Gas Recirculation Fan Motor Overload Sizing

You should never over size the overload heater elements to allow for acceleration, unless you have two separate OL relays and logic to switch between them once the motor is at full speed. OL heater selection is done to prevent a fire. Over sizing them may allow severe insulation damage and / or a fire under running overload conditions.

The typical solution is to have a bypass relay on the overload circuit, controlled by a timer and an acceleration detection system (speed switch typically). On a start command, a timer is energized, which immediately closes a small contactor that bypasses (shorts) the OL CT current so that the relay doesn't see it. The timer is set to the maximum acceleration time of the motor, in your case 18 seconds. If the speed detection system is satisfied within the 18 seconds, the timer is opened and the shorting contactor is opened, so the OL relay is then in the circuit protecting the motor. If the speed detection is not satisfied in 18 seconds, the timer's timed contact changes state and locks out the starting circuit, shutting down the motor starter.

If you are in the US (assuming so by your use of HP instead of kW), the NEC allows for this kind of procedure on high inertia loads. I cannot speak to other countries' codes.

At that HP however, you should consider using a good solid state soft starter. Many of them have this kind of functionality built in to their solid state overload protection scheme since they are often used to control hard to accelerate loads like yours.