A refrigeration system controller for controlling operation of a free-cooling system and a mechanical cooling system , the refrigeration system controller comprising: - a memory configured to store instructions for operating the free-cooling system and the mechanical cooling system ; - a processor configured to execute the instructions, wherein the instructions comprise: - determining one or both of a temperature of ambient air, by means of a feedback received from a temperature sensor configured to monitor the temperature of ambient air, and a cooling load demand; - operating a fan of an air-cooled heat exchanger of the free-cooling system at a first speed based at least on one or both of the temperature of ambient air and the cooling load demand; - adjusting the first speed of the fan based on changes of at least one or both of the temperature of ambient air and the cooling load demand; and - initiating, when the first speed reaches a threshold speed, operation of a variable speed compressor of the mechanical cooling system at a compressor speed based on one or more of the temperature of ambient air, the cooling load demand, and the first speed of the fan before operating the fan at a maximum speed, wherein the threshold speed is less than the maximum speed. - a memory configured to store instructions for operating the free-cooling system and the mechanical cooling system ; - a processor configured to execute the instructions, wherein the instructions comprise: - determining one or both of a temperature of ambient air, by means of a feedback received from a temperature sensor configured to monitor the temperature of ambient air, and a cooling load demand; - operating a fan of an air-cooled heat exchanger of the free-cooling system at a first speed based at least on one or both of the temperature of ambient air and the cooling load demand; - adjusting the first speed of the fan based on changes of at least one or both of the temperature of ambient air and the cooling load demand; and - initiating, when the first speed reaches a threshold speed, operation of a variable speed compressor of the mechanical cooling system at a compressor speed based on one or more of the temperature of ambient air, the cooling load demand, and the first speed of the fan before operating the fan at a maximum speed, wherein the threshold speed is less than the maximum speed. - determining one or both of a temperature of ambient air, by means of a feedback received from a temperature sensor configured to monitor the temperature of ambient air, and a cooling load demand; - operating a fan of an air-cooled heat exchanger of the free-cooling system at a first speed based at least on one or both of the temperature of ambient air and the cooling load demand; - adjusting the first speed of the fan based on changes of at least one or both of the temperature of ambient air and the cooling load demand; and - initiating, when the first speed reaches a threshold speed, operation of a variable speed compressor of the mechanical cooling system at a compressor speed based on one or more of the temperature of ambient air, the cooling load demand, and the first speed of the fan before operating the fan at a maximum speed, wherein the threshold speed is less than the maximum speed. <EOS>