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Home » Evacuating a system (continued)

Evacuating a system (continued)

By Grant Laidlaw

In the last Support article published in the RACA Journal, Jabo asked: Mr Grant, you say that we should use a vac pump, but many do not use this, can you explain? This article continues as part 2 of answering this question.

Jabu, it is extremely important that a refrigeration system is completely free of moisture and non- condensables (such as air, nitrogen, etc). 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. In the last issue I discussed the why. Let us move forward and look at how.

We know that evacuation is the process used to remove non-condensables and moisture from a refrigeration system. This is accomplished by the use of vacuum pumps specially designed for this purpose. A new system or a system that has been opened for repair must be completely evacuated before (re)charging. Prior to evacuation the system must be pressure tested to ensure that the system is leak proof.

The two methods of removing moisture and non-condensables from the system are:

  • The deep vacuum method, and
  • The triple evacuation method

In the “deep vacuum method”, reducing the pressure in the system boils off the moisture in the system which the vacuum pump then removes. In order to make sure that all the moisture is boiled off, the pressure must be reduced to well below the pressure corresponding to the saturation temperature of the water at the plant temperature (plant room ambient temperature).

The non-condensables (air) in the system are removed by reducing the pressure to about 500 microns. After the system has been evacuated to 500 microns it is allowed to stand for at least two hours – depending on the size of the system. Any moisture present in the system will boil off and increase the system pressure (decrease the vacuum) which you can observe on your vacuum gauge.

The system pressure will always increase to a certain extent because of pressure equalization in the system. If, however, the pressure in the system does not rise further than 500 microns, the system may be assumed to be dry and leak proof.

As the boiling point of water at 500 microns is very low, this means that if any water is still present in the system it will boil off as long as the temperature of the plant is above this level (which in South Africa it usually is).

The deep vacuum method is used on systems containing only a minimum of contaminants (for instance when a system has only been open to the atmosphere for a short period); whereas the triple evacuation method is used on systems containing a greater amount of contaminants. 

With the triple evacuation method there are three stages. Typically we only use this method for problematic systems known to have moisture. This method is effective for moisture removal as dry nitrogen contains no moisture and therefore will absorb more moisture before becoming saturated. In addition, at each stage fresh dry nitrogen is introduced. 

Stage 1

The plant is evacuated to an absolute pressure of 5 000μ; this vacuum is broken with dry nitrogen and the pressure in the plant is increased with dry nitrogen to approximately 50 kPa. This pressure is maintained for 15 minutes to disperse the nitrogen throughout the system and for the nitrogen to absorb the water vapour. The nitrogen is very dry and therefore will absorb the moisture very quickly. After the 15 minutes the nitrogen is released together with the moisture it has absorbed. 

Stage 2

This stage is basically the same as stage 1. Before connecting the vacuum pump to the system, the nitrogen must be released. The vacuum pump may only be connected once the pressure inside the plant is 0 kPa (g).

The plant is again evacuated to 2 000 microns and then pressurized again to 50 kPa with dry nitrogen. The nitrogen is left in the plant for 15 minutes and then released.

Stage 3

In stage three of this method the system must be evacuated to the same level as the deep vacuum method (500 microns). The reason for this is that in this stage all the non-condensables as well as the remaining water vapour must be removed. This evacuation will take much less time than the evacuation in the deep vacuum method because there will be very little, if any, moisture left inside the plant.

A four-hose manifold with large evacuation hoses. Image supplied

A four-hose manifold with large evacuation hoses. Image supplied

Jabu, the following represents the equipment that you will require in order to draw a vacuum on a system: 

  • A two stage vacuum pump that can evacuate down to 50 to 100 microns
  • Manifold gauges
  • Refrigerant type hoses
  • A special vacuum gauge that can indicate very low absolute pressures. (Typically we use a micron gauge) The very deep vacuum (or the very low absolute pressure) required for evacuation cannot be read on the normal manifold gauges.
  • Mechanical gauges to measure vacuum are available, however, the more modern electronic gauges are more popular because of their ease of use and the fact that they are not as susceptible to damage.

Three types of vacuum gauges are available, these are: 

  1. The gauge with LEDs to show the vacuum reached, 
  2. The digital vacuum gauge, and
  3. The analogue gauge

The units used for indicating the absolute pressure on the vacuum gauge can be in microns, Pascal, mbar, Torr or mm Hg (mercury).

Jabu, let us have a look at the manifold sets that most technicians use.

We are all familiar with the standard three-pipe manifold sets, but they are limited in their efficiency for evacuation requirements. The long 6mm lines are restrictive and offer resistance which increases effective evacuation times.

We can improve on the standard manifold gauge lines by keeping your manifold lines as short as possible. There are shorter lines available (± 250mm long).

On some manifolds there is an additional port for evacuation, with a larger 9.5mm yellow hose and hose connection. Internal ports are also larger on these sets. This is specifically for evacuation and reduces evacuation times considerably.

Although it is very convenient to use the manifold gauge to connect the pump and the gauge to the system it is preferable to use solid copper piping for the connection (6 mm for small systems and larger for bigger systems). The advantage of using copper piping is that it has a much lower resistance.

  • The correct procedure to evacuate a system is as follows: 
  • Check that all equipment is suitable and safe to use.
  • Check oil with regards to level and clarity.
  • Test your vacuum pump with a micron meter attached to the suction port of the vacuum pump. Take note of the vacuum attained. The pump should achieve a vacuum of 500 microns or better.
  • Connect the vacuum pump to the system.
  • Start the vacuum pump, slowly open the valve on the vacuum pump and on the manifold with two full turns. 
  • Operate the vacuum pump until at least 500 microns of vacuum is obtained. Close the manifold valves and stop the vacuum pump. (NB: in this order). Observe the vacuum gauge for a few minutes. A rise in pressure would indicate a possible leak or moisture remaining in the system. (Leak test, repair leak if needed and evaluate again).
  • When a vacuum is obtained, close all valves, stop the vacuum pump and leave it to stand for at least two hours (during this period watch the vacuum gauge if the pressure increases rapidly then there is a leak that must be found and repaired). 
  • If the system does hold its vacuum, remove the yellow hose from the vacuum pump and connect it to the charging cylinder.
  • Jabu, now how do you evaluate the system vacuum? 

Just obtaining a vacuum of 500 microns does not mean that the system is dry and airtight. If there is still water present in the system this will evaporate and increase the pressure to the vapour pressure of the water at the plant temperature. If there is a small leak in the system, the pressure will rise and eventually come up to atmospheric pressure (0 kPa g). As I have said before, the above reason is why the system must be left standing for at least two hours or longer if possible. 

  • The following is the correct procedure to evaluate the system vacuum: 
  • After the system has been standing for a minimum of two hours observe the vacuum gauge. 
  • If the system pressure stabilises at or below a pressure of 500 microns the system may be assumed to be dry and leak proof. 
  • If the system pressure stabilises at the vapour pressure of water at the system temperature (this will be between 500 microns atmospheric pressure) the system still contains moisture but is leak proof. 
  • If the system pressure increases to above the vapour pressure of water at the system temperature and continues to lose vacuum to atmospheric pressure the system has a leak, which must be repaired before charging the system with refrigerant. 
  • If the pressure increases rapidly the system has a big leak; if the system pressure increases slowly the system has a small leak.

Finally, it is very important to remove the vacuum pump and instrumentation from the system without loss of vacuum, I have seen technicians draw a vacuum only to allow air into the system when disconnecting the vacuum pump.

Remember: close your manifold valves before turning off the vacuum pump. In addition, all the valves on the system must be closed before any of the hoses are disconnected. 

If the hoses are connected to the compressor service valves these valves must be back seated before the hoses are removed.

If the system is to be charged immediately the vacuum can be broken with the correct refrigerant. If not, all valves must be tightly shut and capped. 

Jabu, thank you for the question, I hope that this helps you moving forward. This is a very important technical aspect in the refrigeration and air conditioning industry that is often ignored.

References: 

  1. ACRA
  2. Imperial USA
  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. 

About author

Grant Laidlaw

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale.

He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.