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haq
08-01-2015, 05:58 PM
Hello! I wanted to ask any professional here about vacuum timings a household refrigerator requires before being charged with refrigerant.
I have checked pressure drop in refrigerator when vaccuming and it drops to near zero in about 7-10 mins however i was told that vaccuming must be done atleast 20 mins.

The Viking
08-01-2015, 11:41 PM
Oh dear....

When you check the tyre pressures on your car and find they are to low, how many minutes do you pump them up for?
No, you don't time it? You actually use a pressure gauge designed for the task???

So, now then... The vacuum in any refrigiration system prior to (re-)commissioning is more critical than your tyre pressures, don't you think that process also deserves to be measured using the correct tool?

Vacuum is measured in Torr or microns thereof, never time.

:cool:

Glenn Moore
09-01-2015, 06:04 PM
Hi haq
as Viking says its down to using a proper Torr guage not a time function.
With domestic machinery using a capillary tube as an expansion device, it can make pulling a vacuum on the system difficult. To get a good vacuum on a domestic unit it is best to pull a vacuum on the high side and the low side, one on the process stub of the compressor (Suction) and one on the drier (Discharge/Liquid). This gives a better chance of removing air and moisture from the system. When pulling a vacuum only from the process stub (Suction) the compressor discharge valve shuts of the condenser side of the system and the vacuum must then pull any air or moisture in the high side through the capillary tube. Any moisture can then cause a blockage in the cap tube and the vacuum then looks OK as only half of the system is actually being evacuated.
So when repairing a domestic fridge or freezer it is always better to fit a dual entry copper spun drier to enable you to vacuum from both sides of the system.

xxargs
10-01-2015, 03:44 AM
You must use mBar (in absolute), micron or torr gauge or you have no idea at all how deep you vacuum equipment or if you have steady and not rising pressure vacuum in part of mBar range absolute pressure over time...

Ie. gauge have very good resolution in range of 3 - 0.01 mBar absolute pressure (2000 - 10 micron or 2 - 0.01 torr) and you find out very quickly is hard and take time to pull steady vacuum in 0.2 mBar (absolute) range for domestic equipment.

Important process beside vacuum out humidity, is vacuum out non-condensable gas and air and biggest storage for them is solved air/nitrogen in oil inside compressor (or if new opened used compressor - have big amount of refrigerant solved in oil and this take also long time to evaporate completely before you can receive low enough steady vacuum).

Air or nitrogen is solved in oil same way as carbon dioxide in soft drinks and this take lot of time to evaporate. (try vacuum some amount motor oil in sturdy glass vessel (jam container as apple sauce hold for full vacuum in most cases ie. have almost full vacuum (10-100 mBar absolute pressure) in store and before opening first time) and see you self how much oil 'cook' and bubble from solved air in oil and continue gassing long time after stops bubble.)

For low pressure R600a equipment is very important to hold low residure of non condensate gas in evaporator and condense side ie. even low residue of non condensate gas take down efficiently on condenser and evaporator.

if you using torr gauge - you find out to receive low enough steady vacuum level, we not talk vacuum pumping time in 10 or 20 minutes - we talk pumping couple of hour if equipment stand open to air couple of time or/and filled with new oil...

if possible, leaving vacuum pump running over night to next day is not bad idea...