By Phil Boudreau
One of the main challenges with using carbon dioxide as a refrigerant is its low critical point of approximately 87.8°F or 31°C. Historically, when the critical point of a refrigerant is lower than the ambient air, it was dismissed as a viable refrigerant. However, Gustav Lorentzen, a professor at the Norwegian University of Science and Technology, proposed that refrigerant carbon dioxide could be used in its supercritical state at heat sink temperatures which exceed the critical point. During 1988 to 1991, he developed a transcritical cycle using supercritical CO2.
The primary reason for exploiting CO2 in this manner is that it has no ozone depletion properties and has a global warming potential (GWP) of 1. Additionally, it is non-toxic in the classic sense and non-flammable. This makes CO2 a great alternative in situations where a flammable, natural refrigerant such as ammonia cannot be used. There are different grades of carbon dioxide. Carbon dioxide that has an extremely low moisture content and is very pure, is used for refrigeration and air conditioning applications. Carbon dioxide used in these applications is designated as R744 refrigerant.
Supercritical fluid is not a liquid, and it is not a vapour. As R744’s pressure and temperature and/or temperature reach the critical point, the density of the vapour and liquid become equal. As the refrigerant is forced beyond its critical point, we can say that the R744 transcends the critical point and becomes a supercritical fluid. This is where the term transcritical was derived from. Supercritical fluid has the high density of a liquid and the diffusivity of a vapour. Note that a vapour expands to fill the container which surrounds it. Supercritical fluid has this same property.
Fluids have been used in their supercritical fluid state for several years. One example of an application that employs a substance in this state is a carbon dioxide supercritical fluid extraction process in which the supercritical fluid is used to extract essential oils from plants and such. Another application or supercritical fluid CO2 is the decaffeination of coffee beans, tea leaves, and so on.
Transcending the critical point
In a refrigeration process involving vapour compression, transcending the critical point requires a compressor that can increase the discharge pressure to a point somewhat above the critical point. Since the critical pressure of carbon dioxide is 1,070 psia, this means the compressor must be able to lift the pressure to some level above this. For example, with a gas cooler outlet temperature of 80°F, the pressure will be approximately 1,088 psia. When the gas cooler outlet temperature is 90°F, the discharge pressure will be approximately 1,155 psia. If the ambient temperature was such that the gas cooler outlet temperature increases to 100°F, then the discharge pressure would be approximately 1,350 psia.
The discharge pressure is not set to some arbitrary value, but instead, an optimum pressure that results in the highest coefficient of performance for a given ambient temperature. This optimum pressure is determined by the gas cooler operating temperature difference (TD) and the ambient temperature. Therefore, the sum of both the ambient temperature and the gas cooler TD equals the gas cooler outlet temperature.
Between the outlet of the gas cooler and the receiver, a stepper-motor-driven high-pressure regulator is used to regulate the discharge pressure. As the pressure is reduced, and once the pressure drops to a point that lies on the saturated liquid curve of the pressure-enthalpy diagram, the supercritical fluid changes state. Specifically, flash gas and liquid begin to form. As we would expect, more flash gas will form as the pressure is further reduced as the liquid R744 cools itself. The higher the pressure drop, the higher the percentage of flash gas that will enter the receiver. Liquid of course accumulates in the bottom of the receiver while the vapour occupies the space above the liquid. Since the volume occupied by the flash gas is determined by the remaining space in the receiver that is not occupied by liquid, the pressure will increase as the system operates.
At some point, we must remove some flash gas from the receiver. This is to prevent the pressure within the receiver from exceeding a certain point. Typically, the receiver pressure is at a level that corresponds to a saturation temperature that is 10 to 25°R above the saturated suction temperature (SST) of the medium temperature (MT) suction group. This often corresponds to a receiver pressure that is typically within the 500 to 600 psia range. Aside from the high-pressure valve between the gas cooler outlet and the receiver inlet, a flash-gas bypass valve is used for the purpose of venting the receiver to the suction of the MT compressors.
As we know, higher suction pressures always increase the efficiency and mass flow rate of compressors. Since the flash gas bypass valve reduces the receiver pressure to the MT suction pressure, more compressor displacement is required for a given flash gas flow rate.
A modified version of the basic flash gas bypass system described earlier is the flash gas bypass process, which also utilizes a parallel compressor group. These parallel compressors are sized such that they will be able to handle the total flash gas load at design conditions. This means that the parallel compressors will be loaded up when the ambient temperature is at its highest and the load on the evaporators is at its maximum.
Since the parallel compressors are dedicated to removing the flash gas from the receiver, this suction group will operate at a higher pressure than the medium temperature. This of course increases the overall efficiency of the booster system. In fact, utilizing parallel compressors in a system with a 95°F gas cooler outlet temperature is expected to increase the efficiency of the booster system by 19 to 20 per cent.
Parallel compression is one way to increase the efficiency of a transcritical compression process. An evaporative gas cooler is another form. Ejectors may also be used in conjunction with parallel compressors. With this approach, some of the flow to the MT compressors is diverted to the parallel compressors as the ejector lifts the MT pressure to the receiver tank pressure. This will be explored in a future article.