CO2 Refrigeration Fundamentals: System Operation

Feb 22, 22 | Cold Chain

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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.


C02 Refrigeration Fundamentals

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