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Evacuating a system

By Grant Laidlaw

This essential step avoids various operating issues and potential system damage.

Jabo asks: Mr Grant, you say that we should use a vac pump, but many do not use this, can you explain?

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Hi Jabo, it is extremely important that a refrigeration system is completely free of moisture and non-condensables. Recently I have been involved with the training of refrigeration technicians who are currently working in the field with more than one year’s  experience. It is honestly quite concerning as to how many do not draw a vacuum when working on a refrigeration system.

It has been quite a while since I looked at evacuation, so having said that let us look into your question.

Let me begin by saying: A new system or a system that has been opened for repair must be completely evacuated before charging or re-charging.

The reasons for evacuation are to remove moisture and non-condensables (such as air, nitrogen, etc) from the system.

  • Moisture in the system will combine with the refrigerant and form acids when the plant is in operation, and
  • Non-condensables will cause an increase in the head pressure.
    The acids attack the internal parts of the system and will, in time, destroy the compressor. Hermetic or semi-hermetic type compressors will fail earlier because the acids will attack the windings of the electric motor. This will result in a burnout, which creates products of combustion such as moisture, acid, soot, varnish and hard carbon.

    Air or any other non-condensable gas in the refrigeration system will accumulate in the condenser. This reduces the condenser area available for condensing the refrigerant and hence will cause an increase in head pressure. This can cause overheating of the compressor motor, which can also result in a burnout. It will also increase the amperage drawn by the compressor and increase running costs.

    Air and moisture can be left behind in the system during installation or service, or enter the system during operation through a leak in a part of the system, which is under a negative pressure.

    Nitrogen, used during brazing or for leak testing must be removed before the system is charged with refrigerant. In order to ensure that the refrigeration system is leak-proof it must be leak-tested prior to evacuation and charging.

    Now that we understand the issues let us move on to what level should a refrigeration system be evacuated, and the reasons for this.

The main purpose of evacuating a system is to remove the moisture and the non-condensable (air).

To remove all the air from a system is virtually impossible. If a system is evacuated to 6,65 kPa absolute pressure, or 96,6 kPa below standard atmospheric pressure at sea level, it is estimated that only 94% of the air is removed from the system.

As with refrigerants, the boiling point (saturation temperature) of water will decrease with decreasing absolute pressure. At a pressure of 9203 microns (abs) or 1227 Pascal (abs), water will boil at 10 degrees Celsius. If the plant temperature is 20 degrees Celsius and the pressure inside the plant is 9203 microns, the water in the system will start boiling because heat flows from the plant into the water.

The bigger the temperature difference, the faster the water will boil. Thus, the lower the pressure in the system, the lower the boiling point of the water and the quicker the plant will be dehydrated.

In order to make sure that all the moisture inside the system is evaporated and removed, and that we remove as much air as possible it has been found that the system must be evacuated to an absolute pressure of 5000 microns; this is equivalent to 667 Pascal(abs) or 6,67 mbar.

At this pressure water will boil at ± 2°C. For South African conditions, where the ambient temperature is normally well in excess of 2°C this provides enough heat to evaporate any water that may be inside the system.

Jabo, it is very important to note that these pressures are so low they cannot be measured with the normal manifold pressure gauges used by the refrigeration mechanic. Special (usually electronic) pressure (vacuum) gauges must be used to measure these pressures.

Jabo, it is also important to understand that in order to achieve the required vacuum throughout the system the pressure at the vacuum pump must be reduced to 500 microns absolute pressure.

In practice, in order to ensure that the final pressure inside the entire system is evacuated to the correct level, the plant must be evacuated to 500 microns and then left for at least two hours. If any water is present in the system it will evaporate during this time. This will increase the pressure and further evacuation will be required. If, however, the pressure does not increase to more than 500 microns during this time, the plant can be considered “dry”.

Jabo, let us move on to the vacuum pump.

The vacuum pump for evacuating the system has to be able to reach the vacuum required. To reach a deep vacuum, a two-stage pump capable of reaching a vacuum of 50-100 micron is required (Minimum 500 microns).

The vacuum pump must always be checked prior to use. The pump must be able to reach the vacuum required and the oil level must be correct.

Some manufacturers of vacuum pumps recommend that the oil be changed after each evacuation. The oil should definitely be changed if the pump cannot reach the correct vacuum anymore or when it is not going to be used for a long time. Any water in the oil will attack the inside surfaces of the pump which will reduce its effectiveness.

Jabo, prior to using a vacuum pump perform the following checks and test:

  • The vacuum pump must be suitable for the application and refrigerant.
  • The vacuum pump must be checked for safety including the electrical cord, the plug and the extension (if used).
  • Check oil condition and level when running (correct if necessary). The oil should be clear, not milky (moisture) or dark-black (carbon).
  • Using a micron gauge connected directly to the pump, the pump should reach a minimum of 500μ (microns) within approximately two minutes.

The pump must be able to reach at least 500μ (Microns). If the pump does not reach the required vacuum replace the oil and retest. If the pump is still unable to reach the required vacuum, overhaul or replace it.

Jabo, please note that a hermetic, semi-hermetic refrigeration or air conditioning compressor is not suitable for evacuation as it cannot produce the high vacuum required. Whilst I appreciate the enthusiasm, do not “make your own vacuum pump”.

Jabo, in the next issue I will look at the types of vacuum we draw and go into more detail on this subject.

References:

  1. SARACCA
  2. ACRA
  3. SANS 10147

Thank you for all your questions. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with ‘Solutions Page’ in the subject line. You may include pictures.

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