Operating principle

Ejectors are based on the functional principle of a jet pump and can be used in different arrangements in a system. They generate negative pressure according to the Venturi principle (see following figure). An ejector has a characteristic curve which is similar to that of a pump; as the pressure lift increases, the delivered mass flow drops sharply.

• At the inlet of the nozzle, a refrigerant mass flow under high pressure (high potential energy) – also called motive mass flow – is accelerated.
• The conversion of the potential energy into kinetic energy decreases the pressure of the motive mass flow while increasing its velocity (see figure below).
• The pressure reduction / acceleration continues until the pressure at the outlet of the nozzle has dropped so far that suction / negative pressure is created at an inlet located there.
• A secondary refrigerant mass flow − also called suction mass flow − is thus sucked in and carried along.
• The mass flows mix and the kinetic energy of the motive mass flow is converted into potential energy of the accumulated mass flow – also called outlet mass flow.
  This causes an increase in pressure above the pressure of the suction mass flow.

Applied to transcritical applications with R744, this means that the ejector uses the potential and kinetic energy present in the refrigerant at the gas cooler outlet to suck in another partial mass flow and bring it to a higher pressure level (see following figure).

• The R744 [2] leaving the compressor at high pressure level is accelerated in the ejector nozzle [3] after gas cooling / heat dissipation in the gas cooler [2].
• As a result, the static pressure drops and the pressure of the flow leaving the nozzle is lower than the suction gas pressure of the medium temperature compressor stage [4].
• This allows gas and/or liquid to be selectively extracted from a low pressure level [5].
• Both partial flows mix in the mixing chamber upstream of the diffuser [6].
• In the diffuser, the flow is decelerated again, thus increasing the pressure to intermediate pressure level [6].
• Downstream of the diffuser, the mixture is fed into the intermediate pressure vessel, the gas phase is separated [1] and compressed to a high pressure level [2].