Why we do vaccum before refrigerant charging in air conditioner or refrigerator.

 What is vaccum

Atmospheric pressure.

The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact and it is known as atmospheric pressure. It is also known as barometric pressure. The atmospheric pressure at sea level (above absolute zero) is called standard atmospheric pressure. Value of atmospheric pressure is given below.

Standard atmospheric pressure = 101.3 kN/m² or kPa (1kN/m² = 1kPa)

= 10.3 m of water 

= 760 mm of Hg
= 76   cm of Hg

Gauge pressure

The pressure measured with the help of a pressure gauge is known as gauge pressure, in which atmospheric pressure is taken as datum. Pressure gauge read difference between the actual pressure and atmospheric pressure.

Absolute pleasure = Atmospheric pressure + Gauge pressure



Absolute pleasure = Atmospheric pressure - Negative gauge pressure (vaccum pressure)

Vaccum pressure.

The negative gauge pressure is called vaccum pressure. Vaccum pressure is the pressure less than atmospheric pressure. 

Vaccum meaning in refrigeration system.

In refrigeration system after braze all joint we check leakage and strength of joint to fill nitrogen and helium gas mixture at more than design presure. Then we release the pressure of nitrogen and helium. But some gases remains inside system. 

Nitrogen and helium are non condensable gases.


Non-condensables in the system will result in high head pressure/condensing temperature. They may also cause high side pressure fluctuations. The end result is decreased cooling capacity and efficiency due to higher compression ratios.

The only way to remove non-condensables  in a small air conditioning or refrigeration system is vaccum of the  system. You can recover the charge, let it sit in the tank for a while, and then recover the vapor from the top into another tank. Then, you can recharge with liquid only to remove most of the non-condensables, but it's a pretty inexact science.

You can't remove non-condensables with a line drier. While you do remove air with a vacuum pump, you only remove the air that entered the system once you open it. The vacuum does nothing for the refrigerant you already pumped down or recovered; the non-condensables remain mixed with the refrigerant unless you are dealing with large volumes. Then, the refrigerant and Non condensable gases can actually be separated (and the latter removed).
There are three steps in room air conditioner
1. Initial pressure 
2. Arrival pressure.
3. Leaving pressure 







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