This patent is directed to an external air conditioning system connected to a parked airplane. The system has multiple refrigeration units. Each refrigeration unit has a controller. Each refrigeration unit's controlled is linked to a common controller. A single stream of air flows through all of the evaporators in series. Each of the refrigeration units has multiple compressors. Additional compressors are activated or deactivated based on refrigerant pressure. The activation or deactivation pressures are staggered across the refrigeration units, thus coordinating operation of the multiple refrigeration units.