"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.