Screw chillers have economizer systems that include a low pressure economizer and a high pressure economizer. According to certain embodiments, the low pressure economizer includes a flash tank, an expansion device and a flow control valve while the high pressure economizer includes a heat exchanger and an expansion device. The screw chillers also include a screw compressor that compresses refrigerant. The screw compressor includes a low pressure economizer port designed to receive lower pressure refrigerant from the flash tank within the low pressure economizer and a high pressure economizer port designed to receive higher pressure refrigerant from the heat exchanger within the higher pressure economizer. The screw compressor is designed to compress the refrigerant received from the evaporator, the refrigerant received through the low pressure economizer port, and the refrigerant received through the high pressure economizer port and to discharge the refrigerant through a common discharge.