# Thread: One more quiz question

1. Born, I respect your job as a teacher and I applaud anything that helps to create discussion and further the learning process. So please view my comments in that context.

It is amazing how simple a refrigeration system is while being very complicated.

Actually I could argue that the suction pressure is only affected by heat, because by the laws of physics the pressure/temperature will always remain the same relationship. So when you say expanding vapor creates pressure you are right and wrong at the same time. I could say it is the change of state the creates the pressure. But without heat the liquid refrigerant would not change states and would flow back to the compressor as liquid.
Born, while in theory there may be a simple answers, in the field there is not.

You can not change any part of a refrigerant system without affecting the entire system. That is why it is important to look at the whole system.

Head pressure will affect suction pressure. Ambient affect head pressure. Air flow will affect suction pressure. Blowers, filters motor speed affect air flow. Humidity affects load. Any restriction in the refrigerant system can affect suction pressure including the necessary restrictor.
Jax

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COMPRESSER SUCTION controls suction pressure.

We are talking about a saturated refrigerant state pressure in the evaporator. It doesn't matter what heat is put into the refrigerant, if it is saturated the pressure and temperature will not change.

Now if we are talking about a superheated gas, then yes, the heat load determines the pressure of the gas using the formula pv=nrt.

The compressor sets up the temperature that the evaporator operates at.

OK, let's look at it like this:

A system is undercharged. What happens to the pressure and temperature at the evap coil? It goes down. Why?
We are still sending some liquid to the coil so the refrigerant is saturated. But, we are no longer sending in the expected pounds/minute of refrigerant and thus the compresser is able to pull the evaporator pressure down lower than normal. That is why the coil starts to freeze. Because the lower pressure lets the refrigerant boil at a lower temperature.

I don't know how we can say that the load controls evaporator pressure when the pressure is saturated. You can sock as many BTUs into the evaporator and as long as there is any liquid pressent, the pressure will not change.

3. The compressor sets up the temperature that the evaporator operates at.

i dont understand this
as long as there is a colon of liquid at the metering device. the metering device controls the presure by letting droplets of liquid through and lowering the presor so these droplets can change state into a superheated vapor. i thought that as long as there in heat to asorb fron the air this is what controls the evap temp and the compresor compresses the supeaheated vopor to a higher presure and temp
how does the compresor set up the evap temp
by the way these questions have been a great way to convers and learn. i find it amazing how much you can learn from just three question. even if someone such as me doesnt know or inturpets diffenant the answers and all the info that has come out is astounding thank you guys
wayne

4. Originally posted by on call
COMPRESSER SUCTION controls suction pressure.

I don't know how we can say that the load controls evaporator pressure when the pressure is saturated. You can sock as many BTUs into the evaporator and as long as there is any liquid pressent, the pressure will not change.
The lquid refrigerant pressure will change with its temp. So the amount of btu's it has absorbed, will change the pressure, not the compressor.

Its the same as checking a bottle of refrigerant, the pressure will corespond with the temp.

If btu content had no effect, then you wouldn't have subcooling after the condenser, or superheat after the evap.

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Bornriding

I hope that as an instructor this question and the compressor question are not the type of questions you use on your student's exams.

These questions are too subjective as they have a variety of correct answers. For example, there is no single factor that determines what the suction pressure will be.

I hate questions on exams that require the student to guess what is in the instructor's mind in order to get the question correct.

In order to correctly and completely answer your questions I would need to compose a three page essay carefully discussing all the factors in the hope of including the one factor the instructor happens to be looking for. That is not an objective test, it is subjective. Subject to what the teacher is looking for.

That is one of the reasons many teachers use objective multiple choice questions that are carefully crafted. Not only is it important to craft the stem carefully but it is equally important to develop good distractors but only one correct answer.

In your defense, I will say that your questions do spark some good discussion and cause people to think. That is great. But, please don't use subjective questions with your students.

Questions that require an essay answer are fine but even those questions must be carefully crafted. They too are subjective but at least they get the student to think and learn to organize their answer and communicate it in writing. However, there should only be a few subjective questions on any test and they should not require the student to second guess what the teacher is thinking.

6. Hey Norm,

So now York wants the line set changed when switching to 410a on a retro.

What gives?

7. ## Norm

I am no longer teaching as the school decided that it did not care about the quality of the HVACR program and since quality is not a compromise to me, I had to offer my resignation.
And to expound further, I never use multiple choice questions for my exam. Mine were fill in the blank or listing questions. My students learned quickly how my questions were asked and I insured that every question on my exams were directly from material that we had covered in class. My students did not get all 'A's but very few failed. Three of my students went on to pass Hvac excellence exams. All with high marks. They said the test was easy after my class. ( electrical & a/c exams).
And no, I don;t believe that there is a multitude of correct answers for a question. Just one - the answer can be expounded upon greatly, but the answer is still the same.

I know you teach shorter classes ( not a semester ), so I can understand the greater need for multiple choice questions. But I will alwsys believe that multiple choice questions are used primarily to insure that students make higher grades ( mostly 'A')
The full-time instructor at the school where I taught told me when I started that he gave multiple choice questions and that he 'taught-the-test. He said 'you have to', trying to get me to do the same thing. I could not do that.

BTW - I clept college economics ( both semesters ) by guessing on a multiple choice exam - I did not even know what economics was, but I passed both exams. So, no I do not like multiple choice questions as they do not make you think as much or as hard as blanks, listing, or describe.
I must admit, the grading is not easy when yur trying to be fair. But I never had a major problem with any student
over my amswers - they knew me to be fair.

As far as my question: to me, only one thing causes the pressure and that is vapor - no vapor = no pressure ( other than whats in a vacuum ). More vapor = higher press ; less vapor = less pressure.(period)

I am not a very smart person, so to learn this stuff, I had to understand the very basics of how the system works. I have to put it all together in my language. Ya'll may not agree with the way I view things, but it is the way I have to view it to understand it.

Sorry for the long post,
Respectfully,
Richard

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Born;

I can see that we disagree on our philosophies of education in a number of areas. But, this thread is not the place for that discussion

You state that I don't teach semester long classes but rather shorter seminars and that they are different. Yes, they are different but be careful there, I taught semester long HVAC college classes for 18-years before I took my current position.

I have a passion for discussions on educational philosophy, instructional design, curriculum development, testing methods and presentational styles. I completed my California college teaching credentials at UC Berekley and in the process took all the educational development courses I could. The entire field of study really captivates my interest so any such discussion is welcome.

Norm

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Originally posted by beenthere

Hey Norm,

So now York wants the line set changed when switching to 410a on a retro.

What gives?

And, where did you get that bit of information? That is not York's standard policy across the board.

10. Originally posted by NormChris
Originally posted by beenthere

Hey Norm,

So now York wants the line set changed when switching to 410a on a retro.

What gives?

And, where did you get that bit of information? That is not York's standard policy across the board.
John Huntsberger.

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In our R-410A factory taught classes we do not teach that the old lines used with R-22 have to be changed out. We "recommend" that they be replaced if possible. We allow reusing the existing lines if they are in good condition, are of the correct size for the system's capacity and length of run and we require that they be cleaned out.

Cleaning them out amounts to blowing them out with nitrogen or using a commercial line cleaning solvent made for that purpose. Small amounts of pre-existing oil from the R-22 system are usually not enough to create a problem unless a burnout has occurred. In the case of a burnout, the suction line should be replaced anyway.

This is York's policy and is what we teach in our factory classes.

12. Thanks.

Thats what I was told in the original seminar, then last month it was different, so I had to ask.

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I don't think that the analogy that since pressure increases in a jug of refrigerant necessarily disproves that the compressor doesn't set up the suction perssure. For one thing that is an enclosed volume and has no where to go so pressure has to go up.

The vapor compression system is a dynamic system and in my opinion shouldn't be reduced in theory down to a refrigerant jug model. That is a closed system. I agree that the vapor causes the pressure that is read on the gauge. Also, to say that liquid has no pressure is wrong. Hydrosatatic pressure is a real and dangerous thing and is why cylinders (and not just refrigerant) aren't to be filled past 80%.

If you take a Mollier diagram and plot the entire vapor compression cycle, you will see that pressure/temperature are horizontal lines in both the evaporator and condenser. The compressor suction allows the vapor to be pulled away at a rate that keeps a constant state in the coils.

All I am saying is that the compressor suction determines what that pressure is going to be because of the pumping action. Yes, there are other parameters that affect the pressure, but the compressor suction is the driving force.

If the load was the only thing that determined the coil temperature, then how come it is possible to freeze a coil with correct airflow across it when the system is low on charge?

Again, if this is not the case then we have an organization called RSES that is misinforming its students. RSES Tech Manuals and their Heat Pump Training Course states that "The compressor suction determines evaporator pressure".

[Edited by on call on 08-08-2005 at 08:16 AM]

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