• SkillCat Team

Refrigerant Properties

EPA 608 Type 1 Chapter 5 (Take full course for free)


In this module, we will take a look at how to read Pressure-Temperature charts. We will also discuss the pressure- temperature relationship of refrigerants and talk about how to use it to see whether there are non-condensables present. Skip to quiz!


1. Reading PT Charts


Each substance has a unique saturation pressure-temperature relationship. By this, we mean that for a known refrigerant, we know its saturation pressure given a temperature, and vice versa.


If we list out all the saturation pressures of R-22 or HCFC-22 at different temperatures, we get a PT chart that looks like this. A PT chart is a saturation pressure-temperature chart.

To read this chart, let’s zoom in. The units for temperature are either °F or °C, as we can see up here.


The units for saturation pressure is given under where it says Pressure. In this case, the pressure will be given in units of psig.


To find the pressure at a temperature, we look for the temperature, then look across that row for the corresponding pressure. For example, at 10°F, we see that the saturation pressure at the temperature is 32.8 psig.


And at 50°F, its saturation pressure would be 84.1 psig. This means that the refrigerant R-22 will vaporize at a pressure of 84.1 psig if the temperature is 50°F. And vice versa, if the temperature is known to be 50°F, we know the pressure that the refrigerant vaporizes at is 84.1 psig.


Remember that this chart is specific to the refrigerant R-22. If we wanted to do this for another refrigerant, we would have to look up the PT chart for that refrigerant. For example, for the refrigerant R-134a, we would look up “R-134a PT Chart”


For the EPA exam, we need to know how to reference PT charts to:

  • Find a saturation pressure given a temperature, or

  • Find a temperature given a saturation pressure

For refrigerant blends, we will see two pressures in their PT charts:

  • Vapor Pressure, and

  • Liquid Pressure

This is because the different components of the blend may be in different states at a given temperature. This is the PT chart for R-404a, which is a near-azeotropic blend.


Recall that near-azeotropic refrigerant blends act almost like azeotropic blends. So the components in the blend have a small difference between their boiling points. We see this reflected in the small difference between liquid and vapor pressure in R-404a’s PT Chart.


Recall that zeotropic refrigerant blends contain components that have different boiling points. For refrigerant blends that are zeotropic, we will see a larger difference between the two pressures. For example, we see a much larger difference in the pressures with R-407c, which is zeotropic.


And recall that azeotropic blends contain components that have the same boiling point. For azeotropic, or near azeotropic blends, the difference between the liquid pressure and vapor pressure is much smaller, if there is one at all. For example, we see a much small difference in pressures in the near-azeotropic refrigerant R-404a.


2. Non-Condensables


The pressure-temperature relationship is kind of like the fingerprint of a refrigerant. We can look at a refrigerant’s PT chart to see what the pressure of a refrigerant has to be at a specific temperature.


We can use this pressure-temperature relationship to see if there are non-condensables mixed in with refrigerant. To use this relationship, we need to measure both the pressure and temperature of the refrigerant.


If we measure the temperature and pressure values of a refrigerant and they are not the same as in its PT chart, then there are non condensables present in the cylinder. Non-condensables in the refrigerant cause the refrigerant pressure to be different than what it’s supposed to be.


For example, let’s say we know that the refrigerant is R-22 and the temperature is 70°F. Then, the pressure of the refrigerant should be 121.4 psig.


If we measure the pressure to be 130 psig, then that means there are non-condensables present because the pressure does not match. If the pressure is 122 or 120 psig, this is close enough and the difference may be due to human error.

In order to use pressure and temperature readings to check for non-condensables, we have to know that the pressure and temperature are both stable in their readings. For example, if the temperature of the room is changing, from 70°F to 75°F, we cannot use those temperature values to compare against the refrigerant’s PT chart.


Let’s say we recover R-22 from a system into a recovery cylinder. And then we want to check if there are any non-condensables in the recovery cylinder.


We need to first make sure the pressure and temperature are stable. We need to first let the recovery cylinder rest and come to room temperature. This lets us know that the temperature won’t change in between readings because it is in equilibrium with the room temperature.


After the recovery cylinder comes to a temperature that is the same as your room temperature, you can then perform the following steps to verify its pressure:

  • Look at the system’s high side pressure, and

  • Use your PT chart to check what the pressure should be.

If the pressures are close, you do not have non-condensables in your refrigerant. If it’s not close, there are non-condensables present and you need to recover, evacuate, and recharge the system. We evacuate to get rid of the non-condensables.


3. Conclusion


In this module, we learned how to make sense of and use PT charts. We also learned how to use these charts to see if there are non-condensables in a cylinder of recovered refrigerant.


Question #1: What is the approximate pressure of a storage container of R-22 that is stored at 75°F? Assume there are no impurities.

  1. 131 psig

  2. 308 psig

  3. 78.8 psig

  4. 22 psig

Scroll down for the answer...











131 psig!

Use a R-22 PT chart like this one for reference. At 75°F, we see that the pressure is 132.2 psig. That would be closest to answer choice (a).


Question #2: A near azeotropic blend will contain

  1. One saturation pressure value

  2. One high saturation pressure value

  3. Two saturation pressure values that are close

  4. Two saturation pressure values that have a big difference

Scroll down for the answer...











Two saturation pressure values that have a big difference

Near azeotropic blends have multiple components that go through phase changes at roughly the same temperatures.

This is why their components will have close saturation pressures, if not the same pressure value.


Question #3: A zeotropic blend will contain:

  1. One saturation pressure value

  2. One high saturation pressure value

  3. Two saturation pressure values that are close

  4. Two saturation pressure values that have a big difference

Scroll down for the answer...











Two saturation pressure values that have a big difference

Zeotropic blends have a larger difference between their saturation pressures because their components have different boiling points.


Question #4: What is the approximate pressure of a storage container of R-404a that is stored at 60°F? Assume there are no impurities.

  1. 88.2 psig

  2. 108 psig

  3. 125.8 psig

  4. 269 psig

Scroll down for the answer...











125.8 psig

Use a R-404a PT chart like this one for reference. At 60°F, we see that the two pressures are 126 psig and 124.2 psig. These would be closest to answer choice (c).


Question #5: We can use a known refrigerant’s PT chart to see if there are non-condensables in the refrigerant.

  1. Always true

  2. Only true if we can measure the refrigerant’s temperature

  3. Only true if we can measure the refrigerant’s pressure

  4. Only true if we can measure both the pressure and temperature

Scroll down for the answer...











Only true if we can measure both the pressure and temperature

We can measure a known refrigerant’s pressure and temperature, and compare those values to its PT chart to see if those values match up.

But we first need to know the specific refrigerant, and then measure both its pressure and temperature.

If we cannot measure either its pressure or its pressure, then we cannot compare those values to the values in its PT chart.


Question #6: If a refrigerant’s pressure readings are not stable, we can use its temperature readings to compare to its PT chart to check for non-condensables.

  1. True

  2. False

Scroll down for the answer...











False!

For a known refrigerant, both its pressure and temperature must be stable for us to use them to compare to its PT chart to check for non-condensables.


Question #7: Which of the following values do we check if we suspect that non-condensables are present?

  1. Refrigerant pressure

  2. Humidity

  3. Refrigerant type

  4. Refrigerant weight

Scroll down for the answer...











Refrigerant pressure

We can take the refrigerant’s pressure to check if non-condensables are present.


Question #8: If the pressure reading and expected pressure are far apart, which of the following are possibly contained in the refrigerant?

  1. Non-condensables

  2. Water vapor

  3. Nitrogen

  4. All of these

Scroll down for the answer...











All of these

If the pressure reading is not close to the expected pressure, then non-condensables are likely in the refrigerant. Remember that water vapor and nitrogen are examples of non-condensables so (d) would be correct.


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