What happens when capacity of system is exceeded?

[Tim]

No need to kill the thread.

[DetroitAC]

The thermal power supplied to the condenser is the total of the thermal power absorbed by the evaporator and the power transferred to the refrigerant by the compression process, plus all the other tiny heat gains.

The condenser pressure will continue to rise if more and more thermal power must be rejected, until the HPCO or pressure transducer reaches it's setpoint, or the relief valve pops. The condenser could keep going, but hoses will burst.

Evaporator cooling power is the enthalpy change of the refrigerant from evap in to evap out multiplied by mass flow. More volume flow of suction gas through the evaporator means more mass flow of refrigerant, therefore more refrigerating power.

Your system is not what I'd call "normal", though right? you have basically a suction line pressure regulator. The behavior is different, the rules are different.

A bigger TXV will not increase the flow. I was trying to explain this. Let's say the "stock" TXV from a "normal system is operating at a given set of conditions, the evap capacity is 8kW, the superheat is 5K, the valve is 80% open. We drop in a larger size TXV, adjust the bias spring nut so we get again 5K superheat. The evap capacity will be 8kW, the valve might only be at 60% open, but it's a bigger valve, the effective opening of these two valves is the same, performance is the same.

A bigger TXV is ABLE to handle more flow. It does not just flow more by itself. It flows exactly what the evaporator thermal load demands.

[cyfi66]

The thermal power supplied to the condenser is the total of the thermal power absorbed by the evaporator and the power transferred to the refrigerant by the compression process, plus all the other tiny heat gains.

The condenser pressure will continue to rise if more and more thermal power must be rejected, until the HPCO or pressure transducer reaches it's setpoint, or the relief valve pops. The condenser could keep going, but hoses will burst.

Evaporator cooling power is the enthalpy change of the refrigerant from evap in to evap out multiplied by mass flow. More volume flow of suction gas through the evaporator means more mass flow of refrigerant, therefore more refrigerating power.

Your system is not what I'd call "normal", though right? you have basically a suction line pressure regulator. The behavior is different, the rules are different.

A bigger TXV will not increase the flow. I was trying to explain this. Let's say the "stock" TXV from a "normal system is operating at a given set of conditions, the evap capacity is 8kW, the superheat is 5K, the valve is 80% open. We drop in a larger size TXV, adjust the bias spring nut so we get again 5K superheat. The evap capacity will be 8kW, the valve might only be at 60% open, but it's a bigger valve, the effective opening of these two valves is the same, performance is the same.

A bigger TXV is ABLE to handle more flow. It does not just flow more by itself. It flows exactly what the evaporator thermal load demands.

Thanks Detroit I am following most of what you are saying. What I dont understand is why you assume that the TXV I have installed has the capacity to meet the desired system performance.

You keep mentioning that a smaller TXV open at 80% is equivalent to a larger one open 60% which I understand, but how do you know my TXV is not open 100%? What can I do to rule in or rule out that the TXV is or is not open to 100% when I have the AC maxed out on a scorching day? I do not have physical access to the sensing bulb with the system running otherwise I'd drop it in warm water and observe while the system is running.
Also I have eliminated the evaporator pressure regulator.

[tbirdtbird]

If your pipe into evap is same temp as pipe out, the TXV is doing all it can. The pipe out might be a bit colder, which is OK. If warmer, then TXV is not open enough. You can do the 'feel' test.
Same deal to test how your condenser is doing. Measure temp of pipe in and pipe out. Should be at least 20-25° temp drop. A well installed clean air cooled coil will give a 30° temp drop. Although a thermocouple is preferred, if the pipes are flat black or can be painted black, you can use an IR gun. Check several times for reproducibility. Report back.
Your car's radiator should have a 30° drop as another example . This is also the standard for residential AC, and every freezer box I have worked on.

Depending on access, you can use your temp gun to go down the rows of a radiator or condenser and see the temps drop.
In this shop we do this all the time for diagnosis, as I am sure the other consultants on here do.

I don't recall a lot of diagnostics being used in this very long thread, mostly theoretical discussion and musings

[cyfi66]

Unfortunately the evaporator, TXV, sensing bulb are all burried deep in the dashboard. In order to access them the blower must be removed, so I can't run the system and access them simultaneously.

I can say almost for certain the inlet is colder than the outlet significantly because the outlet pipe is only cool to warm while the saturation temperature in the evap is 45 degrees F. The outlet pipe is certainly not that cold. This happens when blasting the AC when it's scorching outside 5 mins after startup.

This is the reason I keep thinking the TXV might be inadequate.

[tbirdtbird]

OK, post back once you have evaluated the condenser as mentioned. If the condenser has not removed sufficient heat from the refrigerant then you are starting off on a bad foot at the evap.

This is a package deal, the evap and condensor working together....

[DetroitAC]

What I dont understand is why you assume that the TXV I have installed has the capacity to meet the desired system performance.

Yes, guilty as charged, I've done a very poor job of reading this entire thread to keep up to date on what you've changed.

A TXV that is in control of the superheat is able to keep a mostly constant superheat with changing loads. I say mostly constant because a TXV is not a linear device, there are nonlinearities.

If the evap exit superheat is 5K at a set of conditions like OSA, full blower, 40C ambient, panel, full cold, windows open, 2 vents closed. Then same conditions, all vents open and the superheat is still ~5K with a little variation, she's in control, not 100% open.

If you find a condition where you have low superheat (cold suction line) and then if the cooling capacity increases, the superheat steadily increases until the suction gas temperature is pretty much the evap incoming air temperature, then TXV is 100% open, not in control, just a fixed orifice.

[cyfi66]

Sorry to revive an old thread, did some thinking though!

If the TXV loses control and is 100% open, one would expect the low side pressure to start to go down, correct? This would mean the TXV is bottle necking the system and the compressor is overcoming the capacity of the evaporator. As long as the low side/evaporator pressure remains in the normal range I can safely say the TXV is still in control and modulating refrigerant flow.

I would expect that once the TXV is open max and the superheat starts rising the evaporator pressure would start going down since the compressor is pumping but unable to push any more refrigerant through the orifice. Similar to if you had a plugged TXV, since a plugged TXV would mimmick an undersized TXV.

[Mark86]

Add a mister system to the front of the condenser. Use distilled water and a half dozen mister nozzles.