So you have just completed a repair or installation on Refrigeration system. You have pressure tested with dry nitrogen and are satisfied the system is leak free.You are now required to evacuate the system to prepare it for charging with refrigerant.
No doubt you will be told many different methods by many different people over the course of your career on how to do this, or perhaps your just starting out in the trade. The following is the best way to ensure this procedure has been done correctly.
The whole point of evacuating a system is to ensure any non-condensibles and moisture are removed before charging system with refrigerant. There are two methods to do this and they are the deep vacuum method and the triple vacuum method.
Presuming the system is still under pressure, dump nitrogen. Connect your valve core removers to system and remove valve cores. Connect your vacuum hoses or copper lines to valve core removers and vacuum pump.
The bigger the lines the faster the vacuum so always use 3/8 or 1/2 inch lines.
Connect your Vacuum Gauge as far from the vacuum pump as possible, preferably on the system.
Deep vacuum method will require you to pull vacuum down to 500 microns or less (ideally less then 200 microns to ensure no moisture at all).
Once you have reached your vacuum isolate vacuum by shutting ball valves on your core removers and perform a rise test for 30 minutes.
It should not rise more then 100 microns. If it does break vacuum with nitrogen, then perform evacuation again to ensure removal off all contaminants. You may need to change oil in vacuum pump, I recommend changing after every use! This is why it is best to have a variety of vacuum pumps as you don't need a 230 litre per min pump on a domestic fridge.
The triple vacuum method is similar to the above but can help you achieve a much lower vacuum. You will need to evacuate system to 500 microns, then break vacuum with dry nitrogen. Dump nitrogen then evacuate down to 500 microns once again, repeat this process one last time after this. The nitrogen helps with dehydrating system.
If you follow the above you can be sure your systems will be correctly evacuated and dehydrated ready for charging. Using the correct equipment is absolutely critical to achieving the desired vacuum.