Direct-expansion (DDX) air-conditioning or heat-pump systems have an outside condensing unit with finned coils. Air is drawn past these coil banks to extract heat from the refrigerant flowing inside the coiled loops of tubing. The fins on the tubing increase the effective area of the heat- exchange surface so that more heat can be transferred to the airstream. Anything that affects this surface area reduces the heat-transfer rate and therefore lowers the efficiency of the unit.
It’s common for the fins to be damaged or disfigured. This can be fixed by combing the fins back to their original conditions and spacing. If the condensing unit is in a busy spot, such as near a walkway or other location, where the fins are constantly deformed, a metal guard should be installed to prevent this damage. These so-called hail guards are available as factory accessories from most manufacturers.
The coils can also become fouled and dirty. This reduces the efficiency of the unit, so the coils should be kept clean. This includes the fan blade, as a dash of mud solidified on the blade imbalances the fan and can cause the bearings to wear and the motor to work harder to overcome the extra friction.
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Excerpt from Energy Conservation Projects.