In the previous installment of our CO2 Refrigeration Fundamentals blog series, we explored how to manage CO2’s high pressures during normal operation and system shutdowns. Its unique properties also impact system design, resulting in CO2 refrigeration architectures that differ greatly from traditional hydrofluorocarbon (HFC)-based systems. In this blog, we’ll review two leading CO2 architectures and explore how to expand the potential of a CO2 system. Remember, you can also learn more about these CO2 topics in our new CO2 Chats video series.
What is a CO2 transcritical booster system, and how does it work?
A CO2 transcritical booster system is quite different than a standard HFC refrigeration rack. Medium-temperature (MT) compressors discharge into a stainless-steel discharge line designed to handle R-744’s high pressures and carry the refrigerant to a gas cooler. The pressure could reach 1,400 psi on a hot summer day, and refrigerant must be cooled before it can be condensed. So it circulates back into the building and is passed through a high-pressure valve, which drops the pressure to a useable pressure (550 psi) and deposits refrigerant inside the flash tank at 40 °F equivalent saturation.
Now the system circulates the 40 °F liquid through insulated liquid lines to feed all the MT and low-temperature (LT) cases and provide the cooling loads. The MT cases are equipped with an electronic expansion valve (EEV), and the MT suction gas feeds the three MT compressors. On the LT side, the liquid expansion valve and LT loads — which could be -20 °F — are supported by a separate set of LT compressors that discharge into the MT suction group.
In addition, the system utilizes a bypass line that’s designed to relieve the pressure on the flash tank. As ambient temperatures rise and fall, flash tank pressures can also fluctuate. Thus, the bypass line helps to release the flash tank pressure and stabilize the pressure at 550 psi. It is also designed to discharge into the MT suction group. In effect, the three MT compressors are being fed by three sources:
- The total heat of rejection from the LT compressors
- The MT suction from the evaporators
- The bypass line with the excess flash gas from the flash tank
The system is called a booster system because the LT compressors are not going directly to the gas cooler, like they would on a typical HFC system. It’s called transcritical booster because the LT compressors discharge into the MT compressors, thereby allowing the MT compressors to boost the refrigerant to the gas cooler.
What is a CO2 cascade system, and how does it work?
A CO2 cascade system offers an alternate architecture for retailers who want to deploy a low-global warming potential (GWP) option but may not want a full CO2 transcritical booster system. In a CO2 cascade system, the high (MT) and low (LT) stages are completely independent of each other, except for one heat exchanger that connects them.
Typically, the high stage would use a lower-GWP, medium-pressure HFC such as R-513A. Not only does it serve the MT loads, but it is also used to condense the CO2 in the low stage. In the low stage, CO2 is discharged, condensed in a condenser, and then recirculated for LT loads. R-744 is a very effective refrigerant for LT loads; and with a GWP of 1, it contributes to a cascade architecture that could meet many retailers’ sustainability objectives.
Can you reclaim heat in a CO2 system?
CO2 systems are excellent candidates for heat reclamation strategies. In fact, many modern CO2 systems utilize some form of heat reclaim strategy — whether it’s for providing heated air, hot water, or even heating slabs beneath freezers. Rather than burning fossil fuel to generate heat, there are a variety of scenarios whereby the heat generated from a CO2 system can be leveraged.
Compared to HFC refrigerants, another advantage of CO2 is the fact the liquid quality of R-744 is not affected when head pressures are raised to generate more heating capacity. Instead, the flash tank or receiver is designed to keep refrigerant at a consistent pressure, even while head pressures may fluctuate.
How do you optimize the coefficient of performance (COP) in CO2 systems?
To understand how to optimize COP in a CO2 transcritical booster system, we must first evaluate the function of the gas cooler and the characteristics of R-744 as it enters the supercritical zone. R-744 has a low critical point of 87.8 °F, above which the refrigerant enters a supercritical phase where the relationship between pressure and temperature becomes unpredictable.
Below the critical point, the gas cooler acts like a condenser, where the liquid-vapor interface of R-744 exists in a state of saturation. At saturation, the temperature-pressure relationship is predictable: if you know the pressure, you can determine the temperature (and vice versa). Above 87.8 °F, the pressure can change without having an impact on the temperature (and vice versa). When optimizing COP, we can take advantage of this phenomenon.
Based on the gas cooler outlet temperature, technicians can adjust the pressure to achieve the most optimum COP — or the most efficient BTUs per watt input. This is done by modulating the high-pressure valve on the system, which connects the gas cooler to the flash tank. And since this can change throughout the day as ambient temperatures change, technicians may have to continually modulate pressure accordingly. If they need to increase or decrease the system’s head pressure, simply close off or open the high-pressure valve, respectively.
To learn more about the topic of CO2 system design, please view the companion installments in our CO2 Chats video series. The next installment of the CO2 Refrigeration Fundamentals blog series will focus on energy efficiency strategies in warm climates. For more information about Emerson’s comprehensive CO2 products and capabilities, please visit Climate.Emerson.com/CO2Solutions.