Method for controlling suction pressure of a vapour compression system

A method for controlling a vapour compression system, the vapour compression system comprising a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, an ejector, a receiver, at least one expansion device and at least one evaporator arranged in a refrigerant path, an outlet of the heat rejecting heat exchanger being connected to a primary inlet of the ejector, an outlet of the ejector being connected to an inlet of the receiver, and an outlet of the evaporator being connected to a secondary inlet of the ejector and to an inlet of the compressor unit, wherein the vapour compression system further comprises a non-return valve arranged in the refrigerant path between the outlet of the evaporator and the inlet of the compressor unit, in such a manner that a refrigerant flow from the outlet of the evaporator towards the inlet of the compressor unit is allowed, while a fluid flow from the inlet of the compressor unit towards the outlet of the evaporator is prevented, wherein a gaseous outlet of the receiver is connected to the inlet of the compressor unit via a bypass valve, the method comprising the steps of:

• measuring a pressure, P0, of refrigerant leaving the evaporator,
• obtaining a value being representative for a pressure, Psuc, of refrigerant entering the compressor unit,
• comparing the pressures, P0 and Psuc, to respective reference pressure values, P0,ref and Psuc,ref,
• determining ε0=P0−P0,ref and determining εsuc=Psuc−Psuc,ref,
• in the case that ε0>εsuc, controlling the compressor unit based on P0, and
• in the case that εsuc>ε0, controlling the compressor unit based on Psuc.