What is the relationship between temperature in cold room and evaporation temperature?

The temperature controller shows the cold room temperature, and the low pressure gauge corresponds to the evaporation temperature.

For the cold room with static evaporator, the air in the cold room is in a state of natural convection, and the evaporation temperature is required to be 10~15℃ lower than the cold room temperature. For the cold room with air cooler, the air in the cold room is in the state of forced convection, and the evaporation temperature is required to be 5-10 ℃ lower than the cold room temperature.