For the most part, refrigeration is simple.
First of all, the gas laws work. If you have a closed system, with no artificial drivers, such as a running compressor, and there is some liquid in the system, then the pressure at all points in the system is directly controlled by the temperature.
In other words, suppose you have a tank of a gas that is about half liquid and half gas. The pressure in that tank depends on the average temp of the liquid/gas in the tank and its boiling point.
If you increase the temp, a small amount of the liquid will boil off and increase the pressure.
If you decrease the temp, a small amount of the gas will condense and lower the pressure.
Note: The temp is the
average temp of the system. So, when you were reading the pressure before startup, probably somewhere in the system, it was hotter than the outside ambient temp. So average was higher.
After you shut it off, a lot of the system was still cold, so the average was way lower than the outside temp. Thus the lower pressures.
This will always be true unless there is no liquid in the system to maintain the pressure, or if the system were completely full of liquid.
In which case the compressor would instantly lock up when you turned it on.
Couple of more things:
Make sure the frost on the output side of the evaporator never reaches the compressor. In general, frost means there is liquid Freon present. Compressors cannot compress liquids, and if any gets into the compressor, it will hydrolock and be destroyed.
Also, there are several types of systems. No idea of which system you have.
In many cases, especially with expansion valves, the compressor runs all the time. The expansion valve throttles the liquid, letting more or less through according to the temp of the evaporator.
When the valve starts restricting the flow, the excess liquid from the compressor is stored in the accumulator tank.
I see a couple of things about your setup that just look.......wrong. In theory, the expansion valve should not let much frost past its bulb. In fact, in an AC system, I would not expect to see frost at the bulb. The evaporator should not really get cold enough to frost up much at all.
With really high humidity, and low air flow, maybe a small amount, but not enough to restrict the air flow.
This is an air conditioner, not an ice machine.
With the ice machines I work on, I expect to see frost at the bulb. I want the evaporator to freeze so it can make ice.
But, with an AC, the coils should be wet, not frosty. Any frost will cause air blockage, which will cause more frost, and so on.
Having said that, I have seen the return lines frost on truck ACs before, but not much, and not on hot days with full fan.
The amount in your picture is just too much. The expansion valve is not doing its job.
Either because its got problems, its the wrong one for the application, or its sensing wrong.
The bulb placement can be critical. Its the only way the valve has to sense what is going on.
The bulb senses the average temp it sees. If it were uninsulated, it would be reading the line temp as well as the outside air temp.
It
must be reading the line temp only. Personally I don't like it being outside the air plenum.
The first thing I would do, unless you are going to replace it, in which case wait until you get the new one, is remove all the insulation. Unclamp the bulb from the line. Clean the line and the bulb really well. There must not be anything between them that could cause the bulb to not sense the line temp as fast as possible.
Then, if you can, put the bulb on the bottom of the line.
As the boiling freon comes out, it will be running along the bottom of the line, and you want the bulb to see it as soon as it shows up.
Then, clamp the bulb tight to the line. Almost tight enough to crimp the line or bulb. They make copper strap clamps for that purpose, but if the lines are aluminum, you probably don't want to use copper. Anything you do use needs to be tight.
Then, if you go back with the black cork insulation, mold it carefully into every seam. Not only do you not want any outside temp to affect the bulb, but you want no air in there at all.
Air can let moisture form, and liquid water will absorb and release heat at different rates than the Freon.
After you get it sealed, add more insulation around the outside. Maybe some of the black foam. You want to make sure that the bulb never sees the outside air.
Then, go back and check air flow. Remember, the amount out the vents means nothing. Its the amount going through the evaporator fins that is important.
If the air is mostly bypassing the evaporator, you will get good cool air flow out the vents, but frost on the evaporator.
Also, a good rule of thumb, is 20 degrees temp drop. In other words, measure the air temp going into the evaporator out of the cab, and the air out the vents should be about 20 degrees cooler.
80 degrees in , 60 degrees out is good.
As the air inside cools, so will the air coming out.
More drop than that, you run the risk of frost, and high inside humidity.
For instance, if you put in 80 degree air, and got 40 degree out, you would cool off things really fast, but you would probably get fog out the vents.
Ideally, you want the air to make several passes through the evaporator losing moisture every time before things get to the cool you want.
Also remember, your system may be set up for compressor cycling, but with an expansion valve, it may not. Lots of systems never cycle the compressor except for extreme conditions.
Just depends on how your system is designed.
Here is a chart showing 134A pressures vs temperatures in a static system with liquid in the system.
View attachment 391396
