Reply · Sep 11, 2026
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In standard R744 transcritical systems, dropping pressure from the gas cooler to the receiver through a standard high-pressure expansion valve (HPV) wastes a massive amount of energy in the form of expansion work. Ejectors capture that wasted kinetic energy and use it to act as a "free" thermal compressor, lifting suction gas to a higher pressure.
Here is exactly what happens inside the ejector at a thermodynamic and fluid mechanics level.
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## The 4 Stages of Ejector Physics
An ejector contains no moving parts; it relies entirely on flow geometry to manipulate pressure and velocity.
### 1. The Motive Nozzle (Isentropic Expansion)
High-pressure, supercritical CO2 from the gas cooler (the "motive flow") enters a converging-diverging nozzle. As the nozzle narrows at the throat, the fluid accelerates to sonic velocity (Mach 1). As the nozzle diverges, the CO2 expands into a two-phase mixture (liquid and vapor) and accelerates to *supersonic* speeds.
**The Physics:** Potential energy (pressure) is being converted into kinetic energy (velocity). By the First Law of Thermodynamics, assuming adiabatic conditions:
$$h_{in}+\frac{1}{2}v_{in}^2=h_{out}+\frac{1}{2}v_{out}^2$$
Because enthalpy ($h$) drops drastically during this expansion, the velocity ($v$) skyrockets. The static pressure of the fluid exiting the nozzle drops significantly below the rack's suction pressure.
### 2. Suction Chamber (Entrainment)
The supersonic jet creates a localized low-pressure zone (a vacuum). This pressure differential physically pulls vapor from the medium-temperature (MT) suction header or the liquid separator into the ejector.
**The Physics:** This relies on the Venturi effect and the conservation of mass. The mass flow rate of the motive fluid ($\dot{m}_m$) and the entrained fluid ($\dot{m}_s$) are drawn together into the same channel.
### 3. Mixing Chamber (Momentum Transfer)
The high-velocity motive fluid and the low-velocity entrained vapor meet. The fast fluid drags the slow fluid along, transferring momentum and accelerating the suction gas.
**The Physics:** This is an inelastic collision governed by the conservation of momentum:
$$\dot{m}_m v_m+\dot{m}_s v_s=(\dot{m}_m+\dot{m}_s)v_{mix}$$
During this mixing phase, a complex aerodynamic phenomenon called a "shockwave" often occurs. The flow violently decelerates from supersonic back to subsonic, causing a sudden, sharp rise in static pressure.
### 4. The Diffuser (Pressure Recovery)
The mixed, subsonic fluid enters a steadily widening chamber. The flow slows down, and the remaining kinetic energy is converted back into static pressure.
**The Physics:** This is Bernoulli's principle in reverse. The fluid exits the ejector and enters the receiver at a pressure significantly higher than the original MT suction pressure, but lower than the gas cooler pressure.
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## The System Impact on the P-h Diagram
By discharging into the receiver instead of the MT suction group, the ejector moves that vapor load to the **parallel compressors** (IT compressors).
Since the receiver sits at a higher pressure (e.g., 550 psig / 38 bar) than the MT suction (e.g., 400 psig / 28 bar), the parallel compressors have a much smaller compression ratio to pump that gas back up to discharge pressure. This reduction in the required pressure lift ($\Delta P$) is what yields the massive 20-30% energy savings in warm ambient conditions, allowing R744 racks to operate efficiently even in hot climates.
