What happens when capacity of system is exceeded?

[DetroitAC]

OK, I'll just echo what everyone else is saying, but I'll put them in the order I think they **usually** go in, some vehicles will be different, but this is a generalization, I guess. I'm assuming that for every change made, Evap out Superheat is set to an optimum, Condenser out subcooling is set to an optimum.

1. Increase compressor capacity - i.e. bigger displacement, bigger pulley ratio
2. Increase evaporator capacity - i.e. bigger size or more airflow
3. Increase condenser capacity - i.e. bigger size or more airflow
4. Decrease suction line pressure drop - i.e. bigger size pipes, hoses, fewer bends, fewer fittings, no right angle fittings
5. Exchange heat between suction line and liquid line - i.e. add an IHX
6. Decrease Oil amount

Lots of other ways, but they are hard to implement or tricky.

In general, I would say trying to tweak more performance from an existing vehicle, it's easiest and most common to do #3 first, but the 5 ton unit running off generators can't be beat

Vent temperatures can also be a misleading way to judge a system's performance. There are many different preferences for how AC works. Some people like icy cold vents, some people just hate that. Some people want a jet of air hitting them, some people want to never feel any airflow. There are some really great AC systems that get you comfortable quickly, and without icy vent temps and air jets.

[bohica2xo]

The short answer to the OP's question is it simply will not cool when overwhelmed.

As DetroitAC knows very well, the big issue today is marketing and the bean counters making engineering decisions. They wind up building 75% cars. Good for 75% of the country. Great in coastal California. Useless in Laughlin NV. It was not always this way.

I pulled out the original window sticker for my 1970 Monte Carlo. The base price for that car was $3208 The Air Conditioning option was $376.00 Over 10% of the cost of the car for that option. Given the price point, the GM engineers made damn sure the system was up to the task. A6 compressor, TXV and POA valves. Big heat exchangers. Bigger engine fan.

In today's dollars that would have been a $22,463 car with an option cost of $2632.00 But air conditioning became "standard". And the bean counters took over. We wound up with an orifice tube and a clutch clacking on and off. CAFE became important, so every component was tinkered with to arrive at some magical mileage number. And we got 75% systems.

We just took delivery on another 75% car a couple of months ago. A 2021 Escape Hybrid. A CAFE beater. The little dash readout shows impressive economy numbers when the local climate mimics San Diego. The electric compressor runs at full speed regardless of engine speed, and the HVAC system is actually quite impressive - up to about 95f ambient in the sun. At 100f in the sun it struggles if heat soaked. It it has been parked for 3 hours in direct sun at 110f - the system is completely overwhelmed. It can NOT reduce cabin temperature below 100f even after driving for 45 minutes. If you park it in the shade and idle for half an hour the cabin temp drops, but driving in the sun again it heats back up.

The best part in the heat is the magic battery can't take it either. You lose the added torque on acceleration, regenerative braking vanishes. The 2.5L engine runs constantly, and the dashboard says you are getting 7 miles per gallon in traffic. But the whole system was great on paper, and in NYC, San Diego and San Francisco. But in PHX or ABQ? Forget it.

I now know WHY all of the Tesla's, Prius, etc vanish from the roadways around here above 110f. See them all night long, but at noon? Never. The electric compressor could have been made larger, the heat exchanger and fans too. They are all controlled by the computer, would only need to run wide open part of their life. But that would cost more. And when you make throw away cars any extra cost is forbidden.

[Tim]

The short answer to the OP's question is it simply will not cool when overwhelmed.

As DetroitAC knows very well, the big issue today is marketing and the bean counters making engineering decisions. They wind up building 75% cars. Good for 75% of the country. Great in coastal California. Useless in Laughlin NV. It was not always this way.

I pulled out the original window sticker for my 1970 Monte Carlo. The base price for that car was $3208 The Air Conditioning option was $376.00 Over 10% of the cost of the car for that option. Given the price point, the GM engineers made damn sure the system was up to the task. A6 compressor, TXV and POA valves. Big heat exchangers. Bigger engine fan.

In today's dollars that would have been a $22,463 car with an option cost of $2632.00 But air conditioning became "standard". And the bean counters took over. We wound up with an orifice tube and a clutch clacking on and off. CAFE became important, so every component was tinkered with to arrive at some magical mileage number. And we got 75% systems.

We just took delivery on another 75% car a couple of months ago. A 2021 Escape Hybrid. A CAFE beater. The little dash readout shows impressive economy numbers when the local climate mimics San Diego. The electric compressor runs at full speed regardless of engine speed, and the HVAC system is actually quite impressive - up to about 95f ambient in the sun. At 100f in the sun it struggles if heat soaked. It it has been parked for 3 hours in direct sun at 110f - the system is completely overwhelmed. It can NOT reduce cabin temperature below 100f even after driving for 45 minutes. If you park it in the shade and idle for half an hour the cabin temp drops, but driving in the sun again it heats back up.

The best part in the heat is the magic battery can't take it either. You lose the added torque on acceleration, regenerative braking vanishes. The 2.5L engine runs constantly, and the dashboard says you are getting 7 miles per gallon in traffic. But the whole system was great on paper, and in NYC, San Diego and San Francisco. But in PHX or ABQ? Forget it.

I now know WHY all of the Tesla's, Prius, etc vanish from the roadways around here above 110f. See them all night long, but at noon? Never. The electric compressor could have been made larger, the heat exchanger and fans too. They are all controlled by the computer, would only need to run wide open part of their life. But that would cost more. And when you make throw away cars any extra cost is forbidden.

and import TXV valves.

[bohica2xo]

Dang Tim, I wish I had thought of that! I could just swap the OEM expansion valve for some magic Denso part and pick up 30% more capacity! Wow!

[cyfi66]

I appreciate all the explanations. I am curious about the cross charged TXV and skimmed through the patents you mentioned but they are extremely verbose and hard to read.

For discussions sake the vehicle in question is a 1999 Lexus LS400. The AC sllooowwwly cools down when the vehicle is heat soaked (160degree cabin temperature). Most of my driving includes parking in the sun and driving for <15 minutes. This means I am always hot. It drops down to 55 degrees rather quickly but from 55 degrees down to 40 degrees takes a long time (10- 15 minutes).

Compare this to a friends 2017 Ford Edge. That sucker on full blast is blowing 40 degree air within 5 minutes of driving. And this is under the exact same starting and driving conditions (160 degree interior). Visually the compressor and condenser appear to be approximately the same size. So what gives? How can one work so much better?

I care less about the cabin getting cold and more about the air hitting my face being cold. Obviously though, colder air out the vents will cool the cabin faster.

In the Lexus, I dont believe it is an evaporator core/size problem. This is because the vent temperature correlates very close to the suction line temperature. If the suction line is warm so is the vent temp, if the suction line is cold so is the vent temp. This tells me all I need to do is get the entire evap cold and the air will be cold. If the evap was too small I would expect the evap is cold but not able to transfer the heat effectively enough to make the air cold. I can also douse the condenser with water and it doesn't make the vent temperature come down fast. Additionally, if I run the car in low gear at high RPM (cruise at 3K RPM) it doesn't make any noticeable difference in cool down time.

I also have a Nissan 370Z that gives crisp cold air within minutes of turning the AC on even after heat soaked - this is with the windows OPEN so it has nothing to do with cabin volume. All black interior. Doesnt matter if on fresh or or recirc, it gets cold FAST. The Lexus just doesn't, it takes 10, 15+ minutes before you get crisp cold air. The Lexus is charged with precisely 24oz of refrigerant per the label.

[cyfi66]

The short answer to the OP's question is it simply will not cool when overwhelmed.

So I understand that, but more precisely my question is what do pressures and temperatures do? Does superheat go up? Does superheat remain correct, but low side pressure goes up (warmer evap)? In other words how can you look at pressures and temperatures and say "this system is overloaded" vs "this system has something wrong with it"?

[Tim]

Simply answer, high high side pressure will limit the ability of refrigerant to turn from a gas to a liquid.

I'll let the scientist get into a more detailed point of view.

[bohica2xo]

With the Lexus part of the blame goes to the NVH engineers. Can't have all of that blower noise in a luxury vehicle. God forbid you ever hear a refrigerant gurgle...

Overwhelmed system will keep pumping to the HPCO range. So let's say you have 375 psi liquid headed for the expansion device. That would mean the liquid refrigerant was at about 175f when you boil it off in the evaporator...

[cyfi66]

So if I have solid liquid at the exit of the condenser (sight glass) and my high side pressure is below 250PSI does that mean I have not overloaded the system, and that my higher vent temps are not a result of exceeding the system capacity?

Lets say I dont give AF about gurgling, hissing or any other noise, what can I change to make the system work better for me? I am willing to do custom fab to fit different TXV's, compressors ETC. I am happy with the blower volume I just need it to get colder faster.

[DetroitAC]

I wouldn't worry too much about cross charged TXVs, they are not going to fix your 99 Lexus performance.

Seems like the Lexus has a 10PA20 compressor (200cc/rev swashplate/piston type), that's good news, but what is the pulley drive ratio? and the Idle and engine running speed of your Lexus?

The volume of suction gas your Lexus is moving absolutely matters. If the Lexus has a 200cc compressor, 1.5 drive ratio, idles at 800 RPM, 1800 RPM at 30 mph, great performance should be possible (with everything else in good order). If it has a 1.0 drive ratio, idles at 400 RPM, 1100 RPM at 30 mph, then suffering will normal. I'm just making up numbers, trying to make a point.

Compressor suction gas displacement in cc/minute will correlate directly to A/C performance. The car with the biggest number performs the best.

It's almost like the glory days in the 60's and 70's with muscle cars. The car with the most engine displacement probably wins the most street races. You show up with your 302 Ford and pull up next to a 429 Ford, guess who's going to win.

Edit: by the way "solid liquid" is exactly how I say it too, and then non refrigeration people are like "Whaaaaat?"