System design B
with ejector, parallel compression and mechanical high-pressure control valve for emergency operation

• Parallel to the ejector, a mechanical high-pressure control valve can optionally be installed for emergency operation.
• In the intermediate pressure vessel, the gas and liquid phases are separated ① (see information). The liquid from the intermediate pressure vessel is fed as usual to the evaporators of the low temperature and medium temperature compressor stages.
  After expansion and heat absorption in the evaporators, the mass flow of the evaporators of the low temperature compressor stage is absorbed by the low temperature compressor stage and re-compressed to the suction pressure level of the medium temperature compressor stage.
• On the suction side of the medium temperature compressor stage, either:
• a superheat control is used and the suction mass flow of the ejector can be sucked in directly in gaseous form or
• a separator is used for operation with a semi flooded evaporator, which enables operation of the medium temperature cooling points at low superheat.
  In this case, the ejector is a liquid ejector that delivers the share of the excess liquid from the evaporators of the medium temperature compressor stage back into the intermediate pressure vessel. An oil return line must be connected to the separator!

During operation, also ensure that:

• the ejector controls the optimum high pressure via a control characteristic even during part load operation with very low load and at low outside temperatures.
• the mass flow ratio (entrainment) is influenced by this high pressure control. This can result in no suction mass flow being delivered by the ejector during part load operation!

Two different operating modes of the system are possible:

1. Operation with high pressure ejector(s) with high load requirements and gas cooler outlet temperatures > 25°C:
• The evaporator mass flow of the medium temperature compressor stage is absorbed by the ejector(s) after expansion and heat absorption in the evaporators and forms the suction mass flow. This flow is then compressed from medium temperature suction pressure to intermediate pressure.
• The flash gas bypass valve between the intermediate pressure vessel and the medium temperature suction side is closed, which relieves the medium temperature compressor stage.
• The pressure lift generated by the ejector compression stage reduces the pressure ratio for the compression of the mass flow (in terms of thermodynamics, this is referred to as a "temperature lift"). At the same time, the suction gas density increases, allowing the energetic advantage of parallel compression to take effect. The pressure in the intermediate pressure vessel is controlled by the parallel compressor stage.
• In systems with semi flooded evaporators and a separator, the ejector only has to transport the excess liquid back to the intermediate pressure vessel. The energetic advantage here lies mainly in the increased evaporation temperature due to the full utilisation of the heat transfer surface. A superheat control is not necessary in this case.
2. Standard operation without high pressure ejector(s) with low load requirements and gas cooler outlet temperatures < 25°C:
• During standard flash gas bypass operation, the control valve (e.g. solenoid valve or motor valve) is closed upstream of the ejector's suction connection.
• The parallel compressor stage is out of operation and the flash gas bypass valve controls the pressure in the intermediate pressure vessel by expanding the flash gas to medium temperature suction pressure.
• The mass flow of the medium temperature evaporator and the flash gas mass flow are picked up and compressed by the medium temperature compressor stage.
• To enable operation of the medium temperature compressor stage at low loads and low ambient temperatures at lower high pressures, it may be advantageous to install a high-pressure control valve in parallel with ejector(s) (see the following simplified diagram).