The vacuum process

Vacuum is a practice that needs to follow different steps to perform correctly. Below we share some effective recommendations for succeeding in this process.

by Gildardo Yáñez*

Many of the technicians in the field do not know how harmful it can be for the system and for the quality of the service that they themselves provide by not making the vacuum to the system in the right way, coupled with this there is no awareness of the potential failures that could be had after the start-up of the equipment, causing the technician to return for one or more warranty calls from the customer, and in the most serious cases the compressor will be required to change. 

Many of the technicians who execute the vacuum process do it with another refrigeration compressor that is made to pump refrigerant gas or they do it with the same refrigeration compressor of the system and it will be necessary to add that generally there is not the correct measurement equipment to be able to know if we take our refrigeration system to the correct vacuum, according to the type of lubricant with which we are working without having any reference.

Let's analyze what happens if we do not make the correct vacuum to the system. The first thing that could happen would be the presence of non-condensable gases in the system, these will cause the following:

1. Raise the temperature on the high-pressure side of the system

2. The discharge valve becomes hotter than it should be

3. Organic solids are formed that cause compressor failures

The second thing that can happen is that we have the presence of moisture in the system, this will cause the following:

1. That we could have the presence of ice in the system

2. This situation may cause the control element of the system to be clogged.

to. Capillary

b. Expansion valve

3. This condition can damage parts of the compressor

Let's see if this moisture froze inside the expansion valve and blocked the internal mechanism we would have two possible symptoms:

1. The expansion valve will not supply enough refrigerant gas. This condition will occur if our valve is blocked when it is closed and the symptoms in the system will be:

to. The temperature of the load we are cooling is going to be high. (Air or water coming out of the evaporator)

b. Overheating in the system will be high.

c. The suction pressure will be lower than normal.

2. The expansion valve supplies too much refrigerant gas. This condition will occur if our valve is blocked when it is open and the symptoms in the system will be:

to. Return of liquid refrigerant to the compressor.

b. Overheating will be too low.

c. The suction pressure will be normal or higher than expected.

But if we have air and humidity trapped in the system we will have the conditions for the following to happen to our system, by combining these two elements and combining them in turn with a refrigerant gas with chlorine, for example the R-22 gas and through a chemical process known as hydrolysis *, with this situation we will obtain hydrochloric acids or hydrofluoric acids depending on the type of gas,  in addition to sludge in the system. This combination is lethal for semi-hermetic and hermetic compressors, since these cause premature failures in the electric motors contained inside, attacking the insulating varnish, to the extent of making it fail until it goes to earth.

At the beginning we mentioned that many of the times the vacuum is done with the compressor itself, now let's review what happens to our compressor if we do it.

1. To begin with, it damages the compressor insulation significantly, as one of the characteristics that hermetic compressors and hermetic compressors of more than 5 HP have.  is that electric motors are cooled by cooling gas, so if we make them work without their cooling medium, the windings of the engine are going to heat up, and there we start to damage our compressor and we are just starting our system.

2. The second thing that happens is that by physical law electric coils produce electric arcs, only by the fact that an electric current circulates through them when they are in a vacuum condition.

As we have analyzed previously it is not correct and it is not recommended to vacuum with the compressor of the system, since apart from the fact that we will not be able to reach the correct vacuum, we will heat the motor coil and we will also leave very likely some of the conditions described above, which will cause the premature failure of our compressor. 

Vacuum pump selection

Compressor manufacturers do not provide any type of guarantee if the compressor failure is caused or derived from the presence of moisture in the system. Then we have to buy a vacuum pump. What is the right size? How will I select it? They would be the questions we would ask ourselves to choose the vacuum pump that meets our needs.

1. We will choose the vacuum pump according to the tons of cooling of the system, it will not affect the length of the system to select the right one, and the manufacturers of the pumps do not specify their equipment based on how long or short the system is, regardless of whether we make vacuums in large self-service stores or in small refrigeration chambers, since the speed with which we are going to make the vacuum will depend on several factors and that some of them are the following:

to. A factor will be the height above sea level at which we make the vacuum process, since if we make a vacuum in the Port of Veracruz the time it will take us to make our vacuum will be short because we do it at sea level, and if we do it for example in the city of Toluca the time of the vacuum process will be long, because this city is located at an altitude of 2600 meters above sea level.

b. A second factor will be the ambient temperature to which our system is exposed, a known technique to accelerate the vacuum time is precisely to raise the temperature of the system by an external means, either by means of incandescent lamps, or by some other method that can increase the temperature of our equipment or our installation.

c. Remember that the purpose of vacuum is to remove the presence of moisture and non-condensable gases from the system.

Illustration  Vacuum pump

2. For example if we know that our system is 40 tons of refrigeration. One way to know which pump we need is that we know that for every cfm we can effectively evacuate 7 tons of cooling from a system, then we apply a simple formula: (Tons of system cooling / 7) = CFM required to evacuate the system

From our example it would be (40 Tons of refrigeration / 7) = 5.7 CFM'S that would be equivalent to a 6 cfm pump, these data can vary from brand to brand, and are just an approximation that can help us determine the most appropriate size of the vacuum pump.

But to know that we reach the correct vacuum, we need a vacuum gauge, to be able to measure the vacuum in an effective way, since the pressure gauges used in the service maninums cannot measure the microns of vacuum. At present and with the presence especially of Polyol Ester oil, it has become very important to achieve the correct level of vacuum.

There are several types of vacuum gauges that we can use, but the most common currently is the electronic, which among other advantages it has, is that some of them are very resistant, and do not require any type of calibration. We have to measure our vacuum as we know that water boils and evaporates at a temperature of 100°C (212°F) with an atmospheric pressure of 1.03 Kg. /cm2 (214.7lbs/in2).

When we vacuum the system, what happens inside is that the internal pressure of the system begins to drop to the point that we modify the boiling point of the water and make it boil or make it evaporate at room temperature, and our pump will be responsible for sucking the gases outside; on the other hand if we do not have our vacuum gauge to know how much we are lowering the pressure of the system, we run the risk of lowering the pressure so much that we could modify the boiling point of the oil, causing it to begin to boil or under certain conditions we could suck it out of the system. 

That is why it is very important to emphasize: the correct vacuum is achieved by measuring, not by the time we leave the pump working in the system.

To be able to make our vacuum and eliminate moisture quickly, we can use certain simple procedures to execute:

- The first of these will be to make a sweep with gaseous nitrogen to be able to expel as much moisture as possible basically blowing it to the outside of the system. This process must be done with a cylinder of gaseous nitrogen and a nitrogen regulator in between to avoid accidents, because it is only a small sweep with a pressure of 2 to 3 pounds.

- The second of them will be that at the beginning of the vacuum process it is done on both sides of the manifold, the pump is connected to the center, and the two valves must remain open, high and low pressure until the desired vacuum is achieved.

- The third point will be the one I have already mentioned, to heat the system itself, with lamps or with some other means to heat the tubes, evaporators, condensers, etc. so that the humidity evaporates.

Illustration  Connecting the pump to make vacuum

This process is useful when, for example, this system was a chiller, and we had the presence of water inside because our evaporator had crashed or the system was very long. Once we are ready to start the process itself we suggest this sequence of operations:

1. The vacuum pump is connected to the system

2. The pump starts

3. We stop when we have a reading of 1,500 microns

4. Break the vacuum with nitrogen and pressurize the system with 2 pounds

5. We release the nitrogen

6. The pump starts

7. We stop when we have a reading of 1,500 microns

8. Break the vacuum with nitrogen and pressurize the system with 2 pounds.

9. We release the nitrogen

10. The pump starts

11. We stop when we have a reading of 500 or 250 microns depending on the type of lubricant

12. We break the vacuum with the refrigerant gas

13. And we gas our system

The following gaps that we must reach:

- 500 microns if we work with mineral oil or alkylbenzene oil

- 250 microns if we work with Polyol Ester oil

If during the process the oil from the vacuum pump turns white or takes on a milky appearance, what will have to be done is to turn a quarter of a turn the ballast gas of the vacuum pump so that that moisture that is in the oil is released, once the oil takes its normal appearance, the ballast is closed again. 

The vacuum pump should not be stopped, so as not to lose the progress of the work, in case this procedure was not enough, then it is recommended to stop the process, the pump oil is changed when it is hot, the pump is recharged and continued.

It is advisable to change the oil of the pump after each vacuum while the latter is hot, because if we do not, the subsequent voids will be slower and slower, in addition to the useful life of our pump will be reduced, because the internal valves begin to oxidize, losing these the seal.

*(Decomposition of chemical compounds by the action of water)

* Gildardo Yáñez is a Refrigeration Specialist and you can visit his website www.gildardoyanez.com