# Thread: One more quiz question

1. ## On Call

"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."

Richard - A 'jug' of refrigerant is exactly what a refrigeration system is, a closed circuit. What happens in the jug is exactly what happens in the refrigeration 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%."

Richard - Liquid does not exert any pressure other than by weight.
Liquid can pass pressure - if you put pressure behind the liquid it will pass this pressure on, pushed by the pressure behind it.
Ok look at it this way, if I put liquid into a container, how much pressure will be on top of the liquid ?? If I put a top on the container, will there be any pressure on top ??
Now let me heat up the closed container, Trying to absorb the heat, the liquid will boil creating vapor. This vapor will expand and will push on the sides ( & top ) trying to expand further - this is pressure, created by vapor. The greater the heat, the more vapor produced that will be contained within what space there is, this will increase pressure. If I let the container cool down, what happens??
The vapor will condense back to a liquid and the pressure will reduce, all the back to zero, when all vapor has been condensed.

"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."

Richard - The compressor, at start up, will try to pull the evap. coil into a vacuum, This does lower the boiling point of the refrigerant, and thats all. If the refrigerant does not boil, the system would still be in a vacuum. The compressor only reduces the boiling point making it easier for the refrigerant to boil, but without this boiling, where would pressure come from?? It only comes from the vapor that is created by the boiling, as the vapor tries to expand, just like the 'jug'.

"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."

Richard - Look at it this way, What happens when the evap is a solid block of ice. No air flow across coil to provide heat, just 32 degree ice around the coil. What does the pressure do. Is the compressor still running - if so , then how did the compressor cause the pressure to go down?? I've seen frozen evap with suction pressure less than 20#.

"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?"

Richard - With low charge,The refrigerant temperature would be lower than 32 'f' and the humidity from the air would freeze on the coil.

"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".

Can't answer that as I know very little of RSES.

Hope this helps,
I thouroughly enjoy these discussions.
Richard

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Liquid can and does sometimes have pressure!

Hydrostatic pressure is not pressure that is passed on by any force outside of the container when I'm talking about it in this forum. The field of hydraulics uses liquids to transmit force due to fact that liquids are basically uncompressible. However, liquids do expand and contract due to density changes in it caused by temperature changes. It is not caused by the weight of the liquid. Hydrostatic pressure is the same here on Earth as it is in space where the mass of the liquid would have not any weight caused by gravity. It is a real, internal pressure caused by the expansion of the liquid inside the container. If the container doesn't yeild to the pressure (basically expand with it); failure of the cylinder is certain.

The 80% fill limit is so that the volume of the cylinder cannot be full of liquid and thus create a hydrostatic force (at expected ambient temps <130 degress) of several thousand PSI.

I'm not denying the fact that it's the vapor that causes the pressure. If I said that or implied anything else, it was unintentional and wrong. What I'm saying is it's the compressor suction that sets up the pressure due to the regulation of the volume of vapor removed from the coil. The regulation of the pressure by the compressor dictates the saturated boil pressure and temperature in the evap coil. If you would vary the suction of the compressor and keep the discharge pressure constant, the evaporator pressure would have no choice but to go down and therefore temperature would also. This would happen regardless of heat load on the coil.

BTW where did you teach and what are you doing now?

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The jug of refrigerant compared to the closed loop of an A/C system seems incompatable to me. The difference being the jug of refrigerant does not have a metering device with one side receiving positive pressure and the other side being pulled (suction).

4. Ok, I'm going to change my answer to both the evap load, and the compressor.

5. ## On Call

"The field of hydraulics uses liquids to transmit force due to fact that liquids are basically uncompressible."

Richard: Sounds like exactly what I am saying - liquid cannot be pressurized - the liquid can only transfer the pressure put upon it by another force. If I fill a pipe, 3/4 full with water ( and cap both ends - laying horizontally ), and put a pressure gage on the top of the pipe, what would the pressure gage read?

"What I'm saying is it's the compressor suction that sets up the pressure due to the regulation of the volume of vapor removed from the coil."

Richard: Ok, then, if there is not any refrigerant in the system, what does the compressor do?? - pulls into a vacuum - yes?? How is the compressor setting the suction pressure or regulating it. The compressor is a machine, it only pumps up what is put into it. The compressor just 'sucks' from the suction line - no regulation - it just sucks as hard as it can.

"The regulation of the pressure by the compressor dictates the saturated boil pressure and temperature in the evap coil."

Richard: This, I must admit, I cannot comprehend.

"If you would vary the suction of the compressor and keep the discharge pressure constant,the evaporator pressure would have no choice but to go down and therefore temperature would also. This would happen regardless of heat load on the coil."

Richard: The compressor is a "ratio pump", it pumps up whatever comes in to a certain ratioed output ( which can be varied some ). How can the compressor determine what goes into it?? If there is no liquid / vapor, then nothing goes in, where yur suction pressure then??

"BTW where did you teach and what are you doing now?"

Richard: I taught at Wallace,HVACR night courses,for
the last three years, am in business for myself
for 14 years

Now to Jack David
"The jug of refrigerant compared to the closed loop of an A/C system seems incompatable to me. The difference being the jug of refrigerant does not have a metering device with one side receiving positive pressure and the other side being pulled (suction)."

Richard: The 'jug', David, shows what happening inside the coils of the system. As far as the boiling and condensing of the refrigerant. You used to could meter the refrigerant out of a 'jug', by cracking open the valve.Of course thats illegal now!!
As the 'jug' is a closed system as is the refrigeration system , it follows the same characteristics ( for lack of better word ). I did not mean to insinuate that the jug had a metering device as it does not have a compressor either.

To all, I have thououghly enjoyed this, even in the midst of our disagreeing. Thanks ya'll

Richard

[Edited by bornriding on 08-08-2005 at 06:26 PM]

6. I was always taught that the compressor determines suction pressure.

7. ok now i am confused and i am sure all the diys are even more so.
two instructors and two diferant opinuns( spelling sorry)
if the compressor sets up the pressure in the evaporator then what does the vapor do.
the compressor is just what it is called a compressor
it pulls in vapor compresses it to a higher pressure. in the process the tmp also goes up with the pressure. then it is pushed into the condensor coil where the heat is obsorbed and the vapor condenes to a liquid because of the higher pressure and temp. liquid is then metered into the evap coil with a presure drop where it can absorb heat and turn into and vapor. it would seem to me in layman turms the pressure is controled buy the amount of liquid going through the meter and the amout of heat absorbed
how does the compressor control any of this when all it does is compress. even the flow is a biproduct of compression. i have found this also a great discussion

8. ## Re: Norm

Originally posted by bornriding
And to expound further, I never use multiple choice questions for my exam. Mine were fill in the blank or listing questions.
Not that I've ever formally taught hvac/r, I would consider well crafted multiple choice questions to be a fair measure of a test taker's knowledge. If one were responsible for measuring the knowledge of large numbers of hvac/r students, I would consider well designed multiple choice tests to be much more efficent and effective than alternative tests.

Originally posted by bornriding
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.
Poorly crafted multiple choice questions perhaps? Or perhaps questions asked that border on the common sense? Having taken graduate courses in economics, I suspect I could still develop fair multiple choice type questions that would terrify an economics newbie.

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Yes, I agree most DIYers are probably saying "Who cares all I want is cooling."

This thread would have been served better in the Pro Section.

We are getting nowhere fast.

This is my last post on the subject and have enjoyed the discussion, but since neither me nor bornriding have been able to change the other's mind, there is no reason to belate the issue. It would have been nice if some of the regular experts voiced their oppinions on this matter.

So, I'll leave some references than can be checked:

From Modern Refrigeration and Air Conditioning, 18th edition, pg. 181, 5th paragraph... (they're talking about TXVs here)

"The thermostatic expansion valve does not regulate the low side pressure. It controlls the filling of the evaporator with refrigerant. The pumping action of the compressor establishes the low-side pressure.."

For those who have the RSES Technical Manual series. Manual #1, Lesson #3, pg. 12, under the Title "Evaporating Temperature"

"In a mechanical refrigeration system, the pressure on the refrigerant determines the temperature at which it evaporates. This pressure is maintained by the compressor. The compressor is basically a pump that compresses vapor. It draws vapor out of the evaporator. The evaporator is where the liquid refrigerant absorbs heat and turns to a vapor. While the compressor draws out vapor, more is being formed by vaporization of the liquid. If more vapor is removed than is being formed, the pressure is lowered. This occurs according to the pressure-temperature relationship for the refrigerant."

There are numerous other statements in the RSES Manual with the same statement.

The vapor causes the pressure. No doubt. Yes. I agree. BUT the compressor suction determines at what pressure the liquid will boil at and thus determines evaporator pressure and sets the temperature.

The jug is not an accurate explanation for the evaportor for the simple fact that the jug volume is constant and cannot change. Whereas the volume of the evaporator isn't.

Let me explain:

An analogy of the suction would be like a popoff valve. Assume you replace the compressor with a popoff valve and set the cracking pressure at approx 68 PSI. Also suppose there was a container that provided a source of liquid refrigerant to the metering device. As the refrigerant boiled in the evaporator, the only pressure regulating device in the circuit is the popoff valve. As the vapor pressure increases due to more liquid converted to gas, the popoff valve starts to crack open venting some of the vapor. Evaporator temperature is 40 degrees. As a result the pressure starts to go down again.
So the suction of the compressor does the same thing. The pumping action in the suction draws vapor from the evap coil and lowers the pressure again. Of course, venting refrigerant is a waste (and illegal) so we recompresses the vapor for condensing in the condensor to start the cycle over.

What makes pressure? Vapor. What determines the pressure and ultimately temperature of the evaporator? Compressor suction.

[Edited by on call on 08-09-2005 at 07:23 AM]

10. "In a mechanical refrigeration system, the pressure on the refrigerant determines the temperature at which it evaporates. This pressure is maintained by the compressor. The compressor is basically a pump that compresses vapor. It draws vapor out of the evaporator. The evaporator is where the liquid refrigerant absorbs heat and turns to a vapor. While the compressor draws out vapor, more is being formed by vaporization of the liquid. If more vapor is removed than is being formed, the pressure is lowered. This occurs according to the pressure-temperature relationship for the refrigerant."
oncall
thank you for this info and i do stand corrected. interpatation is a wonderful thing
as far as this going no where this is not true as long as someone can learn somthing even the diy as long as they want to then it is going some where. i have always thought the vapor controled the pressure and temp and the compressor just pushed. as far as you two not seeing eye to eye believe it or not differant ways of seeing something is how others learn. you have both been helpful more then you relize while sharing what you know. i only hope it doesnt stop. got me thinking and i am sure a lot of others to. also all the diy"s in here even if they got lost realize there is more to heating and ac then just looking at gauges. we need to know more then what meets the eye. you and bornriding have been extremly well versed and gave out a lot of great info. thank you

11. What causes suction pressure?

The entire refrigeration cycle.

Compressor, condenser, liquid line, metering device, evaporator, suction line, back to compressor.

Suction pressure would be useless without a metering device to impose a pressure drop between liquid side and vapor side.
Refrigeration would not occur for a given heat load if the refrigerant could not flash into a vapor at the right pressure due to both the metering device and the compressor reducing the pressure on the downstream side of the metering device.
Compressor could not send proper amount of absorbed heat to be rejected in condenser if refrigerant could not flash into vapor at correct pressure.

Hmm...sounds like I'm listing off reasons for the significance of suction pressure vs. what causes suction pressure. But it bolsters my point...the entire refrigeration cycle causes suction pressure.

To address the question more directly, my focus would be on metering device, refrigerant vapor pressure, and compressor suction (not suction "pressure", but compressor suction...the evacuating effect the compressor imposes upon the evaporator).

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Shophound, you are right. There are a lot of dependancies in the system. There is a cause and effect in the whole refrigerant system and changing one variable affects everything else. Nothing about this is really cut-and-dry and open-ended questions can and usually spark debate.

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METERING DEVICE!!! Change orfice..... pressure changes. Defective TXV.....Defective pressuers. The owner of a company ultimately controls what happens in that company because he is First in Line! The TXV is First inLine by using superheat to control pressures. On a cap tube system, the amount of pressure before the metering device controls how much refrigerant passes through. The compressor and charge controls suction presure in these systems.

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