Compression ratio question

[coolguy4]

Hello everyone, I have a basic question about compression ratio. I understand what compression ratio is and that running a compressor at a higher compression ratio from a higher temp differential, plugged filter, etc generally results in a lower flow rate and I understand why due to increased internal leakage and the re expansion of the gas.

However, my understanding is that the reason low ambient temps without proper condenser fan control can be a problem is that the head pressure may drop too low to push enough refrigerant through the expansion device to keep the system working properly, which makes sense. But wouldn't the lower compression ratio mean that the compressor is able to move more refrigerant?

If someone would be able to explain to me under what conditions a lower compression ratio would result in more or less refrigerant flow I would greatly appreciate it as I am a bit confused. Thank you very much in advance!

[RANGER1]

Compressor with lower compression ratio works more efficiently & moves more refrigerant.
All good if expansion can operate at lower differential pressure.
Can’t see a downside if system components designed for it.
VSD on condensers & compressors good
EEV cover all situations.
If compressor more efficient without VSD or capacity control, then could cause lower than normal suction pressures or system imbalance, short cycling, the list goes on

[coolguy4]

Compressor with lower compression ratio works more efficiently & moves more refrigerant.
All good if expansion can operate at lower differential pressure.
Can’t see a downside if system components designed for it.
VSD on condensers & compressors good
EEV cover all situations.
If compressor more efficient without VSD or capacity control, then could cause lower than normal suction pressures or system imbalance, short cycling, the list goes on

That's what I thought, thank you for clarifying! So the potential problem with low ambient operation is actually that the compressor could move too much refrigerant due to the increase in efficiency from running at a lower compression ratio?

[RANGER1]

That's what I thought, thank you for clarifying! So the potential problem with low ambient operation is actually that the compressor could move too much refrigerant due to the increase in efficiency from running at a lower compression ratio?

Everything has it’s design & design limits.
Depends what you want to achieve & how much you want to spend.

[coolguy4]

Everything has it’s design & design limits.
Depends what you want to achieve & how much you want to spend.

I wasn't asking because there is something specific I wanted to achieve, I am just trying to learn about the effects different operating conditions has on the system and the reasons why. One of the things I am confused about is why low ambient temps can be problematic. I was told that lower ambient temps result in a lower head pressure and therefore less refrigerant being forced through the metering device due to the lower pressure differential across it, which makes sense. However, that logic also goes against what I have been told about a lower compression ratio will increase the compressor's capacity and therefore result in more refrigerant being pumped.

[NH3LVR]

RANGER1 explained it.
"All good if expansion can operate at lower differential pressure"
Imagine this situation. You have a Mechanical Thermal Expansion Valve. It is sized to feed sufficient liquid to an evaporator to maintain the correct superheat. Now if you drop the head pressure from 150lbs to 100lbs the valve (Think of it a a variable orifice) cannot feed enough refrigerant to the evaporator. Superheat climbs. If the system was designed to operate at the lower head pressure all would be fine. However there are limits. I had customers in years past that were told by the energy folks to turn down the head pressure and their systems lost capacity.
Low head can also cause oil carryover. But that we can discuss later. Going to watch the Nascar Race!