It is hard for me to explain this to those with little understanding of physics, this forum seems to have folks with above average understandings so I will give it a fast shot.
THE COMPRESSOR (caps lock) runs at a fixed RPM, if things have not changed in recent years, that number remains 3,500 rpm, the piston has X cubic inches of space, therefore compressors (at least large ones) are rated in cubic feet per minute.
vapors when heated expand, however if they are contained and can not expand such as when contained within copper tubing, they increase pressure instead of expanding the vessel they are contained within.
so if you know a refrigerant will return at 30 psi and you lift it to 150 psi, the compressor is operating at a 5 to 1 ratio. you would indeed match that with a 5 to 1 ratio metering device.
Now that 150 number represents an approximate pressure one would find on r-134a operating at about 115 degrees (no corrections, I am doing this crap from memory) that would be equal to about an 85 degree space temp. You have to lift the refrigerant temp above the space temp because heat only moves from warm to cold so if the refrigerant is not above the space temp it can not release its heat to the space.
now the evaporator inside the fridge must be colder than the fridge temp or it will not absorb any heat from the fridge. 30 psi equates to about 35 degrees (same rule about memory applies)
Now when the space temp drops the compressor will not lift the refrigerant o as high which lowers the power required at the compressor which is good to a point.
once the space temp drops far
enough you might only be lifting the refrigerant the same 30 degrees over space temp so if it were 40 degrees in the space, your compressor would no lift to about 70 degrees or about 70 psi. Now do not forget the metering device is a ratio device of 5 to 1 so your evaporator pressure will drop somewhat at the same rate aka 14 psi.
Now since you understand basic physics, the WEIGHT of refrigerant entering into the cylinder at 14psi is significantly less than the weight pushed in at 30 it is easy to see you have significantly lower amounts of MASS FLOW aka fewer pounds per minute since the pump moves CFM not pounds,but the refrigerating capacity is indeed per pound.
So as the temp drops, you move less refrigerant. This is not a bad thing as when the temp drops, less heat enters the refrigerator. The problem is, the mass flow rate drops FASTER than the refrigerating capacity does so your compressor runs LONGER to do the same amount of work just as it does when the space gets so warm that the refrigerator compressor can not move enough refrigerant to keep up.
The point I was making to Paul in the other thread is simply that he has lowered the operational temp of the refrigerator beyond expectations and it will be running longer to move enough refrigerant to get the job done and by dropping the pressures in this manner he will also possibly lose oil circulation since the refrigerant is now moving at about half of the designed velocities.
This is about as basic as I can make it on a forum. I teach these heat transfer, thermal expansion, and velocity LAWS on a daily basis except this week where I am testifying about them which I despise beyond belief. I said laws in caps, because sometimes the laymen do not accurately represent theory from law and the principles that apply to the above discussed are often referred to as the "theory" of refrigerating capacity, but in fact all it is well established laws of physics and on the science level, there is a huge difference.
in other words at x psi and x temp, r-134a is X cubic feet and that NEVER changes, if it ws theory it would be open to change based upon other forces, it indeed is not.
Ned
PS hope this helped someone, going to bed myself
