Carbon dioxide (CO2) is a natural component of the Earth’s environment, that's needed by plants in order to carry out photosynthesis, making it a necessary component of the atmosphere. In addition to its purpose in nature, CO2 has many applications used in commercial industries such as food and chemical. For instance, CO2 has been used in the refrigeration business for well over 100 years. But its use in commercial applications was significantly reduced after the introduction of synthetic refrigerants. Today, however, the popularity of CO2 is reemerging among supermarket refrigeration equipment users throughout North America.

So why the new sudden interest? Well, there are several reasons for this and we’ll discuss them a little later in this article. But for now, let’s start by first explaining how CO2 can be used in a refrigeration system.

There are two ways CO2 can be used in supermarket refrigeration.
One is that of a secondary application (also known as "flooded coil" or "liquid overfeed"). Here, CO2 is circulated through and partially evaporated in the refrigerated fixtures by means of a liquid CO2 pump. This method uses the latent heat of CO2 for cooling thereby requiring very little, or insignificant, pumping energy. This is applicable to both low temperature loads (those loads below 0F) and medium temperature loads (those loads above 0F).

The second method for utilizing CO2 for refrigeration is by direct expansion. In this method, CO2 is circulated by means of a vapor compression cycle either for low temperature loads only (in a sub-critical cascade cycle) or for both low and medium temperature loads in a transcritical cycle. The latter is more prevalent in Europe and is not currently in use in the U.S. Chances are you're more likely to see CO2 in a cascade system in either a secondary low and/or medium temperature system or low temperature direct expansion system. With this reemerging technology, it’s important that refrigeration equipment technicians prepare for installing and servicing these type systems sooner than later. It’s no secret that today’s contractor needs to be more technically astute with the increasing use of electronics, floating control logic, variable capacity control and the ever emerging refrigerant choices. Well this is no different when it comes to CO2.

The operating pressures are higher than what you would typically see in a supermarket. This is nothing to be afraid of.

So where should you start? First, it’s very important that an individual looking to improve their knowledge of systems and CO2 should at least have a sound understanding of how a standard direct expansion system operates. Someone who is a beginner or novice should focus on improving their abilities with standard direct expansion systems and then move on to CO2. With that said, the Internet is the best place to start. You can find hundreds of documents, articles, books and other resources that offer theoretical, practical, safety, and even start up guidelines for systems using CO2. There are a lot of resources that may have an ‘industrial’ slant to them or be based on experiences in Europe, but the principles and applications are virtually the same to that of a commercial installation. I would also recommend starting with professional organizations that have subject matter expertise in CO2.

Here's a short list of organizations you can start with:

  • American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
  • International Institute of Ammonia Refrigeration (IIAR)
  • Institute of Refrigeration (IOR)
  • Australian institute of Refrigeration, Air Conditioning and Heating (AIRAH)
  • International Journal of Refrigeration.

    Next, I would recommend studying various component manufacturers’ literature on their CO2 product offerings to gain a better understanding of the differences of how their CO2 components differ from their standard HFC components. This is especially true of the CO2 compressors. There’s no substitute for practical experience. Once you've built a solid foundation of knowledge about CO2, you need to get some hands-on experience with it in an actual operating system.

    Most end users have a list of approved contractors that they work with. If you have a customer that you do regular business with, I encourage you to work with that end user to do a test store. This will be a great opportunity to gain practical experience, improve your company’s capabilities and keep up with the latest trends in the industry. I also recommend that you view your first project as a learning opportunity. Sure it’s a little different, but from what you studied, you'll find that it operates much the same way as a standard DX system or a glycol system does (depending on how the CO2 is used to refrigerate). And of course, you will have the support of the equipment manufacturer to help during the start-up process since it is still relatively new to the industry. I would also recommend networking with other contractors to gain access to an installation. With the growing number of CO2 installations across the U.S., chances are that a system will be, is, or is going to be installed in a location that you could get to relatively easily.

    Differences When Compared to Standard Systems
    With a CO2 system in operation, you can expect a few things to be different from a standard system. There are several major differences that you can expect to see:

    • First, your line sizes are going to be much smaller and typically of a thicker material. This is because of the excellent heat transfer characteristics of CO2 and the relatively higher working pressures. The system requires less amount of refrigerant than a typical HFC to accomplish the same amount of refrigeration resulting in higher efficiency.
    • Secondly, you'll see that the operating pressures are higher than what you would typically see in a supermarket. This is nothing to be afraid of as these pressures are going to be very similar to the pressures you will see in new air-conditioning systems that use R-410A. Most CO2 systems will operate below 500 PSIG.
    • Third, the pressure relief system is different in that there are two sets of reliefs. One relief system is a regulating system to keep the pressures below the maximum operating pressure by allowing small amounts of CO2 to be vented in the case of a pressure spike that will result in a cooling effect of the CO2 and lowering of the pressure. The second relief system is the standard relief system used today for rapid release of refrigerant in case of a sudden increase of pressure as from a fire.
    • The fourth main difference will be the heat exchangers. Subcritical CO2 systems use a refrigeration circuit from one refrigeration system (commonly referred to as a ‘Rack’ or ‘Top Cycle’) to condense or liquefy the CO2 in order to make it useful for refrigerating. These heat exchangers will commonly be plate-to-plate type heat exchangers similar to those used on propylene glycol systems.

    One key item to keep in mind with CO2 is that it must be refrigerated at all times, or the pressures will exceed that of the relief valves and you will lose the charge. So, a backup emergency system is highly recommended, to keep the CO2 vapor’s condensed in case of a power outage.

    The Benefits of Using CO2
    CO2 is an efficient natural refrigerant that is environmentally-friendly. CO2 that's used as a refrigerant doesn't cause any harm to the environment, as it is already produced in some other process (e.g. hydrogen production) and it is effectively a deferred release. Therefore, you can vent it directly to the atmosphere with no requirements for reclamation. It is much lower in cost than the commonly used HFC’s. By using CO2, you can significantly reduce the amount of HFC refrigerant used in a store. And the big benefit with CO2 is the improved environmental impact of the system. Moderate to significant improvements in carbon footprint and energy performance improvements are achievable with CO2. At Kysor/Warren, we are committed to developing solutions to the refrigeration challenges our industry faces today. That’s why we have developed Systems that utilize CO2 for low temperature DX and MT secondary refrigeration applications. With the environmental awareness within our industry and its proven commitment to sustainable technologies, the demand for CO2 refrigeration will no doubt increase. With the growing concern over the impact of refrigerant leaks into the environment, it is important that grocery retailers and equipment manufacturers partner to create ‘green’ solutions within the supermarket industry. By utilizing a natural refrigerant such as CO2, the effective global warming potential can be lowered by approximately 40% to even 75%, creating a significantly less harmful impact on the environment – a win-win situation for both the refrigeration industry and more importantly, our eco-system.

    Travis D. Lumpkin is vice president and systems business unit leader for Kysor/Warren. Kysor/Warren, a division of Manitowoc Foodservice, is a top manufacturer of refrigerated merchandise display cases and refrigeration systems for supermarkets and grocery retailers throughout North America. Built on a tradition of excellence, Kysor/Warren has been a part of the commercial refrigeration industry for more than 125 years and is a leader in innovative, sustainable refrigeration technologies. In October 2009, Kysor/Warren unveiled its first CO2 cascading system with grocery retailer, Food Lion, at a store in Columbus, GA. More recently, the manufacturers’ environmentally-friendly CO2 refrigeration system helped Fresh & Easy Neighborhood Market, Rosemead, CA, to open its first GreenChill certified store. In September 2010, Kysor/Warren was presented with the 2009-2010 GreenChill Distinguished Partner Award by the U.S. Environmental Protection Agency. The award recognized the company's commitment to protecting the environment through the manufacturing of eco-friendly products.

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