Superheated vapors and Gay-Lussac's Gas Law

  • #1
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I attend a trade school, majoring in HVAC (Heating, Ventilation, and Air-Conditioning). My instructor has given me information that seemingly contradicts what my textbooks tell me about Charles' Law. My instructor is not a scientist. I asked my instructor to reconcile the two seemingly contradictory facts, and I did not understand his answer. I was hoping you people could explain this to me. Furthermore, I learn better by reading information than by hearing a person speak information.

My instructor said that superheated refrigerant vapor (vapor is the same as gas) in an air-conditioner does not follow a pressure/temperature relationship. Normally, if you know the temperature of a given gas, you can use a pressure/temperature chart to know the pressure of the gas at that temperature. My understanding of my instructor's statement that "superheated vapors don't follow a pressure/temperature relationship" is that if a refrigerant gas is superheated, you cannot use a pressure/temperature chart to know the pressure of a superheated gas is if you know the temperature of the superheated gas, and you cannot use a pressure/temperature chart to know the temperature of a superheated gas if you know the pressure of the superheated gas.

However, the statement that superheated refrigerant vapor in an air-conditioner does not follow a pressure/temperature relationship seems to violate Gay-Lussac's Gas Law. Gay-Lussac's Gas Law states that "the pressure of a gas of fixed mass and fixed volume is directly proportional to the gas's absolute temperature". Isn't the refrigerant in an air-conditioner in a fixed mass and at a fixed volume? Therefore, Gay-Lussac's Gas Law seems to imply that as the refrigerant is heated, the pressure of the refrigerant gas would always increase directly proportionately to an increase in temperature of the refrigerant, but my instructor says that superheated refrigerant gas does not follow a pressure/temperature relationship.

My idea that the Gay-Lussac's Gas Law contradicts my instructor's statement that superheated gases do not follow a pressure temperature relationship relies on my supposition that the refrigerant in an air-conditioner is at a fixed mass at a fixed volume. Is this where there is a flaw in my reasoning? If refrigerant gas in an air-conditioner is not at a fixed mass and a fixed volume, why does my textbook have a chart that shows the pressure/temperature relationship of refrigerant gases? I mean, if refrigerant gas in an air-conditioner is not at a fixed mass and a fixed volume, why do the refrigerant gases follow a pressure/temperature relationship when the refrigerant gases are not superheated?

My instructor's statement also seems to contradict the Combined Gas Law.

Since Gay-Lussac's Gas Law implies that as the refrigerant is heated, the pressure of the refrigerant gas would always increase directly proportionately to an increase in temperature of the refrigerant gas, how can superheated refrigerant gases not follow a pressure/temperature relationship?
 

Answers and Replies

  • #2
mathman
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I thin this is in the wrong forum. It has nothing to do with computer technology. Try one of the engineering technology forums.
 
  • #3
jrmichler
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The pressure/temperature charts apply only to the case where a single gas and liquid are in equilibrium. In that case, a change in volume (at constant temperature) affects the amount of gas vs liquid, but does not affect the pressure. A change in temperature (at constant volume) results in a large change in pressure, as shown in the pressure/temperature charts.

If there is only the gas, and no liquid, then the gas is superheated, and the pressure/temperature charts do not apply. The superheated gas pressure (at constant volume) with then be proportional to the absolute temperature. The absolute temperature is in degrees Kelvin or Rankine.

The amount of superheat in an refrigeration system is typically small - a few tens of degrees at most. Calculate the ratio of the absolute superheat temperature at a pressure to the absolute saturation temperature for that pressure, and you will see that the ratio of pressures is quite small. This is a case where small differences can be ignored. That is what your instructor is telling you.

Also, while the total volume of a refrigeration system is constant, the compressor is moving the refrigerant, so the pressure in this part of the system is different from the pressure in that part of the system. When you fully wrap your mind around that concept, you will be well on the way to understanding refrigeration systems and heat pumps.
 
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  • #4
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jrmichler, you seem knowledgeable about this sort of thing. Your post contained many truisms about air-conditioning, but the truisms were things that I already knew. Then you made other statements in which I don't really see how they answer my question. Perhaps the implications of your points answered my question, but if so, it went over my head. I hope you will bear with me and help me understand this.

If there is only the gas, and no liquid, then the gas is superheated, and the pressure/temperature charts do not apply.
What I don't understand is why the pressure/temperature charts do not apply when the gas is superheated. I know that you said that the pressure/temperature charts only apply when the gas and liquid are in equilibrium. But why do the pressure/temperature charts only apply when the gases and liquids are in equilibrium?

The superheated gas pressure (at constant volume) with then be proportional to the absolute temperature. The absolute temperature is in degrees Kelvin or Rankine.
"The superheated gas pressure will be proportional to the absolute temperature." -----Isn't this the same as saying "superheated gases follows a pressure/temperature relationship" ?

By the way, "superheated gases follow a pressure/temperature relationship" totally contradicts my instructor's statement "superheated refrigerant vapors do not follow a pressure/temperature relationship".

Please reconcile this.

The amount of superheat in an refrigeration system is typically small - a few tens of degrees at most. Calculate the ratio of the absolute superheat temperature at a pressure to the absolute saturation temperature for that pressure, and you will see that the ratio of pressures is quite small. This is a case where small differences can be ignored. That is what your instructor is telling you.
This is the part where I am very confused. I don't see what your point is.

Let's say my superheat temperature (the actual temperature on the suction line) is 55. And the saturation temperature for that pressure on the low pressure side of the system is 45. The ratio is 55:45. The ratio of pressures is approximately 1.1 to 1. That's a low number. Therefore, you are saying "small differences can be ignored". I am just totally baffled by this. I have no clue whatsoever what you mean. You seem to be implying that there is some sort of implications of the fact that this ratio is a small number, and I have no idea what that implication is. Please explain it to me like I am a kindergartener. I am not an engineer.



Also, while the total volume of a refrigeration system is constant, the compressor is moving the refrigerant, so the pressure in this part of the system is different from the pressure in that part of the system. When you fully wrap your mind around that concept, you will be well on the way to understanding refrigeration systems and heat pumps.
I know how air-conditioners and heat pumps work. I am not trying to find out how air-conditioners and heat pumps work. I created this thread to try to understand how my instructors statement ("Superheated refrigerant vapors do not follow pressure/temperature relationship") and Gay-Lussac's Law could BOTH be true. I still don't know how they could both be true.

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P.S. I made a typo in the OP. In the second sentence of the original post I wrote "My instructor has given me information that seemingly contradicts what my textbooks tell me about Charles' Law. I should have wrote "My instructor has given me information that seemingly contradicts what my textbooks tell me about Gay-Lussac's Law.
 
  • #5
PeterDonis
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My understanding of my instructor's statement that "superheated vapors don't follow a pressure/temperature relationship" is that if a refrigerant gas is superheated, you cannot use a pressure/temperature chart to know the pressure of a superheated gas is if you know the temperature of the superheated gas, and you cannot use a pressure/temperature chart to know the temperature of a superheated gas if you know the pressure of the superheated gas.
As I understand it, a pressure/temperature chart gives the saturation pressure vs. temperature, i.e., the pressure at which the phase transition between liquid and gas occurs at a given temperature. So it's not telling you the actual pressure vs. temperature unless the fluid is in fact saturated--i.e., unless it in fact consists of both phases in equilibrium, while the phase transition is taking place. (There can't be a single relationship between temperature and pressure anyway, because it depends on the amount of the substance present, if the volume is held fixed, or on the volume if the amount of substance is held fixed. See below.)

Gay-Lussac's Gas Law states that "the pressure of a gas of fixed mass and fixed volume is directly proportional to the gas's absolute temperature". Isn't the refrigerant in an air-conditioner in a fixed mass and at a fixed volume?
The total amount of refrigerant is, yes. But the total amount of refrigerant is not at a single temperature and pressure: the temperature and pressure of refrigerant varies drastically from one part of the system to another. So you cannot think of the total amount of refrigerant as a single "parcel" at a single temperature and pressure, which is what you would need to do to apply Gay-Lussac's law.

The usual way of thinking about fluids in this kind of situation is to think about a "parcel" of fluid containing a fixed number of molecules (or number of moles) that flows through the system. As the parcel flows, its pressure, temperature, and volume can all vary. So there is not a single relationship between pressure and temperature that always describes the refrigerant; there are three variables that can vary, not two.

It's possible that your instructor was thinking of something along the above lines, but I would need more details about the context of your discussion to be able to give an opinion on that.
 
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  • #6
jrmichler
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Let's say my superheat temperature (the actual temperature on the suction line) is 55. And the saturation temperature for that pressure on the low pressure side of the system is 45. The ratio is 55:45. The ratio of pressures is approximately 1.1 to 1.
Wrong. The pressure / temperature relationship of an ideal gas is related to the absolute temperature. For example, 55 deg F is 515 degrees absolute (more correctly, degrees Rankine). Then 45 degrees F is 505 degrees Rankine. If an amount of gas is in a sealed container at 505 degrees R, and then heated to 515 degrees R, the pressure will change by the ratio 515/505 = 1.02. Given the range of pressures and temperatures in an HVAC system, and the accuracy of the thermometers and pressures gauges used by HVAC technicians, a pressure change of 2% can be neglected. I believe that is what your instructor means.

HVAC people are technicians, not physicists. As such, they focus on what is important to setting up and troubleshooting HVAC systems, and ignore details that are important only to physicists.

Which begs the question: Are you sure you want to be a technician? Many of the technicians that I have known are smart enough to get a degree in engineering or physics, but have chosen to work with their hands.
 
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  • #7
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As I understand it, a pressure/temperature chart gives the saturation pressure vs. temperature, i.e., the pressure at which the phase transition between liquid and gas occurs at a given temperature. So it's not telling you the actual pressure vs. temperature unless the fluid is in fact saturated--i.e., unless it in fact consists of both phases in equilibrium, while the phase transition is taking place.
All this is true, but it does not really answer my question.

(There can't be a single relationship between temperature and pressure anyway, because it depends on the amount of the substance present, if the volume is held fixed, or on the volume if the amount of substance is held fixed. See below.)
There can be a single relationship between temperature and pressure if both the amount of substance present and the volume are fixed. Correct?

The substance in the situation we're discussing is refrigerant. Is both the amount of refrigerant and the volume fixed in an air-conditioner?

The total amount of refrigerant is, yes. But the total amount of refrigerant is not at a single temperature and pressure: the temperature and pressure of refrigerant varies drastically from one part of the system to another. So you cannot think of the total amount of refrigerant as a single "parcel" at a single temperature and pressure, which is what you would need to do to apply Gay-Lussac's law.

The usual way of thinking about fluids in this kind of situation is to think about a "parcel" of fluid containing a fixed number of molecules (or number of moles) that flows through the system. As the parcel flows, its pressure, temperature, and volume can all vary. So there is not a single relationship between pressure and temperature that always describes the refrigerant; there are three variables that can vary, not two.
I think that of all the reponses on this thread so far, the two above paragraphs by you go the farthest to answering my question.

It's possible that your instructor was thinking of something along the above lines, but I would need more details about the context of your discussion to be able to give an opinion on that.
All I remember about the context is that my instructor was contrasting superheated refrigerants with saturated refrigerants. My instructor said something like the following: "Saturated refrigerants follow a pressure-temperature relationship. Superheated refrigerants do not follow a pressure-temperature relationship."
 
  • #8
PeterDonis
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Is both the amount of refrigerant and the volume fixed in an air-conditioner?
You already quoted my answer to this question, in the very next quote in your post.

My instructor said something like the following: "Saturated refrigerants follow a pressure-temperature relationship. Superheated refrigerants do not follow a pressure-temperature relationship."
As you've stated this, it's still too ambiguous for me to make any reasonable hypothesis about what your instructor was thinking.
 

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