CO2 natural refrigerant trans-critical operation

By Mike Squires

Pressure put on the refrigeration industry to use environmentally friendly and non-toxic refrigerants has resulted in carbon dioxide (CO2) R-744 being resurrected from almost full obscurity to a new normal or commonly specified refrigerant in applications such as grocery store refrigeration systems. R-744 is classified as an “A1” refrigerant, “A” for non-flammable and “1” for non-toxic. This is the same classification as many conventional legacy and current synthetic refrigerants. The bonus for R-744/ CO2 is it is considered environmentally friendly when compared with other A1 synthetic refrigerants.

CO2‘s critical challenge

Despite being environmentally friendly, CO2 has some challenges – one being its low critical temperature of 31⁰C (87.8⁰F). Critical temperature is the temperature at which the fluid cannot condense to become a liquid. The efficient evaporation of liquid refrigerant at specific target temperatures is the main objective in a typical direct expansion (DX) system vapour compression refrigeration cycle.

When the outdoor air is near or above 31⁰C (87.8⁰F) you cannot produce (condense) the liquid refrigerant in the condenser. This liquid refrigerant would travel to the system’s refrigerant receiver by using the air-cooled system’s outdoor condenser coil fans alone. In a CO2 system the outdoor fan/coil cannot always condense the refrigerant. This is why the air-cooled condenser in a CO2 system is commonly referred to as a gas cooler. When the system is operating above the critical temperature of the refrigerant it is in a transcritical state. The system would now be operating in what is called a “trans-critical” or “TC” state of operation.

In colder climates, reducing the pressure of the receiver is a common way to overcome the inability of using outdoor air to condense the refrigerant. This is usually done using electric regulation valves. One will control the refrigerant pressure in the gas cooler and another regulates the pressure in the receiver, also known as a “flash tank” in a TC CO2 system. The gas cooler outlet pressure regulating valve, also known as a high-pressure valve (HPV) or throttle valve, controls or throttles the flow of the CO2 into the flash tank/receiver. The flash tank/receiver pressure regulation valve, also called a flash gas bypass valve (FGBV), regulates the pressure of the flash tank by venting vapour off the top of the flash tank to the suction side of the refrigeration compressors. When this venting takes place, the pressure drops in the flash tank and the refrigerant in the tank condenses to a liquid that can be used for the refrigeration cycle.

To simplify, liquid refrigerant cannot form in the gas cooler so as the super-critical CO2 enters the flash tank compressors are required to lower the pressure of the flash tank/receiver to form useful liquid CO2 refrigerant. This is always accomplished by programmed computer controllers using temperature sensors and pressure transmitters to manage the requirements to control the system.

Penalty … 5 minutes for instigating extra energy use

Using a hockey penalty analogy for TC operation of a refrigeration system is fair. You can see that instead of using smaller HP/kW fans outside in a typical air-cooled condenser, we now rely on the use of higher HP/kW refrigeration compressors to help produce the liquid refrigerant required. That in itself is one of the penalties of CO2. There are things you can do to reduce the penalty to a two-minute minor by helping reduce the amount of refrigerant that is directed through the flash gas bypass valve to the compressors. Equipment such as adiabatic condensers or evaporative condensers could help lower the condensing air temperature below the critical point. Mechanical devices such as ejectors can be used to lower pressures.

Another option is the implementation of parallel compressor groups operating at a higher saturated compression temperature/pressure for efficiency or having additional local chilling sources. Even with some energy penalties CO2 systems can offer additional savings on an annual basis since the BTU per pound of CO2 is very high compared to many synthetic refrigerants. CO2 systems will quite often outperform many older legacy systems.

New materials, new skills

There are things to learn about the installation and upkeep of CO2 systems. TC-CO2 systems often have sections of the systems with pressure ratings up to 1,305 psi/90 bar or 1,740 psi/130 bar. With this comes the need for different materials and different skills than were commonly used and required in traditional synthetic
refrigeration systems.

Regular “Type-L” and “Type-K” copper tubing is no longer satisfactory for the higher pressures in many applications. Steel pipe, stainless steel pipe and stainless steel tube are commonplace in TC-CO2 systems. Installation of those materials requires different skill sets such as TIG welding or robotic orbital welding. There are modern copper-iron alloy pipe and fittings tested to withstand higher pressures, which allow for the use of standard copper joint brazing. You do need to verify the materials selected have the proper certifications and approvals for use in the regulatory jurisdiction of the installation.

Problems and break downs

I recall receiving a call for technical assistance for an odd alarm problem. The problem was abnormally high discharge pressure, which caused the stoppage of the refrigeration compressors. The field technician found that the gas-cooler/condenser outlet pressure regulator was in a fully-closed state and concluded this valve must be the problem. It appeared a new HPV valve was required. However, after working with the tech and asking questions based on my experience, I asked the tech to have a look at the receiver/flash gas pressure and see how it was.

They found that the receiver pressure was up and down and a bit irregular. With that I offered advice for the tech to look at the FGBV and check its operation, thinking that valve was likely the culprit causing the high pressure in the discharge. It took a bit of convincing but after all was done it was the worn FGBV that caused the issue. The FGBV is not directly related to the discharge pressure but if the FGBV cannot regulate the pressure properly, the computer HPV senses the pressure is too high. It will close off and not allow any more refrigerant to enter the flash tank. What looked to be a problem HPV valve, was actually OK. It was doing its job. The FGBV was the villain and was replaced.

Mike Squires, RSE, is manager, service accounts and technical training at Neelands Group Limited in Burlington, ON, where he works with customers and in-house staff on all matters relating to refrigeration. He can be reached at [email protected].

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