• SkillCat Team

Evacuation & Charging Type 3 HVAC Equipment

Updated: Apr 8

EPA Type 3 Chapter 5

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Evacuation


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!


Recovery Procedure

Evacuation is when we remove all water vapor and air from the refrigeration 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.


Recall that a vacuum pump is used to evacuate the system to the required levels. In chillers, the cooling water or moisture from the outside air can leak into the system. Evacuation is the final step to remove all these before filling the system back with refrigerant that we already recovered.


The required levels of evacuation for low-pressure systems are:

  1. 25 mm Hg for equipment manufactured before November 15, 1993.

  2. 25 mm Hg absolute for equipment manufactured after November 15, 1993.

Recall that the term absolute means pressure measured with respect to vacuum.


When repaired for any leaks, all low-pressure systems and chillers must be evacuated till the mentioned levels of pressure. 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.


Before disposing of a low-pressure appliance, it must be evacuated as per the regulations to prevent venting of refrigerants in the atmosphere. The equipment must be evacuated to 25 mm Hg absolute before disposal.


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 water gets converted into ice at very low temperatures. If ice is formed, it clogs the refrigerant piping and must be avoided.


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 increases the pressure and temperature before freezing the does not allow the moisture to accumulate in one place. The gas, along with water, 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.



Charging


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


P-T Chart Uses

Recall that P-T Charts have a 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.

Consider this P-T Chart for R-123. The value 40° Fahrenheit is marked in the chart, and it corresponds to 18.1 inches Hg vacuum. It means that R-123 boils at 40°F when the pressure is 18.1 inches Hg absolute. Recall that the red pressures are vacuum pressures given in the units ‘inches of Hg absolute.'


Charging a System

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. But as major Type 3 appliances use water, this method is not directly usable.


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 saturation pressure of R123 at 0°Celsius is around 20 inches of Hg absolute.


In the chart, the value 20 inches (around 19.5) of Hg vacuum is highlighted, and it corresponds to 35 °Fahrenheit. It means R123 will boil at 35° Fahrenheit when the pressure is 20 inches ( around 19.5) of the Hg vacuum. Recall that water boils at 212° Fahrenheit when it is at atmospheric pressure.


The refrigerant can boil easily because it only needs a temperature lower than 35° Fahrenheit to boil when the pressure is 20 inches ( around 19.5) of Hg vacuum. 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. This is the reason that a chiller system is always first charged with vapor refrigerant.

Vapor charging is done till the pressure in the system rises to the pressure corresponding to a saturation temperature of 35° Fahrenheit or 0° Celsius. Similar PT charts will be available in the EPA exam and one should be able to read the charts to find saturation temperatures and pressures.

Charging Requirements

It is advisable to always charge the vapor refrigerant from the lowest point of the system. The reason behind this is that the refrigerant can easily flow from a high-pressure refrigerant cylinder to the comparatively low-pressure evaporator charging valve of the chiller.

The evaporator charging valve is the lowest access point on all centrifugal chillers. It is the valve from where the refrigerant should be filled back into the system.


In this module, we learned the correct methods of charging a system back with refrigerant. We also learned a few uses of pressure-temperature relations that are important for charging a chiller system.


Question #1: Identify the recovery requirements for equipment disposal

  1. 0 psig

  2. 15 inches of Hg vacuum

  3. 25 inches of Hg vacuum

  4. 25 mm of Hg absolute

Scroll down for the answer.....


Answer: 25 mm of Hg absolute Before disposing of a low-pressure appliance, it must be evacuated as per the regulations to prevent venting of refrigerants in the atmosphere. The equipment must be evacuated to 25 mm Hg absolute before disposal. Question #2: 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 #3: 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 #4: How do you know when enough vapor has entered the appliance before you charge refrigerant liquid?

  1. Refrigerant saturation temperature increases above 32°F.

  2. Recovery unit liquid level drops to lower pressure.

  3. Vapor is charged for 15 minutes and then the liquid is added.

  4. Recovery unit pressure drops to a lower pressure

Scroll down for the answer...


Answer: Refrigerant saturation temperature increases above 32°F. When the refrigerant saturation temperature increases above 32°F (freezing point of water) is the point when you know that the system has been charged with enough amount of vapor refrigerant. You can start with liquid charging after this point. Question #5: Charging liquid R-245fa into a low-pressure refrigeration system under a vacuum greater than 18 inches Hg vacuum can cause the;

  1. liquid to absorb excess moisture.

  2. purge unit to operate.

  3. system water to freeze.

  4. system water to boil.

Scroll down for the answer...


Answer: system water to freeze. If the system is under a vacuum greater than 18 inches Hg vacuum, refrigerant liquid filled into the system will boil. 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. Question #6: Where is the lowest access point on a low-pressure centrifugal air-conditioning appliance?

  1. Evaporator charging valve

  2. Condenser service valve

  3. Compressor cooler jacket

  4. Purge unit exhaust port

Scroll down for the answer...


Answer: Evaporator charging valve The evaporator charging valve is the lowest access point on all centrifugal chillers and air-conditioning appliances.

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