scope103

Doh! Of course!

But (blame it on my lack of AC savvy) it is still is hard for me to understand. The PT chart gives the boiling/condensing temperature at various pressures for a given refrigerant. With 206psi, the refrigerant will have turned to liquid at any temperature below 140°F. Since ambient doesn't get much over 120°F too often where I live, that sounds like too much pressure (meaning: too much refrigerant). A reasonable temperature on exit from the condensor would be only a few degrees above ambient, which isn't likely to hit 140° too often.

The same thinking ought to apply to the low side; at a pressure as low as 28psi, the refrigerant would have turned to gas (using up the "cold") long before it got to the exit of the evaporator.

And for a little empirical discussion, my new R134a system is icy cold with a high side pressure of 130psi (R134a requires about 6% higher pressure at a given temperature).

Someone out there may actually know something about refrigeration, and can set me straight.

cursmello

Checked it again, hi 200, low 20 -22. Temp 65 after running for a few minutes. I do need a new fan motor as the windings are going out on the existing one, so i know its not pushing as much cfm as it should which would effect the heat transfer at the evap coil. Its also a 87 four runner and ambient was 85 with high humidity in the huge passenger area.Compressor was rebuilt two yrs ago. Total system rebuild including new txv. Had freeze 12 in it before. Not happy w freeze 12 so i leak checked and vac to 430 microns before filling w r12. Didnt replace dryer but w that low of a vac and no leaks its prolly as good as its gonna get until i put a new fan motor in.

rustypigeon

The pressure drop occurs right after the expansion valve, which is BEFORE the evaporator. The refrigerant is a low pressure cold liquid going into the evaporator and exits as a low pressure vapor. This pressure drop right before the evaporator is what causes the sudden pressure and temperature change. So at 28psi, the refrigerant is colder than 30F as it enters the evaporator, and warmer than 30F as it exits the evaporator.


This is exactly what is happening. The refrigerant exits the compressor as a high pressure high temperature vapor. The temperature is high not only from the heat removed from the cabin, but also because it was compressed. The condenser then removes the heat and it exits as a high pressure liquid. So at 206psi, the refrigerant enters the condenser as a vapor at a temperature higher than 140F. This heat is removed in the condenser, and exits as a liquid at a temperature lower than 140F. It remains a high pressure liquid until it reaches the expansion valve.

Heat flows to the cooler object. Heat from the cabin is transferred to the evaporator since the evaporator is colder. Heat from the condenser is transferred to the outside air since the outside air is colder.

RJR

Scope, I think where you might have gotten slightly off track is assuming that the P/T chart gives a 1:1 relation between pressure and temperature for the fluid. As rustypigeon correctly points out, that's not the case. The chart gives the boundaries between the liquid and vapor phases. So, at 206psi, the fluid will be liquid at any temperature below 140deg, and vapor at any temperature above 140deg. Same at 28psi, it will be liquid at all temps below 30F, and vapor at all temps above 30F.

So, you can't predict the temperature or the liquid/vapor state based on pressure alone. You have to know both pressure and temp to know where you are on the chart.

Pressure changes happen in the compressor and expansion valve. Temperature changes (and vapor/liquid phase changes) happen in the condenser and the evaporator. This is the same thing rustypigeon pointed out, only stated a little differently.

scope103

Well, not the first time, but I didn't make myself very clear.

You COULD use just air in a refrigeration circuit. It gets hot when you compress it, it cools off in the "condensor" (it wouldn't condense at achievable pressures, but it would cool), it gets cooler when expanded through the TXV into the "evaporator." But it would work very poorly.

So you use refrigerant, which moves heat around due to phase change, not just pressure. The compressor increases the pressure and temperature, and in the condensor (at that pressure) it cools off and condenses, so that the output from the condensor is all liquid. If some gas is still mixed in with the liquid (you can see it in the sight glass), you don't have enough refrigerant to all condense at that pressure and the cooling the condensor can achieve. So, ideally, the temperature (for a given pressure) at the output of the condensor (basically, the input to the expansion valve) is just under the boiling point. Any higher means you have gas in the liquid line, any lower means you are wasting cooling (or have too much refrigerant.) So if you have 206psi on the high side, any exit temperature below 140° is "wasted" cooling in the condensor (on a 75° degree day, my condensor exit temp was 103°). I suppose you might want to plan on 206psi if you were expecting ambient temps of 115°. The record in North America is supposedly 134°, so that might not be crazy.)

Inside the evaporator, it's the reverse. As it goes through the expansion valve, the pressure drops, and the the liquid becomes very cold. But at that pressure it gives up it's cold and evaporates. Optimally, it will be evaporating through the length of the evaporator, so that you get ALL GAS exiting the evaporator. Now, you want a temperature at the pressure set by the expansion valve) that is just above the boiling point for that pressure. If it's higher, the gas evaporated well before the length of the evaporator tubing, because you don't have enough refrigerant.

If the expansion valve allowed the refrigerant to be colder than 30° in any significant part of the evaporator (as suggested by RustyPigeon), ice would form, blocking the air flow around the evaporator. If the temperature at the exit was much higher than 30° for that pressure, that would mean all the refrigerant evaporated long before it went through the whole evaporator tube, and there would be much less cooling.

In my installation, the air coming out of the evaporator (inside the cab) was 46°, and refrigerant coming out of the evaporator was 50°F.

As I said at the very beginning, this refrigeration stuff is treated by many as black magic, and I'm not very good at it. I recognize that the FSM (which I quote as much as anyone) gives those numbers, but based on what else I've read (and followed) those pressures are difficult for me to understand.

RJR

I think you have it correct, scope. The high side pressure, as you say, is set by the maximum expected ambient temperature, to make sure the refrigerant liquifies fully in the condenser. Keep in mind that underhood temps can be pretty high, although the condenser is in front of the radiator so should pretty much experience ambient temps. But, the only downside of a bit too high a pressure is a few extra fractions of a HP to turn the compressor. The downside of too little pressure is poor cooling on a blazing hot day, not the direction most consumers want to go. Best to error in the other direction. (If the pressure gets high enough so that the refrigerant condenses inside the compressor, that's another issue. That will cause major hardware damage. But I think that's pretty unlikely.)

As you correctly point out, on the evaporator side it's a question of keeping the coils from accumulating ice. When that happens, again cooling goes pretty much to zero because no air can flow through the evaporator.

Mid-80's GM cars had a low-side pressure switch which would cycle the compressor off when the pressure got too low, indicating potential freezing problems. As an experiment I adjusted that switch downward, and sure enough in the middle of eastern Nebraska on a hot muggy morning the evaporator stopped blowing cold air due to ice. My wife and kids weren't too pleased with my scientific research, but fortunately I was able to readjust the regulator on the side of the road and regain cooling and domestic tranquility. (I personally thought it was a highly successful experiment...)

rustypigeon

In a properly operating system, air movement over the evaporator provides the heat necessary to prevent ice buildup.

The evaporator is designed to be long enough to ensure that all liquid is vaporized. You don't want liquid refrigerant going through the compressor.

scope103

If the temperature of the coils is well below 32°F, the moisture in the air will condense, and then freeze. How else could you make ice cubes? If the air were so hot (and moving so fast) that it could "heat up" the coils and refrigerant to above 32°, it would also raise the pressure in the low-side to above 30 psi (R12). If the low side pressure is below 30 psi, then the refrigerant is boiling at a lower temperature, and freezing the moisture in the air.

You are correct: a properly operating system will not ice up (that's what "properly" means). If my math is correct, that means the refrigerant is converting from liquid to gas at a temperature above 32°F, which the PT chart tells us is 30 psi or higher. THAT is why I can't understand how a low-side pressure could be as low as 28psi, without icing.

You are also correct (IMO) about all the refrigerant evaporating before it leaves the evaporator. But a properly designed system doesn't have "too much" evaporator (more relevant: too little refrigerant), where all the liquid turns to gas long before the end of the evaporator, allowing the refrigerant (and the air flow) to end up MUCH warmer than the boiling point (for the measured temperature).

I can't argue with the numbers in the FSM, but they are very difficult for me to understand.

rustypigeon

The refrigerant is below freezing, but the external surface of the evaporator is not. It has warm air blowing over it. This constant warm air over the coils keeps them from freezing. When the thermistor senses the air is too cold going through the evaporator, the compressor shuts off. Heat transfers from hot to cold. Think of it as heat being sucked out of the air, not cold being pushed out of the evaporator. The air is only cold because the heat was removed from it.