When a system is being serviced, naturally air enters the system. Detrimental to the system, the oxygen, nitrogen, and water vaper contained in air needs to be removed along with other non-condensable gases. This is where a vacuum pump comes into play – we call this degassing the system.
The with what
With a vacuum pump connected to both, low and high air conditioning system sides pull a deep vacuum for around 20 minutes. It is important to pul to 10 microns, measured with a vac gauge to test the performance of the pump.
It is also good practice that when pulling a deep vacuum, that it is then combined with the replacement of the receiver drier - that will help to keep moisture inside the system under control.
The oil in the vacuum pump needs to be changed as it retains the moisture and debrie that has been pulled from previous services. It is good practice to change the oil every 6-12 services to increase the lifetime of the machine.
The what if you don’t
The ‘non-condensable gases’ have a serious impact on the performance of an air conditioning system, the operating conditions, and ultimately the lifetime of a system.
The high-pressure side, after the compressor has done its job compressing and raising the temperature and pressure of the refrigerant... this refrigerant in a vapor state is supposed to condense in the condenser.
With a loss in heat transfer capability of the condenser and the inability of the refrigerant to cool down properly, the refrigerant temperature will get higher compared to the air temperature, which results in higher discharge pressures. With the system operating at a higher pressure, extra load will put on the compressor which will decrease overall energy efficiency for the system.
Discharge pressure and compressor temperature will go up which can lead to increased lubricant breakdown and shorter compressor lifetime. With a system working under these conditions, pressures will get beyond a default safe point of some refrigerant control devices in the system, probably shutting down the system to prevent any irreversible damage, like compressor seized up or a hose explosion.
Oxygen promotes bad chemistry in the system creating corrosion of all ac aluminum parts, especially evaporators. Moisture can cause freeze-up at the TX Valve/Orifice Tube, which restricts refrigerant flow or blocks it completely.