration systems are common in Europe and Asia, and they’re gaining popularity in the Americas. In the northern U.S. and Canada, transcritical CO2 systems have become a cost-effective choice in many food processing, cold storage and commercial facilities. Subcritical CO2 cascade and volatile brine systems are best used for low-temperature freezing and higher temperature secondary refrigerant applications.



Mechanical refrigeration using CO2 can be an attractive alternative to other refrigerants, but many have written off the refrigerant due to these common misperceptions:

Myth #1: CO2 is too dangerous

Given the right conditions, all refrigerants have the potential to cause harm. Some of the characteristics that drive fear of CO2 are the same that provide its benefits. High operating pressure is “scary,” but it’s the feature that enables lower equipment costs and greater energy efficiency. With proper design, construction and commissioning, a mechanical CO2 system is just as safe as any other.

Myth #2: CO2 refrigeration requires leak detection that other refrigerants don’t

Like many refrigerants, CO2 is a colorless and odorless vapor. Thus, it does require leak detection in the machine room as well as the cold rooms. However, leak detection instrumentation is required in unmanned machine rooms regardless of the refrigerant. In fact, many operators require leak detection for ammonia in the cold rooms too, despite its self-alarming odor. Cryogenic CO2 is widely used in processing facilities with ventilation and leak detection at the points of use. It is important to note that the right leak-detection technology is required with CO2 because of its presence in the human body. Instruments that only measure oxygen levels are not adequate.

Myth #3: A CO2 system is more expensive to install and operate

In most cases, CO2 cascade systems are less expensive to install and operate than two-stage ammonia systems for low-temperature applications.

The unique physical properties of CO2 provide an advantage when used as a secondary refrigerant for higher-temperature applications, too. Its high vapor density and volatility combine to achieve much smaller piping and pumping requirements when compared to chilled glycol systems, thus reducing capital and operating costs. As a volatile brine, CO2 can provide energy savings of up to 10-20 percent for high temperature systems and 20 to 30 percent for medium temperature systems.

When investing in a new refrigeration system, owners and operators should consider using CO2 cascade or volatile brine systems. Whether looking to minimize ammonia charge, reduce carbon footprint, or both, CO2 is often a viable and cost-effective option, providing:

  • Cost-effective installation and operation
  • Improved energy efficiency when compared to glycol
  • Reduction in ammonia charge and PSM compliance costs
  • Sustainability with 0 ozone depletion potential (ODP) and 1 global warming potential (GWP)

Are you attending ATMOsphere 2017? Be sure to connect with the Stellar group who will be there. Want to learn more about the viability of implementing a CO2 system in your food processing or distribution facility? Feel free to email me at rpeterson@stellar.net

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Comments

  • Hello, great article. Can you explain what you mean by “volatile brine” systems? I am not familiar with that term. I am also wondering if you can elaborate on CO2 viability under two scenarios: 1) a facility that is completely dedicated to cooler temperatures (i.e. no freezers at all) and 2) one that has a combination of coolers and freezers. Thanks.

    • Stellar says:

      Good questions Peter, we’ll be responding shortly.

    • Stellar says:

      Thanks for your question Peter.

      Volatile brine refers to liquid co2 when used as a secondary refrigerant. The application is similar to chiller systems using glycol as the secondary refrigerant, except the co2 is volatile, meaning at or near its boiling point. This feature makes it absorb lots more heat per unit mass than water/glycol can.

      For storage coolers less than 40F, volatile brine systems are more efficient and less expensive than glycol chillers, because they don’t require large flow rates, large pipes, pumps, and pump energy. Direct expansion of ammonia for storage coolers is still less expensive to construct. In cases where ammonia minimization is a primary objective, CO2 brine is often a better choice than glycol chillers.

      However, in cold stores where both freezers and coolers are involved, volatile brine is often combined with CO2 cascade systems to serve both. In general, the lower the temperature, the better CO2 performs.

      -Randy

  • Andrew G Carr says:

    SIr:
    What is meant by transcritical and subcritical in reference to CO2?

    What about fish processing applications like blast freezers, plate freezers and storage freezers?

    How do the pressures compare to R22 and Ammonia?

    • Stellar says:

      Hi Andrew,

      Thanks for reading my post!

      The terms “transcritical” and “subcritical” both refer to the “critical point” of a substance. Critical point is a technical term used to describe the pressure above which the properties of a vapor and a liquid are indistinguishable. All refrigerants have a critical point.

      The term “transcritical” (sometimes called “supercritical”) describes refrigeration systems where part of the refrigeration cycle (compressor discharge) operates above the critical pressure. “Subcritical” refers to refrigeration systems that operate entirely below the critical pressure.

      Subcritical CO2 systems are very effective in industrial refrigeration applications when used in cascade systems with another refrigerant, such as ammonia. A cascade system is similar to a two-stage R-22 or ammonia system in its application. The difference? The CO2 handles the low-temperature part, and the ammonia only refrigerates (condenses) the CO2. The two refrigerants are separated by a special heat exchanger. In cascade systems, the ammonia charge is much lower than standard ammonia systems, and it stays in the machine room. CO2 goes to where the cold is needed.

      The seafood applications you mention are excellent CO2/NH3 cascade examples. There are already working examples of CO2/NH3 cascade systems for plate freezers, blast freezers and storage freezers in North America. I actually wrote another blog post on this titled, Six Reasons to Consider a CO2/NH3 Cascade Refrigeration System, which you may find helpful.

      Here are common CO2 refrigerant temperatures and pressures:

      Blast Freezers
      -55°F = 106 psia
      -40°F = 146 psia

      Storage
      -25°F = 196 psia
      +20°F = 422 psia

      Co2 Condensing temperatures
      +20°F
      +25°F = 455 psia

      I hope this was helpful!
      -Randy

  • Swayam Prakash says:

    hello sir, can you explain me why we are using refrigerants like R-134a, R-407c , R-410A etc which have more global warming potential , and not using co2 which has GWP onlu 1 still now.

  • Ramgopal says:

    Can you kindly explain what practical problems may be encountered when CO2 is used as volatile brine, in place of, say glycol? Are there any problems related to unstable operation or pump failures with CO2?

    • Stellar says:

      Thanks for reading! The volatile brine system would be as stable as any other pumped liquid system, so long as the electrical power was also stable. In a power failure of extended duration, the loss of the CO2 condensing system can eventually result in CO2 overpressure and a release through the safety pressure relief valves. Many large CO2 refrigeration systems include a small standby refrigeration system whose sole purpose is to condense CO2 in case of a power failure. They are usually powered by an emergency power generator.

  • Gary Hearn says:

    CAN YOU USE CO2 IN EXISTING COMPRESSORS IF NOT WHY NOT

    • Stellar says:

      Hi Gary. Thanks for your questions! Due to the properties of CO2 vapor, converting an existing compression system to CO2 is not feasible. The valves and vessels (and possibly the compressor casting) would be the incorrect pressure rating, and the existing compressor’s displacement and motor size probably wouldn’t be adequate for the same load.

      In addition, the oil management is different: CO2 requires special rectification and recovery systems that may not be present in the existing system. It isn’t just the compressor that’s the issue, but the rest of the system, too.

  • Gary Hearn says:

    CON YOU USE LARGE 2 STAGE IN A CO2 SYSTEM

    • Stellar says:

      A CO2 cascade system is already a two-stage system: the CO2 cycle handles the lower temperatures and the other refrigerant handles condensing of the CO2 stage. The lower the process temperature requirement, the more efficient CO2 is, and the lower construction cost is compared to other refrigerants.

      You could add a CO2 stage to an existing two-stage system to handle new low-temperature loads, but it’s not typically feasible to retrofit an existing industrial refrigeration system to CO2.

      I hope this answers your questions, and thanks for reading!

  • Gary Hearn says:

    In reading your blogs and information ,I get the impression that your preferred system would be cascade with ammonia as moderate and CO2 as low temp could you use CO2 in a DX system. what would be the down side compared to cascade

    • Stellar says:

      All refrigeration cycles have an expansion step, regardless of the refrigerant. DX stands for “direct expansion,” which refers to the type of expansion where condensing pressure is high enough to force liquid refrigerant through the expansion device. Perhaps a better way to ask your question would be: “Can you use DX in a CO2 refrigeration system?” The answer is yes, you can.

      A CO2 cascade system, usually with ammonia as the high temperature refrigerant, isn’t just a preference. It’s really the only way to get the benefits of CO2 into a large, industrial refrigeration system because CO2 saturation pressure gets pretty high above 40°F. In large systems, the pressure would exceed the working pressure ratings of commercially available refrigeration valves and components. That’s why we need the cascade heat exchanger and another refrigerant for the higher temperatures. Ammonia is most often used as the second refrigerant because it is a natural refrigerant — like CO2 — and has zero ODP and GWP. Man-made refrigerants can be used, but most of those are under intense scrutiny by environmental regulators around the world.

      Perhaps by “DX” you mean single-stage CO2 refrigeration systems? Trans-critical CO2 systems are often single-stage, and can use direct expansion. These systems can have refrigerant pressure above 1,000 psig. Thus, they are limited to commercial sizes where the pipes and components are small enough that high-pressure valves are readily available. Thanks for your question!

  • Gary Hearn says:

    thank you for answering my previous questions very helpful. Her’s one more I have 1 large plate freezer (17 plates presently run with an 80hp 2 stage sabro compressor, 2 50hp blast freezers 1 25hp blast freezer and 2 ice machines 1 12 ton capacity 35hp 1 18 ton capacity 50hp The 2 50hp blasts and the 25hp blasts run on HP 80. The rest of the equipment use R 22.Can I construct a CO2 package to service all my needs and what would that look like.

    • Stellar says:

      Yes, we could design a CO2 cascade system to serve all of those needs. What it would look like depends on the process loads and the desires of the owner.

      It could be a centralized industrial refrigeration system with multiple loads, very similar to that in any food processing facility. Compressors and condensing would be located in a centralized machine room in your building (which must be designed to comply with building codes) or it could be modular and located on a skid outside of the building.

      In either case, new piping systems would be needed to interconnect the machinery with the loads, all of which would likely be new and designed for CO2 pressures.

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