Combined unit refrigerating system and control method

The invention discloses a combined unit refrigerating system and a control method, and belongs to the technical field of ultralow-temperature refrigeration. The refrigerating system comprises a main control system, a liquid nitrogen refrigerating system and a plurality of double-unit multi-falling refrigerating units connected in parallel; the double-unit multi-falling refrigerating unit and the liquid nitrogen refrigerating system are both connected with the main control system and are controlled by the main control system in a unified mode. Each double-unit multi-falling refrigerating unit comprises a high-temperature compressor, a low-temperature compressor, a high-low temperature condenser, an oil-liquid separator, a stop valve, a liquid storage device, an expansion container, an exhaust pipeline and an air return pipeline. According to the refrigeration control method, the refrigeration system is adopted to enable the temperature in the box to reach the specified temperature and maintain the temperature