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CO2 Refrigeration Fundamentals: System Operation

Andre Patenaude | Director – Solutions Strategy

Emerson’s Commercial and Residential Solution’s Business

Welcome to the third installment in our CO2 Refrigeration Fundamentals blog series, based on our companion CO2 Chats educational videos. Previously, we reviewed the distinguishing properties of CO2 (or R-744) and servicing tips for technicians working on CO2 transcritical booster systems. Today, we’ll explore some of the basic principles that end-users and contractors need to know when operating these systems.

What do “CO2 subcritical” and “transcritical” modes mean?

When evaluating CO2 systems, it’s important to first understand the difference between subcritical and transcritical modes of operation. Subcritical refers to when R-744 is at saturation and below the critical point of 87.8 °F. Above this temperature, R-744 is no longer at saturation and is operating in transcritical mode. Unlike traditional hydrofluorocarbon (HFC) systems — like those using R-404A with a critical point of 162 °F and thus, no chance of occurring — it’s possible that CO2 systems could operate for extended periods above 87.8 °F in a typical year.

Why is high-pressure CO2 management so important?

As a refrigerant, R-744 is very dynamic and reacts quickly to changes in pressures and temperatures. When setting up a CO2 transcritical booster system, it’s important to establish a stable baseline of performance during the commissioning process. To achieve this, technicians should focus their efforts on the configuration of the gas cooler, high-pressure valve and flash tank, including:

  • Controlling variable fan speeds
  • Modulating the high-pressure valve
  • Optimizing the coefficient of performance (COP)
  • Maintaining a consistent flash tank pressure

In a CO2 transcritical booster system, all of these aspects are managed by electronic controls. To ensure successful system operation, technicians will need to take their time and fine-tune these controls such that the gas cooler, high-pressure valve and flash tank are all set up perfectly.

How can you preserve CO2 system charge during a shutdown?

Power outages and system shutdowns can have tremendous impacts on a CO2 transcritical booster system. In the event of an extended shutdown, CO2 transcritical booster systems are designed with pressure-relief valves in each zone to allow for the charge to be released. But for brief shutdowns, system pressures typically don’t rise quickly enough to cause concern. This is because the system’s existing thermal inertia — from the full flash tank, insulated liquid lines and still-cold evaporators — will sustain it for a period of time, especially if it’s not during a hot summer day.

Another method for keeping pressures in check during extended power outages — and preventing the release of system charge in a CO2 transcritical booster system — is through an auxiliary condensing unit powered by a backup generator. The condensing unit utilizes a special plate heat exchanger coil that is connected to the system’s flash tank or receiver. When the system is powered down for a long time, the warm gas from the receiver circulates through the cold plate heat exchanger, cools the flash tank vapor, condenses it into a liquid, and returns it to the flash tank at a colder temperature. This keeps the system running properly while preventing the loss of refrigerant through pressure-relief valves.

How do you prevent CO2 evaporators from flooding during a power outage?

To prevent CO2 evaporators from flooding during a power outage, CO2 transcritical booster systems are designed with specific mitigation strategies. Because mechanical expansion valves are not well suited for CO2’s high pressures and variable nature, systems are equipped with either stepper-motor electronic expansion valves (EEVs) or pulse-width modulated EEVs.

A stepper-motor EEV will pause in the exact position it is in when the power drops. So if it’s partially open, it could conceivably flood the evaporator. Strategies to prevent this include placing a solenoid valve at the inlet of the evaporator that closes upon losing power or installing a battery backup on the case control to shut off the flow.

The pulse-width modulated EEV is a direct-acting valve that is based on a redesign of a solenoid valve intended for millions of cycles. When the power drops, it automatically closes and prevents liquid from flooding the evaporator, thereby protecting the compressor when power comes back on. Before the system is up and running again, service contractors should make sure that they clearly understand how the valves will operate and/or close — and that any floodback mitigation strategies have worked as expected.

For more information on any of aspect of CO2 system operation, please view the companion topics in our CO2 Chats video series. The next installment of the CO2 Refrigeration Fundamentals blog series will focus on system design principles. To learn more about Emerson’s comprehensive CO2 products and capabilities, please visit









Keys to Servicing A2L Refrigerants

         Don Gillis | Lead Technical Trainer

          Emerson’s Educational Services

The refrigerant transition is underway, and HVACR service technicians find themselves at the leading edge. As the commercial refrigeration and AC industries move from high-global warming potential (GWP) hydrofluorocarbon (HFC) refrigerants to lower-GWP alternatives, mildly flammable A2L refrigerants are viewed as viable alternatives. But the technician community is largely unfamiliar with A2L servicing requirements and has many questions that need to be answered. I recently participated in an article for RSES Journal, in which we discussed the emergence of A2Ls and reviewed key servicing best practices. You can also view the article here.

Regulatory efforts to approve A2L refrigerants took several steps forward in 2021. The Environmental Protection Agency’s (EPA) Significant New Alternatives Policy (SNAP) Rule 23 approved R-454B and R-32 for use in residential AC applications, subject to use conditions. In addition, the UL approved the second edition of its UL 60335-2-89 standard that included A2L charge limit guidelines for self-contained and remote refrigeration systems. Although the UL 2-89 update was a major development, more regulatory approvals will be required to roll out A2Ls on a broader scale. Industry stakeholders expect EPA guidance and SNAP approvals for the use of A2L refrigerants in commercial refrigeration to happen soon.

But if you’re an HVACR technician, the chances of encountering A2Ls are on the rise. To maximize safety and assist your customers with installation and service calls, now is the time to gain a better understanding of A2Ls.

Back to basics with best practices

Thankfully, the transition from existing refrigerants to A2Ls won’t require a fundamental shift in the way you conduct service calls. But it will require more rigorous attention to basic servicing fundamentals. Existing recommended best practices for A1 refrigerants will apply — with the addition of a few special considerations and A2L-rated tools.

The potential for flammability makes the use of leak sensors and detection equipment a more important system consideration with A2Ls. Otherwise, A2Ls have very similar characteristics and pressures as common A1 HFC refrigerants, such as R-410A. It’s also important to be aware that some blended refrigerants, such as R-454B, will have a degree of glide.

When installing or repairing A2L refrigerant-based equipment, technicians will need to use A2L-rated gauges and tools and wear proper personal protective equipment (PPE). Compared to A1 procedures, there are some required steps when dealing with A2Ls that are considered best practices for A1 systems:

  • Purge the circuit with inert gas (i.e., oxygen-free nitrogen).
  • Evacuate the refrigerant.
  • Leak-test and pressure-test the unit.

A2L cylinders have the same rated pressure as current R-410 cylinders. To make sure an A2L refrigerant is not mistaken for an A1, A2L tanks have several distinguishing characteristics, including:

  • Pressure relief valve is designed to release only enough refrigerant to reduce the cylinder pressure.
  • Red band/stripe (or the entire top painted red) indicates the presence of a mildly flammable refrigerant.
  • Left-hand (LH) thread indicates the presence of an A2L refrigerant.

It’s important to remember that all HVACR equipment must be designed and rated for the use of A2L refrigerants. As such, A2Ls are not to be used as drop-in replacements for A1s in existing HFC systems. When charging refrigeration systems with an A2L, technicians must ensure that they do not exceed the maximum allowable charge rate.

Look for safety labels on A2L-based HVACR equipment to alert you of additional precautions. Some may also include a panel designed to cover service ports. For more information, please visit the Air-Conditioning, Heating, & Refrigeration Institute’s (AHRI) Safe Refrigerant Transition Task Force website (

A2L training is available

As A2L refrigerants make their way into U.S. AC and commercial refrigeration applications, industry organizations, manufacturers and stakeholders are working together to prepare for their wider adoption. At Emerson, we are actively developing A2L-certified compressors, condensing units and components to support the transition to lower-GWP refrigerants in commercial refrigeration and residential AC applications.

In addition, Emerson Educational Services is developing and conducting A2L training seminars as part of our “Fit for the Future” initiative. To prepare your service team to safely install, service and recover A2L refrigerants, please visit our course schedule.





CO2 Refrigeration Fundamentals: Servicing Tips

Andre Patenaude | Director – Solutions Strategy

Emerson’s Commercial and Residential Solution’s Business

In the first blog in this series, we discussed the many distinguishing properties of CO2 (or refrigerant R-744) — including its high system pressures, low critical point and triple point. These characteristics introduce a multitude of unique servicing considerations that differ significantly from traditional hydrofluorocarbon (HFC)-based systems. In this installment, we’ll review some key tips that technicians need to be aware of when servicing CO2 transcritical booster systems. You can also learn more about a variety of related CO2 topics in our new CO2 Chats video series.

How do you store CO2 refrigerant?

From a refrigerant storage best practices perspective, R-744 tank storage is similar to standard HFC storage, including stacking procedures, safety precautions and keeping them chained off in a designated storage area. But that’s where the similarities end. Because CO2 tanks are designed to handle its high pressures, they weigh significantly more than standard HFC bottles. Empty CO2 tanks can weigh close to 150 lbs.; when loaded with 50 lbs. of refrigerant, each cylinder can potentially weigh nearly 200 lbs.

Many supermarkets prefer to have an entire system charge on hand, which could potentially be up to 2,000 lbs. Storing that would require 40 cylinders totaling a weight of 8,000 lbs. — or 4 tons. It’s important for contractors to understand where to store the reserve refrigerant and if it will affect building codes by having that much CO2 in one space. And if stored on a mezzanine, it must be capable of handling the total storage weight.

How do you charge a CO2 refrigeration system?

When charging a CO2 refrigeration system, the most important consideration a technician should keep in mind is the triple point pressure of CO2. 60.4 psi is the pressure at which CO2 will turn to dry ice. As a result, contractors must be careful not to charge with liquid CO2 when the system is below this pressure, and instead charge with vapor until the system reaches triple point. Failure to do so will result in the formation of dry ice. There are various anecdotes about technicians — who are more familiar with charging HFC systems — charging a CO2 system with liquid and causing the formation of dry ice.

Begin charging by introducing CO2 vapor into the system, and then build system pressure to 60.4 psi and beyond based on equipment manufacturer recommendations — up to 145 lbs. Then, it will be safe to switch to liquid CO2 to finish charging the system quickly and effectively without the risk of dry ice formation.

What is trapped liquid in a CO2 refrigeration system?

CO2’s coefficient of expansion (COE) is higher than a typical HFC refrigerant. One potential scenario that can occur in a CO2 system is when liquid refrigerant gets trapped in between two valves. In this instance, the pressure can increase 145 psi for every 1.8 °F increase in temperature. As a result, some systems may need to be fitted with appropriate pressure relief valves at the location of the trapped liquid to assist with system operation and service.

How do you detect leaks in CO2 systems?

Since there is an abundance of CO2 already present in the atmosphere, R-744 refrigerant can be difficult to detect and requires the use of a capable leak detection system. CO2 is colorless, odorless and heavier than air, requiring leak detectors to be mounted 18 inches off the ground and below the breathing level. Like HFC systems, it’s important to immediately detect and mitigate CO2 leaks as they occur.

Manufacturers such as Emerson have designed CO2-specific leak detection technology that quickly can sense the presence of higher levels of carbon dioxide in a machine room or a walk-in box. Emerson offers both a stand-alone CO2 leak detection solution as well as devices that can be seamlessly integrated into a building management system (BMS), such as the Lumity™ supervisory control platform.

Are there safety issues to be aware of when handling CO2 refrigerant?

Because CO2 refrigeration systems operate at extremely high pressures, technicians should take precautions when handling CO2. Even when the system is shut off, standstill pressures are extremely high and need to be handled carefully. In addition, CO2 can displace oxygen and release it in excessive amounts because it’s heavier than air. As a result, technicians should avoid handling it in confined spaces. But with proper training and equipment design, CO2 can be used safely.

For more information on CO2 servicing tips and best practices, please view the companion topic in our CO2 Chats video series. The next installment of the CO2 Refrigeration Fundamentals blog series will focus on system operation. To learn more about Emerson’s comprehensive CO2 products and capabilities, please visit




CO2 Refrigeration Fundamentals: Properties of R-744

Andre Patenaude | Director – Solutions Strategy,

Emerson’s Commercial and Residential Solution’s Business

The use of CO2 (or R-744) in commercial refrigeration is expected to rise significantly over the next several years. With a global warming potential (GWP) of 1 and zero ozone depletion potential (GWP), CO2 can help food retailers to meet their sustainability goals and comply with emerging regulations. Because the fundamentals of CO2 refrigeration are quite different from traditional hydrofluorocarbon (HFC)-based systems, Emerson has compiled a video series that seeks to address common questions and misperceptions about them. As a companion to the CO2 Chats Series — which features candid interviews with me and Derek Langenkamp, Emerson’s product manager of semi-hermetic and CO2 — we’re launching a five-part CO2 Refrigeration Fundamentals blog series. In this first installment, we’ll look at R-744’s unique properties and how it differs from HFC refrigerants.

What is CO2’s critical point?

Among its many distinguishing characteristics, R-744 has a low critical point of 87.8 °F. Although that may not seem like a low temperature, it is from a refrigeration perspective. What this means is that when the temperature rises above 87.8 °F, the refrigerant enters a supercritical fluid phase. In this supercritical zone, its pressure and temperature relationships no longer match — where each can rise and fall independently of the other. R-744 is at saturation when it is below the critical point, which is referred to as subcritical mode; above 87.8 °F, R-744 is no longer at saturation and operates in transcritical mode.

Does CO2 have high operating pressures?

CO2 system operating pressures are significantly higher than those with HFCs such as R-404A or even R-410A. Because of this, service technicians often have reservations when working on CO2 transcritical booster systems. Understanding where those pressures occur within a system can give contractors a greater comfort level. Medium-temperature (MT) compressors discharge onto the roof at the gas cooler. Thus, on a 95 °F ambient day, that pressure will be around 1,400 psi typically within a stainless-steel discharge line coming back into the building.

Once in the building, this high-pressure refrigerant goes through an expansion valve into a receiver flash tank, reducing the pressure to around 550 lbs. It is then carried through a liquid line to an MT evaporator, which at about 23 °F is 425 psi. The low-temperature (LT) evaporator operates near -20 °F and will be at an even more reduced pressure of 200 psi. Although system pressures are higher than what most technicians are accustomed to, proper training and tools should relieve concerns some have related to CO2’s high operating pressures.

What is CO2’s triple point?

CO2’s triple point is the point at which the refrigerant’s gas, liquid, and solid-state coexist — which occurs at -69.8 °F and 60.4 psi. Since -69.8 °F is well outside of normal operating ranges, you might wonder why the triple point is a consideration. If the proper steps and cautions are not taken, a CO2 system can reach 60.4 psi or lower. When this occurs, R-744 is likely to turn into dry ice. Once dry ice forms in a refrigeration system, it stops the refrigerant flow and may cause a variety of potential problems.

What are the differences between CO2 and HFCs?

R-744 has other unique performance and operating characteristics that differentiate it from HFCs and dictate system design. It has a higher density than a typical HFC refrigerant, which translates into the use of smaller compressors. However, the motor is similar in size since the work being done is approximately the same. CO2’s higher density means that smaller pipe diameters can be used, especially on the suction side of the system. Due to its high pressures, system components need to be rated to tolerate a higher maximum pressure rating.

For more information on any of CO2’s unique properties, please view the companion topic in our CO2 Chats video series. The next installment of the CO2 Refrigeration Fundamentals blog series will focus on servicing tips. To learn more about Emerson’s comprehensive CO2 products and capabilities, please visit


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