The discharge of refrigerant from the compressor causes some small amount of compressor oil to be circulated with it. In heavier commercial units, the amount may be great enough to cause damage to the compressor if it runs with insufficient lubrication. This oil must be collected and returned to the compressor to maintain proper lubrication. An oil separator is installed between the compressor and the condenser. The separator screens out and collects the oil, and when it reaches a certain level, the float mechanism permits the oil to return to the compressor. This happens because the pressure in the oil separator is considerably higher than the pressure in the compressor crankcase. It is returned to the compressor through the oil return line connected directly to the compressor crankcase (see fig. 99). Some types of separators are serviceable in which case screens and/or elements can be replaced without having to replace the unit.