A refrigeration system includes a controllable valve positioned downstream from one or more low-temperature compressors. The controllable valve receives compressed refrigerant from the low-temperature compressors and directs flow of the refrigerant to one or both of (i) medium-temperature compressors downstream from the controllable valve and (ii) one or more evaporators based on an operation mode of the evaporators. A pressure-regulating valve is disposed in refrigerant conduit coupling a first flash tank to a second flash tank. After determining to operate a first evaporator in a defrost mode, a controller adjusts the controllable valve to direct a portion of refrigerant to the first evaporator and adjusts the pressure-regulating valve to increase a pressure of the first flash tank relative to that of the second flash tank.