"220V AC Cooling Fan" on occasion to poor ventilation conditions: heat sink surface density of less than 0.024W / cm2, can be naturally cooled. For better ventilation of the occasion, the heat flux "220V AC Cooling Fan" radiator surface is greater than 0.039W / cm2 and less than 0.078W / cm2, must use forced air cooling. On occasion to poor ventilation conditions: "220V AC Cooling Fan" radiator surface heat flux density greater than 0.024W / cm2 and less than 0.078W / cm2, must be forced air cooling."