Read the following technical passage carefully. Based on the provided text, what minimum distance must technicians maintain around the external exhaust louvers to ensure proper airflow?
Combat field refrigeration units (CFRUs) maintain critical medical supplies and whole blood within a strict temperature range of 2°C to 6°C during tactical deployments. When ambient temperatures exceed 40°C, the unit's secondary cooling condenser activates automatically to prevent thermal overload. To ensure proper airflow, technicians must maintain a minimum clearance of 18 inches around the external exhaust louvers. If the condenser fan encounters an obstruction, the unit sounds an audible alert and switches to auxiliary power to protect internal inventory.