• SkillCat Team

System Evacuation (Dehydration)

EPA 608 Type 2 Chapter 10 (Take full course for free)


In this module, we will discuss the process of evacuating a system. We will also learn many tips and techniques to perform a successful evacuation. Skip to quiz!


1. Evacuation-Basic Concepts

Evacuation is when we remove all water vapor and air from the system. We evacuate a system before filling it back with refrigerant to avoid mixing refrigerant with air. We do this by sucking out all the air and creating a vacuum inside the appliance.


Vacuum essentially means the absence of any matter or air. The highest level of vacuum is found in outer space and called absolute vacuum. We generally want to replicate absolute vacuum during evacuation.


A deep vacuum in a system is a vacuum level very close to an absolute vacuum and means that there is very little air in the system. The amount of air present is so little that we need to measure the vacuum in a unit called microns.


2. Evacuation Process


While pulling a deep vacuum, it is impossible to bring the pressure down to 0 microns. We consider 500 microns to be an acceptable deep vacuum level for evacuation.


At a deep vacuum of 500 microns, almost all moisture in the system can evaporate and be thrown out of the system. It is also a standard set and accepted by EPA and all manufacturers. Similarly, different levels of deep vacuum are accepted for various systems.

Any high-pressure appliance, or a component of such appliance, having

  • Less than 200 lbs. of refrigerant - Should be evacuated to 0 inches of Hg (500 microns).

  • 200 or more lbs. of refrigerant - Should be evacuated to 10 inches of Hg (25000 microns).

Any medium-pressure appliance, or a component of such appliance, having

  • Less than 200 lbs. of refrigerant - Should be evacuated to 10 inches of Hg (25000 microns).

  • 200 or more lbs. of refrigerant - Should be evacuated to 15 inches of Hg (37500 microns).

After reaching the required vacuum on an appliance, a technician should wait for at least a few minutes. If the system holds the level of vacuum, it confirms that the system is not leaking anymore.


If the appliance can hold a required vacuum for a few minutes, it confirms that the system does not have any leaks. If there are leaks, the outside air will rush into the system, and the vacuum pressure would start to increase.


Note that depending on the question, the EPA exam may use the word Evacuation in place of Dehydration or in other questions in place of Recovery.


3. Effect of Non-Condensables


Recall that non-condensables refer to the gases that can’t be converted into liquid by the refrigeration system. Air and nitrogen gas are common examples of non-condensable gases.

Recall that the discharge side of a system consists of liquid refrigerant. We can confirm the presence of non-condensable gases if the discharge side pressure in the refrigeration system is higher than normal.


The reason for a higher discharge pressure is;

  • The gases remain in the vapor phase and do not convert to liquid.

  • This exerts pressure on the refrigerant liquid, in turn increasing its pressure in the discharge side of the appliance.


4. Factors Affecting Evacuation


Recall the pressure-temperature relations for any fluid. The pressure and temperature of any system are directly related to each other. A reduction in pressure results in a drop in the temperature of the system.


While evacuation, as the pressure is decreasing, the temperature of the system is also falling. There is a possibility that the moisture gets converted into ice at low temperatures. If ice is formed, it clogs the refrigerant piping and must be avoided.

The risk of freezing is high if a system contains a large amount of water/moisture. As large amounts of water cannot be removed in one go, some water is always left that may slowly start freezing during evacuation.


Technicians can use a special technique to avoid freezing. We stop evacuation in between and increase the pressure by releasing some nitrogen gas into the system.


The nitrogen gas carries the moisture along with it and does not allow the water to accumulate in one place. The gas, along with moisture, is then removed from the system by the evacuation process. It prevents freezing of the moisture inside the system.


Using a large vacuum pump leads to a faster evacuation, but ice formation is a peculiar problem. The pressure (and temperature) in the system reduces very quickly during evacuation and can cause the water in the system to freeze and form ice.


5. Conclusion


In this module, we learned many things about evacuating a system. We also discussed some tips and techniques to perform a successful evacuation. Understanding these concepts will help us in perfectly evacuating a system.


Question #1: Which units are used to measure deep vacuum?

  1. Psig

  2. Psia

  3. Inches of Hg

  4. Microns

Scroll down for the answer...











Microns

The pressure in a deep vacuum is usually measured in a unit called microns.


Question #2: A technician has to make a major repair in an appliance having 1000 pounds of R407C. He must evacuate the system to which of the following levels?

  1. 0 psig

  2. 25 mm Hg absolute

  3. 10 inches of Hg vacuum

  4. 15 inches of Hg vacuum

Scroll down for the answer...











10 inches of Hg vacuum

Recall that R407C is a high-pressure refrigerant.

As the system to be repaired uses more than 200 lbs. of high-pressure refrigerant, it must be evacuated to 10 inches of Hg vacuum.


Question #3: A ____ pressure on the ____ side can confirm the presence of non-condensable gases in the system.

  1. lower; suction

  2. higher; suction

  3. lower; discharge

  4. higher; discharge

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higher; discharge

We can confirm the presence of non-condensable gases if the discharge side pressure in the refrigeration system is higher.


Question #4: Which gas is used to counteract freezing while evacuating a system with large amounts of moisture;

  1. R-410A.

  2. R-22.

  3. Air.

  4. Nitrogen.

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Nitrogen.

The nitrogen gas released in a system during evacuation carries the moisture along with it.

This does not allow the water to accumulate in one place.

It prevents freezing of the moisture inside the system.


Question #5: Water present in the system can freeze if;

  1. A large vacuum pump is used for evacuation

  2. A recovery unit is used for evacuation

  3. The system’s compressor is used for evacuation

  4. A house vacuum cleaner is used for evacuation

Scroll down for the answer...
















A large vacuum pump is used for evacuation

The system's pressure (and temperature) can reduce very quickly while evacuating with a large vacuum pump.

This sudden decrease in temperature can cause the water in the system to freeze and form ice.


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