Multiple stage refrigeration system and control method thereof

A multi-stage refrigeration system, comprising:

• a refrigeration loop, which includes a gas suction port of a multi-stage compressor, a condenser, a first throttling element, an evaporator and an exhaust port of the multi-stage compressor which are sequentially connected through pipelines;
• an economizer branch, which includes an economizer, a second throttling element and a first control valve, the economizer having an economizer liquid inlet connected to the condenser via the first throttling element, an economizer liquid outlet connected to the evaporator via the second throttling element, and an economizer exhaust port connected to an intermediate stage of the multi-stage compressor via the first control valve;
• and
• a bypass branch, which is joined to the evaporator from the downstream of the second throttling element and connected to the condenser via the first throttling element, and on which a second control valve is arranged;
• characterized in that the first control valve and the second control valve are controlled in a linked manner, wherein when the first control valve switches on the economizer branch, the second control valve switches off the bypass branch;
• and when the first control valve switches off the economizer branch, the second control valve switches on the bypass branch;
• wherein one of the first control valve and the second control valve is a normally open control valve and the other of the first control valve and the second control valve is a normally closed control valve, wherein upon applying power, the normally open control valve is closed and the normally closed control valve is opened;
• a plurality of temperature sensors, which are respectively used for detecting an evaporation temperature and/or an exhaust temperature of the multi-stage compressor and/or an outflow water temperature of the condenser;
• wherein the multi-stage refrigeration system is configured to operate in:
• a normal mode, in which the economizer branch is switched on, the bypass branch is switched off, and the multi-stage refrigeration system operates in a multi-stage refrigeration mode;
• and
• a bypass mode, in which the bypass branch is switched on, the economizer branch is switched off, and the multi-stage refrigeration system operates in a single-stage refrigeration mode;
• when the multi-stage refrigeration system operates in the bypass mode, if a difference between an exhaust temperature of the multi-stage compressor and an outflow water temperature of the condenser is smaller than a first preset temperature difference, the multi-stage refrigeration system exists in the normal mode;
• and if the difference between the exhaust temperature of the multi-stage compressor and the outflow water temperature of the condenser is greater than the first preset temperature difference, the multi-stage refrigeration system exists in the bypass mode.