• SkillCat Team

Evacuation & Charging for High Pressure HVAC Equipment

Updated: Apr 7

EPA Type 2 Chapter 4

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System Evacuation (Dehydration)


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!



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.


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.


Effect of Non-Condensables

Recall that non-condensables refer to the gases that can’t be converted into a 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.

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.


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.



Refrigerant Charging


In this module, we will discuss the correct methods of charging a system back with refrigerant. Skip to quiz!



Pressure-Temperature Relations


Recall that boiling temperature is the temperature at which a liquid changes its state to vapor by boiling. The temperature at which vaporization (boiling) starts to occur for a given pressure is called the saturation temperature or boiling point.


P-T Charts have the relationship between the pressure and saturation temperature of any substance. For any refrigerant, we can know its saturation temperature at a given pressure and vice versa. P-T mean Pressure-Temperature Charts. Let us learn to read these charts.

Consider this P-T Chart for R-410A. The units for temperature are either °Fahrenheit or °Celsius, as we can see here. To find the pressure at a temperature, we look for the temperature, then look across that row for the corresponding pressure.


In the chart, the value 80 °Fahrenheit is highlighted, and it corresponds to 235 psig. For example, if an R-410A appliance is at room temperature (80 °Fahrenheit), the pressure of the refrigerant in the appliance will be 235 psig.

In the chart for R134A, the value close to 0 °Celsius is 1.7 °Celsius. The pressure of R134A at 1.7 °Celsius is 30.4 psig and is highlighted in the chart. We will discuss the use and importance of these P-T values later on in the topic.


Charging a System


Once you recover any refrigerant, you can fill it back in the system after the repair works.

Recall the three Rs,

  • Recover

  • Recycle

  • Reclaim

Sometimes it may be necessary to recycle the refrigerant before filling it back.

You should recycle the refrigerant if you find that the refrigerant has many impurities. Many testing kits are available that instantly tell if the recovered refrigerant needs to be recycled. However, if the repairs are only minor ones, recycling is not needed.


Liquid charging means filling the system back with liquid refrigerant. It is faster and saves time. We can initiate charging a system with liquid refrigerants in appliances that do not use water in any part/component. These are air-cooled systems.


Recall that service valves are used to access the refrigerant inside the system without opening it. We also charge the system with refrigerant via these service valves. Liquid charging is always done via the liquid-line service valve. Vapor charging is done via the suction service valve.


In systems that use water in any part or component, it becomes necessary to initiate the charging process with vapor charging. Let us understand why!

Recall that it is necessary to evacuate a system before filling it back with refrigerant. After the evacuation process, the pressure inside a system is low (about 500 microns). The evaporation point corresponding to the evacuation pressure is much lower than 0°Celsius (Freezing point of water).


If the liquid refrigerant is filled in an evacuated system, it can boil into vapor even if the temperature is less than 0°Celsius. For example, the evaporation temperature of R134a at 0 psi is -15°Fahrenheit.


Recall that when a liquid boils, it absorbs heat. While boiling, the liquid refrigerant will absorb heat from the water in the system and convert the water to ice. It is difficult to melt the ice, and at times the evaporator/condenser coils rupture and burst due to ice. This is dangerous and must be avoided.

For this reason, in systems that use water in any part/component, we initiate the charging process with vapor charging. We charge such systems with vapor till the pressure inside the system increase.

Vapor charging is done till the pressure rises to a corresponding boiling temperature of 0°Celsius or more.

The refrigerant R-134A can boil at 0 °Celsius when the pressure is less than 30 psig. While charging, an R-134A system is filled with vapor refrigerant until the pressure rises to 30 psig. Similarly, the refrigerant R410A can boil at 0 °Celsius when the pressure is less than 110 psig.


In this module, we learned the correct methods of charging a system back with refrigerant. We also learned a few pressure-temperature values that are expected to be memorized by the student.




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...




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...




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

Scroll down for the answer...




Answer: 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.

Scroll down for the answer...




Answer: 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...




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.


Question #6: What is the approximate refrigerant pressure of an R-410A machine at 80 °Fahrenheit while the machine is idle?

  1. 143 psig

  2. 212 psig

  3. 238 psig

  4. 260 psig

Scroll down for the answer...




Answer: 238 psig If an R-410A appliance is at 80 °Fahrenheit, the pressure of the refrigerant in the appliance will be 235 psig. The most approximate answer would be 238 psig. Question #7: The refrigerant has been recovered to replace the evaporator coil in an appliance. After the repairs, the refrigerant;

  1. can be filled back into the system.

  2. can be sold as it is to another equipment owner.

  3. must be reclaimed.

  4. must be destroyed.

Scroll down for the answer...




Answer: can be filled back into the system. Any recovered refrigerant can be filled back into the system once the repairs are done. Question #8: How can we charge a system having 80 pounds of R407C and has both an air-cooled condenser and evaporator?

  1. Vapor charge through the suction service valve.

  2. Vapor charge through the liquid-line service valve.

  3. Liquid charge through the suction service valve.

  4. Liquid charge through the liquid-line service valve.

Scroll down for the answer...




Answer: Liquid charge through the liquid-line service valve. We can use liquid charging in a system that has an air-cooled condenser and evaporator coils. As these systems do not have water, there is no risk of freezing during liquid charging. Liquid charging is always done via the liquid-line service valve. Question #9: What is the evaporation temperature of R-134a at 0 psig?

  1. -21°Fahrenheit

  2. -15°Fahrenheit

  3. -5°Fahrenheit

  4. -1°Fahrenheit

Scroll down for the answer...




Answer: -15°Fahrenheit The evaporation temperature of R134a at 0 psig is -15°Fahrenheit. Question #10: The risk of freezing while charging an R-410A appliance will remain till from a vacuum level to a pressure of approximately;

  1. 32 psig.

  2. 63 psig.

  3. 90 psig.

  4. 110 psig.

Scroll down for the answer...




Answer: 110 psig. The refrigerant R410A can boil at 0°Celsius when the pressure is less than 110 psig. Once the pressure reaches 110 psig, there is no risk that the water in the appliance will freeze.

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