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One of the most important tasks during installation or repair of air conditioning and refrigeration systems is evacuation. Many people ignore this, so let us have a look at what evacuation is all about.
Water (moisture) in the system causes the problems discussed below. Air in the system is not condensable and has the effect of increasing heat pressures. This in turn places unnecessary strain on the compressor and if the situation is not corrected, failure may result.
Purpose of the vacuum
It is most undesirable to have any ‘foreign’ gas present in a refrigeration system. The most likely foreign gas is air (which is a mixture of gases, to be exact). Air is non-condensable in terms of the working pressures and temperatures of a refrigeration system. When various gases share a space, all their pressures add together to make the total pressure of the volume. This means that the pressure of any air (or other ‘non-condensable’ that may be present) is added to the working discharge pressure of the refrigerant. Thus more power is used; refrigerant discharge pressure rises higher than necessary and less refrigeration work is done. Temperature, particularly of the oil and at the discharge valves, builds up excessively.
But even more harmful to the system is moisture. This may enter as atmospheric humidity, or in many repair cases, a chilled water or condenser water tube could have fractured, and water could have entered by those means. Moisture may be present in two forms in the system:
- Visible moisture (water)
- Invisible moisture (water vapour)
Refrigerant oil is extremely hygroscopic (i.e. it readily absorbs water from the atmospheric humidity). Even if great care is taken, moisture can enter the system in the oil which is supplied ready charged in some compressors or is charged into the compressor before final evacuation.
Moisture reacts in the most damaging way with refrigerant oils and also with refrigerant itself, particularly if the system runs hot. This causes chemical reactions between refrigerant, oil and the water, building up some extremely powerful acids, including hydrofluoric acid, which dissolves glass. These acids attack and corrode system metals, creating foulants which add to the sludges which form in the oil, seriously harming the compressor lubrication. The compressor can literally be torn apart.
A good vacuum goes nearly all the way towards removing all the moisture from the system, particularly if the system is kept warm during evacuation. As we lower the pressure in the system, we lower the boiling point of the water in the system. At sea-level, water boils at 100°C. The barometric pressure at sea-level is 101kPa. We know that lowering the pressure, lowers the boiling point of a substance. If we lower the pressure from 101kPa to 1kPa, we lower the boiling point of water from 100°C to 7°C. If the plant is exposed to an ambient temperature of 20°C, then there is sufficient heat to boil off the water. As we wish to remove the water from the system as quickly as possible and taking into account the pressure drop over long pipe runs, it is desirable to draw a vacuum of 500 microns.
This table shows the boiling point of water at some low and very low pressures.
| Absolute pressure | Boiling point (°C) |
| 500 microns | – 24° |
| 5 000 microns | 0° |
| 1 kPa | 7° |
| 2 kPa | 18° |
| 3 kPa | 24° |
Deep Vacuum versus Triple Evacuation
A normal system is treated by pulling a deep vacuum down to 500 microns. The vacuum is then observed for a period of time (at least two hours). If the vacuum remains at 500 microns, the system is deemed to be leak and moisture free. This is called ‘deep evacuation method’.
In the case of a wet system, the ‘triple evacuation’ method should be used.
This process involves firstly evacuating the system to a ‘reasonable’ vacuum. (e.g. 5 000 microns). This vacuum is then broken with dry nitrogen. Nitrogen is usually supplied technically dry. The previously evacuated system is pressurised to about 60 or 70 kPa (gauge) with dry nitrogen and allowed to stand in this state for an hour, after which it is re-evacuated.
The dry nitrogen will absorb water by evaporation, just as will dry air. This is sometimes referred to as a ‘blotting’ process.
Then, the nitrogen pressure is released and the vacuum pump is restarted, there is a strong flow of this now moist nitrogen towards the pump suction connection.
This process is repeated a second time but drawing a vacuum of 1 000 microns, to get a further handle on moisture that may be loitering in the system.
Triple Evacuation System
| DRAW VACUUM TO 5 000 MICRONS |
↓
| BREAK VACUUM WITH DRY NITROGEN |
↓
| WAIT |
↓
| EVACUATE TO 1 000 MICRONS |
↓
| BREAK VACUUM WITH DRY NITROGEN |
↓
| WAIT |
↓
| EVACUATE TO 500 MICRONS |
↓
| CHECK FOR TIGHT, DRY SYSTEM
(MUST HOLD DEEP VACUUM) |
↓
| CHARGE SYSTEM |
After this, it should be possible to get down to the desired 500 microns vacuum.
Vacuum Gauges
Vacuum gauges are used to measure any pressure below atmospheric pressure. Many gauges are available. Vacuum may be expressed in kPa vac and not –kPa. The vacuum scale on the compound gauge is not sufficiently accurate when working with dehydrating vacuums. A micron meter or similar gauge should be used.
Vacuum readings are normally in Micron’s or mbar, there are:
- Electronic Vacuum gauges with LED indication.
- Electronic vacuum gauges with digital indication.
- Analogue vacuum gauges.
Testing of the Vacuum Pump
Before using a vacuum pump, check the safety aspects, the electrical cord, the suitability of the pump for use. Check the oil level and colour of the oil (the oil should be clear, not black or milky white). Test the capability of the pump, using a micron meter. A vacuum of at least 500 microns is necessary. If it does not achieve 500 microns, replace oil and retest. If after oil replacement has being carried out and the pump still fails to achieve the required vacuum, the pump should be serviced.
The vacuum pump’s oil should be changed regularly and especially after drawing vacuum on a contaminated system.
Drawing Vacuum
It is best to purge a system through with nitrogen prior to drawing a vacuum for best results, as nitrogen absorbs moisture better than air.
In order to draw a dehydrating vacuum:
- Test your vacuum pump and take note of the vacuum attained.
- The pump should achieve a vacuum of 500 microns or better.
- Make sure that the system is at 0 kPa pressure before connecting the vacuum pump.
- Connect vacuum pump.
- Connect micron meter.
- Open all valves between pump and system.
- Run pump until a vacuum of 500 microns or lower has been reached.
- Make a note of the reading.
- Close valves and stop vacuum pump.
- After two hours the reading should still be the same.
Vacuum Pump Oil
The oil in the pump is very subject to picking up moisture which is being drawn from the system. If the oil becomes saturated, it will be impossible to achieve a good vacuum. It is good practice to ensure that the vacuum pump is capable of drawing 500 microns before each use. Vacuum pumps use specialised vacuum pump oil, consult with the vacuum pump manufacturer for the correct oil.
Storage Of The Vacuum Pump
Apart from not storing the vacuum pump with moist oil, remember to break the vacuum on the pump before putting it away. Otherwise oil will draw into the pumping chambers and make it very difficult to turn the pump.
Drawing vacuum is as you can see, very important and constitutes good practice.
Grant K Laidlaw F.S.A.I.R.A.C.
References:
ACRA
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