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# Charging

## 1. P-T Chart Uses

Recall that 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.

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.'

## 2. 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 a 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 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.

## 3. 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.

## 4. Conclusion

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: 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 a lower pressure.

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

4. Recovery unit pressure drops to a lower pressure

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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 #2: 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.

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system water to freeze.

If the system is under 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 #3: 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...

Evaporator charging valve

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