Motor external type, can be used for fire smoke exhaust, also can be used for general ventilation. If the two-speed motor is used, the fan can be operated at a low speed and used for general ventilation. In case of fire, it automatically turns into high speed operation, which is used for fire fighting and smoke exhaust. Motor built-in, can only be used for general ventilation. Because the noise of type B machine is lower than that of type A machine, so only when the fan is only used for ventilation, type B machine can be preferred.